Field Instruments Questions and Answers<\/h2>\n\n\n\n![\"Field](\"https:\/\/instrumentationtools.com\/wp-content\/uploads\/2016\/01\/instrumentationtools.com_control-room-and-field-instruments-questions-answers.jpg\")
<\/figure>\n<\/div>\n\n\nControl Room Questions and Answers<\/span><\/h2>\n\n\n\nWhat is DCS & PLC?<\/span><\/h3>\n\n\n\nDistributed control system(DCS)<\/strong><\/a> and Programmable logic controller<\/strong><\/a>. These are the control systems which handles fields I\/Os.<\/p>\n\n\n\nBasic difference between DCS & PLC is<\/strong><\/p>\n\n\n\n1) DCS handles more nos of I\/Os rather than PLC.<\/p>\n\n\n\n
2) PLC is faster system than DCS. 3) DCS can handles handsome quantity of I\/Os so that can be used for total plant automation. Where as PLC has own limitations so it generally used for small but for important(Safety point of view) units, like boiler automations, Make-up compressor automations Etc.<\/p>\n\n\n\n
4) In the above mentioned case the these PLC\u2019s can be get connected with the DCS with the help of soft link. Generally this is used to make alert to both the operator.<\/p>\n\n\n\n
5) As I heard the PLC<\/a> used to handle the DI\/DO signals so it can take fast actions. Some of the time it is used to handle few nos of AI\/AO.<\/p>\n\n\n\n6) DCS & PLC\u2019s speed depends on the scan rate of I\/Os.<\/p>\n\n\n\n
7) For both the system Marshalling panels, Consoles and other faculties of Ethernet Etc can be used according to the need.<\/p>\n\n\n\n
8) According to the Cause and effects diagrams the System programmer assigns the control action block into the system, we can call them as memory assigning.<\/p>\n\n\n\n
While making Datasheets which things are to be considered?<\/span><\/h3>\n\n\n\nBasically it depends on the instrument item for which you are preparing the datasheet.<\/p>\n\n\n\n
As an Example. Temperature Element. We have collect following information to prepare D\/s.<\/p>\n\n\n\n
What is potential free contact? What is the significance and application of this contact?<\/span><\/h3>\n\n\n\nContacts having not potential. E.g. Relay contacts\/ field switches contacts. They are used in logic circuits.<\/p>\n\n\n\n
A potential free contact is usually wired into an electrical circuit.<\/p>\n\n\n\n
However it must be ensured that the contact ratings are suitable for the service in which it is used.<\/p>\n\n\n\n
In split range control, whether the signal is splitted through I\/P convertor or the convertor itself?<\/span><\/h3>\n\n\n\nThis can be typically achieved by two ways:<\/p>\n\n\n\n
By connecting o\/p of one I\/P converter<\/a> to two positioners adjusted suitably for split range operation of control valves.<\/p>\n\n\n\nTaking two Analog Outputs (AO) from DCS. Split range to be defined in DCS. Both I\/P converters and positioners to be calibrated with input as 4to20 ma dc and 3to15 psi respectively.<\/p>\n\n\n\n
When do we use SOV of rating 110vac and 110vdc?<\/span><\/h3>\n\n\n\nPrimarily depends upon the availability of reliable power supply source.<\/p>\n\n\n\n
How is cold junction compensation in thermocouple carried?<\/span><\/h3>\n\n\n\nThis is typically performed in modern programmable instruments by means of measuring actual reference junction temperature using a temperature sensor mounted close to the ref. junction and compensating for the same using appropriate look-up table stored within the instrument\u2019s memory.<\/p>\n\n\n\n
In which applications do we use 4 wire RTD?<\/span><\/h3>\n\n\n\nThree wire is a better alternative. Primary objective of 3-wire and 4-wire arrangements is to eliminate effects of lead resistance on temp. measurement.<\/p>\n\n\n\n
What is the sensor used in coriollis mass flow meter to measure density?<\/span><\/h3>\n\n\n\nDensity is measured here by measuring the resonant frequency of a vibrating U-tube.<\/p>\n\n\n\n
What if thermocouple wire is opened in the field? What signal goes to DCS?<\/strong><\/span><\/h3>\n\n\n\nIn most modern instruments the signal may be programmed to go to either maximum or minimum depending upon end user\u2019s requirement.<\/p>\n\n\n\n
if the power supply connections to a two-wire transmitter get interchanged? What signal will go to DCS?<\/strong><\/span><\/h3>\n\n\n\nUsually there is a blocking diode to protect the transmitter<\/a> against supply reversal and almost zero current signal should be transmitted.<\/p>\n\n\n\nWhat are the possible reasons for the failure of barrier? !?!<\/strong><\/span><\/h3>\n\n\n\nFuse<\/a> blowing sometimes. Power circuits are most likely to fail.<\/p>\n\n\n\nHow can we say that the given RTD or Thermocouple is correct?<\/span><\/h3>\n\n\n\nWe can only measure sensor output (resistance \/ maillots ) accurately and look-up corresponding temperature in reference tables.<\/p>\n\n\n\n
The accuracy depends upon quality \/ condition of the sensor. Degraded sensors may not give accurate readings and must be replaced.<\/p>\n\n\n\n
To test a sensor, the sensor response may be tested using a high quality temperature calibrator and compared with reference tables.<\/p>\n\n\n\n
In some cases we have to select the cam position in a control valve for different application? How do we select that?<\/span><\/h3>\n\n\n\nRefer to instruction manual for the positioner<\/a> \/ control valve.<\/p>\n\n\n\nThe cams are often marked with limited amount of information, which may help an experienced person.<\/p>\n\n\n\n
What is the difference between a protocol and a field bus?<\/span><\/h3>\n\n\n\nA protocol defines a standard method for communications.<\/p>\n\n\n\n
A fieldbus<\/a> is a multidropping arrangement where multiple instruments communicate with special interface hardware using the same pair of wires and in most of the cases draw power from the same pair of wires.<\/p>\n\n\n\nWhat happens if transmitter wires get shorted?<\/span><\/h3>\n\n\n\nThe barrier<\/a> if installed limits electrical energy flowing into hazardous area. If there is no barrier, typically a fuse in the power distribution system will blow.<\/p>\n\n\n\nWhat will happen if thermocouple wires get shorted?<\/span><\/h3>\n\n\n\nA cold junction compensated instrument will typically indicate temperature of the location where the T\/C wires are shorted.<\/p>\n\n\n\n
Why do we require loop-terminating resistor in any digital communication loop?<\/span><\/h3>\n\n\n\nA minimum loop resistance<\/a> is required so that modulated current signal produces a modulated voltage signal, which may be detected by the receiving equipment.<\/p>\n\n\n\nWhat is adapter flange?<\/span><\/h3>\n\n\n\nIt is a transmitter part for allowing process connection to pipe\/tube.<\/p>\n\n\n\n
How to calculate the safe distance between cables to avoid electromagnetic interference of each other ?<\/span><\/h3>\n\n\n\nThe design engineers \/ equipment manufacturers follow\/publish certain guidelines w.r.t. different types of cables and the voltages\/currents and types of signals carried by them.<\/p>\n\n\n\n
How Control loop should be tuned in process loop?<\/span><\/h3>\n\n\n\nYou may use Ziegler-Nichol\u2019s<\/a> method ( open loop \/ closed loop ) or special tuning software tools.<\/p>\n\n\n\nWhat is the significance of single ended & differential ended input for PLC? Application wise comparison of these two types of inputs?<\/span><\/h3>\n\n\n\nDifferential inputs provide better common mode rejection and signal-to-noise ratio.<\/p>\n\n\n\n
What is Ground Loop? Preventive steps to avoid ground loop?<\/span><\/h3>\n\n\n\nWhen ground wiring is not done properly, grounding<\/a> of various points is not effective and potential differences exist between them resulting in currents flowing between them.<\/p>\n\n\n\nThis leads to measurement errors and is not desirable. It can be eliminated by proper ground wiring.<\/p>\n\n\n\n
In a globe type control valve, what is the importance of flow direction (top to bottom or bottom to top)?<\/span><\/h3>\n\n\n\nControl valves must be installed as per direction marking provided by the manufacturers or instruction manuals. Though people tend to generalize, this is often misleading.<\/p>\n\n\n\n
What is ATEX directives \/ FM Approval \/ CSA approved \/ CE certified ? What is the importance of individual certification? Are all these certification required for each instruments? Which certification do we prefer?<\/span><\/h3>\n\n\n\nATEX\/FM\/CSA certifications generally refer to certification for suitability of instruments for use in hazardous area when installed in accordance with recommended guidelines.<\/p>\n\n\n\n
Any certification, which is locally acceptable as per statutory requirements, may be used. We typically accept American\/European\/Indian certifications\/approvals in India.<\/p>\n\n\n\n
Advantages of tachometer as speed measuring device compared to inductive type proximity switches?<\/span><\/h3>\n\n\n\nSome tachometers provide analog output with almost instantaneous response time.<\/p>\n\n\n\n
They are highly suitable for speed control in some applications.<\/p>\n\n\n\n
Application wise advantages of Inductive type proximity switches over capacitive type switches?<\/span><\/h3>\n\n\n\nInductive proximity switches<\/a> are better suited for detection of conducting metal objects and are easily tested for proper operation.<\/p>\n\n\n\nCapacitive switches are typically used for detecting non-conductive materials.<\/p>\n\n\n\n
What is cable tray? Its type? Its size? Its support?<\/strong><\/span><\/h3>\n\n\n\nCable tray<\/a> is nothing but the way or media through which we lay the field cables in plant.<\/p>\n\n\n\nThere are two basic types<\/p>\n\n\n\n
\n- Ladder type(made in Rungs type construction)<\/li>\n\n\n\n
- Perforated Type(Solid sheath consist of Holes for ventilation).<\/li>\n<\/ol>\n\n\n\n
Basically discussing about tray support than we could say it depends on the site conditions. Only care has to be taken considering adequate space for laying cable, considering their bends.Etc.<\/p>\n\n\n\n
How to decide cable tray size?<\/span><\/h3>\n\n\n\nAccording to the no. of cable occupancy in the cable tray<\/a> and available tray size we have to choose it. they are available in foll. types 80,150,300,450,600 & 900.<\/p>\n\n\n\nWhat is meant by instrument location & JB location?<\/span><\/h3>\n\n\n\nThis consist of Instrument location<\/a> considering the piping drawing given from Piping dept. We identifies the locations of the instrument in the equipment layouts and put the bubbles and elevation and JB nos for the location.<\/p>\n\n\n\nSame way depending upon the accessibility we decide the location of JB and marking of it into the instrument location plan is JB location. There is no need to make a different drawing for this.<\/p>\n\n\n\n
While locating Instrument & JB which things we have to consider?<\/span><\/h3>\n\n\n\nTransmitter :<\/strong> Tube routing, maintenance area, man approach, Valve : Hand wheel operations, Maintenance area, Etc.<\/p>\n\n\n\nLoop power indicators :<\/strong> Man approach Illumination from Electrical if instrument is not going to provide.<\/p>\n\n\n\nWhat is the use for cable entry in control room? (sleeves & MCT)<\/span><\/h3>\n\n\n\nIn the process plant Control room built considering the non-hazardous area.<\/p>\n\n\n\n
So in case if fire\/Explosion takes place in the plant than that has to be restricted from entering into the control room. So MCT (Multiple cable transient)<\/a> blocks are used.<\/p>\n\n\n\nThey are designed to sustain the fire for a fixed time duration. That block hold the cables which are entering into the CCR.<\/p>\n\n\n\n
What are Analog Input\/output & Digital Input\/output?<\/span><\/h3>\n\n\n\n\n- 4-20 mA<\/a> signals from instrument (transmitters) are analog input to control system.<\/li>\n\n\n\n
- 4-20 mA signals to instrument (I\/p & electro pneumatic positioner) are analog output from control system.<\/li>\n<\/ul>\n\n\n\n
Volt free (24V) contact (NO\/NC) by instruments (all type of switches, ex. Limit switch, press,temp,flow,level switch) are digital input to control system.<\/span><\/h3>\n\n\n\nAll powered signals (24V,48V,110V\u2026.) from system to instruments (SOV) are Digital outputs.<\/p>\n\n\n\n
Types of instrument cables?<\/span><\/h3>\n\n\n\n\n- IS cables &<\/li>\n\n\n\n
- NIS cables<\/li>\n<\/ul>\n\n\n\n
IS \u2013 Intrinsic safety & NIS \u2013 Non Intrinsic safety<\/p>\n\n\n\n
Which cable to use, depends upon hazards condition.<\/p>\n\n\n\n
Instrument JB\u2019s?<\/span><\/h3>\n\n\n\n\n- Instrument JB\u2019s depend upon hazards area<\/a> classification.<\/li>\n\n\n\n
- JB\u2019s also can be IS or NIS<\/li>\n\n\n\n
- For IS signal IS JB\u2019s used for NIS signal NIS JB\u2019s used<\/li>\n\n\n\n
- For analog I\/p & o\/p signal we can use same JB. But for Digital I\/p & o\/p we have to use separate JB\u2019s. Because digital outputs are powered signal, by wrong connection there may be chances to damage the card. For DCS I\/p & o\/p and PLC I\/p & o\/p we used separate JB\u2019s.<\/li>\n<\/ul>\n\n\n\n
What is open loop & close loop?<\/span><\/h3>\n\n\n\nOPEN LOOP :<\/strong> This is nothing but to sense the process signals from the field and to send it to the control room for operator observation.<\/p>\n\n\n\nCLOSE LOOP :<\/strong> This is measuring the process signals for operator\u2019s action. Means Transmitter sense the process and send it to the control room. Where the operators takes action i.e. control action and that given to the final control element as per the process requirement.<\/p>\n\n\n\nWhat is a PID controller? <\/strong><\/span><\/h3>\n\n\n\nPID stands for Proportional-Integral-Derivative. This is a type of feedback controller whose output, a control variable (CV), is generally based on the error (e) between some user-defined set point (SP) and some measured process variable (PV). Each element of the PID controller<\/a> refers to a particular action taken on the error:<\/p>\n\n\n\n\n- Proportional :<\/strong> error multiplied by a gain, Kp<\/sub>. This is an adjustable amplifier. In many systems Kp<\/sub> is responsible for process stability: too low and the PV can drift away; too high and the PV can oscillate.<\/li>\n\n\n\n
- Integral: the integral of error multiplied by a gain, Ki<\/sub>. In many systems Ki<\/sub> is responsible for driving error to zero, but to set Ki<\/sub> too high is to invite oscillation or instability or integrator windup or actuator saturation.<\/li>\n\n\n\n
- Derivative :<\/strong> the rate of change of error multiplied by a gain, Kd<\/sub>. In many systems Kd<\/sub> is responsible for system response: too high and the PV will oscillate; too low and the PV will respond sluggishly. The designer should also note that derivative action amplifies any noise in the error signal.<\/li>\n<\/ul>\n\n\n\n
Tuning of a PID involves the adjustment of Kp<\/sub>, Ki<\/sub>, and Kd<\/sub> to achieve some user-defined “optimal” character of system response.<\/p>\n\n\n\nAlarms Questions and Answers<\/h2>\n\n\n\nOn limit alarms, what is meant by “failsafe”?<\/span><\/h3>\n\n\n\nWhen a limit alarm is configured for “failsafe”<\/a> operation, the contact logic is reversed. This means that the coil, which controls the contact(s), will de-energize when the unit is in alarm, and energize when the unit is not in alarm. Thus, if the limit alarm looses power, the contacts will revert to the alarm condition.<\/p>\n\n\n\nThis functionality is desirable if the user wants an alarm whenever the measured process is beyond acceptable values, or whenever power is lost to the limit alarm.<\/p>\n\n\n\n
On limit alarms, what is meant by “deadband”? <\/span><\/h3>\n\n\n\nDeadband is the difference between the alarm “on” and alarm “off” setpoints.<\/p>\n\n\n\n
For example, if a limit alarm<\/a> is calibrated to trip at a 14.7 mA input, and un-trip at 13.5 mA, the deadband, (or differential) is 1.2 mA.<\/p>\n\n\n\nWhat input range is used for the dc input limit alarms to accept a 4\/20 mA signal? <\/span><\/h3>\n\n\n\nAll the input ranges for the Action Instruments’ field configurable devices are zero based. One of the input ranges is 20 mA. This is the input range to use for all signals up to 20 mA.<\/p>\n\n\n\n
Accuracy is not affected. Having the input range begin at zero allows a limit alarm to be used for open circuit detection.<\/p>\n\n\n\n
Intrinsically Safe Systems<\/strong><\/span><\/h2>\n\n\n\nWhat is “Intrinsically Safe”?<\/span><\/strong><\/h3>\n\n\n\nA protection method employed in potentially explosive atmospheres. Certificate IS tools are designed to prevent the release of sufficient energy to cause ignition of flammable<\/a> material.<\/p>\n\n\n\nIS standards apply to all equipment that can create one or more of a range of defined potential explosion sources.<\/p>\n\n\n\n
\n- Electrical sparks<\/li>\n\n\n\n
- Electrical arcs<\/li>\n\n\n\n
- Flames<\/li>\n\n\n\n
- Hot surfaces<\/li>\n\n\n\n
- Static electricity<\/li>\n\n\n\n
- Electromagnetic radiation<\/li>\n\n\n\n
- Chemical reactions<\/li>\n\n\n\n
- Mechanical impact<\/li>\n\n\n\n
- Mechanical friction<\/li>\n\n\n\n
- Compression ignition<\/li>\n\n\n\n
- Acoustic energy<\/li>\n\n\n\n
- Ionizing radiation<\/li>\n<\/ul>\n\n\n\n
What Industries are Intrinsically Safe Products Designed for?<\/strong><\/span><\/h3>\n\n\n\n\n- Petrochemical<\/li>\n\n\n\n
- Oil platforms and refineries<\/li>\n\n\n\n
- Pharmaceutical<\/li>\n\n\n\n
- Pipelines<\/li>\n\n\n\n
- Any environment where explosive gases or vapors could be present<\/li>\n<\/ul>\n\n\n\n
The Three Key Elements of Combustion are:<\/strong><\/span><\/h3>\n\n\n\n\n- Inflammable material (gases, particles\/dust)<\/li>\n\n\n\n
- Oxygen\/air<\/li>\n\n\n\n
- Ignition source<\/li>\n<\/ul>\n\n\n\n
This combination is very common in chemical, petro-chemical, and pharmaceutical industries.<\/p>\n\n\n\n
Examples of the amount of inflammable material necessary for ignition below show how small of an amount it takes to present a danger to workers.<\/p>\n\n\n\n
What are the Regulations and Guidelines for IS?<\/strong><\/span><\/h3>\n\n\n\nATEX (Europe)<\/h4>\n\n\n\n
European Union’s 94\/9\/EC Directive, commonly called ATEX<\/a> (“Atmospheres Explosibles”) Europe’s primary regulation for protection systems and equipment intended for use in potentially explosive atmospheres. Indended to serve as total harmonization directive, laying down essential health and safety requirements, and replace existing divergent national and European legislation.<\/p>\n\n\n\nThis directive became mandatory on electical and electronic equipment for use in environments subject to explosion hazard sold in the EU on 1 July 2003.<\/p>\n\n\n\n
Derivatives of ATEX are being adopted across the world.<\/p>\n\n\n\n
NEC (United States)<\/h4>\n\n\n\n
National Electrical Code (NEC), is the basis for all electrical codes in the United States.<\/p>\n\n\n\n
Classifications and related product markings for hazardous areas are covered in NEC 500 and 505. Interpretations of NEC 500, a longstanding regulation are utilized throughout the world (outside Europe). NEC 505 is similar to ATEX.<\/p>\n\n\n\n
Field Instruments <\/strong><\/span>Questions and Answers<\/h2>\n\n\n\nWhat is the measurement principle of an vortex flowmeter?<\/strong><\/span><\/h3>\n\n\n\nWhen a shedder bar is placed in a fluid, Karman vortices are generated on the downstream side.<\/p>\n\n\n\n
The vortex frequency is proportional to the flow velocity, and corresponds to a specific range of Reynolds<\/a> numbers.<\/p>\n\n\n\nThe vortex frequency (f) is calculated by the formula; f=St*v\/d<\/strong><\/p>\n\n\n\nv: flow velocity, d: width of shedder bar, St: Strouhal number<\/p>\n\n\n\n
What are the advantages of a vortex flowmeter?<\/strong><\/span><\/h3>\n\n\n\n\n- Frequency output is proportional to flowrate.<\/li>\n\n\n\n
- No moving parts are required because of vibration created by Karman vortices.<\/li>\n\n\n\n
- Wide measurement range<\/li>\n\n\n\n
- Can measure gas, steam and liquid<\/li>\n\n\n\n
- Outputs volumetric flowrate without influence of density, temperature, or pressure<\/li>\n<\/ul>\n\n\n\n
What does “cold junction compensation” mean?<\/span><\/strong><\/h3>\n\n\n\nCold junction compensation allows accurate temperature measurement when using a thermocouple. What’s a thermocouple? Well, a thermocouple is created when wires made from two different types of metal are connected together, (such as Iron and Copper-Nickel). Thermocouples generate a small Voltage, which increases when the thermocouple junction gets hotter.<\/p>\n\n\n\n
When the thermocouple wires are connected to an instrument, two more thermocouple junctions<\/a> are created, because the terminals are made of a different material than the thermocouple wires. These “extra” junctions, (called cold junctions), create their own Voltage, which alters the Voltage generated by the actual thermocouple.<\/p>\n\n\n\nCold junction compensation negates the voltage created by these cold junctions, allowing only the Voltage created by the thermocouple to be sensed by the instrument.<\/p>\n\n\n\n
Why does an RTD need an excitation source?<\/strong> <\/span><\/h3>\n\n\n\nUsually an electrical reference or excitation source is required for sensors whose electrical properties are measured.<\/p>\n\n\n\n
A thermocouple produces a potential difference between the two dissimilar metals that varies with temperature and is repeatable. The measurement is therefore a voltage.<\/p>\n\n\n\n
The resistance of an RTD<\/a> changes with temperature, therefore, a constant DC current excitation source is used as a reference to produce a proportionally changing voltage.<\/p>\n\n\n\nThus a signal conditioner which measures an RTD input provides a current reference as excitation and measures the voltage produced.<\/p>\n\n\n\n
What is a thermocouple sensor?<\/strong><\/span><\/h3>\n\n\n\nA thermocouple is a sensor for measuring temperature. It consists of two dissimilar metals, joined together at one end.<\/p>\n\n\n\n
When the junction of the two metals is heated or cooled a voltage is produced that can be correlated back to the temperature. The thermocouple alloys are commonly available as wire.<\/p>\n\n\n\n
Explain different thermocouple types<\/span><\/h3>\n\n\n\nA thermocouple is available in different combinations of metals or calibrations. The four most common calibrations are J, K, T and E<\/p>\n\n\n\n
How do I choose a thermocouple?<\/span><\/h3>\n\n\n\nBecause a thermocouple<\/a> measures in wide temperature ranges and can be relatively rugged, thermocouples are very often used in industry.<\/p>\n\n\n\nThe following criteria are used in selecting a thermocouple:<\/p>\n\n\n\n
\n- Temperature range<\/li>\n\n\n\n
- Chemical resistance of the thermocouple or sheath material<\/li>\n\n\n\n
- Abrasion and vibration resistance<\/li>\n\n\n\n
- Installation requirements (may need to be compatible with existing equipment; existing holes may determine probe diameter)<\/li>\n<\/ul>\n\n\n\n
Explain Thermocouple Ranges and Limits of Error<\/span><\/h3>\n\n\n\n![\"Thermocouple](\"https:\/\/instrumentationtools.com\/wp-content\/uploads\/2016\/01\/instrumentationtools.com_thermocouple-ranges-and-limits-of-error.gif\")
<\/figure>\n<\/div>\n\n\nTransmitters <\/strong>Questions and Answers<\/h2>\n\n\n\nWhat is the maximum load I can connect to the current output of a 4-wire transmitter? <\/span><\/h3>\n\n\n\nThe maximum load that can be connected to a current output depends on the output compliance.<\/p>\n\n\n\n
Each product has an output compliance specification that can usually be found on the last page of the data sheet. We can determine the maximum allowable load by using Ohm’s Law<\/a>.<\/p>\n\n\n\nFor example, if the compliance of a 4-20 mA output signal is 15 V, then the maximum load is 750 ohms. (15 V \/ 0.02 A)=750 ohms.<\/p>\n\n\n\n
What is the maximum load I can connect to the output of a 2-wire transmitter?<\/span><\/h3>\n\n\n\n
<\/figure>\n<\/div>\n\n\nControl Room Questions and Answers<\/span><\/h2>\n\n\n\nWhat is DCS & PLC?<\/span><\/h3>\n\n\n\nDistributed control system(DCS)<\/strong><\/a> and Programmable logic controller<\/strong><\/a>. These are the control systems which handles fields I\/Os.<\/p>\n\n\n\nBasic difference between DCS & PLC is<\/strong><\/p>\n\n\n\n1) DCS handles more nos of I\/Os rather than PLC.<\/p>\n\n\n\n
2) PLC is faster system than DCS. 3) DCS can handles handsome quantity of I\/Os so that can be used for total plant automation. Where as PLC has own limitations so it generally used for small but for important(Safety point of view) units, like boiler automations, Make-up compressor automations Etc.<\/p>\n\n\n\n
4) In the above mentioned case the these PLC\u2019s can be get connected with the DCS with the help of soft link. Generally this is used to make alert to both the operator.<\/p>\n\n\n\n
5) As I heard the PLC<\/a> used to handle the DI\/DO signals so it can take fast actions. Some of the time it is used to handle few nos of AI\/AO.<\/p>\n\n\n\n6) DCS & PLC\u2019s speed depends on the scan rate of I\/Os.<\/p>\n\n\n\n
7) For both the system Marshalling panels, Consoles and other faculties of Ethernet Etc can be used according to the need.<\/p>\n\n\n\n
8) According to the Cause and effects diagrams the System programmer assigns the control action block into the system, we can call them as memory assigning.<\/p>\n\n\n\n
While making Datasheets which things are to be considered?<\/span><\/h3>\n\n\n\nBasically it depends on the instrument item for which you are preparing the datasheet.<\/p>\n\n\n\n
As an Example. Temperature Element. We have collect following information to prepare D\/s.<\/p>\n\n\n\n
What is potential free contact? What is the significance and application of this contact?<\/span><\/h3>\n\n\n\nContacts having not potential. E.g. Relay contacts\/ field switches contacts. They are used in logic circuits.<\/p>\n\n\n\n
A potential free contact is usually wired into an electrical circuit.<\/p>\n\n\n\n
However it must be ensured that the contact ratings are suitable for the service in which it is used.<\/p>\n\n\n\n
In split range control, whether the signal is splitted through I\/P convertor or the convertor itself?<\/span><\/h3>\n\n\n\nThis can be typically achieved by two ways:<\/p>\n\n\n\n
By connecting o\/p of one I\/P converter<\/a> to two positioners adjusted suitably for split range operation of control valves.<\/p>\n\n\n\nTaking two Analog Outputs (AO) from DCS. Split range to be defined in DCS. Both I\/P converters and positioners to be calibrated with input as 4to20 ma dc and 3to15 psi respectively.<\/p>\n\n\n\n
When do we use SOV of rating 110vac and 110vdc?<\/span><\/h3>\n\n\n\nPrimarily depends upon the availability of reliable power supply source.<\/p>\n\n\n\n
How is cold junction compensation in thermocouple carried?<\/span><\/h3>\n\n\n\nThis is typically performed in modern programmable instruments by means of measuring actual reference junction temperature using a temperature sensor mounted close to the ref. junction and compensating for the same using appropriate look-up table stored within the instrument\u2019s memory.<\/p>\n\n\n\n
In which applications do we use 4 wire RTD?<\/span><\/h3>\n\n\n\nThree wire is a better alternative. Primary objective of 3-wire and 4-wire arrangements is to eliminate effects of lead resistance on temp. measurement.<\/p>\n\n\n\n
What is the sensor used in coriollis mass flow meter to measure density?<\/span><\/h3>\n\n\n\nDensity is measured here by measuring the resonant frequency of a vibrating U-tube.<\/p>\n\n\n\n
What if thermocouple wire is opened in the field? What signal goes to DCS?<\/strong><\/span><\/h3>\n\n\n\nIn most modern instruments the signal may be programmed to go to either maximum or minimum depending upon end user\u2019s requirement.<\/p>\n\n\n\n
if the power supply connections to a two-wire transmitter get interchanged? What signal will go to DCS?<\/strong><\/span><\/h3>\n\n\n\nUsually there is a blocking diode to protect the transmitter<\/a> against supply reversal and almost zero current signal should be transmitted.<\/p>\n\n\n\nWhat are the possible reasons for the failure of barrier? !?!<\/strong><\/span><\/h3>\n\n\n\nFuse<\/a> blowing sometimes. Power circuits are most likely to fail.<\/p>\n\n\n\nHow can we say that the given RTD or Thermocouple is correct?<\/span><\/h3>\n\n\n\nWe can only measure sensor output (resistance \/ maillots ) accurately and look-up corresponding temperature in reference tables.<\/p>\n\n\n\n
The accuracy depends upon quality \/ condition of the sensor. Degraded sensors may not give accurate readings and must be replaced.<\/p>\n\n\n\n
To test a sensor, the sensor response may be tested using a high quality temperature calibrator and compared with reference tables.<\/p>\n\n\n\n
In some cases we have to select the cam position in a control valve for different application? How do we select that?<\/span><\/h3>\n\n\n\nRefer to instruction manual for the positioner<\/a> \/ control valve.<\/p>\n\n\n\nThe cams are often marked with limited amount of information, which may help an experienced person.<\/p>\n\n\n\n
What is the difference between a protocol and a field bus?<\/span><\/h3>\n\n\n\nA protocol defines a standard method for communications.<\/p>\n\n\n\n
A fieldbus<\/a> is a multidropping arrangement where multiple instruments communicate with special interface hardware using the same pair of wires and in most of the cases draw power from the same pair of wires.<\/p>\n\n\n\nWhat happens if transmitter wires get shorted?<\/span><\/h3>\n\n\n\nThe barrier<\/a> if installed limits electrical energy flowing into hazardous area. If there is no barrier, typically a fuse in the power distribution system will blow.<\/p>\n\n\n\nWhat will happen if thermocouple wires get shorted?<\/span><\/h3>\n\n\n\nA cold junction compensated instrument will typically indicate temperature of the location where the T\/C wires are shorted.<\/p>\n\n\n\n
Why do we require loop-terminating resistor in any digital communication loop?<\/span><\/h3>\n\n\n\nA minimum loop resistance<\/a> is required so that modulated current signal produces a modulated voltage signal, which may be detected by the receiving equipment.<\/p>\n\n\n\nWhat is adapter flange?<\/span><\/h3>\n\n\n\nIt is a transmitter part for allowing process connection to pipe\/tube.<\/p>\n\n\n\n
How to calculate the safe distance between cables to avoid electromagnetic interference of each other ?<\/span><\/h3>\n\n\n\nThe design engineers \/ equipment manufacturers follow\/publish certain guidelines w.r.t. different types of cables and the voltages\/currents and types of signals carried by them.<\/p>\n\n\n\n
How Control loop should be tuned in process loop?<\/span><\/h3>\n\n\n\nYou may use Ziegler-Nichol\u2019s<\/a> method ( open loop \/ closed loop ) or special tuning software tools.<\/p>\n\n\n\nWhat is the significance of single ended & differential ended input for PLC? Application wise comparison of these two types of inputs?<\/span><\/h3>\n\n\n\nDifferential inputs provide better common mode rejection and signal-to-noise ratio.<\/p>\n\n\n\n
What is Ground Loop? Preventive steps to avoid ground loop?<\/span><\/h3>\n\n\n\nWhen ground wiring is not done properly, grounding<\/a> of various points is not effective and potential differences exist between them resulting in currents flowing between them.<\/p>\n\n\n\nThis leads to measurement errors and is not desirable. It can be eliminated by proper ground wiring.<\/p>\n\n\n\n
In a globe type control valve, what is the importance of flow direction (top to bottom or bottom to top)?<\/span><\/h3>\n\n\n\nControl valves must be installed as per direction marking provided by the manufacturers or instruction manuals. Though people tend to generalize, this is often misleading.<\/p>\n\n\n\n
What is ATEX directives \/ FM Approval \/ CSA approved \/ CE certified ? What is the importance of individual certification? Are all these certification required for each instruments? Which certification do we prefer?<\/span><\/h3>\n\n\n\nATEX\/FM\/CSA certifications generally refer to certification for suitability of instruments for use in hazardous area when installed in accordance with recommended guidelines.<\/p>\n\n\n\n
Any certification, which is locally acceptable as per statutory requirements, may be used. We typically accept American\/European\/Indian certifications\/approvals in India.<\/p>\n\n\n\n
Advantages of tachometer as speed measuring device compared to inductive type proximity switches?<\/span><\/h3>\n\n\n\nSome tachometers provide analog output with almost instantaneous response time.<\/p>\n\n\n\n
They are highly suitable for speed control in some applications.<\/p>\n\n\n\n
Application wise advantages of Inductive type proximity switches over capacitive type switches?<\/span><\/h3>\n\n\n\nInductive proximity switches<\/a> are better suited for detection of conducting metal objects and are easily tested for proper operation.<\/p>\n\n\n\nCapacitive switches are typically used for detecting non-conductive materials.<\/p>\n\n\n\n
What is cable tray? Its type? Its size? Its support?<\/strong><\/span><\/h3>\n\n\n\nCable tray<\/a> is nothing but the way or media through which we lay the field cables in plant.<\/p>\n\n\n\nThere are two basic types<\/p>\n\n\n\n
\n- Ladder type(made in Rungs type construction)<\/li>\n\n\n\n
- Perforated Type(Solid sheath consist of Holes for ventilation).<\/li>\n<\/ol>\n\n\n\n
Basically discussing about tray support than we could say it depends on the site conditions. Only care has to be taken considering adequate space for laying cable, considering their bends.Etc.<\/p>\n\n\n\n
How to decide cable tray size?<\/span><\/h3>\n\n\n\nAccording to the no. of cable occupancy in the cable tray<\/a> and available tray size we have to choose it. they are available in foll. types 80,150,300,450,600 & 900.<\/p>\n\n\n\nWhat is meant by instrument location & JB location?<\/span><\/h3>\n\n\n\nThis consist of Instrument location<\/a> considering the piping drawing given from Piping dept. We identifies the locations of the instrument in the equipment layouts and put the bubbles and elevation and JB nos for the location.<\/p>\n\n\n\nSame way depending upon the accessibility we decide the location of JB and marking of it into the instrument location plan is JB location. There is no need to make a different drawing for this.<\/p>\n\n\n\n
While locating Instrument & JB which things we have to consider?<\/span><\/h3>\n\n\n\nTransmitter :<\/strong> Tube routing, maintenance area, man approach, Valve : Hand wheel operations, Maintenance area, Etc.<\/p>\n\n\n\nLoop power indicators :<\/strong> Man approach Illumination from Electrical if instrument is not going to provide.<\/p>\n\n\n\nWhat is the use for cable entry in control room? (sleeves & MCT)<\/span><\/h3>\n\n\n\nIn the process plant Control room built considering the non-hazardous area.<\/p>\n\n\n\n
So in case if fire\/Explosion takes place in the plant than that has to be restricted from entering into the control room. So MCT (Multiple cable transient)<\/a> blocks are used.<\/p>\n\n\n\nThey are designed to sustain the fire for a fixed time duration. That block hold the cables which are entering into the CCR.<\/p>\n\n\n\n
What are Analog Input\/output & Digital Input\/output?<\/span><\/h3>\n\n\n\n\n- 4-20 mA<\/a> signals from instrument (transmitters) are analog input to control system.<\/li>\n\n\n\n
- 4-20 mA signals to instrument (I\/p & electro pneumatic positioner) are analog output from control system.<\/li>\n<\/ul>\n\n\n\n
Volt free (24V) contact (NO\/NC) by instruments (all type of switches, ex. Limit switch, press,temp,flow,level switch) are digital input to control system.<\/span><\/h3>\n\n\n\nAll powered signals (24V,48V,110V\u2026.) from system to instruments (SOV) are Digital outputs.<\/p>\n\n\n\n
Types of instrument cables?<\/span><\/h3>\n\n\n\n\n- IS cables &<\/li>\n\n\n\n
- NIS cables<\/li>\n<\/ul>\n\n\n\n
IS \u2013 Intrinsic safety & NIS \u2013 Non Intrinsic safety<\/p>\n\n\n\n
Which cable to use, depends upon hazards condition.<\/p>\n\n\n\n
Instrument JB\u2019s?<\/span><\/h3>\n\n\n\n\n- Instrument JB\u2019s depend upon hazards area<\/a> classification.<\/li>\n\n\n\n
- JB\u2019s also can be IS or NIS<\/li>\n\n\n\n
- For IS signal IS JB\u2019s used for NIS signal NIS JB\u2019s used<\/li>\n\n\n\n
- For analog I\/p & o\/p signal we can use same JB. But for Digital I\/p & o\/p we have to use separate JB\u2019s. Because digital outputs are powered signal, by wrong connection there may be chances to damage the card. For DCS I\/p & o\/p and PLC I\/p & o\/p we used separate JB\u2019s.<\/li>\n<\/ul>\n\n\n\n
What is open loop & close loop?<\/span><\/h3>\n\n\n\nOPEN LOOP :<\/strong> This is nothing but to sense the process signals from the field and to send it to the control room for operator observation.<\/p>\n\n\n\nCLOSE LOOP :<\/strong> This is measuring the process signals for operator\u2019s action. Means Transmitter sense the process and send it to the control room. Where the operators takes action i.e. control action and that given to the final control element as per the process requirement.<\/p>\n\n\n\nWhat is a PID controller? <\/strong><\/span><\/h3>\n\n\n\nPID stands for Proportional-Integral-Derivative. This is a type of feedback controller whose output, a control variable (CV), is generally based on the error (e) between some user-defined set point (SP) and some measured process variable (PV). Each element of the PID controller<\/a> refers to a particular action taken on the error:<\/p>\n\n\n\n\n- Proportional :<\/strong> error multiplied by a gain, Kp<\/sub>. This is an adjustable amplifier. In many systems Kp<\/sub> is responsible for process stability: too low and the PV can drift away; too high and the PV can oscillate.<\/li>\n\n\n\n
- Integral: the integral of error multiplied by a gain, Ki<\/sub>. In many systems Ki<\/sub> is responsible for driving error to zero, but to set Ki<\/sub> too high is to invite oscillation or instability or integrator windup or actuator saturation.<\/li>\n\n\n\n
- Derivative :<\/strong> the rate of change of error multiplied by a gain, Kd<\/sub>. In many systems Kd<\/sub> is responsible for system response: too high and the PV will oscillate; too low and the PV will respond sluggishly. The designer should also note that derivative action amplifies any noise in the error signal.<\/li>\n<\/ul>\n\n\n\n
Tuning of a PID involves the adjustment of Kp<\/sub>, Ki<\/sub>, and Kd<\/sub> to achieve some user-defined “optimal” character of system response.<\/p>\n\n\n\nAlarms Questions and Answers<\/h2>\n\n\n\nOn limit alarms, what is meant by “failsafe”?<\/span><\/h3>\n\n\n\nWhen a limit alarm is configured for “failsafe”<\/a> operation, the contact logic is reversed. This means that the coil, which controls the contact(s), will de-energize when the unit is in alarm, and energize when the unit is not in alarm. Thus, if the limit alarm looses power, the contacts will revert to the alarm condition.<\/p>\n\n\n\nThis functionality is desirable if the user wants an alarm whenever the measured process is beyond acceptable values, or whenever power is lost to the limit alarm.<\/p>\n\n\n\n
On limit alarms, what is meant by “deadband”? <\/span><\/h3>\n\n\n\nDeadband is the difference between the alarm “on” and alarm “off” setpoints.<\/p>\n\n\n\n
For example, if a limit alarm<\/a> is calibrated to trip at a 14.7 mA input, and un-trip at 13.5 mA, the deadband, (or differential) is 1.2 mA.<\/p>\n\n\n\nWhat input range is used for the dc input limit alarms to accept a 4\/20 mA signal? <\/span><\/h3>\n\n\n\nAll the input ranges for the Action Instruments’ field configurable devices are zero based. One of the input ranges is 20 mA. This is the input range to use for all signals up to 20 mA.<\/p>\n\n\n\n
Accuracy is not affected. Having the input range begin at zero allows a limit alarm to be used for open circuit detection.<\/p>\n\n\n\n
Intrinsically Safe Systems<\/strong><\/span><\/h2>\n\n\n\nWhat is “Intrinsically Safe”?<\/span><\/strong><\/h3>\n\n\n\nA protection method employed in potentially explosive atmospheres. Certificate IS tools are designed to prevent the release of sufficient energy to cause ignition of flammable<\/a> material.<\/p>\n\n\n\nIS standards apply to all equipment that can create one or more of a range of defined potential explosion sources.<\/p>\n\n\n\n
\n- Electrical sparks<\/li>\n\n\n\n
- Electrical arcs<\/li>\n\n\n\n
- Flames<\/li>\n\n\n\n
- Hot surfaces<\/li>\n\n\n\n
- Static electricity<\/li>\n\n\n\n
- Electromagnetic radiation<\/li>\n\n\n\n
- Chemical reactions<\/li>\n\n\n\n
- Mechanical impact<\/li>\n\n\n\n
- Mechanical friction<\/li>\n\n\n\n
- Compression ignition<\/li>\n\n\n\n
- Acoustic energy<\/li>\n\n\n\n
- Ionizing radiation<\/li>\n<\/ul>\n\n\n\n
What Industries are Intrinsically Safe Products Designed for?<\/strong><\/span><\/h3>\n\n\n\n\n- Petrochemical<\/li>\n\n\n\n
- Oil platforms and refineries<\/li>\n\n\n\n
- Pharmaceutical<\/li>\n\n\n\n
- Pipelines<\/li>\n\n\n\n
- Any environment where explosive gases or vapors could be present<\/li>\n<\/ul>\n\n\n\n
The Three Key Elements of Combustion are:<\/strong><\/span><\/h3>\n\n\n\n\n- Inflammable material (gases, particles\/dust)<\/li>\n\n\n\n
- Oxygen\/air<\/li>\n\n\n\n
- Ignition source<\/li>\n<\/ul>\n\n\n\n
This combination is very common in chemical, petro-chemical, and pharmaceutical industries.<\/p>\n\n\n\n
Examples of the amount of inflammable material necessary for ignition below show how small of an amount it takes to present a danger to workers.<\/p>\n\n\n\n
What are the Regulations and Guidelines for IS?<\/strong><\/span><\/h3>\n\n\n\nATEX (Europe)<\/h4>\n\n\n\n
European Union’s 94\/9\/EC Directive, commonly called ATEX<\/a> (“Atmospheres Explosibles”) Europe’s primary regulation for protection systems and equipment intended for use in potentially explosive atmospheres. Indended to serve as total harmonization directive, laying down essential health and safety requirements, and replace existing divergent national and European legislation.<\/p>\n\n\n\nThis directive became mandatory on electical and electronic equipment for use in environments subject to explosion hazard sold in the EU on 1 July 2003.<\/p>\n\n\n\n
Derivatives of ATEX are being adopted across the world.<\/p>\n\n\n\n
NEC (United States)<\/h4>\n\n\n\n
National Electrical Code (NEC), is the basis for all electrical codes in the United States.<\/p>\n\n\n\n
Classifications and related product markings for hazardous areas are covered in NEC 500 and 505. Interpretations of NEC 500, a longstanding regulation are utilized throughout the world (outside Europe). NEC 505 is similar to ATEX.<\/p>\n\n\n\n
Field Instruments <\/strong><\/span>Questions and Answers<\/h2>\n\n\n\nWhat is the measurement principle of an vortex flowmeter?<\/strong><\/span><\/h3>\n\n\n\nWhen a shedder bar is placed in a fluid, Karman vortices are generated on the downstream side.<\/p>\n\n\n\n
The vortex frequency is proportional to the flow velocity, and corresponds to a specific range of Reynolds<\/a> numbers.<\/p>\n\n\n\nThe vortex frequency (f) is calculated by the formula; f=St*v\/d<\/strong><\/p>\n\n\n\nv: flow velocity, d: width of shedder bar, St: Strouhal number<\/p>\n\n\n\n
What are the advantages of a vortex flowmeter?<\/strong><\/span><\/h3>\n\n\n\n\n- Frequency output is proportional to flowrate.<\/li>\n\n\n\n
- No moving parts are required because of vibration created by Karman vortices.<\/li>\n\n\n\n
- Wide measurement range<\/li>\n\n\n\n
- Can measure gas, steam and liquid<\/li>\n\n\n\n
- Outputs volumetric flowrate without influence of density, temperature, or pressure<\/li>\n<\/ul>\n\n\n\n
What does “cold junction compensation” mean?<\/span><\/strong><\/h3>\n\n\n\nCold junction compensation allows accurate temperature measurement when using a thermocouple. What’s a thermocouple? Well, a thermocouple is created when wires made from two different types of metal are connected together, (such as Iron and Copper-Nickel). Thermocouples generate a small Voltage, which increases when the thermocouple junction gets hotter.<\/p>\n\n\n\n
When the thermocouple wires are connected to an instrument, two more thermocouple junctions<\/a> are created, because the terminals are made of a different material than the thermocouple wires. These “extra” junctions, (called cold junctions), create their own Voltage, which alters the Voltage generated by the actual thermocouple.<\/p>\n\n\n\nCold junction compensation negates the voltage created by these cold junctions, allowing only the Voltage created by the thermocouple to be sensed by the instrument.<\/p>\n\n\n\n
Why does an RTD need an excitation source?<\/strong> <\/span><\/h3>\n\n\n\nUsually an electrical reference or excitation source is required for sensors whose electrical properties are measured.<\/p>\n\n\n\n
A thermocouple produces a potential difference between the two dissimilar metals that varies with temperature and is repeatable. The measurement is therefore a voltage.<\/p>\n\n\n\n
The resistance of an RTD<\/a> changes with temperature, therefore, a constant DC current excitation source is used as a reference to produce a proportionally changing voltage.<\/p>\n\n\n\nThus a signal conditioner which measures an RTD input provides a current reference as excitation and measures the voltage produced.<\/p>\n\n\n\n
What is a thermocouple sensor?<\/strong><\/span><\/h3>\n\n\n\nA thermocouple is a sensor for measuring temperature. It consists of two dissimilar metals, joined together at one end.<\/p>\n\n\n\n
When the junction of the two metals is heated or cooled a voltage is produced that can be correlated back to the temperature. The thermocouple alloys are commonly available as wire.<\/p>\n\n\n\n
Explain different thermocouple types<\/span><\/h3>\n\n\n\nA thermocouple is available in different combinations of metals or calibrations. The four most common calibrations are J, K, T and E<\/p>\n\n\n\n
How do I choose a thermocouple?<\/span><\/h3>\n\n\n\nBecause a thermocouple<\/a> measures in wide temperature ranges and can be relatively rugged, thermocouples are very often used in industry.<\/p>\n\n\n\nThe following criteria are used in selecting a thermocouple:<\/p>\n\n\n\n
\n- Temperature range<\/li>\n\n\n\n
- Chemical resistance of the thermocouple or sheath material<\/li>\n\n\n\n
- Abrasion and vibration resistance<\/li>\n\n\n\n
- Installation requirements (may need to be compatible with existing equipment; existing holes may determine probe diameter)<\/li>\n<\/ul>\n\n\n\n
Explain Thermocouple Ranges and Limits of Error<\/span><\/h3>\n\n\n\n<\/figure>\n<\/div>\n\n\nTransmitters <\/strong>Questions and Answers<\/h2>\n\n\n\nWhat is the maximum load I can connect to the current output of a 4-wire transmitter? <\/span><\/h3>\n\n\n\nThe maximum load that can be connected to a current output depends on the output compliance.<\/p>\n\n\n\n
Each product has an output compliance specification that can usually be found on the last page of the data sheet. We can determine the maximum allowable load by using Ohm’s Law<\/a>.<\/p>\n\n\n\nFor example, if the compliance of a 4-20 mA output signal is 15 V, then the maximum load is 750 ohms. (15 V \/ 0.02 A)=750 ohms.<\/p>\n\n\n\n
What is the maximum load I can connect to the output of a 2-wire transmitter?<\/span><\/h3>\n\n\n\n
Distributed control system(DCS)<\/strong><\/a> and Programmable logic controller<\/strong><\/a>. These are the control systems which handles fields I\/Os.<\/p>\n\n\n\n Basic difference between DCS & PLC is<\/strong><\/p>\n\n\n\n 1) DCS handles more nos of I\/Os rather than PLC.<\/p>\n\n\n\n 2) PLC is faster system than DCS. 3) DCS can handles handsome quantity of I\/Os so that can be used for total plant automation. Where as PLC has own limitations so it generally used for small but for important(Safety point of view) units, like boiler automations, Make-up compressor automations Etc.<\/p>\n\n\n\n 4) In the above mentioned case the these PLC\u2019s can be get connected with the DCS with the help of soft link. Generally this is used to make alert to both the operator.<\/p>\n\n\n\n 5) As I heard the PLC<\/a> used to handle the DI\/DO signals so it can take fast actions. Some of the time it is used to handle few nos of AI\/AO.<\/p>\n\n\n\n 6) DCS & PLC\u2019s speed depends on the scan rate of I\/Os.<\/p>\n\n\n\n 7) For both the system Marshalling panels, Consoles and other faculties of Ethernet Etc can be used according to the need.<\/p>\n\n\n\n 8) According to the Cause and effects diagrams the System programmer assigns the control action block into the system, we can call them as memory assigning.<\/p>\n\n\n\n Basically it depends on the instrument item for which you are preparing the datasheet.<\/p>\n\n\n\n As an Example. Temperature Element. We have collect following information to prepare D\/s.<\/p>\n\n\n\n Contacts having not potential. E.g. Relay contacts\/ field switches contacts. They are used in logic circuits.<\/p>\n\n\n\n A potential free contact is usually wired into an electrical circuit.<\/p>\n\n\n\n However it must be ensured that the contact ratings are suitable for the service in which it is used.<\/p>\n\n\n\n This can be typically achieved by two ways:<\/p>\n\n\n\n By connecting o\/p of one I\/P converter<\/a> to two positioners adjusted suitably for split range operation of control valves.<\/p>\n\n\n\n Taking two Analog Outputs (AO) from DCS. Split range to be defined in DCS. Both I\/P converters and positioners to be calibrated with input as 4to20 ma dc and 3to15 psi respectively.<\/p>\n\n\n\n Primarily depends upon the availability of reliable power supply source.<\/p>\n\n\n\n This is typically performed in modern programmable instruments by means of measuring actual reference junction temperature using a temperature sensor mounted close to the ref. junction and compensating for the same using appropriate look-up table stored within the instrument\u2019s memory.<\/p>\n\n\n\n Three wire is a better alternative. Primary objective of 3-wire and 4-wire arrangements is to eliminate effects of lead resistance on temp. measurement.<\/p>\n\n\n\n Density is measured here by measuring the resonant frequency of a vibrating U-tube.<\/p>\n\n\n\n In most modern instruments the signal may be programmed to go to either maximum or minimum depending upon end user\u2019s requirement.<\/p>\n\n\n\n Usually there is a blocking diode to protect the transmitter<\/a> against supply reversal and almost zero current signal should be transmitted.<\/p>\n\n\n\n Fuse<\/a> blowing sometimes. Power circuits are most likely to fail.<\/p>\n\n\n\n We can only measure sensor output (resistance \/ maillots ) accurately and look-up corresponding temperature in reference tables.<\/p>\n\n\n\n The accuracy depends upon quality \/ condition of the sensor. Degraded sensors may not give accurate readings and must be replaced.<\/p>\n\n\n\n To test a sensor, the sensor response may be tested using a high quality temperature calibrator and compared with reference tables.<\/p>\n\n\n\n Refer to instruction manual for the positioner<\/a> \/ control valve.<\/p>\n\n\n\n The cams are often marked with limited amount of information, which may help an experienced person.<\/p>\n\n\n\n A protocol defines a standard method for communications.<\/p>\n\n\n\n A fieldbus<\/a> is a multidropping arrangement where multiple instruments communicate with special interface hardware using the same pair of wires and in most of the cases draw power from the same pair of wires.<\/p>\n\n\n\n The barrier<\/a> if installed limits electrical energy flowing into hazardous area. If there is no barrier, typically a fuse in the power distribution system will blow.<\/p>\n\n\n\n A cold junction compensated instrument will typically indicate temperature of the location where the T\/C wires are shorted.<\/p>\n\n\n\n A minimum loop resistance<\/a> is required so that modulated current signal produces a modulated voltage signal, which may be detected by the receiving equipment.<\/p>\n\n\n\n It is a transmitter part for allowing process connection to pipe\/tube.<\/p>\n\n\n\n The design engineers \/ equipment manufacturers follow\/publish certain guidelines w.r.t. different types of cables and the voltages\/currents and types of signals carried by them.<\/p>\n\n\n\n You may use Ziegler-Nichol\u2019s<\/a> method ( open loop \/ closed loop ) or special tuning software tools.<\/p>\n\n\n\n Differential inputs provide better common mode rejection and signal-to-noise ratio.<\/p>\n\n\n\n When ground wiring is not done properly, grounding<\/a> of various points is not effective and potential differences exist between them resulting in currents flowing between them.<\/p>\n\n\n\n This leads to measurement errors and is not desirable. It can be eliminated by proper ground wiring.<\/p>\n\n\n\n Control valves must be installed as per direction marking provided by the manufacturers or instruction manuals. Though people tend to generalize, this is often misleading.<\/p>\n\n\n\n ATEX\/FM\/CSA certifications generally refer to certification for suitability of instruments for use in hazardous area when installed in accordance with recommended guidelines.<\/p>\n\n\n\n Any certification, which is locally acceptable as per statutory requirements, may be used. We typically accept American\/European\/Indian certifications\/approvals in India.<\/p>\n\n\n\n Some tachometers provide analog output with almost instantaneous response time.<\/p>\n\n\n\n They are highly suitable for speed control in some applications.<\/p>\n\n\n\n Inductive proximity switches<\/a> are better suited for detection of conducting metal objects and are easily tested for proper operation.<\/p>\n\n\n\n Capacitive switches are typically used for detecting non-conductive materials.<\/p>\n\n\n\n Cable tray<\/a> is nothing but the way or media through which we lay the field cables in plant.<\/p>\n\n\n\n There are two basic types<\/p>\n\n\n\n Basically discussing about tray support than we could say it depends on the site conditions. Only care has to be taken considering adequate space for laying cable, considering their bends.Etc.<\/p>\n\n\n\n According to the no. of cable occupancy in the cable tray<\/a> and available tray size we have to choose it. they are available in foll. types 80,150,300,450,600 & 900.<\/p>\n\n\n\n This consist of Instrument location<\/a> considering the piping drawing given from Piping dept. We identifies the locations of the instrument in the equipment layouts and put the bubbles and elevation and JB nos for the location.<\/p>\n\n\n\n Same way depending upon the accessibility we decide the location of JB and marking of it into the instrument location plan is JB location. There is no need to make a different drawing for this.<\/p>\n\n\n\n Transmitter :<\/strong> Tube routing, maintenance area, man approach, Valve : Hand wheel operations, Maintenance area, Etc.<\/p>\n\n\n\n Loop power indicators :<\/strong> Man approach Illumination from Electrical if instrument is not going to provide.<\/p>\n\n\n\n In the process plant Control room built considering the non-hazardous area.<\/p>\n\n\n\n So in case if fire\/Explosion takes place in the plant than that has to be restricted from entering into the control room. So MCT (Multiple cable transient)<\/a> blocks are used.<\/p>\n\n\n\n They are designed to sustain the fire for a fixed time duration. That block hold the cables which are entering into the CCR.<\/p>\n\n\n\n All powered signals (24V,48V,110V\u2026.) from system to instruments (SOV) are Digital outputs.<\/p>\n\n\n\n IS \u2013 Intrinsic safety & NIS \u2013 Non Intrinsic safety<\/p>\n\n\n\n Which cable to use, depends upon hazards condition.<\/p>\n\n\n\n OPEN LOOP :<\/strong> This is nothing but to sense the process signals from the field and to send it to the control room for operator observation.<\/p>\n\n\n\n CLOSE LOOP :<\/strong> This is measuring the process signals for operator\u2019s action. Means Transmitter sense the process and send it to the control room. Where the operators takes action i.e. control action and that given to the final control element as per the process requirement.<\/p>\n\n\n\n PID stands for Proportional-Integral-Derivative. This is a type of feedback controller whose output, a control variable (CV), is generally based on the error (e) between some user-defined set point (SP) and some measured process variable (PV). Each element of the PID controller<\/a> refers to a particular action taken on the error:<\/p>\n\n\n\n Tuning of a PID involves the adjustment of Kp<\/sub>, Ki<\/sub>, and Kd<\/sub> to achieve some user-defined “optimal” character of system response.<\/p>\n\n\n\n When a limit alarm is configured for “failsafe”<\/a> operation, the contact logic is reversed. This means that the coil, which controls the contact(s), will de-energize when the unit is in alarm, and energize when the unit is not in alarm. Thus, if the limit alarm looses power, the contacts will revert to the alarm condition.<\/p>\n\n\n\n This functionality is desirable if the user wants an alarm whenever the measured process is beyond acceptable values, or whenever power is lost to the limit alarm.<\/p>\n\n\n\n Deadband is the difference between the alarm “on” and alarm “off” setpoints.<\/p>\n\n\n\n For example, if a limit alarm<\/a> is calibrated to trip at a 14.7 mA input, and un-trip at 13.5 mA, the deadband, (or differential) is 1.2 mA.<\/p>\n\n\n\n All the input ranges for the Action Instruments’ field configurable devices are zero based. One of the input ranges is 20 mA. This is the input range to use for all signals up to 20 mA.<\/p>\n\n\n\n Accuracy is not affected. Having the input range begin at zero allows a limit alarm to be used for open circuit detection.<\/p>\n\n\n\n A protection method employed in potentially explosive atmospheres. Certificate IS tools are designed to prevent the release of sufficient energy to cause ignition of flammable<\/a> material.<\/p>\n\n\n\n IS standards apply to all equipment that can create one or more of a range of defined potential explosion sources.<\/p>\n\n\n\n This combination is very common in chemical, petro-chemical, and pharmaceutical industries.<\/p>\n\n\n\n Examples of the amount of inflammable material necessary for ignition below show how small of an amount it takes to present a danger to workers.<\/p>\n\n\n\n European Union’s 94\/9\/EC Directive, commonly called ATEX<\/a> (“Atmospheres Explosibles”) Europe’s primary regulation for protection systems and equipment intended for use in potentially explosive atmospheres. Indended to serve as total harmonization directive, laying down essential health and safety requirements, and replace existing divergent national and European legislation.<\/p>\n\n\n\n This directive became mandatory on electical and electronic equipment for use in environments subject to explosion hazard sold in the EU on 1 July 2003.<\/p>\n\n\n\n Derivatives of ATEX are being adopted across the world.<\/p>\n\n\n\n National Electrical Code (NEC), is the basis for all electrical codes in the United States.<\/p>\n\n\n\n Classifications and related product markings for hazardous areas are covered in NEC 500 and 505. Interpretations of NEC 500, a longstanding regulation are utilized throughout the world (outside Europe). NEC 505 is similar to ATEX.<\/p>\n\n\n\n When a shedder bar is placed in a fluid, Karman vortices are generated on the downstream side.<\/p>\n\n\n\n The vortex frequency is proportional to the flow velocity, and corresponds to a specific range of Reynolds<\/a> numbers.<\/p>\n\n\n\n The vortex frequency (f) is calculated by the formula; f=St*v\/d<\/strong><\/p>\n\n\n\n v: flow velocity, d: width of shedder bar, St: Strouhal number<\/p>\n\n\n\n Cold junction compensation allows accurate temperature measurement when using a thermocouple. What’s a thermocouple? Well, a thermocouple is created when wires made from two different types of metal are connected together, (such as Iron and Copper-Nickel). Thermocouples generate a small Voltage, which increases when the thermocouple junction gets hotter.<\/p>\n\n\n\n When the thermocouple wires are connected to an instrument, two more thermocouple junctions<\/a> are created, because the terminals are made of a different material than the thermocouple wires. These “extra” junctions, (called cold junctions), create their own Voltage, which alters the Voltage generated by the actual thermocouple.<\/p>\n\n\n\n Cold junction compensation negates the voltage created by these cold junctions, allowing only the Voltage created by the thermocouple to be sensed by the instrument.<\/p>\n\n\n\n Usually an electrical reference or excitation source is required for sensors whose electrical properties are measured.<\/p>\n\n\n\n A thermocouple produces a potential difference between the two dissimilar metals that varies with temperature and is repeatable. The measurement is therefore a voltage.<\/p>\n\n\n\n The resistance of an RTD<\/a> changes with temperature, therefore, a constant DC current excitation source is used as a reference to produce a proportionally changing voltage.<\/p>\n\n\n\n Thus a signal conditioner which measures an RTD input provides a current reference as excitation and measures the voltage produced.<\/p>\n\n\n\n A thermocouple is a sensor for measuring temperature. It consists of two dissimilar metals, joined together at one end.<\/p>\n\n\n\n When the junction of the two metals is heated or cooled a voltage is produced that can be correlated back to the temperature. The thermocouple alloys are commonly available as wire.<\/p>\n\n\n\n A thermocouple is available in different combinations of metals or calibrations. The four most common calibrations are J, K, T and E<\/p>\n\n\n\n Because a thermocouple<\/a> measures in wide temperature ranges and can be relatively rugged, thermocouples are very often used in industry.<\/p>\n\n\n\n The following criteria are used in selecting a thermocouple:<\/p>\n\n\n\n The maximum load that can be connected to a current output depends on the output compliance.<\/p>\n\n\n\n Each product has an output compliance specification that can usually be found on the last page of the data sheet. We can determine the maximum allowable load by using Ohm’s Law<\/a>.<\/p>\n\n\n\n For example, if the compliance of a 4-20 mA output signal is 15 V, then the maximum load is 750 ohms. (15 V \/ 0.02 A)=750 ohms.<\/p>\n\n\n\nWhile making Datasheets which things are to be considered?<\/span><\/h3>\n\n\n\n
What is potential free contact? What is the significance and application of this contact?<\/span><\/h3>\n\n\n\n
In split range control, whether the signal is splitted through I\/P convertor or the convertor itself?<\/span><\/h3>\n\n\n\n
When do we use SOV of rating 110vac and 110vdc?<\/span><\/h3>\n\n\n\n
How is cold junction compensation in thermocouple carried?<\/span><\/h3>\n\n\n\n
In which applications do we use 4 wire RTD?<\/span><\/h3>\n\n\n\n
What is the sensor used in coriollis mass flow meter to measure density?<\/span><\/h3>\n\n\n\n
What if thermocouple wire is opened in the field? What signal goes to DCS?<\/strong><\/span><\/h3>\n\n\n\n
if the power supply connections to a two-wire transmitter get interchanged? What signal will go to DCS?<\/strong><\/span><\/h3>\n\n\n\n
What are the possible reasons for the failure of barrier? !?!<\/strong><\/span><\/h3>\n\n\n\n
How can we say that the given RTD or Thermocouple is correct?<\/span><\/h3>\n\n\n\n
In some cases we have to select the cam position in a control valve for different application? How do we select that?<\/span><\/h3>\n\n\n\n
What is the difference between a protocol and a field bus?<\/span><\/h3>\n\n\n\n
What happens if transmitter wires get shorted?<\/span><\/h3>\n\n\n\n
What will happen if thermocouple wires get shorted?<\/span><\/h3>\n\n\n\n
Why do we require loop-terminating resistor in any digital communication loop?<\/span><\/h3>\n\n\n\n
What is adapter flange?<\/span><\/h3>\n\n\n\n
How to calculate the safe distance between cables to avoid electromagnetic interference of each other ?<\/span><\/h3>\n\n\n\n
How Control loop should be tuned in process loop?<\/span><\/h3>\n\n\n\n
What is the significance of single ended & differential ended input for PLC? Application wise comparison of these two types of inputs?<\/span><\/h3>\n\n\n\n
What is Ground Loop? Preventive steps to avoid ground loop?<\/span><\/h3>\n\n\n\n
In a globe type control valve, what is the importance of flow direction (top to bottom or bottom to top)?<\/span><\/h3>\n\n\n\n
What is ATEX directives \/ FM Approval \/ CSA approved \/ CE certified ? What is the importance of individual certification? Are all these certification required for each instruments? Which certification do we prefer?<\/span><\/h3>\n\n\n\n
Advantages of tachometer as speed measuring device compared to inductive type proximity switches?<\/span><\/h3>\n\n\n\n
Application wise advantages of Inductive type proximity switches over capacitive type switches?<\/span><\/h3>\n\n\n\n
What is cable tray? Its type? Its size? Its support?<\/strong><\/span><\/h3>\n\n\n\n
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How to decide cable tray size?<\/span><\/h3>\n\n\n\n
What is meant by instrument location & JB location?<\/span><\/h3>\n\n\n\n
While locating Instrument & JB which things we have to consider?<\/span><\/h3>\n\n\n\n
What is the use for cable entry in control room? (sleeves & MCT)<\/span><\/h3>\n\n\n\n
What are Analog Input\/output & Digital Input\/output?<\/span><\/h3>\n\n\n\n
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Volt free (24V) contact (NO\/NC) by instruments (all type of switches, ex. Limit switch, press,temp,flow,level switch) are digital input to control system.<\/span><\/h3>\n\n\n\n
Types of instrument cables?<\/span><\/h3>\n\n\n\n
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Instrument JB\u2019s?<\/span><\/h3>\n\n\n\n
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What is open loop & close loop?<\/span><\/h3>\n\n\n\n
What is a PID controller? <\/strong><\/span><\/h3>\n\n\n\n
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Alarms Questions and Answers<\/h2>\n\n\n\n
On limit alarms, what is meant by “failsafe”?<\/span><\/h3>\n\n\n\n
On limit alarms, what is meant by “deadband”? <\/span><\/h3>\n\n\n\n
What input range is used for the dc input limit alarms to accept a 4\/20 mA signal? <\/span><\/h3>\n\n\n\n
Intrinsically Safe Systems<\/strong><\/span><\/h2>\n\n\n\n
What is “Intrinsically Safe”?<\/span><\/strong><\/h3>\n\n\n\n
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What Industries are Intrinsically Safe Products Designed for?<\/strong><\/span><\/h3>\n\n\n\n
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The Three Key Elements of Combustion are:<\/strong><\/span><\/h3>\n\n\n\n
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What are the Regulations and Guidelines for IS?<\/strong><\/span><\/h3>\n\n\n\n
ATEX (Europe)<\/h4>\n\n\n\n
NEC (United States)<\/h4>\n\n\n\n
Field Instruments <\/strong><\/span>Questions and Answers<\/h2>\n\n\n\n
What is the measurement principle of an vortex flowmeter?<\/strong><\/span><\/h3>\n\n\n\n
What are the advantages of a vortex flowmeter?<\/strong><\/span><\/h3>\n\n\n\n
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What does “cold junction compensation” mean?<\/span><\/strong><\/h3>\n\n\n\n
Why does an RTD need an excitation source?<\/strong> <\/span><\/h3>\n\n\n\n
What is a thermocouple sensor?<\/strong><\/span><\/h3>\n\n\n\n
Explain different thermocouple types<\/span><\/h3>\n\n\n\n
How do I choose a thermocouple?<\/span><\/h3>\n\n\n\n
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Explain Thermocouple Ranges and Limits of Error<\/span><\/h3>\n\n\n
<\/figure>\n<\/div>\n\n\nTransmitters <\/strong>Questions and Answers<\/h2>\n\n\n\n
What is the maximum load I can connect to the current output of a 4-wire transmitter? <\/span><\/h3>\n\n\n\n
What is the maximum load I can connect to the output of a 2-wire transmitter?<\/span><\/h3>\n\n\n\n