Saudi Aramco Interview questions: Instrument/DCS Engineer

what is cascade loop and how it work and why it used instead of PID single loop? In cascade, Which loop is faster and slower? Please explain with loop drawing and example…


A.1) In cascade loop the parameter to be controlled is in series with another parameter which is to be measured for normal operating conditions i.e 2 manipulated variables are in series
it is used in fractional distillation system , boiler operation in power plants etc In pid only one variable is measured and controlled at a time .it is used for temperature , pressure , flow control system A.2) cascade loop is the combination of two pid control.where 1st controller output ie mv is the input of 2nd controller.1st contrroler more fater than second controoler. it’s used in boiler three’s control feed water flow,drum level and steam flow.


Difference between PLC and DCS

Cold Loop checking & Hot Loop Checking?

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plc is a programmable logic controller which can handle the binary input and output of the logic statement which are stored in its memory.its a digitally operating system which are logic stored in its memory by programming system. DCS:-Distributed control system its control all types of process variable at a time through this we can control the whole plant,we can see the status of plant,and handling yhe unit. A2)In PLC the Scan time is less.but the scan time of DCS is high.Because it Control the whole plant. So in safety loop we used PLC A 3)First let’s take a quick brief history about controllers: 1 - For analog control loops we have a stand alone PID controller. 2 - Then we get a networked PID controllers through a hub and HMI for configuring and monitoring.The PID itself contain a network port.SIEMENS introduced PID 353 & PROCIDIA software for it. 3 - Then we get DDC - DIRECT DIGITAL CONTROL, single CPU, I/O’S for analog instruments only & networked PC is an option.SIEMENS introduced APOGEE, FISHER introduced ROC. 4 - The next phase is DCS - Distributed Control System, and this is first introduced by HONEYWELL & YOKOGAWA.Ability for handling large scale of I/O’S, discrete and analog, complex control loops, PID algorithms calculations and executions. 5 - PLC is basically designed and produced for discrete on/off control, limited I/O’S. but modern PLC with great progress in h/w technology introduced itself as a less expensive option for PID, with it’s complicated calculations and algorithms. 6 - by the time with great achievements in H/W tech. and networking capabilities the single PLC can handle discrete and analog instruments with wide range. 7 - Now the man can make his own DCS with multible networked PLC’S, but it will need a great engineering and effortsto get near from DCS features and functionality.


Why ues 250 ohms Resistor during hart type instruments calibration with hart protocol in details ?

The 250 ohm resiatance is necessary to support the communication between transmitter and HART because it equalizes

  1. loop resistance
  2. barrier resistance 3.wire resistance
  3. receivers resistance

Wht is Dry Leg Calibration and Wet Leg Calibration?

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Dry leg calibration and Wet Leg Calibration are used for closed and pressurized vessels level measurement. Difference: Wet leg calibration is used for vapourised liquid(which will be condensated by atmosphere)level.In LP side Tapping ,liquid filled . Dry leg calibration for the liquid under pressurised condition which will not be condensated.


Why we use 4 - 20 ma?

To operate a Digital signal we need 1-5Vdc. For Analog Instruments we are using 250 ohms as load resistance. When we converting this signal into current By Ohm’s Law V=IR I=V/R Put, V=1V Then I=1/250=4mA Put, V=2V Then I=2/250=8mA Put, V=3V Then I=3/250=12mA Put, V=4V Then I=4/250=16mA Put, V=5V Then I=5/250=20mA

*Also, since this is linear region *Not possible to cause spark *Since it is a current signal we can transmit it into long distance. *Suppose if the instrument is dead we can identify it.


in earlier days we had used BJT transister in signal conditioning circuits for amplification purposes so in BJT transfer characteristics 4 to 20 milli amps is a linear region,…but now-a-days we re using cmos only… as a convention only we are following this 4-20 range 2}///another reason…for not using 0 milli amp coz if we use like that we cannot distinguish between true signal zero and dead zero(break in wires)


A temperature sensor (RTD) connected via a temperature transmitter is adjusted to give an output of 4ma at 60 deg and 20 ma at 600 deg. This output (4-20ma) from the transmitter is connected to an IGBT-inverter to control a high speed (-12,000 to +12,000 RPM) DC motor. And in the inverter it is programmed as, for 4ma the motor speed will be -3,000 RPM and for 20ma the motor speed will be +10,000 RPM. The question is : If the motor speed is +5,500 (RPM), what will be the temperature? (-sign indicates that the motion is in reverse direction).


413 deg and mA will be 14.64

difference between null and deflecting instrument.


The accuracy of these two instruments depends on different things. For the Null type ins it depends on the linearity and calibration of the spring, while for the second it relies on the calibration of the weights. As calibration of weights is much easier than careful choice and calibration of a linear-characteristic spring, this means that the second type of instrument will normally be the more accurate. This is in accordance with the general rule that null-type instruments are more accurate than deflection types.


what is bevel? why bevel required in orifice plate? Orifice plates which are used in flow measurement are mostly concentric “square-edge” type. The bore of the orifice is not constant when one moves from the upstream side to the downstream side along the cross section. The bore size is constant for a few mm only and then expands till it reaches the downstream face. This inclined cut is called the bevel in an orifice. The main reason to have a bevel in the orifice is to avoid turbulence thereby providing more accuracy to the measurement. Restriction Orifice plates are not beveled since their intent is to reduce the pressure only.

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what is pyrometer? which purpose it has to be used?

Working definition of process ?

A process is anything that changes

A Pyrometer is a type of remote-sensing thermometer used to measure the temperature of a surface, Various forms of Pyrometers have historically existed.