INSTRUMENTATION INTERVIEW QUESTIONS-CONTROL VALVES

Introduction

Hello guys this session contains 25 interview questions about CONTROL VALVES.And if you need to know more about these check out links described below.

 

 

1.What is “quick opening” control valve.

For smaller movement of the stem, there is maximum flow rate.

2..What is “Linear” control valve.

If stem position varies linearly with flow rate, then it is linear.

3.Define Control Valve sizing.

Q=Cv.sqrt(P/Sg)

Q-Flow rate

Cv-Valve coefficient

P-pressure difference across valve.

Sg-Specific gravity of liquid.

4.HOW WILL YOU CHANGE THE ACTION OF A CONTROL VALVE?

IF THE CONTROL VALVE IS WITHOUT BOTTOM FLANGE THE ACTUATOR NEEDS TO BE CHANGED.

IF THE BOTTOM FLANGE IS PROVIDED, DISCONNECT STEM, SEPARATE BODY FROM BONNET.

REMOVE THE BOTTOM FLANGE AND PLUG FROM BODY .

DETACH THE PLUG FROM THE STEM BY REMOVING THE PIN.

FIX THE STEM AT THE OTHER END OF THE PLUG AND FOX THE PIN BACK.

TURN THE BODY UPSIDE DOWN.

CONNECT IT BACK TO THE BONNET AFTER INSERTING THE PLUG AND STEM.

CONNECT BACK THE STEM AND COUPLE IT.

FIX BACK THE BOTTOM FLANGE.

CALIBRATE THE VALVE

5.WHEN CAN A BY- PASS NOT BE USED ON POSITIONER?

SPLIT RANGE OPERATION.

REVERSING ACTION POSITIONER.

VALVE BENCH SET NOT STANTARD

6.WHAT ARE THE DIFFERENT VALVE CHARACTERISTICS?

LINEAR, EQUAL % , QUICK OPENING.

7.AN OPERATOR TELLS YOU THAT FLOW INDICATION IS MORE. HOW WOULD YOU START CHECKING IT?

FIRST FLUSH THE TRANSMITTER ( BOTH IMPULSE LINES ).

ADJUST ZERO BY EQUALISING IF NECESSARY.

IF STILL THE INDICATION IS MORE THEN CHECK LP SIDE FOR CHOKE.

IF THAT IS CLEAR THEN CHECK LEAKS ON THE LP SIDE.

IF NO, CALIBRATE THE TRANSMITTER.

8.AN OPERATOR TELLS YOU THAT A CONTROL VALVE IS STUCK, HOW WILL YOU START CHECKING?

PROCUREMENT OF WORK PERMIT,

CONTROLLER IN MANUAL MODE,

TRIP ENABLE

NECESSARY TOOLS TO BE TAKEN,

BYPASS OF A VALVE ( TO BE DONE BY OPERATOR ).

APPLY AIR SIGNAL TO ACTUATOR, CHECK FOR STROKE.

IF NUMBER DISENGAGE FROM SPLIT CLAMP AFTER TAKING NECESSARY PRECAUTION,

SUPPLY TO ACTUATOR IF MOVING ,

PROBLEM IN BODY OF VALVE – REMOVE AFTER LINE IS DE PRESSURISING / DRAINED

9.WHEN DO YOU USE A VALVE POSITIONER?

IF THE DIAPHRAGM ACTUATOR DOES NOT SUPPLY SUFFICIENT FORCE TO POSITION THE VALVE ACCURATELY AND OVERCOME ANY OPPOSITION THAT FLOWING CONDITIONS CREATE A POSITIONER MAY BE REQUIRED

10.WHAT IS “EQUAL PERCENTAGE” IN THE EQUAL PERCENTAGE VALVE?

FOR EQUAL INCREMENT OF STEM TRAVEL AT CONSTANT PRESSURE DROP AN EQUAL PERCENTAGE CHANGE IN EXISTING FLOW OCCURS.

11.HOW DO YOU CONVERT PSI SIGNAL IN TO LINEAR?

PSI SIGNAL IN TO LINEAR = ( ( A-3) / 12) X 12 + 3

12.HOW DO YOU CALIBRATE A D / P TRANSMITTER IN LINE?

PROCUREMENT OF WORK PERMIT.

CONTROLLER IN MANUAL MODE AND IF TRIP EXISTS,

DISABLE THE SAME BY INFORMING THE OPERATOR.

CLOSE THE MAIN ISOLATION VALVE ( HP & LP),

EQUALISE & DRAIN THE LIQUID IF ANY.

ADJUST ZERO OF THE TRANSMITTER &

REQUIRED SPAN BY APPLYING MAX. CAL. SPAN.

13.WHAT IS THE SEAL LIQUID FOR FILLING IMPULSE LINES ON CRUDE/ VISCOUS LIQUIDS?

GLYCOL.

14.WHAT IS FLASHING IN CONTROL VALVE?

WHEN A LIQUIDS ENTERS A VALVE AND THE STATIC PRESSURE AT THE VENA CONTRACTA

LESS THAN THE FLUID VAPOUR PRESSURE AND THE VALVE OUTLET PRESSURE IS ALSO LESS THE

FLUID VAPOUR PRESSURE THE CONDITION CALLED FLASHING EXISTS

15.WHAT IS AN ACTUATOR?

A DEVICE THAT CREATES AUTOMATIC MOTION BY CONVERTING VARIOUS FORMS OF ENERGY TO ROTARY OR LINEAR MECHANICAL ENERGY. ITS A DEVICE TO CONVERT AN ELECTRICAL CONTROL SIGNAL TO A PHYSICAL ACTION.

16.WHY IS ma SIGNAL PREFERRED FOR SIGNAL TRANSMISSION?

NOISE REDUCTION AND NO CURRENT DROP FOR LONG TRANSMISSION LINE.

17.WHY DOES A TRANSMITTER O/ P SIGNAL START FROM 3 – 15 OR 4 – 20 M. A?

LINEAR AND CAN CHECK UP WHETHER ITS LIVE ZERO OR DEAD ZERO

18.WHAT IS MEANT BY CAVITATIONS IN CONTROL VALVES?

WHEN A LIQUID ENTERS A VALVE AND THE STATIC PRESSURE AT THE VENA CONTRACTA DROPS TO LESS THAN THE FLUID VAPOR PRESSURE AND THE RECOVERING TO ABOVE FLUID VAPOUR PRESSURE, THIS PRESSURE RECOVERY CAUSES AN IMPLOSION OR COLLAPSE OF THE VAPOUR BUBBLES FORMED AT THE VENA CONTRACTA. THIS CONDITION IS CALLED CAVITATION

18.WHAT IS THE PRINCIPLE OF PRESSURE GAUGE?

MEASURING THE STRESS IN AN ELASTIC MEDIUM

19.WHAT ARE THE LIMITATION OF A LEVEL TROL?

MIN – 12” MAX – 72”

20.WHAT HAPPENS IF THE DISPLACER HAS FALLEN OR HAS A HOLE IN IT?

DISPLACER HAS FALLEN, OUTPUT WILL BE MAXIMUM.

HAS A HOLE IN IT, OUTPUT WILL BE MINIMUM

21.TYPES OF ACTION / TYPES OF ACTUATOR?

AIR TO CLOSE ( DIRECT ) AND AIR TO OPEN ( REVERSE )

22.TYPES OF BODY DESIGN?

GLOBE VALVE, SINGLE OR DOUBLE SEATED.

ANGLE VALVE,

BUTTERFLY VALVE,

PLUG VALVE

THREE WAY VALVE,

PINCH VALVE,

SAUNDERS VALVE

BALL VALVE.

NEEDLE VALVES

23.WHAT IS Cv OF A VALVE?

IT IS THE NO. OF U.S GALLONS / MIN. OF H2O WHICH PASS THROUGH A FULLY OPEN VALVE AT A PRESSURE DROP OF 1 PSI AT 60 DEG. F .

24.WHAT TYPE OF BONNETS WOULD YOU USE FOR HIGH TEMPERATURE AND VERY LOW TEMPERATURE?

HIGH TEMPERATURE BONNETS ARE PROVIDED WITH RADIATING FINS.

LOW TEMPERATURE : EXTENDED BONNETS.

25.WHAT IS THE USE OF POSITIONER?

QUICK ACTION AND POSITIONING OF CONTROL VALVE.

VALVE HYSTERSIS.

VALVE USED ON VISCOUS LIQUIDS.

SPLIT RANGE.

LINE PRESSURE CHANGES ON VALVES.

VALVE BENCH SET NOT STANDARD

REVERSING VALVE OPERATION.

SPRING LESS ACTUATORS.

CHANGING VALVE CHARACTERISTICS.

News Reporter

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