TOP PIPING/INSTRUMENTATION INTERVIEW QUESTIONS
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1.What are instrument categories?
Ans: General, flow, temperature, pressure, level, density.etc.
2.Which instruments are handled by piping department ?
Ans: flame arrestor, breather valve.
3.What are basic functions of instruments?
Ans: To sense, transmit, indicate, read and control.
4.Why block valve is used at the root of an instrument like pressure
indicator?
Ans: They are needed to isolate gauges for maintenance during plant
operation and during hydrostatic testing of piping system. They are
called root valve.
5.Where diaphragm assembly is used ?
Ans: For corrosive, abrasive, viscous fluids. Neutral fluid like glycol
issued on instrument side of diaphragm.
6.Where and why bleed valve is used in instruments ?
Ans: If the conveyed fluid is hazardous or under high pressure, a branch
fitted with a bleed valve is inserted between the gauge and its isolating
valve, to relieve pressure and /or drain the liquid before servicing the
gauge. It can also be used to sample or for adding a comparison gauge.
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7.Why control valves are usually flanged?
Ans: For ease of installation and removal during maintenance.
8.Where ball control valve is used?
Ans: Suitable where nature of fluid is slurry form or handling two phase
flow having particle in suspension, for non-critical and critical fluids
having a fluid temperature to suit the soft seat material and where
relatively low pressure drop is required across control valve. They have
side mounted actuator.
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9.Where butterfly control valve is used ?
Ans: Used for large size piping network handling clean fluid for low
pressure drop across control valve and soft seated control valve. Metal
seated valves used for higher temperatures.
10.How control valve block valves should be located ?
Ans: They should be as close to control valve as possible, considering
drain requirement and hand wheel clearance. At least one of the block
valve should be placed in vertical so that spool can be removed allowing
the control valve to be removed.
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11.How TSO valve should be installed ?
Ans: Tight shut off (TSO) valve or reduced port ball control valve
should be installed in vertical for gas service and horizontal ok for liquid
service.
12.How bypass valve in control station should be locate
Ans: Near inlet branch point, not above control valve, with 200mm
clearance between actuator and valve.
13.How control station should be supported ?
Ans: It should be supported such that control station piping is self
supporting even if the control valve is removed. Normally, on exit side
elbows of bottom run of control valve.
14.Why and where drain should be located in control valve
assembly ?
Ans: Drain is used to drain the piping between two block valves on two
sides of control valve before removing control valve for maintenance. It
must be located at lowest point of bottom run. One drain is used if
control valve is fails open, and two drain used if control valve fails
close, as both sides are blocked in this case. For one drain, it should be
upstream side of control valve.
15.Where reducer shall be located in control valve assembly ?
Ans: As close as possible to control valve, preferably directly welded
control valve flanges.
16.How breakup flanges for control valve should be located ?
Ans: One vertical and one horizontal which is achieved by having one
block valve in vertical leg of control valve inlet. Do not place support on
inlet elbow.
17.What are the types of flow measuring devices?
Ans: Orifice plate, Variable area flow meter i.e. rotameter, magnetic
flow meter, turbine flow meter, positive displacement meter.
18.What is piping and instrument scope of supply for orifice meter ?
Ans: Piping provided gaskets and bolts and tapping from orifice plate to
two block valves. Instrument provides orifice plate and flange assembly
and connection down stream of block valves.
19.How orifice flange taps should be oriented ?
Ans: For liquid and steam, it should be horizontal or 45 degree down
from horizontal on both sides. For gas and vapour lines, it should
bevertical or 45 to vertical on both sides. Taps are 0.5 inch.
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20.What are straight run requirements of orifice plate ?
Ans: Inlet straight run requirement can be 15D to 20D and outlet
straight run requirement 5D min. It depends on piping configuration and
d/D ration i.e. ration of inside dia of orifice plate and pipe. It is to be
confirmed with instrument engineer in instrument specification of
project.
21.What are straight run requirements for vortex meters?
Ans: 5D upstream.
22.Where restriction orifice plates are used ?
Ans: For effective distribution of fluid in piping network.
23.What are types of level measurement?
Ans: Direct and inferential
24.What are direct methods of level measurement?
Ans: Gauge glasses, reflex or transparent, calibrated tapes or dip sticks
placed in vessel and calibrated directly in level or volume.
25.What are inferential methods of level measurement?
Ans: Hydrostatic head, buoyancy, conductance, radiation, float.
26.What is hydrostatic head type level transmitter?
Ans: Installed directly on nozzle, uses pressure sensing assembly to get
the level. Diaphragm type used for liquids with solids.
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27. What are pressure instruments used in chemical industries?
Ans: Manometers, bourden tubes, bellows and diaphragms.
28.What are manometers?
Ans: Single or U type glass tube with mercury or other fluid. Inclined
tube for very low pressure measurement used.
29.What are types of bourdon tubes?
Ans: C, the spiral and helical, most widely used since 100 years.
30.What are temperature measuring instruments?
Ans: Thermocouples, resistance temperature detectors, filled
system,bimetallic thermometer, thermowells.
31.What thermocouple metals normally used?
Ans: Iron + Constantine, Chromel + Alumel etc.
32.How resistance thermocouples work ?
Ans : Electrical resistance of a conductor changes with temperature.
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33.How filled system temperature element work ?
Ans : Bourdon tube, filled with liquid, liquid expands, bourdoun
tubeindicates on scale temperature.
34. How bimetallic thermocouple work ?
Ans: Different expansion of different metals bends the bimetallic strip
toindicate temperature. Invar and Nickel pair used.
35.What is minimum pipe size required for thermowell installation?
Ans: 3”
36.What are design considerations for pipe rack ?
Ans: Rack width, no of levels and elevations, bent spacing,
pipeflexibility, Access and maintenance of each item in pipe rack.
37.What are steps in rack design?
Ans: Generate line routine diagram, Calculate bent spacing, set width of
rack, decide elevations and levels.
38.What is pipe bent?
Ans: Pipe bent consists of a vertical column or columns and a horizontal
structural member or members that carry piping systems, usually above
the headroom.
39.What factors affect pipe span?
Ans: Size, Schedule, liquid or vapor, insulated or bare pipe.
40.What are spandrels?
Ans: Horizontal structural members located along the
longitudinalcenterline that are used for structural stability, pipe support
orintermediate pipe bents.
41.Which lines placed where in rack ?
Ans: Process lines on lower level, utility lines on top level, instrument
and cable trays on utility level or separate topmost level, Heavy lines
near columns, Flare line outside rack on cantilever beams or inside rack
above top level , steam lines with expansion loops on one side of rack,
line switch orifice runs on one side of rack beside columns for
maintenance using portable ladder.
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42.How much extra space required for rack ?
Ans: 20 % extra for future lines.
43.How to route steam lines on rack ?
Ans: Steam headers on top level or rack, determine growth by
multiplying coefficient (From nomograph) of expansion by length of
line.Try anchor in middle, determine growth of each branch and see
weather the have enough flexibility, if not anchor at ¼ th distance on
both sides,determine amount of expansion leg from nomograph, and
decide loopsize. Line that require largest expansion leg should be
located on the outside of the loop.
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