Instruments infomartion used in piping system designing

Following points shall be taken care of while routing piping for instruments.

A.   Flow measuring instrument needs certain straight length on upstream & downstream of the instruments. Normally, 15D on the upstream and 5D on the downstream is kept.

B.   The pipe line in which flow meters such as magnetic flow meters, vortex meters, turbinemeters etc are located shall be routed in such a way that the line must be filled with liquid all the time. The pipe line shall be supported with robust support on both side of the meter.

C.   Control valves are located at grade e.g. at about 500mm height from finished ground to provide convenient access for operation and maintenance. Block and bypass valve shall be located to have easy operation/ access from the grade. Locating control valve on the vertical line shall be avoided.

D.   Isolation valves for level gauges and pressure gauges shall be made accessible. All primary and secondary indicators of pressure, temperature, flow, level, positioners etc shall be visible from the operating area.

E.   Rotameter shall be placed on vertical line and the inlet shall be from the bottom of the instrument.

Question related to instruments used in piping

  •  What are instrument categories?
  • Ans : General, flow, temperature, pressure, level, density etc.
  • 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.
  • Which instruments are handled by piping department ?
  • Ans : flame arrestor, breather valve.
  • What are basic functions of instruments?
    Ans : To sense, transmit, indicate, read and control.
  • Why block valve is used at the root of an instrument like pressure indicator?
  • Ans : They are needed to isolate gages for maintenance during plant operation and during hydrostatic testing of piping system. They are called root valve.
  • 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.
  • How control valve block valves should be located ?
  • Ans : They should be as close to control valve as possible, considering drain requirement and handwheel 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.
  • Where diaphragm assembly is used?
  • Ans : For corrosive, abrasive, viscous fluids. Neutral fluid like glycol is used on instrument side of diaphragm.
  • 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 gage and its isolating valve, to relieve pressure and /or drain the liquid before servicing the gage. It can also be used to sample or for adding a comparison gage.
  • Why control valves are usually flanged?
  • Ans: For ease of installation and removal during maintenance.
  • 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.
  • 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.
  • 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.
  • How TSO valve should be installed?
  • Ans: Tight shut off (TSO) valve or reduced port ball contrl valve should be installed in vertical for gas service and horizontal ok for liquid service.
  • How bypass valve in control station should be located?
  • Ans : Near inlet branch point, not above control valve, with 200mm clearance between actuator and valve.
  • 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.
  • Where reducer shall be located in control valve assembly?
  • Ans : As close as possible to control valve, preferably directly welded control valve flanges.
  • 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.
  • 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.
  • 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 vapor lines, it should be vertical or 45 to vertical on both sides. Taps are 0.5 inch.
  • What are straight run requirements for vortex meters?
  • Ans : 5D upstream.
  • Where restriction orifice plates are used ?
  • Ans : For effective distribution of fluid in piping network.
  • What are types of level measurement? 
  • Ans : Direct and inferential
  • 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.
  • What are inferential methods of level measurement? 
  • Ans : Hydrotatic head, buoyancy, conductance, radiation, float.
  • 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.
  • What are pressure instruments used in chemical industries? 
  • Ans : Manometers, bourden tubes, bellows and diaphragms.
  • What are manometers?
  • Ans : Single or U type glass tube with mercury or other fluid. Inclined tube for very low pressure measurement used.
  • What are types of bourdon tubes?
  • Ans : C, the spiral and helical, most widely used since 100 years.
  • What are temperature measuring instruments?
  • Ans : Thermocouples, resistance temperature detectors, filled system, bimetallic thermometer, thermowells.
  • What thermocouple metals normally used? 
  • Ans : Iron + Constantine, Chromel + Alumel etc.
  • How resistance thermocouples work ?
  • Ans : Electrical resistance of a conductor changes with temperature.
  • How filled system temperature element work?
  • Ans : Bourdon tube, filled with liquid, liquid expands, bourdoun tube indicates on scale temperature.

  • How bimetallic thermocouple work?
  • Ans : Different expansion of different metals bends the bimetallic strip to indicate temperature. Invar and Nickel pair used.
  • What is minimum pipe size required for thermowell installation?
  • Ans : 3”

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