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Chemical Process Technology

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Wednesday, July 23, 2008

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A question raised by a young engineer.

Should a Flow element (FE) be located upstream or downstream of a control valve (CV) ?

There is no fix rules governing the location of this flow element. It is very much subject to few factors i.e. fluid condition of the measured fluid, properties fluctuation, etc.

a) Fluid Characteristic
A flow element is general prefer single vapor or liquid phase as compare to two phases gas-liquid (2Ph-GL) flow. First to check is the fluid characteristic downstream of control valve. For example, liquid control valve (LCV) maintaining level of a separator. The fluid feeding the LCV is probably a liquid at bubble point. Due to static head it potentially maintain as liquid from separator outlet nozzle upto LCV. However, there is pressure drop across the LCV and 2Ph-GL flow is likely to occur downstream of LCV. Under this condition, it is always prefer to locate flow element upstream of LCV.

Let take another example for vapor flow line. A pressure control valve (PCV) maintaining a constant saturated gas flow to downstream system i.e. fuel gas system from a separator. In the event there is large pressure drop across the PCV, Joule-Thompson (JT) effect results significant liquid formation downstream of PCV. Similarly, it is always prefer to locate flow element upstream of PCV.

b) Pressure & Temperature Fluctuation
Large pressure and temperature fluctuation would probably leads to properties (i.e density) changes. In general, a vapor system would experience severe changes as compare to liquid system. Thus, large pressure and temperature fluctuation may results a large deviation in properties and lead to difficulties in select a good flow element for the service. It is always prefer to locate flow element in the stream with "less" properties change .

For example, a slug catcher is having a pressure control valve (PCV) feeding gas to booster compression system. The booster compression is targeted to maintain a constant pressure of

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the suction scrubber. As slugcatcher pressure (upstream of PCV) may fluctuate due to slugging and pigging operation whilst the pressure (downstream of PCV) at compressor suction scrubber is rather constant, it is always prefer to locate flow element downstream of PCV. It shall be noted that there is potential liquid formation downstream of PCV which prefer to locate PCV upstream of PCV. Both rule-of-thumbs are contradicting between each and other, a process engineer shall weight both impacts to locate it at right location.

c) Pressure Rating
High pressure rating of flow element will be costly. Thus, it is always prefer to locate the flow element at the system with lower pressure rating, downstream of control valve.

d) Vibration
Pressure drop across control valve would partially convert its energy to noise and vibration. The vibration (higher intensity) would travel along fluid flow direction. Some flow element (i.e. Coriolis,Vortex) is sensitive to vibration. Vibration sensitive flow element is always advisable to locate upstream of control valve.

A process engineer may consider above factors to locate the flow element. Apart, process engineer is always advisable to verify this subject with instrument engineer and vendor for proper location and correct flow meter type.

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posted by Webworm, 3:07 PM


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