DP Flow Measurement

[Scada Tom 0]

I have a pending application to measure flow across a flow control valve using differential pressure (its accurate to +/- 3 %). Traditional method is to use a DP transmitter, but I understand the Compact Logix can do a square root math calculation to get flow rate. If we can use the compact logix then it would require to pressure transmitters. Which would be preferreable - compact logix, square root, two press trans, or a differential pressure transmitter? Is there a difference in accuracy?

[Michael Lloyd 85]

Upside to using the two transmitter approach: You get two pressure values (up and downstream). Downside to using the two transmitter approach: Calibration errors or drifting values will give you erroneous DP readings (the same thing is true of the single transmitter) Flow across a control valve? That's a little tougher than calculating flow through an orifice. For one thing the area is variable. As little as a 0.1% change in position could resulte in a large change in flow if the valve is large and/or DP across the valve is high. Then there are the effects of flashing and cavitation. If the flowing medium reaches sonic velocity at the vena contracta flow does not increase as area increases so your flow calc show increased flow rate when in fact it has not and cannot increase. You would have to do a curve fit for the Cv of the control valve vs position to extrapolated the values for Cv. Typically Cv's are given in 5 - 10% increments.

[ieseberri 0]

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http://instrumentationandcontrol.net/

[DanW 41]

This is now a four year old thread, but given how threads come up in searches I'll give an answer:

>"Which would be preferreable - compact logix, square root, two press trans, or a differential pressure transmitter? Is there a difference in accuracy?"

1) Two or one transmitters

Whether measuring pressure drop across a flow element or a filter, one always, ALWAYS, uses a differential pressure transmitter, one with two ports, and high side and a low side.

Using two separate pressure transmitters is folly.  The combined error can put you in the 10's of percent error.  In fact, at low flow, the errors could combine arithmetically to indicate reverse flow, because the basic calculation is high side minus low side.  The nature of percent full scale accuracy means that the error increases as the signal approaches the low end of the scale and is greatest at the low end.  If the low side transmitter errs on the high side and high side transmitter errs on the low side, the subtraction of the two could drive the result negative, for low DPs.

2)  where to extract the square root

Differential pressure transmitters accuracy is spec'd on the DP, not on the line/static pressure, which is what would be used with two separate Gauge pressure.transmitters.

The instrumentation world is divided as to whether the square root should be extracted in the transmitter or the receiver (PLC).  I favor square root extraction in the transmitter because a) it's the most accurate, it's internal to the transmitter, b) the output signal is linear over the range of determined by the sizing of the primary flow element (the orifice plate).

But there are lots of others who extract in the receiver.  An all too common problem (for people who should know better) is erroneous double extraction, in both the transmitter and the receiver.

3)  flow across a control valve

If getting accurate flow rate measurement across a valve was feasible and reasonably easy, lots of people would do it.  Not only could one avoid the cost of a primary flow element and its installation, measuring flow across a valve would avoid another process pipe intrusion.   It would be a popular measurement technique.   But it isn't easy and I can't even recall where I've ever seen that done and I'm an instrument guy, that's what I do.  

A major problem with measuring across a valve is flow profile.   Established DP Primary Flow elements, orifice plate, venturi, averaging pitot tube, or nozzle all require fairly extensive straight pipe upstream/downstream in order to develop a flow profile at the DP element.   Valve manufacturers design valves to create pressure drop which takes energy out of the system, valves are in now designed to create a clean flow profile suitable for measurement.  All sorts of phenomena will affect the flow profile (if there is one) and the subsequent DP across a valve, like Mr. Lloyd mentions.

Primary flow elements used for flow measurement are over 100 years old with lots of supporting data for the relative accuracy claims.  Probably the least expensive primary flow element including installation is the Averaging pitot tube.   Any of these beat the task/effort to characterize flow across a valve and its questionable reported flow rate.