POSITION OF THE ORIFICE AND THE CONTROL VALVE

[RICARDOARTURO]

THE ORIFICE SHOULD BE LOCATED UPSTREAM OR DOWNSTREAM OF THE CONTROL VALVE??? WHY ?? WHICH POSITION IS MORE ACCURATE?? THE POSITION AFFECTS MORE IF IT IS A LIQUID OR IF IT IS A GAS??? WHERE CAN I FIND SOME INFORMATION??

REGARDS

 

[dcasto]

orifice for what? more pressure drop, for measurement, for fun, for sound abatement, for getting the control valve in its midpoint?

 

[RICARDOARTURO]

The orifice is for fluid measurement.

 

[dcasto]

In liquids the valve is down stream to assure no flashing. On gas streams an upstream control valve can actually shorthen the meter run because its breaks up swirl. Again, flashing and liquid formation means typically downstream.

the API MPM (AGA has information on the industry standards for measurement.

 

[JLSeagull]

Does the control valve control the flow, upstream pressure, downstream pressure, etc. dcasto makes a point not often considered onshore. Generally locate the flow measurement upstream. Also, if you are controlling the pressure upstream then locate the measurement upstream.

 

[RICARDOARTURO]

The function of the control valve is to control the gas flow to a stripping tower.

I've seen a lot of PID's and the flow measurement it's always upstream the control valve (Liquid and gas cases). I wonder the reason and I think it's because we calculate the orifice of measurement with a pressure among other qualities of the fluid (Viscosity, spgr, temperature). The pressure will vary downstream the control valve, but upstream the control valve the pressure will remain constant. I was thinking that maybe we need a constant pressure for the measurement device, the orifice in this case.

Anyway I'll look up in the API MPM that dcasto said.

Regards

 

[JLSeagull]

Most head-type flow elements requires a "fully developed flow profile". This is achieved by long straight pipe or a flow conditioner upstream of the flow element. Elbows within the same plain just upstream of the flow element increase the straight lenght. Multiple elbows in different plains further increases the length.

The API MPMS is a complete manual of standards. I don't have the complete set of the standards. Addressing chapters, the head type flow measurement standards are mostly chapter 14.3. Other standards include chapter 21, chapter 5 for Coriolis, etc.

 

[dcasto]

But a control valve upsteam shortens the meter lenght.

 

[RICARDOARTURO]

If you're designing you have to assure that there is a straight run enough for your meter.

I meant that the orifice should be upstream the control valve. In order to don't short the straight run of the meter you have to assure that there is enough straight pipe (I guess is 5 pipe diameters downstream the orifice and 20 pipes diameters upstream the orifice)

 

[TD2K]

It's an unusual installation where the orifice plate would be located downstream of the control valve in my experience.

The instrumentation engineer wants, as mentioned, a fully developed flow profile into the meter. The straight piping requirements will vary depending on the upstream piping configuration and what type of error you can accept. A process control application is less critical obviously than a custody transfer service.

 

[vzeos]

In natural gas metering &regulating stations, the orifice meter is always upstream of the pressure cut because it is more accurate to meter at higher pressures. To illustrate, take a look at the orifice equation

Q_h = C’[√]h_wP_f

Q_h = Volumetric flow rate, cfh
C’ = Modified orifice constant
h_w = Differential pressure, inches of water
P_f = Metering pressure, psia

Assume P_f is measured with a transducer that has an error of 1 psi. A 1 psi error relative to P_f = 52 psia will introduce much more total error than a 1 psi transducer error relative to, say, P_f = 520 psia.

There are requirements for upstream and downstream lengths unobstructed pipe or flow conditioning. You should get a copy of AGA Report 3 for specifics. By the way, control valves and regulators also have requirements for lengths of unobstructed pipe up and down stream. Check the manufacturer’s specs.

The attached paper is pretty informative.

http://www.ceesi.com/docs_techlib/events/ishm1994/Docs/112.pdf

 

[pipehead]

transducers have error as a percent of range. A 1% error on a 0 to 2000 psi means 20 psi so put your 52 psia meter spanned to 0 to 2000 psia and 20/520 = ? meanwhile I'll put a 0 to 100 psia in on my 52 psia and be off 1/52. Who wins now?

Oh and because pressure is under the radical, the error is is cut in half (approximately).

 

[vzeos]

pipehead:

You must be a technician. Obviously my illustration was not intended for someone of your level of sophistication. If you look at the illustration for what it is, an illustration, you will begin to understand why the natural gas industry installs orifice meters up stream of the pressure regulation. It's not about winning.

 

[djack77494]

Passage through a partially opened valve creates about the least developed flow profile possible. As a result, a long run of straight pipe would be required to return the flow pattern back to "fully developed". This is one of several reasons for generally preferring to have the flow measuring orifice upstream of the control valve.
Doug