Can anyone tell me about this...

What happens when the DP that has been generated by orifice plate is not recovered properly, Is there permanent loss in line pressure? What are the consequences? What are the bad effects that will occur?

 

When designed properly the pressure loss is very low. I am not sure what you mean by not recovered. The diff press that is used to measure flow across an orifice plate is very sensitive and is measuring a very low DP.

Carl Petersen

 

The pressure drop across the orifice plate is never recovered. Usually this is a small amount compared with the pressure in the line. If the process conditions are such that a drop in line pressure does not allow proper process operation, then there can be a problem. If so, you may want to consider using a measurement technology that does not cause as significant pressure drop such as ultrasonic, magnetic, turbine, or vortex shedding flowmeters.

Dick Caro
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CMC Associates
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Tel: +1.978.635.9449 Mobile: +.978.764.4728
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Per ASME estimates the pressure drop that is permanently lost with a sharp edged orifice plate varies from about 45% to nearly 100% of the sensed DP based principally on the Beta ratio (d/D). The smaller the Beta ratio the higher the drop. A properly sized orifice plate will typically lose 40% of the sensed DP to permanent pressure loss. This loss is from the turbulence being converted to heat energy. The main consequence of the loss is the required pressure energy to maintain a certain system pressure. In some cases where a a loss is required anyway, this isn't a problem but a welcomed attribute. In other systems such as a low pressure fan driven system even very small losses will require a bigger and hence more expensive fan. There are other more subtle consequences that may also affect a process system.

Sincerely,
John Catch
http://www.inflowinc.com

 

hi vinod

there two type of funtion of orifice
1> measuring orifice, its design criteria is dp and beta
ration (for measuring loss)
2> restiction flow orifice, its design criteria is permanent pressure loss (PP loss) (for reducing pressure in process)

For measuring orifice if design permanent pressure loss is higer the process allowed, then you can expand pipe dia. to reduce PP loss. So i think that PP loss is not big issue for orifice and PP loss is useful for process also
(pressure reduction). i think it is useful to you.

Shailesh C Patel
FDC - Control Systems And Instrumentation
Reliance Engineering Associates(P) Ltd.
Jamnagar Refinery Complex

 

Thank you very much Mr. John Catch.

Please clear my confusion that the relation between beta ratio and straignt pipe clearance on either side of the orifice plate. (branch free, tapping free)

That is 10D on upstream side and 5D on downstream side. As beta ratio varies is this clearance varies? Because in our application we don't have that much clearance.

Please give reply to this.

 

There is no application in which 15 pipe diameters of straight unobstructed piping is sufficient for an orifice plate. I don't have the ISO requirements in front of me but from memory, at a beta ratio of .6, you need approx 60 diameters. Requirements are much higher if upstream piping includes two elbows in close proximity in different planes.

If all you have to work with is 15 diameters of straight pipe and you need an accurate measurement of flow, you need to select a different metering technology. If a very rouogh measurement is all you need, there are probably cheaper ways of getting it than an orifice plate. If you need some help selecting a meter, send me the process data and the available straight runs of pipe and I'll give you some suggestions. [email protected]

Joe Hohn

 

ISO, etc. are only suggested pipe layouts to minimize flow distortion and to therefore achieve good metering results. A number of papers by Mitsubishi, Vortab, and others go into far greater detail than a few charts. Major factors such as Reynolds number, fluid type, pulsation type and frequency, pipe friction factor, etc. all are in the equation. Our own research shows that even with numerous elbows, etc. good profile can still be acheived in pipe sections far different from that of the "charts". After performing many velocity traverses I can say that there is no majic number that will guarantee perfect flow conditions. I can also say that there are many thousand of applications where the old rule of 10D/5D works perfectly satisfactorily.

What we are discussing is flow velocity profile not meter type. This is a matter of physics not meter brand. A vortex meter, turbine, or other type may be just as corrupted by swirl, and profile distortion. If custody transfer accuracy is required and you have an appropriate
budget, than I would suggest a meter run. Such a meter run would include inlet and perhaps outlet flow conditioning plates, straight honed pipe, meter type of your choice, and a NIST traceable system calibration.

A good reference book on this and other metering subjects is R.W. Miller's "Flow Measurement Engineering Handbook" available through McGraw-Hill.

Sincerely,
John Catch
http://www.inflowinc.com

 

Dear Friend,

According to me if your Orifice plate is sized for your specific requirement and even you have taken care of Beta ratio, you wil be getting the flow exactly by measuring the DP across the orifice plate and as well by calculating through AGA Calculation. If you are not able to get the DP value properly I don't think that you can have a Accurate Flow measurement... and when the Beta ratio you have designed is too low then the Pressure Drop wil be high so try to maintain the Beta ratio b/w 0.28 to 0.65.

All the Best