Orifice Meter Max Differential Pressure
Orifice Meter Max Differential Pressure
(OP)
What is the maximum recommended differential pressure across an orifice meter run in a lean natural gas service?
Normally the meter would be sized for around 75" H2O so you can use a 0-100" H2O transmitter. What if you have an existing meter run and don't want to change anything, but you want to flow a lot more gas? Can you just re-range your transmitter to read upwards of 2000" H2O and howl the gas through it (assuming you have the inlet pressure required)? Is a bent/eroded plate the worst thing that could happen? What would the symptoms be of a bent/eroded orifice plate? What happens to the meter accuracy?
Normally the meter would be sized for around 75" H2O so you can use a 0-100" H2O transmitter. What if you have an existing meter run and don't want to change anything, but you want to flow a lot more gas? Can you just re-range your transmitter to read upwards of 2000" H2O and howl the gas through it (assuming you have the inlet pressure required)? Is a bent/eroded plate the worst thing that could happen? What would the symptoms be of a bent/eroded orifice plate? What happens to the meter accuracy?





RE: Orifice Meter Max Differential Pressure
If you have a 2-inch meter run with a 1.5 plate, you can "measure" upwards of 24 MMCF/d at 1,000 psig and 2000 inH2O at 80F, but your velocity is 190 ft/sec and you are going to scour any passivation layer off the steel.
Thinking back on the assumptions that went into the development of the equations, I'd think that as long as the plate didn't bow you'd be good up to about 0.3 Mach, (somewhere around 300 ft/sec depending on specific fluids and pressure/temperature). If the plate bows then you'll be reporting low by something on the order of 15% (to say nothing of the difficulty of removing the plate from the fitting). I can't remember a specific Reynolds Number upper limit (the example above is over 15 million), but there may be one.
David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
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RE: Orifice Meter Max Differential Pressure
Study the plate thickness selections in the API standards and find that the differential pressure is capped at 1000 inches water.
For gas applications a rule of thumb limits the pressure differential in inches water to the inlet pressure in pounds per square inch. You don't want the meter to drop a significant percentage of the available pressure just to measure the flow rate.
The normally sizing for 100-inch water dp accommodated the 100 inch water mercury manometers. You can buy a smart dp capable of 1000 inch water and select 0-50 inches with suitable accuracy.
If you have an existing meter run you can increase the orifice bore toward a beta ratio of bore and pipe inside diameter toward 0.7.
Other technologies exist. An external clamp-on transit-time (time of flight) ultrasonic meter may be suitable for some non-custody applications.
Apporacing 2000 inches water will bend a large plate. Check into the API MPMS for guidelines. Gas flow is covered in 14.3. API Chapter 14 “Natural Gas Fluids Measurement,” Section 3, “Concentric, Square-Edged
Orifice Meters,” Part 3, “Natural Gas Applications” of the Manual of Petroleum Measurement Standards, Third Edition,
August 1992.
RE: Orifice Meter Max Differential Pressure
If you have an upset condition that causes an offscale high flowrate, I doubt if you would create enough DP to hurt the plate. If you are worried about that, 1/4" thick plates are available. Where you might be concerned is if the high flow picks up particles in the system and the edges of the orifice are eroded. For accuracy, the edges of the orifice must be very sharp and uniform, not eroded or dinged by chunks of weld slag.
DP instruments are almost universally protected against overrange. You are not likely to hurt the transmitter with a flow surge.
RE: Orifice Meter Max Differential Pressure
Orifice plates are not expensive. Could you not change the bore to suit your new flow conditions?
Or, are you needing to change online without breaking the line? In that case, would a senior or rotary changer work?
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RE: Orifice Meter Max Differential Pressure
You just have to ask yourself what is you gas accuracy is worth to you and any related down down for plate retraction.
RE: Orifice Meter Max Differential Pressure
At that extemely high of pressure drop (2000 iWC = 72.22 psig) that you are talking about, I would think that you would reach sonic velocity (but depending on your inlet pressure). At that point all accuracy is shot, because the orifice plate will act as a choke (i.e. decreasing downstream pressure will not increase flow).
RE: Orifice Meter Max Differential Pressure
The 20" to 80" limit is only for paper chart integration and it is really an error caused by the act of integrating the charts. If you read the transmitter specifications they state accuracy (keep it simple here no repeatability, bias, ect analysis) as % of range so it typically .15% of range, so you should always try to run as high in the range as possible for the least amount of error.
RE: Orifice Meter Max Differential Pressure