In natural gas metering ®ulating 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.