It depends on what you are trying to do. Low Beta ratios are associated with dramatically increased uncertainty. For custody transfer I will change out a meter tube before I'll go below 0.34, the uncertainty at 0.2 is just too high.
Now if you are trying to get a +/-15% number for plant balancing then it shouldn't be a problem to go below 0.2, but the uncertainty is an exponential function that gets really ugly in that range. You are basically turning the process into a random number generator.
One of the steps in completing a new gas well is to do a flow back and "measure" the gas. The drillers use a Haliburton table that only requires orifice size (and it goes down to 0.05 Beta I think) and upstream pressure to come up with a volume. The calculation assumes atmospheric pressure (at sea level) downstream of the choke. Most of the chokes are near the wellhead and the exhaust pipe can be 150 ft of 2-inch pipe that jumps around like a frog so instead of the dP being (100 psi - 0 psig) it is actually (100 psi - 96 psi) due to friction and work done to move the pipe. This "measurement" is reported to the state and to management and everyone gets excited about the huge well that just came in. Then we put it on production and struggle to make 1/4 of the driller's number. I bring this up to illustrate that we regularly violate good Engineering practices in gas measurement and then misuse the data from the nonsense. In other words, there are precedents for what you are suggesting.
Now if you are just restricting the maximum flow in a line (without trying to convert the dP to a volume), there really isn't an industry rule about how small an orifice you can use. It just comes down to Engineering judgement.
David Simpson, PE
MuleShoe Engineering
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