METERING GASES AT ELEVATED PRESSURES
by Paul Devlin
Posted by Devtech
Metering gases at pressures other than utilization pressures (usually 1/4 PSI or 7" w.c.) can save a gas company or industrial user by utilizing a smaller meter. The down side of this is that it can cost considerable money if not done properly.
For years an industry notable preached the gospel using the right index for the right pressures, but he is now retired so I am taking the opportunity to share some of the hazards of metering at elevated pressures.
Always keep in mind the safety concern; is the meter body rated for the higher pressure? Most domestic meters are rated at 5 PSI working pressure (some at 10 PSI).
Another safety concern is to make sure the equipment downstream of the meter can handle the elevated pressures as some appliances can only take 14" w.c. or 2 PSI. These limitations may necessitate a second regulator to reduce the pressure to a safe level for the user.
If you use a standard index and apply other than your contract pressure (that's usually in your tariff with the Public Service Commission or your city regulation), you can use a calculated pressure factor in your computer to obtain the correct volume. The following is how this factor is derived:
fp= gauge pressure + atmosphere
(Example) fp= 7.0 PSIG + 14.73 PSIA 14.98 PSIA = 1.45
If it is not feasible or practical to use this factor in your system, you can use a correcting instrument such as a Mini-Max flow corrector by Mercury Instruments. This instrument will perform this function automatically in an accurate manner as it monitors the pressure and constantly adjusts the pressure factor to provide the ultimate in accuracy.
Another method is to use a compensating index, although this method also has limitations. Indexes are only available in increments up to 10 PSI and the atmospheric and base pressures are fixed. It is recommended to contact any meter company for assistance on making adjustments for these pressures. In addition, a pilot operated regulator upstream is recommended to maintain constant pressure for these type indexes.
Our next article will address a related subject that will address the other major concerns of metering at elevated pressures including proper sizing of upstream regulators.