Gas pressure and meter readings

Mike Kusiak_2

When you have gas metered and entering the house at say 2 psi, how do you compensate  when clocking the meter to check the firing rate? Since the density of gas increases with pressure, one cu ft of gas at 2 psi will deliver more BTU than one cu ft after being regulated down to 7" or 8" wc at the boiler. What is the standard pressure at which 1 cu ft of gas delivers about 1020 BTU?



Does the gas company then adjust the measured reading at 2 psi to bill you for therms with  different conversion factor?



What brings this to mind is the discussion in this thread:



 <a href="http://www.heatinghelp.com/forum-thread/139525/System-load-vs-Burner-output">http://www.heatinghelp.com/forum-thread/139525/System-load-vs-Burner-output</a>



The owner was advised to clock the gas meter to check his firing rate, but the results are inconsistent with the specs of the burner and the actual performance of the system. Gas is metered at 2 psi and then regulated down to less than 1/2 psi at the boiler.



So in this case, how do you accurately measure the firing rate by clocking the meter?

Tim McElwain

Mike a good question

Gas pressure and meter readings

When you have gas metered and entering the house at say 2 psi, how do you compensate  when clocking the meter to check the firing rate? Since the density of gas increases with pressure, one cu ft of gas at 2 psi will deliver more BTU than one cu ft after being regulated down to 7" or 8" wc at the boiler. What is the standard pressure at which 1 cu ft of gas delivers about 1020 BTU?





TO  THE BEST OF MY KNOWLEDGE I HAVE ALWAYS CALCULATED A CUBIC FOOT THE SAME NO MATTER WHAT THE PRESSURE PASSING THROUGH THE METER. THE REASON BEING IT IS REDUCED TO WHATEVER THE REQUIRED OUTLET PRESSURE IS ON THE RATING PLATE WHICH CAUSES IT TO FIRE AT RATED INPUT ALSO ON THE RATING PLATE. THIS IS HOW WE SIZE ORIFICES SO WHAT DIFFERENCE DOES THE PRESSURE DELIVERED MATTER. REGULATORS CONTROL PRESSURE ORIFICES CONTROL FLOW.







Does the gas company then adjust the measured reading at 2 psi to bill you for therms with  different conversion factor?







YES A DIFFERENT METER IS REQUIRED ON REQUEST FROM THE UTILITY WHICH AS ALL METERS DO MEASURES AT A COMPENSATED TEMPERATURE OF 60° F. IT IS ALSO CALIBRATED FOR PRESSURE DIFFERENCE AND ALSO MOST METERS IF YOU LOOK AT THEM ARE RATED FOR UP TO 5 PSIG.





What brings this to mind is the discussion in this thread:





 http://www.heatinghelp.com/forum-thread/139525/System-load-vs-Burner-output





The owner was advised to clock the gas meter to check his firing rate, but the results are inconsistent with the specs of the burner and the actual performance of the system. Gas is metered at 2 psi and then regulated down to less than 1/2 psi at the boiler.





So in this case, how do you accurately measure the firing rate by clocking the meter?





THE METER WILL ONLY MEASURE WHAT FLOWS THROUGH THE METER SO CLOCKING SHOULD NOT BE AFFECTED. I FIND MANY CONTRACTORS DO NOT CLOCK BURNERS CORRECTLY AS FAR AS CALCULATIONS GO.







To make sure I am correct I will run this by one of my friends at gas measurement at the utility.



In looking at for example piping tables for sizing gas pipe the tables list on Page 37 of the International Fuel Gas Code Natural gas, inlet pressure of 5 lbs, allowable pressure drop of 3.5 psi at specific gravity of 0.60 in fact in all the charts the specific gravity is .6 regardless of pressure.

VictoriaEnergy

Correct clocking of a 2PSIG Meter

When ever you have a high pressure meter set, you must compensate for the increased volume of gas at low pressure.  So multiply your calculated consumption by 1.13

So say you take 34 secconds on a 1 cu foot dial:



3600 [secs /hr] X 1 [cu ft dial size] X 1020 [local btu value of gas]

________________________________________________

                               34 [secconds]



=108 000 X 1.13 [2 PSIG conv]= 122 040 BTUs / hr actual consumption rate.



Note due to the limit of how accurate you are eyeballing the time, your calculation will only have accuracy of about +/- 3%



The gas utility calculates the difference between differing service pressures in the billing process. 

VictoriaEnergy

z

bob_46

Mike

If you are wondering where the correction factor comes from.

C.F. = meter pressure (PSIA) / sales pressure (PSIA)

Sales pressure is usually 7" W.C. = .25 PSI

Note; to convert to PSIA add atmospheric pressure at the meter location.

Example: for 2 LB. service.

Meter pressure = 2 + 14.735 = 16.735PSIA divided by sales pressure = .25 + 14.735

= 14.985 PSIA = C.F. 1.116

Mike Kusiak_2

Thanks for the info

After I posted the question, I also found this PDF which confirms what was said above regarding the calculation for the correction factor for higher pressures than standard and the difference it makes when clocking the meter.



http://www.elster-americanmeter.com/en/downloads/EAM-OM0001.pdf



Its interesting that the local atmospheric and standard pressure at a given location is taken into account when doing the correction.



In my area gas is always delivered from the street at about 7" to 8" wc, so I never considered the possibility of clocking at higher pressures.

Tim McElwain

Mike and others

I want to thank my good friend Bob Morse (Connecticut Natural Gas Company) for steering me in the right direction on this topic.



If you have a 2009 version of NFPA 54/ANSI Z223.1 National Fuel Gas Code go to page 54-118, 119 and 120 for the formula and also Table A.11.1.1 (revised in the 2009 edition) for a table that gives gas flow rates at 7" W.C.(0.25 psi) 11" W.C.(0.40 psi) and 55.4" W.C.(2 psi). If anyone wants that data and you do not have access to NFPA 54 contact me at gastc@cox.net and I will either fax it to you or for a postal address mail it to you.



I knew I had seen something on this just could not find it so now we know, I am going to actually alter one of my manuals I sell to include this information. So much we needs to know to do this job correctly.