Gas Pressure Regulator Venting 2

[CelsoIII]

We recently replaced an existing single gas pressure regulator feeding that was feeding three boilers with three new dedicated pressure regulators to improve pressure control. The pressure after the new regulators was fluctuating until we disconnected the new ½” vent lines connected each of the regulator vent ports. The boiler technicians indicated that the pressure regulator vent lines were under sized and this was causing pressure fluctuations on the gas lines feeding the boilers.
The three new vent lines are ½” galvanized pipe, 60’ ft long and have about five 90 deg elbows. Is there a “rule of thumb” or local code (California) for sizing gas regulator vent lines based on the length of the pipe?
Can you please point me in the right direction? I am not afraid of reading but I just can’t seem to find the appropriate source for this information.

Thanks,

Celso

 

[CelsoIII]

I made some corrections to the original post. There is no option to edit text after it has been posted...
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We recently replaced an existing single gas pressure regulator that was feeding three boilers with three new dedicated pressure regulators to improve pressure control. The pressure after the new regulators was fluctuating until we disconnected the new ½” vent lines for each of the regulator vent ports. The boiler technicians indicated that the pressure regulator vent lines were under sized and this was causing pressure fluctuations on the gas lines feeding the boilers.
The three new vent lines are ½” galvanized pipe, 60’ ft long and have about five 90 deg elbows. Is there a “rule of thumb” or local code (California) for sizing gas regulator vent lines based on the length of the pipe?
Can you please point me in the right direction? I just can’t seem to find the appropriate source for this information.

Thanks,

Celso

 

[317069]

How did you size the vent pipe as 1/2"?

 

[RossABQ]

If you are talking about the diaphragm vent lines, there should be zero flow thru them unless a diaphragm is ruptured. I can't see why removing them would make any difference provided the diaphragms are intact, and surely you'd notice if they weren't.

I would suspect that your new regulators are operating out of their stable control range, likely too big for stable part load operation. You may need a split-range control setup with a smaller regulator for lower loads with the big regulator set for slightly lower pressure.

 

[PEDARRIN2]

ASHRAE 15-2007 indicates how to size the relief vents of pressure regulators for refrigerant piping. It is applicable to natural gas as well. It will enable you to determine the pipe size for the length of vent piping you have. Use 10%-15% maximum back pressure.

IF the regulator has an internal relief - there should be tables that indicate the flow it will release. Otherwise you have to determine what the appropriate flow.

Also, if the vent pipe is too small, there can be "backpressure" on the diaphragm during normal operation (even with no diaphragm failure) which can cause fluctuations. We had a similar installation where the owner initially fixed the problem by loosening the fittings on the vent piping.

 

[BronYrAur]

Are you combining multiple vents together? See attached for some guidelines.