Tunnel oven gas pressure. 1

[rsbuck]

Hello all, I hope this is the right forum. I am new to this site but hope I may glean some knowledge on a natural gas regulator and outlet pressure. This is not my specialty but this problem has been thrust upon me. The 120 foot tunnel oven gas is supplied by 2" pipe through a regulator to a Maxon valve. The inlet pressure to the regulator is 5.5 lbs. and the outlet pressure must not exceed 28 inches of water. The minimum outlet pressure must not be less than 8 inches of water for proper oven operation. There is a "HI-LIMIT" switch immediatly in front (up stream) of the Maxon valve. This switch has been jumped out since before I worked here. I replaced the hi limit switch and removed the jump but the oven would not start because the switch was open due to 40 inches of water read by gauge at that location. The upper limit of the hi limit switch is 25 inches of water. By jumping out this switch and allowing the Maxon to open, like it has been for many years, the gauge reads only 8 inches of water. My boss thinks that the regulator is bouncing when the Maxon slams shut when the oven is shut down. There is 25 feet of pipe between the regulator and the hi limit switch. As you can see there is no real problem with the regulator doing its job but only when the Maxon valve is closed. The Maxon valve is spring loaded and closes very fast. Then the regulator bounce (bosses theory) causes the gas pressure to rise in this 25 foot section of pipe to 40 inches of water and the hi limit switch opens like it should. Question: Should there be another type of relief valve on this section of pipe when the oven is not in use to bleed off this excess gas pressure when the oven is not in use? Does my description sound like the piping components are correct? The regulator had been replaced in the past but I do not have the reasons for this replacement. Sorry to take up so much space but after reading other threads I wanted to try and provide a complete picture.

 

[josephn16]

From what I can get from your explanation, is seems like the set-up is correct.

What I do not understand is if the hi-limit is only tripped when the safety valve is shut, then where is the problem? It's impossible to start the kiln with the safety valve shut, anyway. If your only question is whether there should be a relief valve, then I have read too much into this, and I would say no, you do not need to worry as long as the pipe is rated to handle 40"wc, which I think it can since this is less than 2psi.

 

[rsbuck]

Hey josephn16, Sorry I took so long to respond. The problem was solved (?) by moving the gas hi-limit switch down stream of the shutoff valve. I do not know it this is kosher but was following orders. The hi-limit switch has a mechanical reset by code. Problem is that with the overpressure in the line between the shutoff and the regulator (where the switch used to be) we cannot start the oven with out jumping out the switch. Which in my view negates its purpose. Once the shutoff valve is opened then the presure is normal in the gas line and everything is OK until the shutoff valve is closed again. The problem in a nutshell is that when the shutoff valve is closed, the regulator allows pressure to rise in the short line between it and the shutoff which caused the hi-limit switch to open. It is hard to describe a problem over the internet but I am doing the best I can. Is the gas hi-limit permitted to be down stream of the shutoff? The hi-limit is set to 16"wc which is the oven manufacturers requirement.

 

[psafety]

rsbuck,
The high-gas pressure switch is correct downstream of the Maxon valves, low-pressure should be upstream.

 

[Combustor1]

rsbuck,

psafety has it right - the high gas pressure switch is alays downstream of the shut-off vvalves (Maxons or other) and the Low gas is before them. When the gas is flowing the regulator is open to maintain pressure - when the burner shuts down - Moxon valvces close but the pressure regulator is still open and takes a few seconds to close or respond to the loss of demand and subsequentially the pressure will rise before the shut off valves. The High pressure switch is Normally Closed forced open the low gas pressure switch is Normally open forced closed.


Check out NFPA 85 or 86

 

[psafety]

Combustor,
Your right on target. I split my time between "Process Safety" and "Combustion Safety". Application of Code is so very often misunderstood or totally ignored.
Typically, I'll shoot for the PSH to be set at 125% of the regulated pressure and the PSL at 75%, if the line is appropriately sized this provides a safe limit.