Banging in steam boiler control manifold

Greg Mahley

I am a hvac/r tech with 20+ years experience, am quite familiar with steam systems. Here is the unusual problem I have been experiencing. One of the facilities I service is an old school with 2 steam boilers, they both have independent controls, in other words there is no separate lead-lag setup. I have the boilers set up to run as follows: On a cold start, both boilers will come on, modulate up to high fire, and as steam starts to build, the lag boiler will modulate to low fire, and eventually shut off. The lead boiler will modulate the firing rate to maintain steam. It runs continuously to maintain just enough steam pressure to get heat to the entire building. I know when it comes to steam pressure the lower the better, so I set the controls to modulate the firing rate to maintain constant steam. My customer has a lot of passing traps, so I have to run the pressure higher than I would like to. My customer has been very pleased with the operation and their energy bills have dropped some. Now on to the problem. The lag boiler remains off during system operation as the lead boiler maintains the load, I leave the control switch on so that in the event of a drop in steam pressure due to failure of the lead boiler the lag boiler will start up. While the lag boiler is dormant, my customer has recently informed me that there is water hammer occuring in the controls area of the lag boiler, which I know is caused by water flashing to steam in the control header piping, this causes the gauge to jump rapidly and the pressuretrols to "chatter" the controls rapidly on and off. There is a trap for the "overflow" to prevent flooding of the boiler, so this is not an issue. The one thing that I don't like which I suspect may be a factor in the equation is this: The pump/cutoff combination control and the secondary cutoff control are manifolded typically on the one side of the boiler, There is a 1" pipe coming off of the top of this manifold, this pipe extends dowmward and connects to a 1" pipe manifold where all of the pressuretrols and the gauge is located. The controls are actually below the boiler water level(this was done to conviently locate them right next to the burner control cabinet). The manifold has no outlet and therefore fills with water. I know that the controls should not be exposed to steam so they need a water "trap", but this seems excessive to me. Could this be the cause of the water hammer? I think that the controls should be mounted typically on top of the boiler, in a manifold with a "pigtail" for each control and for the gauge. Am I correct in this thinking? Would this solve my problem by not allowing a lot of water to build up in the control manifold? When I have pulled the plug and drained the manifold I have gotten approx. 2 pints of water out of it, this seems excessive to me. Could the boiler water temp being obviously lower when the boiler is dormant be a cause? Should I consider putting an aquastat in parallel with the pressuretrols to keep the water at around 200? The controls have been installed and positioned like this for more than 10 years but this problem has shown up recently. On a side note, the system has vacuum breakers on both boilers and they are operational. The lead boiler is a H B Smith 450 Mills about 12 years old and the lag boiler is an old H B Smith 60 series 1950 boiler, obviously the lag boiler has a much larger steam chest. I appreciate any help you guys can give me. Dan, if you read this please let me know about any upcoming "Dead Men Steam Nights" in the Southern NJ / Philadelphia areas. I really want to attend one of your "Dead Men Steam Nights". Thanks

Steamhead (in transit)

What Pressure Are You Running?

I am sorry to say that it may be necessary to rebuild the failing traps. Any pressure above 1 psi is most likely too high.

What is your load in EDR? You may be able to shut and valve off one boiler and keep it in reserve.

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Greg Mahley

reply to Gordo

Gordo, you are correct about the steam traps. I have recommended to my customer that they all need to be replaced or rebuilt. Unfortunately, it is a school district in New Jersey, and in New Jersey all of the big spending in school districts as well as the state gov't itself takes place near the top, in other words they have tried to get funds to properly fix this for years but maintenance budgets always fall short. The people on my level that I deal with such as the facility director understand completely about wasting energy with bad traps, but the people at the top only see dollars and no "sense"! Anyway, I am running the pressure at approx. 2 psi. I came onto the job fairly recently and the first thing I did was set the controls lower, they were at 5 psi and even higher before- I attribute this to a lot of technicians not really knowing steam systems and thinking the pressure is too low- there are not a lot of steamheads out there . I agree that one boiler will handle the load and I may suggest to my customer about closing the valve on the lag boiler supply. The main reason that I have them as they are is an automatic redundancy, especially for during weekends. I'm still wondering, though, about the control manifold piping where the waterhammer is taking place. I will keep working on my customer about the need to fix all the traps. With all of the traps working properly, I could then lower the pressure even more, and the boilers may even cycle. In this case I could put in an alternating lead-lag setup.

Steamhead (in transit)

Ah, yes

Trap repair is a hard sell to those convinced of their innate superiority over lesser mortals from the boiler room.

Could you post pictures of the control manifold piping?

I would go through the whole system. Take a radiation count to find your EDR and an estimate of type and number of traps. Note end-of-main traps. Ask questions on which rooms overheat. Look for open windows. They are most likely where the failed open traps are. Move those radiator valves to almost shut. You've just become what Dan calls "the two-legged zone valve".

Lower the pressure to 1 1/2 psi. See who complains about being cold.

Can an alarm be put on the boiler if it goes down? It'll be cheaper than running both boilers.

Do you have pumped return? How is air getting out of the system?

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Greg Mahley

Will post pics

I will attempt to return to this jobsite in the near future and take my digital camera and take as many pics as I can of the control piping manifold.