1-Pipe Steam, 2 Zone Valves

Craig_8

I just fixed the same problem on a boiler with 3 zone valves. the solution was replacing the pressuretrol with a vaporstat, and setting the cutout at 1.2 LBS and differential at 8 ounces.

the problem with 1 pipe steam and ZVs is that when the ZV is closed, there is no pressure in the off zone to push remaining condensate back into the boiler. at the same time if is single piece of rust sticks in the check valve, water will be push out of the return and into the off zone. The water will collect in the mains, and the boiler feeder will come on. When the water returns the boiler will be overfilled resulting in wet steam the next cycle and water hammer.

Measure the distance between the horizontal dry returns and the maximum normal water line of the boiler. divide by 2.3. This is the maximum opperating pressure that the boiler can be run at. Install a vaporstat and set it a little lower than this pressure.

a time delay or bypass is unnecessary.

Jed_2

Burner Shut Down

Hi, all

got a call Friday about a Slant Fin steam boiler replacement job. The caller did not do the replacement, but was called in to troubleshoot the system operation.
From what I was told, the replacement included installing 2 motorized valves for a duplex building with a 1-pipe system. O.K., 2 dwellings, 2 zones. The Contractor who did the replacement put a check valve on the return. It's a dry return, so he put in in the section below the water line. Of course the HO started complaining.

I don't know any more than this. But the troubleshooter, who has limited steam experience, asked me if there was a some kind of time delay he could use to delay the zone valves' closing to relieve the steam generation after burner shutdown. System works fine during steaming and zone call, but hammers after shut down. I told him to pipe a bypass (½-3/4") around the zone valves to the mains. He said that makes sense, but trying to do that would be very difficult. He suggested modifying the PRV on the boiler with a tee, and running the bypass from there to the mains.

I couldn't think of a reason not to, but now wonder if that is not such a good idea. There is 1 supply riser from the boiler to the header, with 2 main take-offs with the ZV's. But adding a bypass from the PRV might pull wet steam into the mains. In this kind of situation, do the ZV bypasses need to come from the zone main with dry steam? I hope this is clear enough to make sense.

Jed

Jed

Jamie Hall

If the pressure is right

the check vavles should not be needed. Gravity will do the job. I assume that things are pitched so that condensate in the steam mains can return whether the zone valves are open or closed? I hope? Otherwise, condensate could collect downstream of the zone valves and give trouble (like hammering!) too.

Jed_2

Thanks, Craig, Jamie

I'll pass those along. I haven't seen this job, but he tells me elevations and pitch are O.K.

Jed

ttekushan_3

perhaps

a good sized vacuum breaker after the zone valve would help. Closure of a zone valve will allow vacuum to form far more quickly than the radiator vents can break it.

Just an extra thought.

-Terry