High Pressure Steam Boiler Equalization

Darrell Lackey

In a high pressure (125 psig) steam boiler plant of five 250 hp boilers connected to a common header designed for four operating when load is at maximum with one standby, each boiler has ASME outlet piping with non-return and shutoff valves. Non-return valves don't stop all of the backflow from the active boilers feeding the header into boilers that are off. How to solve effect of backflow into the off boilers in an energy efficient and low cost manner. When header pressure enters the off boilers, it causes overflow and activates the boiler feed pumps which leads to ripple affects. Where does the overflow go.

Thanks.

ttekushan_3

Backflow valves

If the backflow valves at the steam outlet are of the proper disk type, they may require rebuilding or replacement. Also make sure there are no irregularities in the boiler feed systems that can allow pressure to back up through the feed piping. Unlikely, but occasion bungles are not unheard of!

It seems to me the best way to set up multiple parallel boilers is with a sequencer that operates a motorized gate valve on the steam outlet of each boiler in addition to its firing controls.

Check this out:

http://www.spiraxsarco.com/learn/pdf/gcm_28.pdf

One thing has me scratching my head:
You said,

When header pressure enters the off boilers, it causes overflow and activates the boiler feed pumps.

Now I've seen pressure push boiler water backward into the feed system through a leaking check valve, activating the feed pump because the LOW water level requires it to feed the boiler. Flooding under these condtions has me mystified. Have the water level controls been blown down regularly?

Anna Conda

Is a double block and bleed arrangement on the boiler piping? This is a normal isolation in high-pressure applications and helps to prevent situations like what you're describing.

Brad White_9

Stop-Check Valves

are another term for what is being described. I am a fan of "double-block and bleed" as Anna Conda says, but only for manual service. I think you are looking for an automatic means of preventing back-pressure of dormant boilers.

I would check the seats.

Josh_10

Sounds like you should have the non-returns re-built. That is what non-returns do. A common mistake people make is not maintaining non-returns. They need to be serviced annually.

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Christian Egli_2

Low cost trick? use duct tape all over the parts you don't like

I'm just deciding here and now that there is nothing wrong with your check valves. Here is what may be happening to you.

You make steam, that's cool, the boiler fills with steam. Then you stop. The check valve stops any backwards reentry of anything back into your boiler, which for now is still full of steam. Then, this steam collapses while it condenses and you get a giant vacuum inside the pressure side of the boiler. The dutiful check valve on the steam main won't let any onlookers through to this collapse scene. Drive on steam, there is nothing to see.

Boilers also have a back door. The hole by which the boiler is fed is still wide open. The check valves on the feed line won't let condensate escape the boiler, that's true, (water naturally wants to escape because it is being boiled alive!) but at the same time, the feed line welcomes all the incoming water that wants to go through its web of ensnaring check valves. It's a trap. The bait: something that sucks. Condensate can never resist the attraction of a collapsing vacuum scene. Admit it! you'd slow down too if you were driving by.

All this stupid water is foolishly trapped inside the steam cavity where there no longer is anything to see but a flooded boiler. Now is time to practice your sardonic laugh.

What to do? Two things.

One, to quell the sucking

Install a small (3/4 like perhaps) pipe from the boiler cavity to the condensate return system with a check valve that lets system air back into the boiler. This check valve also prevents boiling steam from leaving through to the condensate lines during fire time.

It might be possible in your system to just simply install a vacuum breaker to the atmosphere. You'll have to see.

Two, to get rid of the overflow

Pipe (again in small pipes) a float trap right at the water altitude you set your overflow at to take whatever spills over from inside the boiler and dump it into the condensate return system.

Keep the duct tape handy just the same... If your boilers are already fitted with these devices, I'll just tape over my mouth. :[]

Let us know what you found.

larry_15

Best answer yet

Christian:

It sounds like you have worked on these type of systems I know that I have had the boilers flood before and what you decribed was what was happening, a vaccum breaker on high pressure steam boiler, I would be a little concerned about. But the equializing line to the boiler feed is the best solution to the problem.

Christian Egli_2

And who says vacuum sucks?

I'm glad you think the solution will work. I rather prefer that; so I'll put away the duct tape then!

Happy steaming

Brad White_9

Another classic tome, Christian

Christian, you are the Bard of BTU's, clearly.

Yours in entropy,

Brad