Vacuum breaker on two pipe steam

Tommi68 · October 1

Hi all,

I have a new AHU being installed. The unit is getting a new 2", 10psig steam line piped from our high pressure steam plant. The engineers drawings show a vacuum breaker on the unit's supply piping, after the control valve. This should be correct, except that the condensate is being piped into the low pressure heating system which uses vacuum return, so the vacuum breaker whistles when the control valve is closed. This unit will handle 100% FA, all year.

Is there a breaker of trick to help drain condensate out of this unit that won't whistle, like the existing vacuum breaker? We have another low pressure (2psig) coil with a control valve that would benefit from a vacuum breaker installation.

mattmia2 · October 1

Maybe it needs to be a check valve piped to the return instead of a vacuum breaker open to the atmosphere.

Jamie Hall · October 1

https://forum.heatinghelp.com/discussion/comment/1812605#Comment_1812605

This. But note: it has to go to the return!

Tommi68 · October 1

Hmm, without a vacuum relief valve won't the coil be in vacuum when the control valve closes? The check valve would prevent condensate from being drawn into the coil, but the coil won't drain when the valve closes...maybe?

We have several HP coils other that use check valves as vacuum breakers, but these are on a gravity return.

Jamie Hall · October 1

A vacuum breaker is a check valve — of sorts. Just one side (the upstream side) is open to the atmosphere. The suggestion here is to pipe the upstream side to a return — so that when pressure is up, steam won't flow, but when the zone valve closes (and steam condenses) air can come in from the return and equalize the pressure.

jumper · October 1

With correct coil in AHU won't vacuum return drain condensate?

Pumpguy · October 2

I'm wondering if an equalizer line(s) might be more appropriate here?

The goal here is to equalize the pressure between the steam and condensate sides so the condensate can gravity flow down to the vacuum condensate pump's receiving tank, without the vacuum breaker leaking air in from atmosphere.

This would also allow the vacuum pump to pull out to the set point of the vacuum switch and shut off. Another benefit would be no whistling sounds from the vacuum breaker.

The attached file illustrates the details and principles involves.

equalizer lines.pdf

EBEBRATT-Ed · October 2

See this from Sarco" Steam Hook up Book" available to download on line.

"Vacuum Breaker And Trap Location A vacuum breaker ensures that some differential pressure can always exist across a trap that drains by gravity but any elevation of condensate after the trap reduces the hydraulic head available. Heating is done using an atmospheric air/steam mixture so coil air venting is most important. A vacuum breaker should be fitted to the steam supply pipe, between the temperature control valve and the coil inlet. It is not recommended to fit a vacuum breaker on the steam trap where the hydraulic head of water used to push condensate through the trap would hold the vacuum breaker closed.

In systems where the return piping is kept under vacuum, a reversed swing check valve should be used and piped to equalize any coil vacuum not to atmosphere, but to the discharge side of the trap"

This is the fix. No vacuum breaker. A check valve piped in reverse to equalize the pressure between the coil and the trap outlet With the pressure equalized between the coil and the trap outlet the condensate will drain when the control valve is closed.

The check should be up by the control valve in a horizontal position. Use a good quality "Y" pattern check valve. 1/2" check valve should be sufficient. Put a tee between the control valve and the coil and a tee on the outlet of the steam trap. 1/2" pipe in between Both tees should be looking up.

Probably the same fix @Pumpguy mentioned.

Tommi68 · October 2

https://forum.heatinghelp.com/discussion/comment/1812631#Comment_1812631

The vacuum is returning the condensate, but when the valve closes the vacuum relief opens to further drain the coil but also whistles until the valve opens again.

Tommi68 · October 2

https://forum.heatinghelp.com/discussion/comment/1812647#Comment_1812647

I think once my I've read this again, my brain will agree… I will sketch this idea onto the drawing.

Pumpguy · October 2

https://forum.heatinghelp.com/discussion/comment/1812689#Comment_1812689

Looking forward to see what you come up with.

jumper · October 2

One can also allow enough steam to prevent freezing. In addition to whistling vacuum breakers have downside of introducing more air to be eliminated.

EBEBRATT-Ed · October 2

See attached drawing

EQUALIZER LINE.pdf

Pumpguy · October 2

https://forum.heatinghelp.com/discussion/comment/1812729#Comment_1812729

Exactly, and more operating time and wear on the vacuum pumps.

I had a case at a school where vacuum breakers had been installed on heat exchangers for heating swimming pool water. As a result, the vacuum pumps were running continuously and virtually no vacuum was being produced on the return lines.

In this case I had maintenance relocate the vacuum breaker to the steam side and all was good after that.

Tommi68 · October 3

https://forum.heatinghelp.com/discussion/comment/1812729#Comment_1812729

Interesting option, but this AHU is dedicated make up air for the kitchen hoods, they keep the set point lower than the room for the cooks comfort.

jumper · October 3

»keep the set point lower than the room for the cooks comfort.«

When outside air is cold enough to freeze pipes; will a bit of steam be too warm?

BTW it can be a problem to use main control valve for low load.

Vacuum breaker on two pipe steam

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