Inverted P-trap with Vacuum Breaker

[TCmech33]

Hello.
I am working on an open loop cooling water system. The system has an open to atmosphere holding tank with pumps. On the return side just before the atmospheric holding tank I have an inverted P-trap and vacuum breaker to prevent the system from siphoning back when the system is turned off. The inverted P-trap is above the highest point in the system and the vacuum breaker is on top at the highest point of the P-trap.

I need help determining the set point of the vacuum breaker. I am using a Bell and Gosset #26.

http://www.pexsupply.com/Bell-Gossett-113075-26-Vacuum-Breaker

Part Description:
Designed to protect closed vessels and piping systems against collapse when the induced vacuum exceeds design conditions. When used on steam heating systems, the No. 26 Vacuum Breaker controls induced vacuum, permitting normal return of condensate to the boiler. Adjustable range 1/4" to 20" (mercury) vacuum. Factory set to 4".

This is factory set to 4” mercury vacuum which is ~4.5ft water. How do I calculate the vacuum pressure due to the siphoning effect? I would think it would be a based on the down leg length of the P-trap, from the top of the P-trap to the atmospheric tank?

Right now when the system is turned off some water comes back in to the atmospheric tank (as expected from top of P-trap to tank). But then a small amount of water continues to bleed back into the system. I have verified that water is not coming back to the tank on the supply side. I am not sure if the vacuum breaker is set correctly or if it could be undersized. Any thoughts?

 

[katmar]

The top of the P-trap (actually the underside of the pipe at the top of the P-trap) must be >4.5 ft above the highest point to be sure that it will not all siphon out. You are correct that the suction force is developed by the downleg from the P-trap. The suction generated depends on whether this section of pipe runs full, which in turns depends on the pipe diameter and flowrate. But to be safe just assume it runs full and make the P-trap high enough not to allow a siphon.

Katmar Software - Engineering & Risk Analysis Software

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[MikeHalloran]

OR, forget the vacuum breaker and just run a small standpipe, open at the top, up to the ceiling, or to a drain on the floor above if that makes you comfortable, or just straight up from the top of the P-trap high enough to keep the water in with maximum return leg flow



Mike Halloran
Pembroke Pines, FL, USA

 

[TCmech33]

katmar, thanks for the input. My issue may be that i dont have 4.5ft between bottom of P-trap and top of header. If I reduced the vacuum set point to the lower limit of 1/4" mercury/.28 ft (or somewhere closer to the actual height) it should help. Doing this would just introduce air into the return line. Do you think this change would effect any other aspects?

Mike, normally our standard designs do not allow for such standpipes (due to space restraints) so we incorporate vacuum breakers. If adjusting the vacuum breaker doesn't work we will give this a shot.

 

[katmar]

I have always just used open pipes as recommended by MikeHalloran. The only risk with a shorter standpipe is that you might shower the plant from time to time. Where it has been my own design I have always tried to ensure the return leg is of a large enough diameter to be self venting and then there is little risk of wetting your colleagues.

I would be tempted to quietly go and take the innards out of the vacuum breaker.

Katmar Software - Engineering & Risk Analysis Software

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[Compositepro]

If the P-trap is not full of air it cannot prevent siphoning.