Vacuum Pressure in Pipe Drainage

[FraiserCrane]

Does anyone have any references for calculation of vacuum pressure in gravity pipe drainage. I have a piping system that flows throughout a facility. The piping is not straight and sloped. it is layed out with twist and turns. I am wanting to calculate the vacuum pressure when the air/vacuum valves are not in operation and see what the max negative pressure the piping will experience during the drainage condition.

 

[bimr]

There is usually zero vacuum in drainage because the piping is not designed to flow full, but partially full pipe.

When you have a situation with transitions from full pipe flow to two phase low to partially full flow, weird scenarios start to occur as air is entrapped.

Post a sketch and you may get a better response.

 

[FraiserCrane]

i am looking at a scenario that involves all the isolation valves closed. and draining from a smaller pipe.. lets say .. 18" main pipe, gravity drain from a 2" pipe. there is some vacuum occurring. The couplings used at the joints are experiencing suction into the pipe. As the pipe is refilled, leakage occurs. the difference in elevation is a maximum 16 ft. I am attempting to calculate the max vacuum pressure when the draining happens.

 

[bimr]

What is the purpose of this exercise. If you want the pressure for design purposes, assume 0 psia. Exterior pressures from soil or ocean water pressure make the contribution from the atmosphere relatively insignificant, additionally thinking that someday the pipe internal product pressure could easily be reduced to near 0 psig for maintenance or tie-in purposes anyway.

It will be difficult to determine the actual vacuum since air and fluid will be trapped within the various pipe segments and will not be free draining.

 

[LittleInch]

Looking at this is seems as if it is normally full of liquid given you have ARVs everywhere.

hence if it is all shut off and then you open the drain then the lowest pressure you can get is at the high point and is the differential head between the drain and the highest point times the density. so if its 16 ft (5m) then the lowest pressure at the top with water is about 500 mbara.

If you get a pressure less than 0, but realistically below 100mbara, then you're probably drawing a vacuum.

NOTE - this only works if at the start point the pipework is completely full.

And the reason you can't install a vacuum release valve at the high point is?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[FraiserCrane]

The piping has been installed and there was some failures at the couplings, when the piping was drained. When they took the couplings apart, the gaskets seemed to be "sucked" into the pipe. This caused leaks when the pipe was refilled. My initial thought was that the ARVs were not open when they draining occurred, thus causing the vacuum pressure to exceed the max allowable of the gaskets (5 PSI). Static pressure (difference in elevation) the vacuum pressure equates to (6.9 PSI). I was looking for any reference material (quick and dirty) calculation for vacuum pressure in piping during gravity draining.

I did assume 0 PSI during design and used the selection tools offered for the Air Valves. Never again.

 

[LittleInch]

Problem solved?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[dbill74]

Sounds like you want to prevent a vacuum from forming creating sinphonage. To solve the problem you don't really need to know how much vacuum is present, only how to prevent it from forming.

To prevent a vacuum from forming you will need to either modify draining procedure and/or add vacuum relief valves. During draining, the one valve at the top of the diagram should be open and remain open.

Installing Vacuum Breakers at high points near the air relief valves should work. They are designed to let air into the system to prevent the "sucking" of gaskets.

My initial thought was that the ARVs were not open when they draining occurred

Why would they? They are designed to open outward allowing air to escape out of the piping. If the system is under a vacuum, they will be "sucked" closed.