Question on low residual pressure in water main

[Spacemouse]

I've read that the residual pressure in water mains needs to stay above 20 psi in order to prevent pipe collapse. Why would pipe collapse at 20 psi residual?

 

[PEDARRIN2]

If there is a domestic booster pump or a fire pump that is connected into the main, it could "pull" enough suction to create a situation where backflow out of other building services (if not protected by backflow preventers).

Therefore the AWWA and most state EPA branches have requirements that if the pressure goes below 20 psig, a boil order has to be issued.

If the suction goes down even further, it could cavitate the pump, or at worst case, supposedly it could collapse the main, but I do not think a centrifugal pump could create enough suction to do that.

On fire pumps, often there is a requirement for a pressure sensing valve which will actually throttle the discharge of the fire pump so the suction is never less than 20 psi. On domestic booster pump, there is often a sensor that prevents this.

 

[PEDARRIN2]

Spacemouse,

Look at thread184-342887 for more information.

You could also do a search on the site for other threads which could shed light.

 

[rconner]

I realize this is a few weeks old post, but would offer the following. While there are certainly good reasons to maintain meaningful pressure residuals in water mains (such as e.g. insurance against contamination and backflow), as you may suspect there is no history of collapse due to vaccuum and at least the most commonly used main materials of cast iron and ductile iron pipe that have quite high ring stiffness. Pipe materials of lesser stiffness and even very large steel pipes of very high D/t ratios have on the other hand have been known to collapse or present other perhaps non-obvious problems in various vacuum-related events (

http://www.ragsdaleandassociates.com/WaterSystemsCertificationManual/Chapter 06 - Distribution.pdf

, or if you're from Missouri and want to see something at least like this happen yourself

http://www.youtube.com/watch?v=JsoE4F2Pb20

-- don't try this at home!)