PRV Directly After Booster Pump..why? 2

[ziptron]

Hi Everyone,

So I have a system where a building is boosting the domestic cold water from the incoming city pressure up to the 30th level. For this we have some booster pumps (standby and lead) which boost the pressure so that at level 30 the pressure is around 60 PSI. To get 60 PSI on level 30 I need approximately 200PSI at the booster pump level. 200 PSI at the booster pump level is what I have however this 200 PSI is not created by the booster pumps. Directly after the pumps (1 ft away) I have a PRV valve. So, my booster pumps pressurize the system up to some pressure (no gauge so I do not know what) and the PRV valve lowers this back down to 200 PSI.

I'm curious of why they bothered installing a pump that is too big, and then just reducing its pressure. The reason I ask is that this valve keeps failing and I keep on having to replace it because with it failing I am getting pressures that are too high throughout the building. Would it not just make more sense to install a pump with the same flow capacity but lower head capacity? I would then not need to worry about the PRV valve failing so often.

Any input is appreciated, thanks!

 

[3DDave]

Anyone else bothered that 200 psi is available at the ground floor? There must be other regulators in the system, right?

It seems to me like there should be a 200 psig cracking, high flow check valve and a somewhat higher pressure cutoff switch for the pump. The pump needs to produce 200 psig plus the pressure drop required at max expected flow, which depends on how tolerant the tenants are. There should probably be pressure tanks on each floor with 40-60 psig valves to throttle the inlet flow based on downstream pressure.

 

[EngineerTex]

Your Ouija board is as good as my crystal ball, but I would say that the system was put together this way because the overall design is simple, cheap, robust, reliable and keeps irate tenants from calling the building manager/maintenance guy when there's no water. Yes, this overall system could use more power than a VFD, but that's a long-term problem for the person who has to pay the electrical bills for the next 30 years, not the developer whose intent is to build the building as cheaply as possible and then sell it to a new owner.

That said, I think that MintJulep is likely onto the problem.

A pressure reducing valve is not generally considered to be a consumable. Replace it once and shrug your shoulders. Replace it twice and you've probably got an incorrectly-selected reducing valve. Possibly wrong flow rating or possibly the wrong pressure.

Your first job tomorrow is to get a pressure gauge between the pump and the pressure reducing valve. Remember a couple of things:

1) No pump creates pressure. It is only resistance to flow that creates pressure. (Don't believe me? Get a pressure reading at the outlet of a pump where you have the discharge completely disconnected and dumping onto the ground. This is true for centrifugal and positive displacement pumps as well)
2) The pressure with zero flow will not be what you find on the pump curve literature. It will be what is on the pump curve PLUS the pressure from the municipal source coming into the building.
3) Overnight, when there is relatively little demand from surrounding buildings, the municipal pressure is the highest. When a person on the first floor (one of the people experiencing 200 psi in his apartment) abruptly turns off both the hot and cold water going into his bathtub, the sudden closure of the valve puts even higher pressures (via water hammer) on the system, including the area between the pump and the PRV.

The water hammer effect is just a working theory that could have nothing to do with this, but have you gone there at 3:00 a.m. and listened to how the pipes sound when you turn on and off the outside water faucets? Does it sound like someone throwing an anvil into the wall?

Have you taken apart the valves that have failed? What part failed? Spring? Diaphragm?


Engineering is not the science behind building. It is the science behind not building.

 

[EngineerTex]

Oops -- I misread or mis-remembered the original question. The question was about a properly-sized pump vs. an improperly-sized pump. I read it and then answered some question that apparently, I just made up after reading all of the replies. That's where my VFD answer came from. Not untrue, but not applicable to the question originally asked. Yes -- a properly-sized pump would be better.

As for the why, it's possible that the original design failed to consider the additional pressure coming in from the municipal source, which would be a simple mistake on the original designer's part. The rest of my first response does still apply.

Engineering is not the science behind building. It is the science behind not building.