PRV Directly After Booster Pump..why?
PRV Directly After Booster Pump..why?
(OP)
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!
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!





RE: PRV Directly After Booster Pump..why?
RE: PRV Directly After Booster Pump..why?
RE: PRV Directly After Booster Pump..why?
You need a widely varied flow rate to account for water usage at different times. In the middle of the night, the water usage may drop to zero. At half time of the Superbowl, the water usage may spike incredibly high. (You didn't say if this was residential or business) In order to provide consistent water pressures for the end users, you need a constant (or nearly constant) water supply pressure over the full range of flow rates.
The pump is (I assume) centrifugal. The curve for a centrifugal pump provides a range of pressure depending on the flow rate. As the flow rate increases, the pressure drops. As the flow rate decreases, the pressure rises.
In order to get this pump to meet that need (variable flows at constant pressure), you have to do something. You can design the pump to produce a slightly higher pressure than you want at the highest flow rate that you ever expect and then control that pressure using a PRV. That is what we normally do. A VFD may not work well. As you change the speed of a centrifugal pump, the head changes even at a constant flow. And you want a constant pressure. If the problem is as I have described it, I don't think a VFD would work very well. We usually use VFD's for applications where we want a variable pressure, not for a constant pressure application.
You could address the problem with a controlled spill-back system to spill back the excess flow so you can run the pump at constant flow even when you have variable demand. But, this wastes a lot of energy. You could use a fixed head tank (like a city water tower) and cycle the pump to maintain level in the head tank. But, you may not want to build a tank 140 feet above the highest water user. You could use a positive displacement pump with a variable speed drive. This would be the most energy efficient option that occurs to me.
Without telling us more about your pump and your system, you are asking us to guess and speculate. So, that is what I have done. With more information, I could be more helpful.
Johnny Pellin
RE: PRV Directly After Booster Pump..why?
This sounds like the sort of thing we see in the boost/servo circuits of some hydraulic powerpacks, where a requirement for a constant pressure but substantial variation in required flow is satisfied by a small positive displacement pump and a downstream PRV.
The thinking in the hydraulic example is that the PD pump is a constant flow device, not a constant pressure one. When deadheaded, it will produce silly pressures, so something (and the PRV is about as simple as it gets) has to be provided to control the pressure. The downside is that once you start to scale this simple system up, you lose a lot of energy in the PRV.
A.
RE: PRV Directly After Booster Pump..why?
RE: PRV Directly After Booster Pump..why?
Johnny Pellin
RE: PRV Directly After Booster Pump..why?
RE: PRV Directly After Booster Pump..why?
However that does not mean that you have the correct PRV installed.
Rather than constantly repairing or replacing the same valve, take some time to understand why this valve is failing and look for a valve that would not be subject to the same failure mode.
RE: PRV Directly After Booster Pump..why?
I grew up in a rural area, with water coming from a well. Water for the house came from a pressure tank (compressed air plus internal bladder) in the basement. The pressure tank was fed from the pump deep down in the well, which contained a check valve. The pump cycled on and off based on a pressure switch. Nothing exotic, nothing hard to understand, nothing that is wasting energy when there is low/no demand.
RE: PRV Directly After Booster Pump..why?
I think some sort of buffer pressurised tank would be a good idea or if there is one a bigger one or retrofit some smaller pumps to reduce the requirement to throttle the outlet so much.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: PRV Directly After Booster Pump..why?
A.
RE: PRV Directly After Booster Pump..why?
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: PRV Directly After Booster Pump..why?
- throttling type. A variable restriction in the supply line.
- relieving type. Requires a return line to relieve excess pressure to the pump inlet. Energy wasteful.
Simplest solution IMO is a more appropriate pump ie one with the desired pressure and a "flat" characteristic.
Link
je suis charlie
RE: PRV Directly After Booster Pump..why?
The control system setpoint is "pressure", what happens to "flow" will be a function of the system characteristic.
je suis charlie
RE: PRV Directly After Booster Pump..why?
1. How do you know that the PRV (I assume pressure-reducing valve) lowers the pressure back to 200 psi, if you do not have pressure gauge
2. Why do you need 200 psi on the first floor residential? Looks as way too high. 60 psi downstream of the valve seems to be more than adequate
RE: PRV Directly After Booster Pump..why?
Agreed. So, if the flow demand drops by 200 gpm the VFD will have to change the rpm by only 60 rpm to to maintain the target pressure. That is a very small change in speed and may be difficult to control well enough to maintain the required pressure within an acceptable range.
ForeverStudent
He has a pressure gauge after the PRV and knows that the pressure there is 200 psi, which is what he needs. The pressure upstream of the PRV is not known since there is no gauge there. They do not need 200 psi for the first floor residents. They need 200 psi at ground level in order to have 60 psi for the residents on the top floor. If they had 60 psi downstream of the valve, no water would reach the top floor.
Johnny Pellin
RE: PRV Directly After Booster Pump..why?
Thank you for the clarification, I thought that riser to upper floors was upstream of PRV. This is let's say ... unusual setup. Pressure reducing valves (PRVs) should be on each floor. Pump needs to be changed in my opinion.
RE: PRV Directly After Booster Pump..why?
Yes but if the flow demand drops by 200 GPM (say from 600 to 400) the pressure increases by about 7%. Not very far from target pressure and the VFD hasn't done anything yet.
The VFD is controlling pressure and the relationship between output pressure and speed has a sensitivity which is conducive to stable control.
je suis charlie
RE: PRV Directly After Booster Pump..why?
Johnny Pellin
RE: PRV Directly After Booster Pump..why?
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.
RE: PRV Directly After Booster Pump..why?
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.
RE: PRV Directly After Booster Pump..why?
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.