Controlling pressure in the Domestic water system 4

[mirza057]

In the domestic water supply system for a building, how can we control the pressure under limits and as per codes to reach faucets, showers, perennial sprays etc… Will it be ok if we install a PRV to keep pressure under 80 PSI? Will that be sufficient?

 

[BigInch]

In general you should not use a pump that can discharge at pressures higher than your pipe code design limit. If you used a well selected pump, you would not need any pressure control valve at all.

A pressure control valve will not turn off the pump as it closes to keep the pump discharging at your required pressure. The pump would continue to run as the valve closes, causing the pump to get too little cooling flow, overheat and fail.

Usually you would not need anything more than a pump with a pressure switch and accumulator tank. The pressure switch turns the pump on when pressure in the tank drops below normal and turns it off when the pressure is acceptably high again.

 

[LittleInch]

Be aware that "PRV" could mean Pressure Regulating Valve - which is what I assume you mean, but can also mean Pressure Relief Valve, which would then just dump water to reduce pressure.

Are you using a pump?? if so BI is correct, but if this is either the municipal supply or you have a tall building with a water tank at the top then yes, a Pressure regulating valve or more correctly a pressure control valve, will do what you want.

Note however that over time, these valves rarely seal completely, therefore at no flow, the pressure could exceed the set point and not fall until you establich flow.

How much are you throttling this supply?

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

 

[mirza057]

Thank you for the reply.

I meant pressure reducing valve to keep the pressure under 80 PSI. Yes I am using a booster pump. I have water tank at the bottom and it is not a tall building, 4 floor only.
Pump Head is 427 Kpa and residual pressure is 150 Kpa.

 

[LittleInch]

That's the problem with multi- storey buildings - the pressure you need on the top floor from a ground floor pump will deliver much more pressure on the lowest floor. Hence why you need Pressure Regulating Valves at the floors lower than the top floor if you want the same pressure on each floor.

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

 

[mirza057]

Thanks again. I am just using the PRV only where the pressure exceeds 80 Psi and not at all levels/floors.

 

[bimr]

It is common to install a PRV to reduce excessive pressure. 80 psi is too high of a set point. You will need to reduce the pressure to below 60 psi.

 

[BigInch]

The pump has a bit too much head for only domestic water supply to a 4 story building.

 

[PEDARRIN2]

80 psi is the typical plumbing code maximum - although a lot of AHJ will let you get away with a bit more.

Depending on the type of building and expected use, you might want to specify a hi/lo system so you are not opening a large ~2.5" valve to provide 0.5 gpm to a lavatory. If you look on Watts (

http://www.watts.com/pages/literature_cats.asp?catId=69),

they have good information on specifying and designing these valves.

 

[bimr]

Agree with BigInch.

The top floor should be designed for 25 psi pressure. If you have 4 floors, that would be 40 feet elevation difference or 40 * .433 psi/ft = 17 psi. The first floor pressure would have only 42 psi pressure.

You should resize your pump. Many of the inexpensive plumbing fixtures are not capable of operating at 80 psi.

See the link, starting on page 38.

http://www.ctaspe.com/uploads/TS_400-1-1.pdf

 

[DubMac]

Most residential bldgs have a domestic water booster system, or "constant pressure" system, with multiple pumps on VFD control to supply water. It is tough to do with one pump since pump's pressure will vary with the number of demand points being used at any one time. You may well need an 80 psi (or higher) capable pump to deliver 60 psi in the morning when everyone is showering. The pump will run out on it's curve and deliver less than it's maximum head.

You will need a set of pressure regulating valves. As Pedarrin2 said, go look at the WATTS website; they've been doing this for years.

Also go look at Synchroflow or Systecon's website, surely they are still around. They make these packaged systems and I imagine they have a wealth of information on their site to help you understand how to best handle your situation.

 

[PEDARRIN2]

Bimr,

42 psig static would be too low (pump or not).

Current IPC requires 35 psig at top level fixture (static).

So, minimum static, in your instance, would be 52 psig.

But we also have to think of the dynamic, which will include friction losses, which could be 10-15 psig, pressure drop across the backflow, which could be around 13 psig, pressure drop across a water softener (which could be required) of ~15 psig. Say your residual pressure at the building source at 25 gpm (for a toilet) is 50 psig, you have lost 17 for static, 10 for friction, 13 for backflow. You now have 10 psig at the toilet when it flows. Most toilets require about 25 psig dynamic pressure. So you need to start about 15 psig higher static (pump shut off) or 76 psig so your toilets work. It would have to be 91 psig if you have a softener.

So for the 4 story building (and 10 feet per floor is a bit low, but will go with it), a realistic minimum static pressure is 76 psig. If the municipal service cannot provide that, then you need a pump, which will require either splitting the system between pumped and non pumped or providing PRV's in the building.

 

[bimr]

The 25 psi is a minimum under all conditions of flow, not a static head requirement.

Building water system have controls so that the pressure is not going to vary greatly despite changes in demand during the day.

"While the table identifies both minimum flow rates and minimum pressures, these values are used independently of one another. When sizing a water distribution system, the flow rate values are used in determining the peak demand of the system.
The pressure values are the minimum requirements for the most demanding fixture operating under a peak demand condition."

I fail to understand the logic for designing water for static head.

"All water mains, including those not designed to provide fire protection, shall be sized after a hydraulic analysis based on flow demands and pressure requirements. The system shall be designed to maintain a minimum pressure of 20 psi (140 kPa) at ground level at all points in the distribution system under all conditions of flow. The normal working pressure in the distribution system shall be at least 35 psi (240 kPa) and should be approximately 60 to 80 psi (410 - 550 kPa) and not less than 35 psi (240 kPa).

 

[PEDARRIN2]

bimr,

You are correct in that the table is not indicating a static pressure. But, if you do not have sufficient static pressure, you will not have sufficient dynamic pressure due to friction losses in the pipe and equipment, unless you have a pump. So we typically use the value in the table as a static requirement, to begin with.

Also, in the IPC, the value is now 35 psig to help low flow (1.1 gpf - 1.28 gpf) water closets work better with less flow.

Other than a PRV and pressure balancing shower valves, I am not sure what controls a building water system has which help control the pressure, irrespective of flow.

I agree with your last statement.

 

[mirza057]

Thank you guys

 

[nabeel3]

U can also use a Globe valve to throttle the flow for each level. I think Globe valve is cheaper than PRV.