Pressure Booster Pump Calculation
Pressure Booster Pump Calculation
(OP)
Hi
I am working on selecting a pressure booster pump for a small residential apartment building. I am estimating my peak cold water demand based in the Water Supply Fixture Units, from the American Society of Plumbing Engineering Design Handbook Vol2, Tables 5-18 & 5-19. I'm using the values for combined the hot & cold water because cold water will be fed into standalone water heaters in each apartment, so the hot water demand is ultimately cold water demand. (hope that doesn't sound too confusing)
I have 11 identical flats, each has the following water demand outlets:
2 water closets: 2 x 2.2 = 4.4 WSFU
2 Wash Basins: 2 x 3 = 6.0 WSFU
1 Bathtub: 1.4 WSFU
2 Hand Bidet: 2 x 1 = 2 WSFU (Not in the table, estimated)
1 Kitchen Sink: 1.4 WSFU
Total for 1 apartment = 15.2 WSFU
Total for building = 15.2 x 11 = 167.2 WSFU
From the next table I get: Estimated demand (after doing some interpolation) = 58.4 GPM
As for the pump head, I am designing for 21 psi at the outlet with the least pressure (highest floor that is)
So my required head would be:
(21psi residual + friction losses + dynamic losses) MINUS elevation head between pump and outlet (pump is placed on the roof next to the roof water tanks)
Thus my pump is selected for: 58.4 GPM @ calculated head as shown above.
Am I doing this correctly? Any corrections/comments are highly appreciated.
Best Regards
I am working on selecting a pressure booster pump for a small residential apartment building. I am estimating my peak cold water demand based in the Water Supply Fixture Units, from the American Society of Plumbing Engineering Design Handbook Vol2, Tables 5-18 & 5-19. I'm using the values for combined the hot & cold water because cold water will be fed into standalone water heaters in each apartment, so the hot water demand is ultimately cold water demand. (hope that doesn't sound too confusing)
I have 11 identical flats, each has the following water demand outlets:
2 water closets: 2 x 2.2 = 4.4 WSFU
2 Wash Basins: 2 x 3 = 6.0 WSFU
1 Bathtub: 1.4 WSFU
2 Hand Bidet: 2 x 1 = 2 WSFU (Not in the table, estimated)
1 Kitchen Sink: 1.4 WSFU
Total for 1 apartment = 15.2 WSFU
Total for building = 15.2 x 11 = 167.2 WSFU
From the next table I get: Estimated demand (after doing some interpolation) = 58.4 GPM
As for the pump head, I am designing for 21 psi at the outlet with the least pressure (highest floor that is)
So my required head would be:
(21psi residual + friction losses + dynamic losses) MINUS elevation head between pump and outlet (pump is placed on the roof next to the roof water tanks)
Thus my pump is selected for: 58.4 GPM @ calculated head as shown above.
Am I doing this correctly? Any corrections/comments are highly appreciated.
Best Regards





RE: Pressure Booster Pump Calculation
Independent events are seldomly independent.
RE: Pressure Booster Pump Calculation
You need to think how you intend to start and stop this pump and the effect it has on the apartment on the lowest level but with low friction losses. As the pump starts they could get a much higher pressure than before which could be very bad. You could think instead about a central large diam header which you keep at constant pressure so that the pump starts and stops on a pressure setting and have pressure limiting valves to each apartment so they all get the same pressure regardless if the pumps is running or not. I would also think you're much better off with a set of pumps rather than one big pump. Most booster systems of this type use some sort of accumulator so that a small flow doesn't cause the pump to start and stop many times a minute. For 11 apartments this might be quite large so allow enough space on the roof or in the service shaft for it.
Whatever you do it will probably need some adjustment depending on actual usage so allow for other pumps and fit the largest central pipe you can before the branch to each apartment, my guess would be 6" to 8".
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Pressure Booster Pump Calculation
These are sets of 3-4 pumps usually with one of them being somewhat smaller and used as a jockey pump. They are controlled by PLC logic and VFD's to keep the pressure constant at all flows.
You cannot assume that everyone is going to turn all of the faucets on or off at the same time; and while the ASPE are pretty good, you must be able to handle all situations encountered: provide constant pressure regardless of the flow. You can't do this with one centrifugal pump.
RE: Pressure Booster Pump Calculation
The formula of simultaneity depends on the number of apparatus used in the building. Something that I've used myself in the past and which is given on page 4/13 of the link hereby :
http://www.ksb.com/linkableblob/ksb-fr/942342-1344...
RE: Pressure Booster Pump Calculation
Total = 8 x 11 flats = 88 apparatus
Y = 0.8 / (88 - 1)^0.5 = 0.09
Base Flowrate for building = approx. 12 l/s
Calculated flow = 12 x 0.09 = 1.03 l/s
It seems to me that you are overestimating the flow.
I don't say mine is correct but at least a reason for double check.
RE: Pressure Booster Pump Calculation
BigInch: I think 21 psig is fairly reasonable at a faucet or a bathtub right? As for the pump inlet, the pressure won't actually be zero as the water level is higher than the inlet, but I want to design for the case where the water level drops. Also the pressure caused by the water level (static head) will work as safety factor as well.
LittleInch: Dynamic Losses are losses caused by the fittings in the network according to my terminology. I'm guessing you call them something else? I am using a variable speed pump that varies flow to maintain a constant pressure in the network regardless of the flow demanded. The pump also comes coupled with a pressure vessel to feed the system with small amounts of water so that the pump doesn't have to start whenever a single tap is opened. I'm not sure about the pressure valves, are they necessary? Won't the pump variable flow be enough to regulate the pressure in the system? I'd understand the need for them in a high rise tower. Regardless, I'm not that experienced yet but I think I'll do some calculations and get some pressures at the lowest level outlets.
DumbMac: I'm not using one centrifugal pump. As I mentioned, I'm using a pair of variable speed pumps. The demand is split 50-50 between them.
rotaryworld: I thought the ASPE tables take into account "simultaneity". My understanding is that what I am getting is the most probable flow demanded. Not the flow resulting from all the outlets flowing simultaneously. The tables already have a sort of diversity factor built it. Is this not the case? Thank you for the link. I will look into it.
RE: Pressure Booster Pump Calculation
Glad you're designing it that way - should work well, but as we don't know the height or layout of your apartment block it's difficult to say what the highest pressure will be compared to the lowest, but you always need to check the extremes of the system (lowest connection) at no flow as this will be the worst condition without any friction losses. I'm with the others here that 58 GPM seems quite high and you may want to think about making your pumps more of a 70/30 split or putting in three 33% units instead of two as most of the time you will be in low flow conditions and don't really want your pumps starting and stopping fequently or operating at minimum speed for long duration.
Glad we've been of some help to you.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Pressure Booster Pump Calculation
The height of the pump above the lowest outlet is ~33 ft. So there will be an additional 14 psi of static pressure at the lowest outlet
I'm going to assume that a two taps were opened at the lowest level demanding 5 gpm.
From the pump curve: The pump pressure that is provided at a flow of 2.5 gpm is 65 psi.
Now the total pressure arriving at the outlet (ignoring friction losses) is 65+14 = 79 psi
Deducting approx 4 psi for friction losses I end up with 75 psi.
This is just a quick calculation, hopefully it is "in the right ballpark"
Why did you consider the extreme point to be the lowest outlet at no flow? Doesn't this mean that there is no pressure boosting from the pump, making it not the most extreme scenario?
RE: Pressure Booster Pump Calculation
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Pressure Booster Pump Calculation
Sorry I made a bad mistake up there, I took the pressure corresponding to the flow from the pump performance graph as if its a constant speed pump. It is the only graph provided in the catalog for this pump so I am guessing it shows the pump's performance at the maximum operating speed. Now I remembered that the variable speed drive will produce the required flow at whatever pressure required (within the operating limits).
My pressure at the lowest outlet should be 14 psi(static) + 21 psi (boosted main) = 35 psi. If the tap is opened this pressure should drop a little due to the friction losses incurred by the incoming flow. Now if the static pressure becomes too high, then a pressure regulating valve is required at this floor to reduce the static pressure such that the sum of the static and boosted pressure becomes something reasonable. Am I correct?
I apologize if I violated the forum policies or anything :/.
In this economy, when you get a job offer, you take it :D
RE: Pressure Booster Pump Calculation
Yes looks you are correct - I would allow space to put a presure reducing valve in and suitbale isolation if it becomes necesaary, until the whle thing starts working then you won't really know, but you've opened up something else with your comments about "will produce the required flow at whatever pressure required". If I was you I would search this forum for VFD pumps as that statement is not correct. Variable speed is good, but the pump will only do what is can at a certain speed and no more when it meets your system curve (flow resistance). Bigger flow will mean higher pressure at the pump outlet.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Pressure Booster Pump Calculation
I did not mean that it can blatantly produce any flow at any flow. I meant within certain limits. What I know is that at reduced flow the operating point will slide down the system curve, and then as the pump speed is reduced the pump curve will "shrink" on both the flow and the pressure axes till it intersects the new operating point on the system curve, if an intersection is possible.
Thanks again for the help.
RE: Pressure Booster Pump Calculation
I thought the ASPE tables take into account "simultaneity". My understanding is that what I am getting is the most probable flow demanded. Not the flow resulting from all the outlets flowing simultaneously. The tables already have a sort of diversity factor built it. Is this not the case? Thank you for the link. I will look into it
unquoted
EnOm,
Ok clear, but I am just surprised that the handbook you refer to (ASPE) and the KSB rules (based on NF EN 806-3) end up in a such big difference in flow. dont you agree ?
RE: Pressure Booster Pump Calculation
You are right. There is quite a large difference. The ASPE tables are based on Hunter's Curve, which was developed in 1924. So it's quite outdated and can overestimate water demand. But I see it in use in some of the local codes so it must have some credibility I guess. The code allows other "good engineering" methods to be used. I am still looking for some better alternative especially that now I'm about to do the pipe sizing as well. Any modernized fixture units tables that you know of?
The document you pointed me to seems very interesting but I can't yet find an English version :/
Best Regards.
RE: Pressure Booster Pump Calculation
Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.
RE: Pressure Booster Pump Calculation
Thank you for the response.
So you say I should design for 45 ot 50 psig at the tap/shower?
What I know now is that pressure for at a standard shower or tap should be around 20 to 25 psig. Is this incorrect?
RE: Pressure Booster Pump Calculation
Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.
RE: Pressure Booster Pump Calculation
I never specify anything but ball valves for all of the several isolating valves needed in a system such as you seem to be describing. Gate valves are perfectly good and usually less costly, but almost invariably, they get over-tightened and then become very troublesome. Cheap gate valves seem to be inclined to developing bonnet or stem leaks.
Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.
RE: Pressure Booster Pump Calculation
Thank you for your elaborate reply.
Regards