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Water Booster Pump Calculations

Water Booster Pump Calculations

(OP)
Hi,

I am specifying my first water booster pump, and would like to hear how you other engineers determine whether a booster pump is needed and the amount of pressure boost that is needed.

My calculations are as follows:

Water flow test
static pressure: 58psi
residual pressure: 54psi
test flow: 1800 gpm
available pressure at building peak flow: 57 psi @ 200 gpm

Pressure losses:
10 psi - 4" Reduced Pressure Zone Backflow Preventer - Watts LF-909 - 200 gpm
8 psi - 3" compound meter - AWWA tables - 200 gpm
17 psi - 39ft static head (4th floor)
20 psi - 400 ft of pipe @ 4psi/100ft + fittings + accessories
5 psi - Leonard Valve Master Mixing Valve TM-2020B-3PS-L @ 200 gpm
1.5 psi - tap & 50ft 4inch water service pipe
-----------------
61.5 psi

Pressure Required at the fixture
20 psi - pressure balanced shower valve (per IPC 2012)
10 psi - ASSE 1070 point of use temperature limiting valve (per local code)
-----------------
30 psi

Required Pressure: 91.5 psi @ 200 gpm
Available Pressure: 57 psi @ 200 gpm

Pressure Boost: (91.5psi-57psi) = 34.5 psi + 5psi booster loss = 40 psi

A 40 psi, 200 gpm booster pump set is required.

Happy Thanksgiving!,

Thanks!

RE: Water Booster Pump Calculations

First work out the flow requirement.

You don't say where you get 200 gpm from. That's quite a lot of water, but over how many apartments? Lots of your losses are flow relates, so a bit less flow, even say 150 gpm and you're ok.

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

RE: Water Booster Pump Calculations

(OP)
Thanks for the response, LittleInch.!

The building has 130 units (a mix of 1-bedroom, 2-bedroom and studio units), a commercial kitchen, a spa, a beauty salon and several offices.

The peak flow was calculated using Hunter's method (IPC 2012 Appendix E with local amendments):
Total number of water suply fixture units: 930 wsfu
Flow rate from Hunter's curve: 196 gpm

Thanks!

RE: Water Booster Pump Calculations

Ok, that's starting to make sense.

However there is never a right answer here.

Simply putting a 40 psi booster pump at the inlet would give the nearest unit of the ground floor 70 psi at the tap. That's quite a lot for a domestic fitting and you would probably need to insert a pressure or flow reducer in each outlet on the ground floor.

You need to look at this on a more floor by floor basis I think.

Then your losses might come down.

From your list, only the static lift is fixed.

If the flow is equal to each floor, then the top floor only has 50 gpm??
Does that "master mixing valve" have all 200 gpm going through it?

So by my reckoning you've got about 20psi at the top floor to play with. probably not enough, but you probably only need a booster pump for floors 3 and 4, so a smaller pump ( less head and less flow), hence lower cost and less power.

Also try and figure out what the range of flows is. It might mean you are better off with 2 or 3 pumps so you don't get a big pump continually starting and stopping for the 80% of the time the flow is < 40% of max flow... The pump won't last very long like that or you end up with a large pressure tank to handle the low flow scenarios.

There's a lot more to it that your somewhat simplistic calculation above.

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

RE: Water Booster Pump Calculations

With Hunter, you do not get altogether good peak flow rates - so I would question your flow rate.

You have 130 residential units and a commercial kitchen - which I would assume would be open for breakfast. These are heavy users requiring higher than anticipated flows.

You do not say if the commercial kitchen is located on a lower floor. if it is, you could deduct this flow from the booster pump. You would need to have a separate water heater (never mix pressure systems and hot water systems).

Assuming peak operation at 7AM-9AM to account for the breakfast crowd and people showering prior to leaving for work, you will likely have the commercial kitchen filling 3 bowl sinks, maybe starting the dishwasher to fill the tank, and other prep items, so you could have a flow of ~40 gpm (short duration, though). With 130 residential units, assuming 80% maximum use and a 2 gpm shower head, you could have ~250 gpm of flow. This does not include the other occupancies, which might contribute water closet (assuming flush valve) flow of 25-50 gpm, depending on how many flush nearly simultaneously. So you are up to ~300 gpm, which may not last long at all, still needs to be accounted for. If you think this is too conservative, you could go down to ~250 gpm (only one flush valve flushing). If you are using lower flow showers, you could get to the 200 gpm, but you would need higher pressure. You decide.

For the pressure, you seem to have covered the bases well, but remember flush valves for low flow fixtures (if that is what you are using) require 35 psig static to operate correctly. Also, if you are using lower flow showers, experience has shown, you need 40 psig flowing pressure. So you have starting pressure and how much you are losing, and what the top pressure is to be. If this is in the USA, remember you cannot go below 20 psig at the inlet to the booster pump per EPA requirements.

I would also recommend using a variable speed booster pump so it could hit the peak flow, but not run there all the time. This saves energy and is also compliant with ASHRAE 90.1.

RE: Water Booster Pump Calculations

(OP)
Thanks for the response, LittleInch & PEDARRIN2!

I am considering using a booster pump set with 3 pumps sized at 50% capacity (each pump provides a 40 psi pressure boost @100gpm, one pump is redundant). The set comes with Variable Frequency Drives on each pump and a small hydro-pneumatic tank. The VFDs will allow keeping a constant pressure in the system under different flow conditions. (Pressure transducer located at the top of the most remote riser) and the hydro-pneumatic tank will reduce cycling of the pumps under low flow conditions.

The building can more closely be described as two buildings: a 4 story building (retirement home) with a 1 story building (nursing home) attached to it, the commercial kitchen is on the first floor of the high rise building and it shared by both buildings. The main water distribution pipes are run on the first floor ceiling with several down-feed risers supplying the residential units on the upper floors. (Due to this configuration, having a booster pump only for the upper floors would be impractical)

PEDARRIN2, following your example, the peak flow will have to be revised to 225 gpm: The retirement home has about 80 units and the nursing home has 40 units (total of 120 units).The residents in the nursing home are expected to require constant assistance; hence, they will take showers in shifts with the assistance of on-site staff. (I've assumed 3 shifts). The shower heads have a 2.5 gpm flow rate. 80x0.7x2.5+40/3*2.5=175gpm + 40 gpm kitchen consumption = ~225 gpm.

With regards to the system layout, the whole building shares the same water distribution system and master mixing valve, and the kitchen has a dedicated 140 gpm water supply line (for the dishwasher and the three-compartment sink) that bypasses the mixing valve.

Now, for the distribution pressure, the IPC only requires a Pressure Reducing Valve (PRV) when the distribution pressure exceeds 80 psi static, which I don't anticipate occurring in the system. The difference in pressure at the tap between the fixtures that are closer to the booster set and those that are farther away is going to be about 35 psi (static head + friction losses) at peak flow. Would you recommend adding PRVs?

PEDARRIN2, for the 30 psi static pressure required at the flush valves, do you include friction losses (dynamic losses?) in the calculation or only the pressure loss due to change in elevation? I usually run my calculations with flowing or residual pressure. And with regards to the shower head, I think the 2.5 gpm wouldn't be considered a low flow fixture. Please correct me if I am wrong.

Thanks for taking the time to read the post and to reply!.

RE: Water Booster Pump Calculations

daaguirre,

Your description of the facility changes things. You could likely go back to the Hunter curve peak flow, because retirement/nursing residents use the showers a lot less intensively than say a hotel or apartment building. If they take a shower, it will be based on the employees ability to get to them and there will not be that many employees. Or you can just leave it as is, since you have variable speed pumps, it will pump what you need. Another thing to think about is to use a duplex pump set with each pump set at 100% load. That way, if one goes down, you still have full load. The cost difference between the two would be the deciding factor.

You have 58 psi static and you are providing 40 psig boost. That equals 98 psig at lower levels when the pump is not on. If you keep a single system, you will need PRV's - which is not an ideal design with a booster pump.

My suggestion would be to route a non boosted line to the kitchen and nursing home (with separate water heater(s)) and route the boosted line to the high rise. You would also have to have separate TMV, which might not be feasible for space and cost.

Since you are using a top down (stacked) approach for the high rise, you will need to adjust the set pressure of the booster pump so you do not exceed 80 psig at the lower floors. You have to make sure you have sufficient pressure at the top to operate the flush valves (showers are not low flow so only need the 20 psig required). With 35 psig loss due to elevation, you would have ~45 psig a the top level if your lowest level is 80 psig. Something you can do, is once the final plumbing inspection has passed, the facility owner can reset the pressures so they are more in line with operation (exceed 80 psig by a bit)

With a stacked design, it is difficult to use PRV because you have to have a separate PRV for the hot and the cold to make sure you do not have different pressures in the hot/cold at the faucets.

Also, due to the nature of the occupants, I would recommend a T/P shower valve, not just a P. They are more expensive, but will ensure the shower temp never gets above set point.

Static pressure does not include friction since there is no flow. You will have a bit of a pressure drop at the backflow at no flow conditions due to the spring force in the check valves.

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