Sizing a Pressure (Bladder) Tank Question

[brazilianjj]

I will be needing a booster pump station with a supply tank and a pressure tank to supply potable water for an emergency shower. Unfortunately, I have to provide this system because there is not a reliable source of potable water. I’ve sized the booster pump station to deliver 20 gpm against 119 feet (51.6 psig) of total head at the outlet of the pressure tank. I would like to determine how to size the pressure (bladder) tank.

I’ve had two manufacturer’s provide me with there recommendations. But they’re methology used to size the pump is different, thus resulting in two different pressure tanks sizes. The vendors sized the tanks with the following approach:

Vendor 1: Used the criteria of 1 minute run time to determine a drawdown or an available water storage volume of 20 gallons (20 gpm x 1 min.). With a cut-in pressure of 40 psig and a cut-out pressure of 50 psig, he determined a drawdown factor of 0.155. The total tank volume = 20 / 0.155 – 129 gallons. He recommended a 132 gallon tank.

Vendor 2: I’m still not clear about this vendor’s exact methology. He uses a something called a leak load and mentions a minimum off time. They indicated that the based it on a ½ gallon leak load and a 15 minutes minimum off time and indicated the minimum tank size would be 39 gallons. They recommended a 62 gallon tank.

At a website (

http://www.drilleronline.com),

there is an article titled “The Fundamentals of Pumped Water Systems). It stated that bladder tanks are sized according to the flow capacity of the pump, with enough drawdown to allow the pump to be off for a minimum of one minute between run cycles, letting the motor cool, as recommended by the motor manufacturers. A 10 gpm pump would require a pressure tank with 10 gallons of drawdown and so on. In appears Vendor 1 uses the same sizing methology.

Would someone know the correct method of calculating the size of the pressure tank?

 

[hydrae]

There are rule of thumb for this application accross the board. Ten state standards recomends a size of 10 gallons for every gpm so in your case 200 gallons. Note in most states a tank larger 119 gallons requires a ASME code stamp in water service applications. So use more smaller tanks...

If the shower is the only application, say use once a month for testing and all leaks will be fixed right away, and power is very reliable, then a small tank to ensure water hammer is not going to destroy the piping is also a solution say 20 gallons, depending upon pipe diameters, flows, pressure surges values and pipe pressure ratings.


Hydrae

 

[BobPE]

brazilianjj:

You gave us two vendor approaches but where is the engineers approach?

BobPE

 

[VER]

Brazilianjj,

I used the following formula for a long time without any problem. I can't remember whether it was from Grundfos Denmark or from an Italian Bladder Tank Manufacturer. Anyway here it is:

Vt=16.5 X Q/Zmax X (PUXPI)/(DPXPc)

Where:
Vt = The volume of the Bladder tank, in liters
Q = Pump flow, in l/Min.
Zmax. =Maximum allowable number of start/stop of the pump per hour, usually @10 to 20 times.(You can get this Data from the pump maunfacturer/vendor).
PU = Absolute pressure setting at which the pump stops, in Bar
PI = Absolute pressure setting at which the pump starts, in Bar
DP = PU-PI
Pc = Initial air charge of the bladder tank, absolute pressure in Bar, normally PI-0.5 Bar

Hope this helps.

VSV...