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#1

# Drawdown factor, precharge pressure and tank volume?

can anyone help me to find the drawdown factor, precharge pressure and tank volume ?This is for pressure boosting system with presure tankCut in pressure is 7 bar and cutout pressure is at 11.5 bar
01-17-2014 02:49 AM
Top #2
I would like to introduce to you Variable speed and constant pressure booster pump system for further reliable system.
01-17-2014 05:24 AM
Top #3
Haji - I am not an expert in this sizing as my experience has been more into retrofitting existing systems and diagnosing design issues. Let us say that I'm more a critic in this aspect than an innovator although I have retrofit numerous systems to VFD control before some manufacturers had starting selling them. There are manufacturer's guidelines for system sizing, the following is a start, there are others available via Internet:

You will need to offer more information for anyone to assist you. System size, demand, etc...what you have provided is not sufficient. Please review the armstrong article and possibly a few other resources and then perhaps repost?

Kind regards,
01-17-2014 07:38 AM
Top #4
Let P_1 be the precharge pressure
Let V_1 be the pressure tank volume during precharge

Let P_2 be the tank pressure when pump cuts in @ 7 bar
Let V_2 be the air volume in pressure tank when pump cuts in @ 7 bar

Let P_3 be the tank pressure when pump cuts out @ 11.5 bar
Let V_3 be the air volume in pressure tank when pump cuts out @ 11.5 bar

Using Ideal gas law, assuming isothermal process (i.e tank content temperature remains same):

P_1*V_1=P_2*V_2....equation 1

P_1*V_1=P_3*V_3....equation 2

Tank drawdown = V_2 - V_3 (substituting equations 1 and 2)
= [(P_1*V_1)/P_2] - [(P_1*V_1)/P_3]
= V_1*[(P_1/P_2) - (P_1/P_3)]

where (P_1/P_2)-(P_1/P_3) is the "acceptance factor" and pressure values are absolute.

As a rule of thumb the pre-charge pressure should be enough so that the pressure does not drop to zero when the pump starts again. I looked online and typically pre-charge pressure of 2 psi below the cut-in pressure is used.

I have no experience sizing pressure tanks, my commentary is purely theoretical and I am not 100% sure on the methodology. But I hope this gives you some hints.

Goodluck,
Waleed
01-17-2014 09:41 AM
Top #5
Dears,
pls explain with tank sizing formulas and solution. please let me know what more details you want
01-17-2014 12:10 PM
Top #6
First you have to select the purpose of the tank. Is it only for pressure stabilzation (mostly VFD) or also to prevent the the pumps of starting to much per hour. If that is the case you first have to know the q @ 11.5 bar and the q @ 7 bar.
Also you need the max alowwelble starts of the pump per hour and how many pumps are on the booster set.

With these values you can calculated the minimum run time for the pump, en with that value you can calculated the size of the tank.

Pre-charge pressure in the pressure tank is 0.5 bar below cut in.
If you need the calculation details please send an email to d.randsdorp@dp.nl
01-17-2014 02:31 PM
Top #7
PS energy wise is the system w/o vfd and with an extra membrane tank to most efficient system

Normal; 20%, VFD; 45% and pressure tank: 60% eff. only for booster sets in apartment buildings
01-17-2014 05:14 PM
Top #8
For a standard speed motor cycling on a switch, tank pre-charge should be 3-5 psi below cut-in psi. Tank or (tanks) volume & draw-down depends on the flow of the pump, minimum run time & maximum cycles per hour required by the motor manufacturer. What is the flow rate and horsepower requirement?
01-17-2014 07:23 PM
Top #9
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01-17-2014 10:18 PM
Top #10
Refer page 28 onwards on this publication.
http://documentlibrary.xylemappliedwater.com/wp-content/blogs.dir/22/files/documents/2011/12/TEH-1096.pdf

You'll find all your answers with necessary details. Let me know if you find anything confusing.
01-18-2014 01:06 AM
Top #11
Dear Haji .
well , the only one that is practical and very close to the practice is Mr .Danny randsdorp . things are as followes :
Vt = 16.5 - Qmax(Pmax+1) (Pmin+1) / Nmax (Pmax-Pmin) (Pprec+1)
Vt = pressure tank volume that needed for the max. flow of the system / not the pump .
Qmax. =is the maximum flow/ capacity of the system.( l/min)
Pmax. = the pressure that stops the pump .(bar)
Pmin. = the pressure that starts the pumps.(bar)
N = the max. namber of starts per houre of each pump ( aprox. 15 times per pump)
Pprec = the air pressure in the tank ( Pmin. - aprox. 0.5 - 1 bar )
16.5 = K ( a constant number to get the volume in liters).
make shure that when charging the air , the tank mast be empty of water.
make shure that your tank is PN16 standard .
sincerely yours
Amnon Jablonsky - Israel