Pressure Tank Size

The tank sizing needs to consider the maximum number-of-starts-per-hour of the pump and its starter components. From the look of it, it is an 11kW motor or thereabouts.

What does the pump manufacturer recommend?

pump 15 KW , arounr 20 starts / hour

So what does the pump manufacturer recommend? Make the call!

our pumps dealer said one 500 lit tank is enough , but tanks dealer said it should be two tank with at least 500 to 700 liter each .

So I would like to know which is right and how to calculate the right volume , also tanks mostly delivered with initial pressure bet 1.5 - 2.5 bar so why this different

• The tank dealer is there to sell tanks. The pump dealer doesn't care about the tanks. Find out why the tank dealer is offering two larger tanks when the pump dealer suggests only one. Then take a view based upon the operation and maintenance regimes in place at the facility.
• There would be no space in the tank for any water if the empty tank pressure were much higher, at those system pressures.

Savvy?

What is the anticipated outflow rate from the pressure tank?

The pressure tank size has nothing to do with the flow. The smaller the size/bigger the shorter/longer the pump start/stop. If you check booster pump supplier's catalogs you will find these tanks in various sizes ( 12" dia x 24"/ 24" dia x 36"/ 36" dia x 48"). Calculate the volume m3 based on size and that will be stop over of the pump in minutes/cut in by the pressure switch. Some of my 100~150 m3/hr flow requirements with series of 50 m3/hr x 4 sets pumps at 4 bar (1 pump standby) have these pressure tanks even in size 6ft x 10 ft. I have no issue. Each pressure switch starts/stops each pump as per flow demand from the plant.

There are many factors to calculate pressure tank size such as

1 - tank draw down which is the amount of water drawn from tank through pump stopping time ( bet. cut out and cut in )

2 - cut in ,cutout and pre charge pressure

all details are here http://www.nationaldriller.com/Articles/Column/c04c543281b39010VgnVCM100000f932a8c0

Primary charge (usually called pre-charge) absolute pressure is about 1/2 working absolute pressure, so the tank is 1/2 during normal operation.

If you want to limit the pump start frequency by tank size, the change in water capacity (caused by pressure change of the air) = pump flow/start frequency/4. The 4 comes from calculation, which shows that max start frequency occurs when the demand = 1/2 pump flow. As the pump flow and pressure each depend on the other, you'll have to put some thought into it and maybe iterate a few times to find a suitable volume. Also the pump needs a steeply rising curve to minimise tank size.

However, having said that, most booster sets do not limit start frequency by pressure tank size. They have a minimum-run timer, so e.g. if this is set to 10 mins, start frequency cannot exceed 6 per hour. If the flow falls to zero before the timer expires the pump dead-heads for the remainder of the minimum-run time, and that does no harm.

Dear sir

thank you ,but I still confused.

can you kindly give clear formula I can use for my case

Yes. Determine maximum number of starts per hour, N. Divide 60 by N. Set the minimum run timer to that number of minutes. E.g. for N = 6 starts/h, min run time = 10 minutes.

If you want to limit start frequency by the volume between start and stop, for a simple case, e.g. emtying a tank by running a pump at fixed output Q m³/h between 2 levels hence a definite volume V m³, there is a formula, V = Q/N/4. Then in worst case when inflow to sump = Q/2, start frequency = N.

But for a pressure tank it's not possible to give a simple formula. The volume change between start and stop is determined by the pressure switch settings, but you need to apply the gas laws to work it out. Also assuming it's a centrif pump (usually is) the output is not constant but falls as the pressure rises, but in just what way is determined by the pump curve. You could start by taking pump flow as the average of flow at low and high pressure switch settings. So you have to put a bit of thought into it.

Contact reputed pump manufacturers like KSB,Grundfos etc