Pump (With Different TDH) in Parallel

Hello PWSlack,

Thanks for the answer.

Does the answer to Question 1 valid for pumps with very small difference in pressure (let's say pump A is pumping at a pressure of 5.5bar with the flowrate of 5m3/hr into discharge line and what will happen when pump B starts discharging 5 bar at 120m3/h)? Will the flow from Pump B go into the discharge line (Will there be any flow from pump B)?

For Question 2, I don't know how I can look where the place of the pump will be on its performance curve with increasing suction head pressure. Isn't the TDH the sum of the suction pressure (in head) + the pump's pumping pressure (in head)? Then if the suction pressure is increased the TDH should increased linearly. So is it right for me to look at the performance curve of the above mentioned pump at the head of 3 bar (originally at 5 bar - the increased suction head 2 bar) which will show more flowrate?

<...Does the answer to Question 1 valid for pumps with very small difference in pressure (let's say pump A is pumping at a pressure of 5.5bar with the flowrate of 5m3/hr into discharge line and what will happen when pump B starts discharging 5 bar at 120m3/h)? Will the flow from Pump B go into the discharge line (Will there be any flow from pump B)?...>

Pump B cannot discharge 120m3/h at 5barg if the downstream system is operating at 5.5barg!

Consider the case where two identical pumps plumbed in parallel are started. One doesn't get twice the flow. Why? Because the downstream system characteristic determines where on their individual characteristics the pumps will operate. One doesn't even get the same flow through each pump. Why? Because slight variances in the piping introduce flow resistances to one pump that the other one doesn't see.

In order to see what is happening in the question 1 pumps situation, fit some pressure gauges and a time-of-flight ultrasonic flowmeter. Take some readings with pumps on and off, and if the smaller pump ain't doing much, turn the damn thing off.

<...For Question 2, I don't know how I can look where the place of the pump will be on its performance curve with increasing suction head pressure. Isn't the TDH the sum of the suction pressure (in head) + the pump's pumping pressure (in head)? Then if the suction pressure is increased the TDH should increased linearly. So is it right for me to look at the performance curve of the above mentioned pump at the head of 3 bar (originally at 5 bar - the increased suction head 2 bar) which will show more flowrate?...>

What does the downstream system characteristic look like? No pump operates without reference to its piping! The answer can be found by reading a flowrate in the downstream piping system.

I'm sure contributors' replies have been helpful, but you need to study the performance curves of the 2 pumps (mainly head vs flow) and the characteristics of the system you're pumping into, and work out what will happen. The results could differ widely depending on the details, and it's hard to cover all cases on a forum like this.

Cheers.........Codey

Thank you all for your guidance, now I could figure it out.

Wish you all the best.

Previous posters have answered #1 so I won't touch that.

#2-Excluding the potential for casing breakage, seal leakage, etc. for all practical purposes the performance of a pump is measured across the inlet and the outlet. The pump curve is relative to the pressure between the suction and the discharge. For example if a certain type pump has a rating of 50gpm at a discharge pressure of 40psi (that is 40psi greater than the suction side) then five of these pumps in parrallel will give you 250gpm at 40psi. Put them in series and you will have 50gpm at 200psi discharge pressure. This is the principle of a multistage pump except instead of having seperate pumps you combine several impellers in series within the pump body to get the higher pressure pump and design the pump to be strong enough to withstand the highest operating pressure. Many boiler water feed pumps are like this.

Many fire engines have what are called double stage pumps. They are actually two impellers within the pump body. Maxumum volume is obtained by putting the controls in the "volume" mode and it operates the two impellers in parrallel. Put the controls in the pressure mode and it operates the impellers in series, with twice the "volume mode" pressure but only half the volume pumped. In real life they are common and an example would be to operate with a volume of 2,000gpm at 200psi for the "volume mode" operation and 1,000gpm at 400psi for the "pressure mode" operation. This has the advantage of being able to pump water to higher elevations such as high-rise building stand-pipe systems because of the higher outlet pressure. These pumps are made to be able to operate at the higher pressure, whether they are used at it or not.

Your question 1) is not entirely clear.

What is connection scheme of your pumps, a) or b)?