2 expansion tanks in closed system. 1 on suction side of pump. 1 way out in system. Pressure/flow?

[BronYrAur]

This is more of a theoretical question, although I may run into it soon in the field. What happens to flow/pressure in a system with 2 expansion tanks? One tank would be on the suction side of the pump, and the other way out in the system. I have attached some quick sketches of my understanding of the pressures when the tank is on the suction side, discharge side, or at the halfway point in the system. But I can't figure out what would happen with 2 tanks.

Please don't side-track this thread into why I should not have multiple tanks in a system. I am trying to figure out the pressures if I did have 2.

 

[MJCronin]

Centrifugal pumps only know about their suction side pressure and discharge head.

Have you evaluated NPSHa in all scenarios ?

Centrifugal pumps will also have the longest operating life if they operate only at one discharge pressure.

Suction side expansion tanks usually ensure the largest NPSHa of any configuration ....

Respect us and complete this thread ..... tell us of your final decision.


MJCronin
Sr. Process Engineer

 

[LittleInch]

So to answer your questions in the attachment.

Yes both tanks want to stay at 50, but they can't. Expansion tanks vary, but most are simple sealed systems where the volume of fluid moving in and out is realtively small part of the volume and the other part is a gas bladder or space which is pressurized to a set pressure ( in your case 50)

If the pressure falls below 50 the gas will expand and force liquid out to try and pressurise the system or equalize gas pressure with system pressure. If liquid just keeps flowing out then at some point you run out of liquid.

Reverse happens if the pressure goes up - liquid flows in until the gas pressure rises to meet it. If your system relieves at excess pressure then at some point all the gas is gone - not good.

So assuming your expansion tanks are relatively simple, my understating is that the two expansion tanks will balance out. So inlet might fall to say 35 or 40 and other tanks rise to 65 or 70 given it's not directly at pump outlet.

To get any real idea you need to understand how your expansion tanks are working, what their volumes are, gas pressures empty and full, pressure maintainence (won't work) or simple ones.
This is why you only have one....
Make sense?


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

 

[racookpe1978]

Unless the two tanks are at equal heights and volumes, will the higher one not leak through the pump (and any check valves) when the system is shutdown into the lower tank?

 

[BronYrAur]

I'm a little confused by the last comment. Assume there are no elevation differences between the tanks. I am just trying to understand what happens to the pressures throughout the loop with the 2 tanks connected. The fill pressures are the same with the pump off. Then the pump is turned on. What happens to the pressure around the loop?

 

[Compositepro]

When you turn a pump on or off the pressure will change at different points in the system and fluid will flow, in effect, from one tank to the other. This could result in damped or in undamped oscillation in flow.