Running different pumps in series

[Quentin]

I am proposing a system using existing pumps and lines to transport water to an area that now requires this water in our factory.
For simplicity sake, this proposal utilises two centrif pumps in series, 20HP and 10HP although the first pump (20HP) would be sufficient for the task.
The system will be run manually. If the second pump is not running when the system is started, is there a possibility the second pump may start spinning in reverse as the water flows through it from the first pump? What then if the operator starts the second pump? Is there anything else to be wary about with a system like this?
Am I better off just spending more money to bypass the second pump

 

[unclesyd]

Bypass the 10Hp pump with valves. If the 20 Hp is running and spinning the 10 Hp pump backwards all manner of bad things can happen, the impeller comes loose, the 10 Hp motor starts and tears up the 10 Hp pump.
If the 20 Hp can do the job isolated the 10 Hp pump until needed. You overcome all types problems, hydraulic, mechanical, doing this.
Much as I hate to say this you might just use a check valve in the discharge of the 10 Hp. But valves will be better as it allows you to isolated a pump for maintenance.

 

[Quentin]

Cheers mate. Looks like bypassing the second pump with valves is the sensible option.

Quentin

 

[Quentin]

I'm now wondering if running two pumps in series always poses this problem, or if certain impellor types in the second pump will always spin in the correct direction when they have feed introduced to them and they are not powered up.
If not and you do have a two pump system should you always be starting the second pump first to avoid the problems you spoke of?

Quentin

 

[TD2K]

Are these two pumps in parallel or series?

While you say series in your OP, your description what you are concerned could happen sounds more like two pumps in parallel.

 

[arsef]

It would be best to avoid having these pumps in series. They could be installed in parrallel with check valves on the discharge of each pump. I am at a loss as to the reason for having two mismatched pumps in series.

 

[Quentin]

The pumps are in series.

I am only proposing to use the second smaller pump in the system because this water supply will be required very infrequently, and this would be the easiest thing to do. It doesn't involve having to change over too much pipework or make valve changes. If the system was valved, I'd imagine needing proximity switches and software protection to ensure the second pump was actually being bypassed.

Quentin

 

[patrickjmp]

Quentin,

If the pumps are in series, and of the same rotation, one pump will not cause the other pump to run in reverse. Running pumps in series add their respective TDH capacity (head or pressure). It is OK to run two mismatched pumps in series, I would not recommend this for parallel operation.

Patrick

 

[abcmex]

Quentin,

1. If the pumps are connected in series and are similar you probably do not risk a reverse rotation.

2. If the pumps are connected in parallel it is usuall to install check valves to avoid reverse rotation

3. If you want to operate both: check that the pumps flow are compatibles and that the suction pressure of the second pump is in the operational range.

4. If you operate the 20 hp pump and the 10 hp pump is off and (no by-pass) you may:
a)have higher presure drop on the line, the pump will be like an obstruction.
b) there is the risk of induction in the motor by the residual magnetism. this could damage the motor windings .

you could add a check as by pass for the second pump and this may work without any electronics or opening valves ;-)

 

[kcj]

The shaft seal of the second pump might be seeing high pressure, not suction as it may be intended for.

If the flow & pressure ratings mismatch the second pump might be cavitating.

If this is being done to boost pressure, I would look at putting a larger pump, closer to the beginning of the circuit, and deleteing the original one