×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Displacement and centrifugal pump in series?

Displacement and centrifugal pump in series?

Displacement and centrifugal pump in series?

(OP)
This one came up a while ago when talking with a customer, and I fear the issue will pop up again.

Here's the situation: We build a biogas plant, the liquid digestate/slurry is to be applied as fertilizer. We use centrifugal pumps that are sitting directly at the storage tanks to fill the slurry spreader wagons.

Now, our customer has wagons with a rotary lobe pump installed to draw digestate from tanks. Depending on the way we connect the wagons both pumps would be in series. I think this is a bad idea and would like you to check my reasoning:

The lobe pump (that is not specified by us and will be provided by a partner of our customer) will have to work at the same flow as the centrifugal pump, if it is over- or undersized something will be damaged.

My second question is - how ould you actually calculate such a system?
Pumps in series means added head, same flow - but the flow of the centrifugal pump will be far more dependent on the head, so I need to know what "share" of the head each pump will bear. Am I correct so far? How do I arrive at this?

RE: Displacement and centrifugal pump in series?

Can we assume the centrifugal pump is higher capacity than the lobe pump?

This being the case, the centrifugal pump will just see the lobe pump as a restriction in the discharge line and will move along its performance curve to the flowrate that the lobe pump is able to pump at its operating speed. The lobe pump being "positive displacement" can only pump "x" flow at the speed of operation.
The only real problem that first comes to mind is if the centrifugal is forced to run too far left on its curve you may have shaft deflection, seal, vibration problems, internal re-circulation and accelerated wear - how about supplying the performance curves of the centrifugal and the lobe pump, this will give a better understanding.
There might well be a few other minor problems which crop-up as the discussion develops from those with experience with lobe pumps (not my area of expertise).

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Displacement and centrifugal pump in series?

One would expect the centrifugal and rotary lobe pumps connected in series to pump at the flow rate of the rotary lobe pump since the rotary lobe pump is a positive displacement pump.

Obtain the centrifugal pump performance curve and determine where the centrifugal pump will be operating. The farther you get away from the BEP, the more potential there will be for pump problems.

RE: Displacement and centrifugal pump in series?

Is there some reason you can't just bypass the lobe pump and have the centrifugals load the wagons??

RE: Displacement and centrifugal pump in series?

I would recommend by passing the the loble pump and filling the tank with just the centrifugal pump. The lobe pump is likely sized to operated a slurry injection system on the wagons. This requires higher pressures and lower volumes. The maximum flow rate will be based on the lobe pump which is likely operated by a hydrolic motor and then pump speed is dependent on the engine speed and will vary with each truck and operator.

The final issue is that the lobe pump is required to reverse for loading. In the event of operator error failing to reverse the to lobe pump operation may cause catistrophic failure due to over pressure.

Due to these couple of issues that i can think of and the many that i have not yet considered, I restate that it is not a good idea to run the pumps in series. If you insist on using the lobe pumps to load the wagons, do not put pumps on the storrage tanks.

RE: Displacement and centrifugal pump in series?

I wouldn't ask the customer to modify his wagons, e.g. bypass the lobe pumps, because he's the customer.

I have used a somewhat similar arrangement, a centrifugal pump to supercharge the suction of a very large hydraulic pump, so as to provide the hydraulic pump's NPSHR without pressurizing the reservoir. That was 40+ years ago, and I think the systems are still in service. Those pumps were matched, as in checking the OP of the centrifugal throughout the flow rate range of the big swashplate pump.

In this case, the series connection should be checked for corner conditions, but in general, as Artisi and bimr described, the centrifugal pump will adjust, and the world won't come to an end. If the flows are widely disparate, or the combination will drive the centrifugal into an undesirable flow regime, the solution is as simple as switching the centrifugal off, and letting the lobe pump just draw through it.



Mike Halloran
Pembroke Pines, FL, USA

RE: Displacement and centrifugal pump in series?

Nice answer, Mike.

RE: Displacement and centrifugal pump in series?

Just one point for clarification, you are connecting the centrifugal pump to the inlet of the lobe pump, or is the lobe pump reversible?

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Displacement and centrifugal pump in series?

(OP)
Thank all of you for your input.

I think this will be the route to go:

* specify a range of flow rates close to the OP of the centrifugal pump, if the lobe pumps are within it's ok
* else, we will need to bypass the lobe pump

Just to give you some context:
those wagons have an opening on top, the only issue there could be emissions (smell). I don't see a huge issue there.

Again, thanks for the detailed inputs.

RE: Displacement and centrifugal pump in series?

Do you have a curve for the centrifugal pump?

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources