wickedpete

Pros and cons:  Assume a very well designed PD verses a very well designed centrifugal:

I would expect a shorter MTBF for the PD than the centrifugal if for no other reason than pump vibration.  You may want to look comparable installations to obtain expected MTBF for both systems.  You may get lucky and get repair cost too.

I would expect more flow line related failures with the PD if for no other reason than vibration and pulsations.

PD efficiency verses slurry centrifugal efficiency; the PD efficiency will look great.  After a few months operation I would expect the PD efficiency to go down by several percentage points while the slurry pump should remain comparatively constant.  You should be able to make annual power cost estimates for both pumps.

PM time on the PD pump will be a lot higher than the centrifugal.  Talk to both types manufactures and add their recommendations to your cost.

If using municipal employees to perform pump repairs the centrifugal should be simpler for them.  You may require specialized training for both pumps, but the centrifugal should be less for them to remember.

It depends on the total flow, but I would believe that the PD will look better mainly due to power cost.  There is no way to put a price on headaches.

If you are pumping sludges you must consider the thixotropic nature of the sludge i.e the dynamic head is near on impossible to work out.  An exception is EAS and WAS which we use flygt N pumps for) this is less of a problem with PD pumps which is why they are commonly used.  There are no problems using them in parallel.  most common problem is the wear of the rotor and stator (eccentric screw pumps) when suppliers undersize them and run them at >250 RPM
With a centrigugal pump if you calculate the head wrong you could be in big trouble.  having siad that, Hydrostal pumps perfrom well in these situations.

When designing with centrifugal pumps, there is some loss in flow when operating 2 in parallel (achieve ~150-160% of 1 pump operation).  So the PD will be 200% with 2 operating?  We need to acheive flows in the range of 350gpm - 1000gpm.

I don't think that the thixotropic nature will be an issue from a clarifier.  Isn't that more of an issue drawing from the disgester?

Also, we were going to manifold the line to the low pressure flow from the headworks to the aeration basin.  How would you match the head at the manifold?  Is there some way to set the operating head for all pumps to work at?  I've never spec'd a PD pump either.  Thanks for the advice to this point.

P.S. We are looking into the Lobe type PD pump.

Positive displacement pumps commonly have some characteristic of pulsation in their flow rate.  Be sure to fully understand this and allow for it in your design and selection processes for your entire system.  Pulsation dampers are readily available to mitigate these pulsations, but the presence of solids in the flow can impair the functionality of these dampers.

Since there may be some solids in the flow, you may want  to consider progressing cavity pumps for this application if you want to use a positive displacement type of pump.

Positive displacement pumps are just that--positive displacement.  They will pump a fixed volumetric flow with each revolution (or stroke cycle) without regard to line pressure.  Simply stated, running two identical pumps instead of one will double the flow rate.  It is important to include suitable pressure relief provisions to prevent overloading the driver or causing harm to equipment or personnel from damage due to excessive pressure.

As with centrifugal pumps, the pump(s) and all the associated piping and equipment always function as a complete interactive system.

For what its worth I have to agree with ccfowler a "PCP" pump may be another option to consider.

high efficiency
no pulsations
good with solids
readily available
simple maintenance

PCP = high efficiency - you have to be kidding!! - turn one by hand.
I generally agree with most comments above, except that I amtrying to warn of the dangers of undersize pumps, which can be specified by suppliers, who will quote small pumps to win the job, then you will be changing rotors and stators every 18 months as it wears out due to its fast operating speed.  Ask the supplier for the cost of these parts.  In addition to this if you run them in parallel the higher head will mean you will not get double the flow - a single stage for example can slip substantialy and even the pump curves will show you that.
Having said that we are currently designing a WWTP to use about 10 of these PCP pumps, because we are sizing them correctly (ie under 250 RPM).

Try a Hidrostal. They have very good delicate handling properties (good on activated sludges), small footprint, high efficiency & low spares usage.

They also have a very steep Q/H curve, which allows them to cope better than a typical centrifugal pump, when sludge consistency (& thus head changes occur). Plus they can pump upto a 13% dry solids sludge.

They bridge the gap between typical centrifugals & PD pumps for sludge pumping.

Another handy feature is an externally adjustable liner (can re-set impeller clearances without stripping the pump down)

Some General Points

Centrif's give velocity to a liquid and are therefore affected more by variations in Back Pressure than PD Pumps, Hence 2 Centrifs in parallel will have a much reduced performance.

PD Pumps as the name implies move a set volume of liquid in relation to their rpm/stroke as has already been said. If the Backpressure capability of each pump exceeds the total line losses of the system then effectively 200% flow is achievable.

If pulsation is a problem (which may not be so)then PCP pumps are ideal, but Lobe Pumps are not generally affected either.

It is common sense to have the largest displacement possible at the lowest rpm, this will probably be decided by budget rather than design. I prefer <150 rpm to ensure a reasonable return on spare parts life.

Efficiency is not a word commonly used with PD Pumps

Beware of Dry Running Suitability.

Whatever you choose they will be big PD pumps.

Good Luck

You could also look at a progressive cavity pump. I have just done business with Seepex (Germany) and received excellent service. There are however many other suppliers.

Best Regards

Morten