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How good is Twin-Screw pump suction performance ?

Posted by goon2019 
How good is Twin-Screw pump suction performance ?
December 20, 2022 11:55PM
How good is Twin-Screw pump suction performance ?

I was lucky enough to recently take an 18 day vacation from work. During that time myself (and my better half) travelled through India and the UK and spent a good portion of that time in a Dera that requires you to surrender your cell phone. For once unencumbered by constant trivial interruptions, my mind got a chance to contemplate the wonder of this universe and (as is my fate as an Engineer), it soon turned to considering some of the more interesting questions about pumps.To get more news about MONO pump Accessories, you can visit hw-screwpump.com official website.

(For my fellow US citizens I highly recommend making your next vacation slower and longer than the usual 7 day mad dash to cram in as much "recreation" as humanly possible. Such activities are more likely than not going to leave you more stressed than when you left... Those of you in the US know what I'm talking about.)
Onward to suction performance
For as almost as long as I can remember the received wisdom in the pump industry was that Twin-Screw pumps had better suction performance than Centrifugal pumps. The suction performance is more commonly referred to as NPSHr or NPIPr, so please consider that here I use the terms more or less interchangeably.

Having had the privilege to have worked on both Twin-Screw and Centrifugal pumps, when I started to actually look at this piece of "wisdom" it didn't really hold up (except in a few cases we'll discuss later). Let me explain to you what I mean.

NPSHr performance can be tricky to compare between different pumps unless you normalize for flowrate and rotational speed. This is most easily done by computing the Suction Specific Speed - known as Nss according to the formula below. In doing so we create a (mostly) dimensionless number that can be used to easily compare performances.
Keep in mind that when talking about Nss , a higher number indicates lower NPSHr and hence better suction performance. Nss can be expressed in US units (based on USGPM and ft) or in Metric units (based on m3/h and m).

For centrifugal pumps a very average Nss value would be 8000 in US units (9294 metric in Metric units), for a pump that was NOT optimized for suction performance.
For a centrifugal pump that was optimized to minimize NPSHr, 11,000 to 14,000 in US units (12780 to 16265 in Metric units), would be readily achievable, although many would be artificially handicapped to the lower figure due to reasons that I've discussed in several of my blogs.
If I make a review of the Nss achieved by the some major suppliers of Twin Screw pumps on their tank stripping pumps (the names are redacted to protect the guilty), I get the following result:

Supplier EN Nss = 3758 to 12602 in US units (4366 to 14,641 Metric)

Supplier TL Nss = 4476 to 11345 in US units (5200 to 13180 Metric)

The higher Nss values are typically achieved on larger pumps. While they are "OKish" and generally competitive with centrifugal pumps (at least for low viscosity fluids), they are hardly aligned with the industry wisdom of being substantially better.
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