Last of 4-part article

Effects of viscosity on pump performance


QV1  -  Is the net positive suction head required by a centrifugal pump affected by
viscosity?


AV1  -  Viscosity is the property of a liquid that resists deformation under shear
stress. Simply stated, it is the property that resists the force that causes the liquid
to flow. A more viscous liquid has a higher resistance to deformation that causes
the liquid to flow.

A centrifugal pump does not have the capability to force the liquid to enter its
suction nozzle. A positive external force has to be applied to push the liquid into its
suction nozzle. The force required to do the push is called positive suction head
required, or NPSHR. An example of external force is the atmospheric pressure.
Common sense dictates that the higher the liquid's viscosity is, the higher is its
NPSHR.

ANSI/HI Standard 9.6.7-2004 provides updated instructions for correcting a known
water performance into an estimated viscous performance for a given viscosity.
The standard now includes a method for converting a known water NPSHR into a
viscous NPSHR. The method is analytical and, as of this writing, is not supported
by actual test data.


QV2 - I read from a pump book by a widely-known pump expert that Affinity Laws
are applicable to viscous performance. Is this true?


AV2 - If that pump book is the same book we have in mind, the author made that
statement many decades ago on an analytical basis because, back then, there
was very little historical tests from which to form a different conclusion.  

Based on subsequent data, we now know that the Affinity Laws should not be
applied to viscous performance, rather the viscous performance should first be
converted to its equivalent water performance before the Affinity Laws are applied.
Once the new water performance is known, only then should the viscous
correction factors be re-applied to estimate the new viscous performance.

For example, consider a centrifugal pump with BEP conditions of 3000 GPM, 200
Feet head at 1780 RPM, pumping crude oil with 0.80 specific gravity with 250
centistokes viscosity. The viscous correction factors are CQ=0.958, HC=0.958,
and CE=0.794. These correction factors will be applied to get the equivalent
viscous performance.

If the pump were to run at 3560 RPM the new BEP conditions would be 6000 GPM
and 800 FT. If the Affinity Laws were to be applied to get its equivalent viscous
performance, as asserted by the author, then the same viscous correction factors
would apply.

But this is not true. The new viscous correction factors would be CQ=0.982,
CH=0.982, and CE=0.87

The Affinity Laws should not be applied to viscous performance.


QV3  -  The same book also states that only the head and efficiency correction
factors need to be known to estimate an equivalent viscous performance. Please
comment.

Av3 - We also now know that it is not true, as explained in AV2. A capacity
correction factor should also be applied.
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QV4  -  Due to a change in our process our pump will be used to handle a liquid
with much higher viscosity than it was originally designed for but at same capacity
and RPM. What do we check for to ensure that the pump is safe to operate at this
new duty?


AV4  - High viscous liquid usually results in reduced pump efficiency and possibly
reduced head also. Estimate the new viscous pump performance using the
Hydraulic Institute correction chart. Calculate the BHP for the new viscous duty and
check that the driver and the pump shaft is big enough for the required BHP.

If the pump is fitted with mechanical seal and cyclone separator, the cyclone
separator may have to be modified or may not work at all with its existing seal
flush piping. Consult with your pump vendor.


QV5  - I read that a centrifugal pump is not  suitable for highly viscous liquids and
that a positive displacement pumps should be used instead. What is the
maximum viscosity that a centrifugal pump can handle?


AV5  - A centrifugal pump should be able to handle a liquid within the viscosity
range shown in the Hydraulic Institute viscosity correction charts issued prior to
the publication of ANSI/HI Standard 9.6.7-2004. The viscous correction chart show
the maximum viscosity at 3,300 centistokes. This is presuming that the pump is
properly designed to handle the viscosity in terms of adequate shaft size, driver
HP, correct clearances, proper mechanical seal piping, etc.

(One pump manufacturer shows ( * ) centistokes as the highest viscosity in its
reference installation list with trend towards using slower speed ( * ) and bigger
pumps at higher viscosity values.)

ANSI/HI Standard 9.6.7-2004 does not address the issue of maximum viscosity,
thus it implies that any viscosity can be handled by centrifugal pumps which is
misleading. The use of centrifugal pumps are likely to reach their practical limit at
some very high viscosity values.


QV6  -  We have an RFQ for centrifugal pumps in viscous service. We are holding
up the PO because the bidders will guarantee the pumps' water performance only
but not their viscous performance. Please comment.

AV6  -  Pump vendors do not want to guarantee a viscous performance.

Firstly, the pumps can be tested in the test facility using only water as test
medium, hence the actual viscous performance cannot be verified.

Secondly, field testing at the pump site using the actual viscous liquid cannot be
done accurately enough to be used as basis for verifying compliance with the
predicted viscous performance.

Thirdly, pump companies use empirical viscous correction factors that are widely
accepted in the industry as standard. Companies do not want to be accountable
for the accuracy of those empirical factors that are outside of their control.

( * ) Some data are excluded in this article.
Read more.


R: 1111EFOF - viscosity effects
C: design, operation

Beta version
Related topics

Part 1: What is viscosity
Part 2: Correcting for viscous performance
Part 3: Precautions for handling viscous liquids
Part 4: Effects of viscosity