Contributing editor Chikezie Nwaoha
(AMIMechE) interviews Robert
Perez (He has more than 30 years of
rotating equipment experience in the
petrochemical industry and has numerous
machinery reliability articles to his credit.
Mr. Perez holds a BSME degree from Texas
A&M University at College Station, a MSME
degree from the University of Texas at
Austin, and a Texas PE license). The author
of a new book entitled, “Operator’s Guide
to Centrifugal Pumps” and co-creator of
the PumpCalcs.com website on Centrifugal
Pumps in a Nutshell.
Q: H OW HAS CENTRIFUGAL PUMP
TECHNOLOGY EVOLVED OVER THE PAST
DECADE ?
A: In the past decade I’ve seen several
major trends. The first is a trend to lower
and lower fugitive emission requirement
for mechanical seals. This has been a real
challenge to machinery engineers. Luckily
mechanical seal manufacturers have come
to rescue with design improvements, such
as reduced face loading designs and dry gas
seals. Another solution to this problem has
been mag-drive and canned motor pumps,
which address the issue in a different fashion.
The hydraulic range of sealless pumps has
expanded significantly in the last decade.
The second trend has been the offering of
low-cost pump monitoring solutions, which
include wireless systems that can monitor
pump vibration and temperatures remotely.
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PECM Issue 29
The prices of these systems continue to drop
while their capabilities are expanding. Some
of these systems are now viable options for
monitoring critical process pumps. inches, is often used to define shaft flexibility.
An upper limit of 60 for English units (2 in
metric units) is recommended for L3/D4 to
limit shaft deflection.
Q: H OW IS THE CENTRIFUGAL PUMP In the 80’s, multistage and double suction
overhung designs were outlawed in the
API 610 Standard. These overhung designs
were found to be highly unreliable and even
dangerous in flammable services.
TECHNOLOGY OF TODAY MORE EFFECTIVE /
EFFICIENT THAN THE TECHNOLOGY OF
PREVIOUS GENERATIONS ?
A: In the 70’s and 80’s, 3600 rpm (or 3000
rpm in 50 Hz countries) pumps became the
norm in process pump applications due to
pricing pressures. This led to more vibration
problems that were caused by rotordynamic
and hydraulic stability issues. Thanks to a
lot of smart people most of the common
problems were solved by the end of the 80’s.
Many of hydraulic stability issues were
solved by limiting suction specific speeds
(Nss) to less than 11,000 in English units
(9460 in metric units, i.e. m3/h) and limiting
operation to the 80% to 110% of the best
efficiency flow. Aftermarket companies also
began to offer “hydraulic rerates” for poorly
applied pumps. (Rerating a pump means
redesigning the pump internals and fitting
them to an existing pump casing.)
A lot of the rotordynamic issues were
resolved by 1) limiting shaft flexibility, 2)
improving balancing best practices, and 3)
discouraging the use of certain overhung
rotor designs. For overhung impellers, the
term L3/D4, where L is the overhung length
and D is the shaft diameter are both given in
Another huge improvement in the 80’s was
the development and acceptance of the
mechanical cartridge seal. This dramatically
improved seal reliability by reducing early
failures and allowing seal testing before their
i nstallation.
Q: W HEN SELECTING CENTRIFUGAL
PUMP , WHAT ARE SOME KEY
CONSIDERATIONS AN END - USER SHOULD
MAKE TO ENSURE SUCCESS ?
A: Here are 10 tips to help you select efficient
and reliable centrifugal pumps:
Only select pumps with suction specific
speeds (Nss) less than 11,000—an Nss of less
than 9000 is even better.
Never select a pump that will have to operate
below 70% to 80% of its best efficiency point.
Remember that 1800 rpm and slower pumps
are usually more reliable that 3600 rpm
pumps. Hydraulic efficiency peaks at specific
speeds (Ns) between 2000 and 3000 and
drops dramatically below 500. If efficiency