is important, try to select pumps in the Ns
range.
Use double suction impellers sparingly. They
are less stable at off-design conditions than
single suction impellers.
Never select pumps with a maximum
diameter impeller. You may need to increase
the impeller diameter in the future for more
flow or head.
Always provide expected normal, minimum,
and maximum pumping rates and
temperatures in the bid specifications. This
will allow bidders to make pump and seal
recommendations that will meet the true
process needs.
Use hydraulic stability, not temperature
rise as criteria for setting the minimum
acceptable pump flow.
Q: H OW DOES A CENTRIFUGAL PUMP
DIFFER FROM OTHER TYPES OF PUMPS ?
A: Centrifugal pumps are designed to operate
in a narrow operating range, i.e. near the
best efficiency point (BEP). This means that
if they are not properly applied, they will be
highly inefficient and unreliable.
Once the proper pump is selected and
installed, the proper controls are required to
ensure the pump stays in its ideal flow zone.
If the pump control system is not able to
maintain a safe flow, then a safeguard such
as a low flow spillback controller or fixed
spillback flow orifice is highly recommended.
Q: W HAT ARE SOME BEST PRACTICES
YOU CAN OFFER END - USERS IN THE AREAS
OF SPECIFICATION , INSTALLATION AND
MAINTENANCE OF CENTRIFUGAL PUMP
Incorporate a healthy NPSH (net positive
suction head) margin or ratio, i.e. NPSHa/
NPSHr, into your selection. This ratio should
be anywhere from 1.1 to 2.0 depending on
the liquid, criticality, and suction energy level.
A larger NSPH margin is always better. TECHNOLOGY TO ENSURE LONG - TERM
Consider liquid volatility when making your
pump selection. Be more conservative in
your pump selection when the liquid has
a single boiling point (more volatile); as
opposed to a liquid with a wide boiling point
range (less volatile). Monitor your pumps regularly. Monitor
flows, pressures, vibration, and either replace
the lubricating oil regularly or have the oil
tested in a lab for contamination and wear
metals. When it‘s time to repair the pump,
this data will help define the pump repair
workscope and reduce repair costs.
PERFORMANCE ?
A: Always purchase and install heavy duty
pump baseplates that are grouted in and
minimized piping loads on the pump.
Develop detailed repair standards that
contain balance standards and acceptable
limits for critical fits and runouts. These
standards will pay for themselves in
reliability improvements.
Q: W HAT ARE SOME PITFALLS YOU SEE
END - USERS COMMONLY ENCOUNTER
IN CENTRIFUGAL PUMP APPLICATIONS ?
H OW CAN END - USERS BEST AVOID AND /
OR RESPOND TO SUCH APPLICATION
PITFALLS ?
A: Pump control issues are common. If the
process controls permit pump operation
outside the ideal flow range, some type of
flow protection should be implemented.
A pump does not like to be started up with
inadequate liquid level or without sufficient
backpressure. This means that pump
operators must always ensure that the
proper suction liquid level and discharge
backpressure conditions are met before
starting up a pump. Detailed start-up
procedures will help provide low stress pump
start-ups.
Operating centrifugal pumps in parallel with
other pumps can lead to flow problems if
their head-flow curves do not match. Always
check with a pump professional to see if it’s
acceptable to operate pumps in parallel.
Most processing plants view centrifugal
pumps as commodities and sometimes as
throw away items.
Issue 29 PECM
11