Flowserve Mark 3 - Manual (Page 25)

USER INSTRUCTIONS MARK 3 High Silicon Iron ENGLISH 71569249 09-04
®
life, shaft deflection and bearing housing vibration limits
documented in the latest version of ASME B73.1M.
Pumps may be operated at lower flows, but it must be
recognized that the pump may exceed one or more of
these limits. For example, vibration may exceed the
limit set by the ASME standard. The size of the pump,
the energy absorbed, and the liquid pumped are some
of the considerations in determining the minimum
continuous flow (MCF).
The minimum continuous flow (capacity) is established
as a percentage of the best efficiency point (BEP). (See
section 3.4.4.)
5.8.2 Minimum thermal flow
All Mark 3 pumps also have a minimum thermal flow.
This is defined as the minimum flow that will not
cause an excessive temperature rise. Minimum
thermal flow is application dependent.
Do not operate the pump below
minimum thermal flow, as this could cause an excessive
temperature rise. Contact a Flowserve sales engineer
for determination of minimum thermal flow.
Avoid running a centrifugal pump at drastically reduced
capacities or with discharge valve closed for extended
periods of time. This can cause severe temperature
rise and the liquid in the pump may reach its boiling
point. If this occurs, the mechanical seal will be
exposed to vapor, with no lubrication, and may score or
seize to the stationary parts. Continued running under
these conditions when the suction valve is also closed
can create an explosive condition due to the confined
vapor at high pressure and temperature.
Thermostats may be used to safeguard against over
heating by shutting down the pump at a predetermined
temperature.
Safeguards should also be taken against possible
operation with a closed discharge valve, such as
installing a bypass back to the suction source. The size
of the bypass line and the required bypass flow rate is a
function of the input horsepower and the allowable
temperature rise.
5.8.3 Reduced head
Note that when discharge head drops, the pump’s
flow rate usually increases rapidly. Check motor for
temperature rise as this may cause overload. If
overloading occurs, throttle the discharge.
5.8.4 Surging condition
A rapidly closing discharge valve can cause a
damaging pressure surge. A dampening
arrangement should be provided in the piping.
5.8.5 Operation in sub-freezing conditions
When using the pump in sub-freezing conditions
where the pump is periodically idle, the pump should
be properly drained or protected with thermal devices
which will keep the liquid in the pump from freezing.
High Silicon Iron pumps are not recommended for
applications below -29 °C (-20 °F).
5.9 Stopping and shutdown
THERMAL SHOCK
Rapid changes in the temperature of the liquid within
the pump can cause thermal shock, which can result
in damage or breakage of components and should be
avoided. High Silicon Iron should be heated and
cooled at a maximum rate of 55˚C (100˚F) per hour.
5.9.1 Shutdown considerations
When the pump is being shutdown, the procedure
should be the reverse of the start-up procedure.
First, slowly close the discharge valve, shut down the
driver, and then close the suction valve. Remember
that closing the suction valve while the pump is
running is a safety hazard and could seriously
damage the pump and other equipment.
5.10 Hydraulic, mechanical and electrical
duty
5.10.1 Net positive suction head (NPSH)
Net positive suction head - available (NPSH
A
) is the
measure of the energy in a liquid above the vapor
pressure. It is used to determine the likelihood that a
fluid will vaporize in the pump. It is critical because a
centrifugal pump is designed to pump a liquid, not a
vapor. Vaporization in a pump will result in damage to
the pump, deterioration of the Total differential head
(TDH), and possibly a complete stopping of pumping.
Net positive suction head - required (NPSH
R
) is the
decrease of fluid energy between the inlet of the
pump, and the point of lowest pressure in the pump.
This decrease occurs because of friction losses and
fluid accelerations in the inlet region of the pump and
particularly accelerations as the fluid enters the
impeller vanes. The value for NPSH
R
for the specific
pump purchased is given in the pump data sheet, and
on the pump performance curve.
For a pump to operate properly the NPSH
A
must be
greater than the NPSH
R
. Good practice dictates that
this margin should be at least 1.5 m (5 ft) or 20%,
whichever is greater.
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