UNIREX Pump - Manual (Page 39)

MARK 3 USER INSTRUCTIONS ENGLISH 71569102 08-06
Page 39 of 68 flowserve.com
5.9 Stopping and shutdown
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.
5.9.2 Shutdown - Mark 3 Self-Priming
At shutdown, the liquid in the discharge piping falls
back into the priming chamber and washes through
the impeller into the suction. The backflow creates a
siphon effect in the casing until the liquid level falls
below the bottom of the suction nozzle. The inertia of
the flow pulls fluid from the priming chamber to a
level lower than the initial priming fill. Though the
level is lower, there is still sufficient fluid in the
priming chamber to allow the pump to reprime itself.
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.
Ensuring that NPSH
A
is larger than
NPSH
R
by the suggested margin will greatly enhance
pump performance and reliability. It will also reduce
the likelihood of cavitation, which can severely
damage the pump.
5.10.2 Specific gravity (SG)
Pump capacity and total head in meters (feet) of
liquid do not change with SG, however pressure
displayed on a pressure gauge is directly proportional
to SG. Power absorbed is also directly proportional
to SG. It is therefore important to check that any
change in SG will not overload the pump driver or
overpressurize the pump.
5.10.3 Viscosity
For a given flow rate the total head reduces with
increased viscosity and increases with reduced
viscosity. Also for a given flow rate the power
absorbed increases with the increased viscosity, and
reduces with reduced viscosity. It is important that
checks are made with your nearest Flowserve office if
changes in viscosity are planned.
5.10.4 Pump speed
Changing the pump speed affects flow, total head,
power absorbed, NPSH
R
, noise and vibration levels.
Flow varies in direct proportion to pump speed. Head
varies as speed ratio squared. Power varies as speed
ratio cubed. If increasing speed it is important to ensure
the maximum pump working pressure is not exceeded,
the driver is not overloaded, NPSH
A
> NPSH
R
and that
noise and vibration are within local requirements and
regulations.
6 MAINTENANCE
It is the plant operator’s responsibility to ensure
that all maintenance, inspection and assembly work
is carried out by authorized and qualified personnel
who have adequately familiarized themselves with
the subject matter by studying this manual in detail.
(See also section 1.6.2.)
Any work on the machine must be performed when it
is at a standstill. It is imperative that the procedure
for shutting down the machine is followed, as
described in section 5.9.
On completion of work all guards and safety devices
must be re-installed and made operative again.
Before restarting the machine, the relevant
instructions listed in section 5, Commissioning, start
up, operation and shut down must be observed.
Oil and grease leaks may make the ground
slippery. Machine maintenance must always
begin and finish by cleaning the ground and the
exterior of the machine.