UNIREX Pump - Manual (Page 38)

MARK 3 USER INSTRUCTIONS ENGLISH 71569102 08-06
Page 38 of 68 flowserve.com
The Mark 3 Unitized self-priming centrifugal pumps
have a slightly different requirement regarding
priming. The initial priming liquid must be added to
the pump casing until the liquid has reached the
bottom of the suction nozzle. Once the initial prime is
in place, the pump will automatically replenish itself
and additional priming liquids are not normally
needed. If liquid is lost, additional priming liquid may
be needed.
5.7 Starting the pump
a) Open the suction valve to full open position. It is
very important to leave the suction valve open while
the pump is operating. Any throttling or adjusting of
flow must be done through the discharge valve.
Partially closing the suction valve can create
serious NPSH and pump performance problems.
b) Never operate pump with both the
suction and discharge valves closed. This could
cause an explosion.
c) Ensure the pump is primed. (See section 5.6.)
d) All cooling, heating, and flush lines must be
started and regulated.
e) Start the driver (typically, the electric motor).
f) Slowly open the discharge valve until the desired
flow is reached, keeping in mind the minimum
continuous flow listed in section 3.4.
g) It is important that the discharge
valve be opened within a short interval after
starting the driver. Failure to do this could cause
a dangerous build up of heat, and possibly an
explosion.
5.8 Running or operation
5.8.1 Minimum continuous flow
Minimum continuous stable flow is the lowest flow at
which the pump can operate and still meet the bearing
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 chrome iron pumps are not recommended for
applications below -18 °C (0 °F).