Flowserve Twin Screw Rotary - Manual (Page 27)

TWIN SCREW PUMPS. ORIGINAL USER INSTRUCTIONS. ENGLISH. 71569243 – 07/10
Page 24 of 53
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d) On suction lift the piping should be inclined up
towards the pump inlet with eccentric reducers
incorporated to prevent air locks.
e) On positive suction, the inlet piping must have a
constant fall towards the pump.
f) Flow should enter the pump suction with uniform
flow, to minimize noise and wear. This is
particularly important on large or high-speed
pumps which should have a minimum of two
diameters of straight pipe on the pump suction
between the elbow and inlet flange. See section
10.3 Additional sources of information
for more
detail.
g) Inlet strainers, when used, should have a net `free
area' of at least three times the inlet pipe area.
h) Fitting an isolation valve will allow easier
maintenance.
i) Never throttle pump on suction side.
4.7.3 Discharge
piping
A non-return valve should be located in the discharge
pipework to protect the pump from excessive back
pressure and hence reverse rotation when the unit is
stopped.
Pipework reducers should have a maximum total
angle of divergence of 15 degrees. Fitting an isolation
valve will allow easier maintenance.
4.7.4 Relief
Valves
Twin Screw Pumps are of the positive
displacement type and will build up considerable
pressure if discharge line is blocked through closing of
valve, etc. It is therefore necessary for the protection
of the pump and discharge line to provide a relief
valve. This should be piped back to the suction tank
and not to the suction line.
4.7.5 Auxiliary piping
4.7.5.1 Drains
Pipe pump casing drains and gland leakage to a
convenient disposal point.
4.7.5.2 Pumps fitted with gland packing
When suction pressure is below ambient pressure it is
necessary to feed the gland packing with liquid to
provide lubrication and prevent the ingress of air. This
is normally achieved with a supply from the pump
discharge volute to the stuffing box. A control valve is
fitted in the line to enable the pressure to the gland to
be controlled.
If the pumped liquid is dirty and cannot be used for
sealing, a separate clean compatible liquid supply to
the gland at 1 bar (15 psi) above suction pressure is
recommended.
4.7.5.3 Pumps fitted with mechanical seals
Single seals requiring re-circulation will normally be
provided with the auxiliary piping from pump casing
already fitted.
If the seal requires an auxiliary quench then a
connection must be made to a suitable source of
liquid flow, low pressure steam or static pressure from
a header tank. Recommended pressure is 0.35 bar (5
psi) or less. Check General arrangement drawing.
Special seals may require different auxiliary piping to
that described above. Consult separate User
Instructions and/or Flowserve if unsure of correct
method or arrangement.
For pumping hot liquids, to avoid seal damage, it is
recommended that any external flush/cooling supply
be continued after stopping the pump.
4.7.6 Final checks
Check the tightness of all bolts in the suction and
discharge pipework. Check also the tightness of all
foundation bolts.
4.7.7 Protection Against Improper Operation
The user should review the need for special operating
procedures and protective devices peculiar to the
particular installation involved. These may include
special start-up and shut-down procedures, over-
speed protection, temperature, flow and pressure
interlocks, protection against automatic start-up in the
event of power failure, surge protection, protection
from freezing, lack of prime protection, temporary
strainers in the suction line, vacuum breakers, etc.
Great care should be exercised during the erection of
piping to keep lines clean and free of dirt, scale,
threading or welding chips, etc. Such foreign matter
entering pump may cause scoring of body bores and
unnecessary breakdown and costly repairs.
For pumps having jacketed bodies, the jacket inlet
piping should always be made at the lowest inlet point
with outlet at top or highest point. Provide a valve on
the inlet piping so that flow may be regulated to
control temperature.