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TWINRO ("W"
series)
pumps from Plenty Mirrlees are positive displacement rotary twin
screw pumps designed for bulk transfer of liquids.
The TWINRO series is available in five frame sizes with a selection
of different pitch screwsets to match system flow requirements
at 50 Hz. or 60 Hz. direct electric motor speeds. Pumps
may also be driven at other speeds from diesel engines or other
prime movers.
The material and design options available enable the pump to
be offered for most bulk liquid transfer duties across many industries.
In particular the pump is used extensively in bulk loading and
unloading duties in the Oil, Marine, Power Generation and Chemical
Industries.
DESIGN AND
CONSTRUCTION
Designed around cast body and cover shapes the TWINRO pump offers
a low cost unit with minimum material requirements. The
pumping element consists of two contra-rotating shafts from which
right hand and left hand epicycloids screw shapes are accurately
machined. The screwset conveys the fluid being pumped from
each end and out through the center. The screw shafts are
carried in roller bearings at the drive end and ball bearings
at the non drive (gear case) end. The driven (lower) screw
is synchronized from the driving (upper) screw by a pair of hardened
and ground timing gears.
OUTBOARD BEARING
PUMPS
(for non lubricating liquids) are equipped with four mechanical
seals keeping the bearings and timing gears external from the
pumped liquid. Drive end roller bearings are provided grease
packed. An oil bath is provided at the non drive end for
splash lubrication of the timing gears and ball bearings.
INBOARD BEARING
PUMPS
(for lubricating liquids) are provided with one mechanical seal
on the drive shaft only. The liquid being pumped lubricates
the bearings and timing gears.
Relief Valve
Design and Operation
The valve
is of the disc type with an attached dashpot and spring.
Under normal operation a very small proportion of liquid from
the pump discharge leaks post the clearances between the skirt
and cylinder. To prevent pressure building up the liquid
drains back to the suction through orifice "O".
Under pressure build up the relief valve starts to open, against
the spring, exposing slot "S" to discharge pressure.
This allows the pressure to enter area "A" and quickly
complete the opening of the relief valve to fully by pass the
flow.
When the pressure drops the spring pushes the disk back on the
seat forcing the liquid in area "A" back through slot
"S". When the slot "S" is completely
blanked off by the cylinder wall all the liquid is constrained
to flow back through the orifice "O". This constraint
has a dampening effect which prevents the relief valve slamming
onto its seat.
As an option TWINRO pumps can be fitted with a jacking device
to manually lift the relief valve off its seat. This has
the operational advantage of being able to circulate pumped liquid
around the pump to aid extreme discharge or suction conditions..
The device has proved extremely useful in aiding cold start conditions
where the liquid in the discharge line is below normal pumping
temperature. Another useful application is the partial
circulation of discharge liquid back to suction to aid high suction
lift applications at the end of barge or tank emptying.
Operation of the jacking device does not alter pre set relief
valve spring pressure.
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