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• Pumped fluid changes state in contact
with air, i.e. crystallizes, forms a film,
dries out or precipitates solids. The flush
dissolves and rinses away the small
amount of residue which could build up on
the edges of the seal faces.
• Pumped fluid is close to its boiling point,
e.g. water over 80°C (175°F). The
flushing fluid is used to cool the seal faces.
• Pumped fluid is temperature sensitive and
evaporates when heated by the shearing
action of the seal faces. The flushing fluid
is used to cool the seal faces.
• Pump may run “dry”, i.e. no liquid in
pump chamber for very short periods.
• Pump is under high vacuum.
• A low pressure sterile barrier is required.
A low-pressure flushing fluid system must
be installed as follows:
• Liquid must be compatible with the
pumped fluid; water is the most
commonly used liquid.
• Pressure shall typically be 0.5 bar (7 psi)
gauge. Maximum of 1.0bar (14.5 psi)
gauge.
• Temperature shall be maximum of 70ºC
(160ºF) for water, less for volatile liquids
(minimum 20ºC (68ºF) below boiling point
of liquid).
• Flow rate shall preferably be 2 to 3
litres/min. (0.5 to 0.75 US gal/min) per
seal.
• Flush fluid should be connected to enter
at the lowest point on the seal housing
and flow out at the highest point to vent
air pockets.
• Pipework should be as shown in the
diagram (Figure 11).
Figure 11 - Suggested low pressure flushing system
The Double Mechanical Seals fitted to HP &
LH pumps provide the facility to contain a high
pressure fluid behind the primary seal. This allows
the pump to be used for applications where the
single seal is unsuitable.
Double seals code D are run with a fluid between
the primary and secondary mechanical seals to
form a barrier between the pump and the
atmosphere. (Figure 12)
Figure 12 - Double mechanical seal (Size 4-8)
Double Mechanical Seal with a low pressure
liquid flush when:
ATTENTION
• The pumped fluid is toxic or hazardous
and must not escape from the pump even
in minute quantities.
• The system can be installed as in figure 12,
but must be capable of withstanding the
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