The liquid is clean, free from sediment and
chemical precipitation, and compatible with seal
The temperature is above 32°F and below 160°F.
The suction pressure is below 75 psig.
The liquid has adequate lubricating qualities.
The liquid is nontoxic and nonvolatile.
EXTERNAL LIQUID LUBRICATION.
When the liquid being pumped contains solids or is
otherwise not compatible with packing materials, an
outside supply of seal liquid should be furnished. In
general, external-injection liquid (from an outside
source) is required when the following conditions
Liquid being pumped contains dirt, grit, or other
The temperature of the pumped liquid is below
32°F or above 160°F.
The liquid being pumped has nonlubricating
The liquid is toxic or volatile.
The suction pressure is above 75 psig, vacuum,
or high lift.
Mechanical seals are preferred over packing on
some applications because of better sealing qualities
and longer serviceability. Leakage is eliminated when
a seal is properly installed, and normal life is much
greater than that of packing on similar applications. A
mechanical shaft seal is supplied in place of a packed
stuffing box when specifically requested. The change
from packing to an alternate arrangement may be made
in the field. Conversion kits may be ordered from the
SINGLE SEAL.Pumps containing single
mechanical seals normally use pumped liquid to
lubricate the seal faces. This method is preferred when
the pumped liquid is neither abrasive nor corrosive. If
the liquid being pumped is not suitable, an external
flush should be provided. (See External Liquid
DOUBLE SEAL.A double mechanical seal
consists of two single seals mounted back to back and a
suitable sealing liquid that is introduced into the seal
chamber. The sealing liquid (preferably clear water) is
injected into the box at a higher pressure than exists at
the entrance to the seal cavity on the pump side. The
pressure differential isolates the sealing faces from the
pumped liquid. Double mechanical seals are normally
preferred in pumps handling sewage, slurries, or any
other solids suspended in the pumped liquid.
Sealing liquid that is introduced through the tap in
the seal cavity provides lubrication for the double seal.
The sealing liquid pressure must always be higher than
the pressure on the seal closest to the suction side. If
sufficient sealing pressure is not maintained, the
pressure within the pump can force open the lower seal
and allow the pumped liquid to enter the box. This can
damage the seals.
Two methods are used to provide sealing liquid to
the stuffing box. The first method uses a pressure line
installed from a tap on the discharge nozzle to the tap in
the stuffing box cartridge. A filter is installed in the
line to trap the solid particles. The filter must be
capable of screening out all particles above 25 microns
in size. Since the liquid is bypassed from the high-
pressure (discharge) side of the pump and dead-ended
in the stuffing box cartridge, there are no problems in
maintaining a sufficient pressure differential, provided
the filter is not clogged. The second method uses clear,
clean water supplied from an external source. City
water can be used if there is an air break between the
water supply and the water being provided to the
pump. Various municipal ordinances require this
break to prevent contamination of the city water
Operating conditions vary so widely that to
recommend one schedule of preventive maintenance
for all centrifugal pumps is not possible. Yet some sort
of regular inspection must be planned and followed.
You should maintain a permanent record of the
periodic inspections and maintenance performed on a
pump. This procedure will assist you in keeping the
pump in good working condition and prevent costly
One of the best rules to follow in the proper
maintenance of a centrifugal pump is to keep a record
of actual operating hours. Then, after a predetermined
period of operation has elapsed, the pump should be
given a thorough inspection. The length of this
operating period varies with different applications and
can only be determined from experience. New
equipment, however, should be examined after a
relatively short period of operation. The next