SAMCO CP7 Instructions Manual page 17

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More pressure is required, however, for slurries than for clear liquids. Examination of the leakage will
indicate whether to increase or decrease external pressure. If slurry is present in the leakage, increase the
pressure until only clear liquid drips from the box. If the drippage is corrosive or harmful to personnel, it
should be collected and piped away.
A common error is to open the external piping valve wide and then control the drippage by tightening the
packing gland. Actually, a combination of both adjustments is essential to arrive at the optimum
condition. The life of packing and sleeve depends on this careful control more than any other factor.
Mechanical seals:
Mechanical seals are preferred over packing on most 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 by competent service personnel (Standard stuffing box only). Conversion parts
may be ordered from your S.A.M. Engineering outlet.
A mechanical seal consists of a rotating element and a stationary element. The sealing faces are highly
lapped surfaces. The faces run with a very thin film of liquid between them. Since mechanical seals are
made with a wide, variety of designs, the instructions for the specific seal must be carefully studied and
followed. A mechanical seal is a precision device and must be treated accordingly. When the pump is
equipped with a mechanical seal, no attention or adjustment to the seal is normally required. Except for
possible slight initial leakage, the seal should operate with negligible leakage.
Just as with packing, the mechanical seal chamber must be supplied with a source of clean, clear liquid to
flush and lubricate the seal. The most important consideration is to establish the optimum flushing
pressure that will keep contaminants from the seal cavity. If this pressure is too low, contaminants may
enter the stuffing box. If the pressure is too high, excessive seal wear may result. Difficult seal applica-
tions may require the use of various API seal flush piping plans.
Double mechanical seals:
A double mechanical seal consists of two single seals mounted back-to-back and a suitable sealing liquid
which is introduced into the seal chamber. The sealing liquid (preferably clear water) is injected through
the tap in the gland into the box at a higher pressure than that which 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 slurries, or any other solids suspended
in the pumped liquid, or when product dilution by the seal water is a problem.
The sealing liquid pressure must always be higher than the pressure on the seal closer to the suction side
(inside seal). If sufficient sealing pressure is not maintained, the pressure within the pump can force open
the inside seal and allow the pumped liquid to enter the box, which can damage the seals.
A recommended "Controlled Pressure System" for a double mechanical seal is shown in Figure VII.