Mechanical Seals for Zero Leakage

By: Thomas Yoon

A centrifugal water pump contains a few basic components as
follows:

1) The housing for containing the water.
2) The impeller attached to a rotating shaft that moves the
water.
3) The coupling that transmits the rotating action of a motor
(or other prime movers) to the pump shaft.
4) The seal between the housing and the rotating shaft.
5) The bearings for ensuring that the shaft turns smoothly.
6) The inlet and the outlet ports for channeling the water
flow through the pump.

In order for a pump to work, the pump impeller must generate
pressures inside the pump housing. The seal between the housing
and the rotating shaft plays a critical role in making the pump
work.

One of the most common ways of sealing a rotating pump shaft
is by using mechanical seals.

The 3 components of a mechanical seal are:
1) The rotating component that moves together with the rotating
shaft.
2) The stationary component that is fixed to the pump housing.
3) The spring that presses the rotating component onto the
stationary component axially.

Rotating Component

This is a ring, usually consisting of 2 types of materials - a
soft carbon piece with a highly polished axial surface, and a
rubber material or o-ring that is able to grip the shaft for
sealing against water leak through the shaft surface.

Stationary Component

The stationary ring is usually made of stainless steel, although
there are some made of ceramic or similar hard materials.

The
axial surface is also highly polished. At the circumference of
the stationary piece, a rubber o-ring will be installed to seal
against water leak through the pump housing.

Spring

The spring(s) is installed around the shaft and rotates with the
rotating component. The purpose of the spring is to press the
rotating component of the mechanical seal against the stationary
component so that the matching surfaces are in contact with each
other axially to the shaft.

Because the axial rubbing surfaces in contact are very highly
polished, there is almost no leak there. Water does not leak
through the pump housing opening because of the o-ring pressed
between the stationary component and the pump housing. Water will
also not leak through the shaft because of the o-ring in the
rotating component gripping against the shaft.

So, if there is a leak in a mechanical seal of a pump, look for
the sources of leak in those 3 places. Surely one of them is not
sealing properly.

For more information on mechanical seals and pump troubleshootingComputer Technology Articles,
download the free software from
http://www.free-marine.com/diagnoppad.htm

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