MECHANICAL SEALS0 pages

COMMERCIAL MARINE MECHANICAL SEALS TECHNICAL INFORMATION

Mechanical seals: Controlled leakage devices
You need the best overall value with the lowest total cost of ownership from the equipment you buy. That's why Colfax
Fluid Handling delivers performance-based products that decrease maintenance, boost efficiency, increase intervals
between service and reduce energy costs.
We design our products to be of ”heavy duty — industrial grade” construction. Certain models are specifically designed for
“severe duty" service, such as in marine, navy, industrial process or power generation applications.

Mechanical Seals
The amount of acceptable leakage, if any at all, is a
controversial subject for which there are few clear answers.
This document is intended to clarify what to expect on
mechanical seals found in Colfax Fluid Handling products
from Allweiler, Imo, Houttuin and Warren.
Design
A mechanical seal figure 1, consists of a stationary element,
seat, and a rotating element. The rotating element is
statically (or nearly so), sealed to and rotates with the
pump shaft. The seat is statically sealed to a nonmoving
cover. The rotating element has a seal ring, with a highly
polished and a very flat face which runs against the seat
face, also a very flat and highly polished surface. Primary
sealing takes place where the rotating and stationary faces

are in near contact. The force which maintains contact
between the faces is provided by a spring, frequently
supported by additional pressure load.
The seal faces cannot be permitted to run dry (in hard
contact) as the frictional heat build-up will very quickly
cause severe seal face damage and extensive leakage.
Because the faces cannot run dry, they are designed to
allow a very thin film of liquid to exist between and migrate
across the seal faces. In a properly operating mechanical
seal, this thin liquid film, of necessity, leaks to the lowpressure side of the seal, usually outside the pump. In a
single type mechanical seal pump, this liquid is normally
the fluid being handled by the pump. There are more
complex designs with double, tandem or other arrangements,
which place a barrier fluid between seals such that one of
the seals is exposed only to the barrier fluid while the inner
seal also is exposed to the liquid being pumped.

rotating element

Figure 1
Leakage
Many pumped liquids are sufficiently volatile that the
frictional heat gained while traversing the seal face,
causes the leakage to vapourize. Such fluids may display
no apparent or visible leakage. Leakage is, however, still
taking place. Other liquids will not so readily vapourize
and thus may show an accumulation of leaked liquid
outside the pump. Liquid to be sealed, sealing pressure,

face loading, speed, size, sealing face materials, finishes,
temperature, viscosity, vapour pressure, normal
tolerance variations and other parameters all impact how
much apparent leakage will be incurred. Some seal
manufacturers design seals for high face loads that
minimize leakage but also shortens the seal life. Others
use lightly loaded faces for long life but these seals are
prone to greater leakage (figure 2).