Seal
factor
Developments
in mechanical seals are helping to increase seal life, and reduce downtimes
and maintenance costs, says Atul Bivalkar
The
importance of seal design has been recognised by professionals of various
industries and cannot be underestimated in any fluid power process.
Any problems with seal can have serious implications for the operation
of the machinery. For example, seal failure or breakage is considered
nothing less than a disaster, as a considerable amount of money gets
spent in replacing it. In the most extreme cases, process equipment
worth millions may be rendered dysfunctional until the faulty seal is
replaced. Another major problem encountered in seal design is friction,
which has a big impact on the amount of power consumed by the machinery
using it.
The failure of mechanicals seals used in difficult sealing applications
can also be perilous as they provide a safeguard against hazardous materials
escaping into the environment. The biggest problem faced during the
use of mechanical seals, is the constant threat to the seal face due
to several factors that cause wear and tear, making the seals unfit
for use.
Longer seal life
The focus of the users of mechanical seals is constantly towards
ensuring a longer seal life, leading to reduced downtimes and maintenance
costs. Hence industry trends are mostly noted in this area.
Given the possibility of leakage in every mechanical seal, efforts are
on to minimise the seal face wear, while maintaining the required lubrication
between seal faces, thus arresting higher rates of leakages. Several
practices seem to be in vogue with respect to retaining the seal face
and preventing or reducing the damage caused to it.
Pressure balanced seals
Optimising the face loads of seal faces seems to have found priority
with most seal designers. Face loads on the seal faces, owing to spring
force, can be optimised in a simple way. However, once box pressure
comes into the picture, spring force becomes a minor component of the
thrust. This is because the increase in box pressure considerably increases
the face load.
Such an increase results in a state where the lubricating fluid film
is squeezed out, the seal faces start running dry and eventually high
rate of wear leads to premature seal failure.
If the pressure on the seal faces is minimised, then the seal faces
can work properly under optimum spring load. This process is called
pressure balancing and it is different from the conventional hydraulic
forces balancing (as in case of a balanced mechanical seal). Through
pressure balancing, the pressure bearing capability of a normal balanced
seal can be enhanced considerably. In fact, seals with a load capacity
for pressures as high as 200 bars have been designed using this technology.
It is noticed that once both the forces are balanced, the thrust can
be minimised by equating the diameters.
The resultant thrust acting on the rotating seal face is limited to
the spring force. Thus, even the pump bearings are relieved of higher
thrusts, which occur on account of using the mechanical seal in high-pressure
applications. This...
