Resolution for 2017 - Improve Mechanical Seal Life
Mechanical seal failure / dependability is the Achilles Heel of almost
all plant operators and engineers. It could actually be a huge drain on
the operations bottom line. However the most overlooked and under
appreciated aspect of plant maintenance.
The design of pump should be such that the deflection of the shaft at
impeller end is within the limits prescribed by the seal manufacturer.
A pump's dependability and reputation is built on the fact that it has
minimum downtime and least requirement of spares. This can only happen
when the deflection of the shaft at impeller end is within the limits
prescribed by the seal manufacturer. ANTICO pumps are designed with
this in mind.
Almost all instances of mechanical seal failures are due to:
Selection of Seal or Pump
We see in many instances practically zero knowledge of the process. The
plant/system designer has no idea what is a pump or seal. In most cases
engineers do not know what is head, flow – basic fluid mechanics.
Moreover to save costs – plant owners do not take service of
experienced consultants – this leads to wrong selection of seal faces.
There are instance where the actual head requirement is 20% of the
the pump – Single biggest reason for seal failure.
Operators start the pumps without ensuring priming of the pump. Flooded
suction cases where operator opens the suction valve as an
Running the pump at extreme right
side of the curve – Second biggest reason for seal failure.
Centrifugal pump will try to pump liquid in whatever condition that
prevails – that is it's nature. However the design of each pump is
distinct. This means that each pump at each impeller trim as a limit to
the minimum head it can pump liquid without causing failure. This is
called the full open condition. If the system head is lower than the
full open condition – pump tries to find flow that is commensurate with
that head – the pump is not capable of pumping such high flow rates –
this causes cavitation (liquid gets converted to vapour) – this will
definitely DESTROY the mechanical seal.
Improper pump alignment, imbalance, operating the pump too far to the
right or left of the Best Efficiency Point on the performance curve.
Vibration hurts your pump – it will causes shortening of the life of
mechanical seal / bearings / oil seals and the pump on the whole.
Wrong Seal Type or Face Combination -
Lack of knowledge – lack of respect for engineering practices or plain
ignorance accounts for majority of mechanical seal failures. Most
process engineers' knowledge of seal is actually what his local “bag”
seal supplier has told him. We find that engineers / plant owners are
totally closed to the idea of studying or understanding basic seal
principles. Buying soft faces to save money and using that seal for
slurry or abrasive liquids – most common case.
“Hands On” Operators –
Initial start-up procedures and installation errors are big reasons for
mechanical seal failures. Incorrect starting of the pump causes the
motor to trip and the shaft to twist casing orbital movement. This
results in rotary parts coming in contact, seals fail and bearing life
is affected. Not cleaning the seal faces before start-up – a small
amount of dirt or oil, sometimes fingerprints can cause the faces to
mis-align, damaging the seals.