SUBJECT: Pump and seal problems
with no apparent cause 4-5
These problems are the ones that drive you crazy.
No matter how hard you look, the solution keeps evading you. Over the
years I've collected quite a few examples. I offer some of them for
your enjoyment and maybe, in the process, they will help you solve
The pump cavitated every time it
The product temperature would cause it to vaporize very close to
ambient pressure, and when it rained atmospheric pressure dropped
enough to cause the problem.
The pump never cavitated in the summer months,
only during the winter when everything was cooler.
The tank vent froze during the winter months causing the pump to
pull a partial vacuum in the tank.
The cavitation started suddenly.
A plastic pipe liner collapsed at the
suction side of the pump or the gate fell off a gate
The cavitation started after the packing was
converted to a mechanical seal. A careful inspection showed that the
seal was not leaking air into the suction.
The pump had speeded up (increased the rpm) when the packing was
removed. This increase in speed and capacity caused the
The cavitation kept getting worse with time,
nothing obvious had changed in the system.
The product had formed a coating on the
inside of the suction pipe increasing the pressure drop and
resulting in a loss of suction head.
The cavitation only occurred when there was a
higher head at the suction of the pump and stopped cavitating when
the level fell in the tank - just the opposite of what should have
The pump was pumping to a fixed discharge head. The capacity of
the pump increased when the suction level was higher, because the
pump delivers the difference between the suction and discharge
head. When the differential went down, the capacity
Two pumps were installed in parallel, one
cavitated the other did not. They had separate suction lines so that
wasn't the problem.
Some one had installed an oversized section of pipe on the
discharge side of the pump that was cavitating. The lower
discharge resistance caused an increase in capacity which caused
the cavitation. When the proper sized pipe was installed the
The pump had been cavitating for some time, but
after a visual check everything appeared normal.
A globe valve had been substituted for a gate valve on the suction
of the pump. A globe valve can add the equivalent of another 100
feet (30,5 meters) of pipe to the system.
The pump started to cavitate when a flange gasket
was replaced on the suction side of the pump.
The inside diameter of the gasket was too small. It was acting as
an orifice, and restricting the flow.
The pump cavitated about one third of the time it
A close inspection of the system revealed that there was no surge
tank installed between the pump discharge and the multiple outlets
that were using the product. The pump was acting like an
accumulator and started to cavitate when the demand went up and
the discharge head dropped.
The pump cavitated, although here was excessive
suction head available.
There was too much velocity on the suction side of the pump. I saw
this problem in Scandanavia in an application where the pump was
taking a suction on a flow of water coming off of a
SEAL WAS GETTING HOT
The seal was showing evidence of running dry, but
the fluid level was never lost in the pump.
Air was trapped in the stuffing box of a vertical pump after it
was converted from packing to a mechanical seal. Most seal designs
have no facility for venting the stuffing box in a vertical
The seal showed evidence of running
The open impeller had been adjusted backwards and the "pump out
vanes" on the rear of the impeller were pumping the stuffing box
dry. This happens if you are using several brands of pumps and the
maintenance mechanics confuse the impeller adjustment method. Some
pumps adjust towards the volute (Goulds), some adjust towards the
back plate (Flowserve). It's easy to mix them up.
There was little to no fluid circulating between
the two seals.
The pipe fitting had bottomed out in a gland inlet elbow shutting
off the flow. This sometimes happens after the seal has been
repaired several times and the pipe thread shows some wear letting
it protrude further into the elbow fitting.
The mechanic had marked the seal location on the
shaft sleeve before the impeller was installed. When the impeller was
tightened against the shaft shoulder, the sleeve moved and over
compressed the seal.
Almost all Flowserve pump impellers adjust to the
pump back plate. When you make impeller adjustments you over compress
the mechanical seal.
A cooling jacket was being used, but the seal
continued to get hot. I have seen multiple reasons for
- A discharge recirculation line had also been
installed, but it was hidden by some insulation. The cooling
jacket could not keep up with the heat being added by the
- The inside of the cooling jacket had become
coated with a layer of calcium because hard water was being used
as the cooling medium. Condensate should have been
- A thermal bushing had not been installed in
the bottom of the stuffing box.
- The cooling jacket flow changed with
fluctuations in shop water pressure.
- The inner seal of some double seal
applications can get hot if the mechanic installs the cartridge
seal by pushing on the gland and fails to reset the seal
compression with the installation clips. The interference from the
cartridge sleeve elastomer can cause enough resistance to compress
the inner seal and unload the outer seal.
SEAL WOULD LEAK FOR NO APPARENT REASON
The open impeller was being adjusted
without resetting the seal. Many operators make their own impeller
adjustments. Cartridge seals solve this problem
The seal faces were opening because the
equipment's sliding foot had been bolted to the floor allowing the
shaft to grow through the stuffing box when the unit came up to
The cartridge seal had been hydrostatically tested
with water and then put into a hot oil application. It leaked almost
The trapped water vaporized when the unit was started. This could
be a dangerous condition because water trapped in a gasket and
then flashed to steam could blow the equipment apart.
The seal would start leaking about thirty minutes
after the pump started.
The carbon insert would come loose in its holder when the seal
came up to temperature. At shut down the metal holder would shrink
and everything appeared normal.
The seal was tested in the shop, but leaked when
it was installed in a pump that was operating at cryogenic (cold)
The faces had to be lapped at cryogenic temperature to keep them
flat at the seal operating temperature. The cryogenic temperature
can also harden the o-ring and freeze any lubricant that was put
on the seal face.
The seal was found to be leaking every Monday
A utility man did not know about seals. He would loosen the gland
on the weekend so that, what he thought was packing, would drip a
little. The leak was found by the regular maintenance people every
The leakage occurred during the winter
Someone circulated commercial anti freeze between two seals to act
as a barrier fluid. The brand they selected contained a chemical
to plug up radiator leaks and it kept plugging up the
The seal would fail only during the winter months.
The problem was traced to swelling of the dynamic O-ring but no
logical reason could be found for its failure.
During the winter months a worker decided
to oil the bed of his dump truck to make the mined, raw product
slip off easier. The petroleum oil he used attacked the Ethylene
Propylene (EPR) O-ring in the mechanical seal, installed
downstream in the system.
The seal area was wet, but no visible leakage
could be seen.
It turned out that there was a flange leaking above the pump and
dripping the product next to the shaft.
The problem was traced to the fact that the
mechanic was installing the seal at the wrong dimension. The written
instructions were clear and placed in the box and yet the mechanic
continued to do the installation incorrectly.
The mechanic could not read. He had been faking it for many years
and was quite good at it. The same problem occurs with older
mechanics that refuse to wear glasses and as a result cannot see
the funny little lines between the numbers on their measuring
The sealed leaked everytoime the pump ran but
stopped when the pump stoppesd
The centrifugal pump discharge was connected to the bottom of a
surge tank. As the tank filled, the pump operating point shifted
from too much capacity to too much head, deflecting the shaft in
The outside seal in a double seal application
failed suddenly. Nothing had changed in the system.
Routine maintenance included repainting the pump. The paint spray
got into the outside seal springs and stopped them from
The seal ran great for several days and then
started to leak. It tested all right on the test bench after it had
been removed from the pump.
It had been set screwed to a hardened sleeve and the set screws
The seal was changed several times, but the steady
The leak was occurring between the pump
sleeve and the shaft. This is a common problem in double ended
pumps that have been converted to a mechanical seal. You often
have to devise a method of sealing the sleeve to the shaft or the
sleeve to the impeller because the manufacture has not provided
The seal started to leak after many months of
service. A bench vacuum test showed that the seal was all
The seal was fretting the shaft below the Teflon wedge allowing
the leak to come through this groove.
The seal ran approximately six months and then
The lines were steam cleaned and the wrong grade of Viton® was
in the seal. Most Viton® compounds will be attacked by steam,
caustic or other water based solutions.
The seal was installed correctly, but it leaked
The solid, hard face is usually lapped on only one side. The face
had been installed backwards and the rotary unit was running on a
non lapped surface.
COMPONENT DAMAGE IS VISIBLE, BUT WHAT IS THE CAUSE?
It looked like a seal part had come loose in the
stuffing box, but all of the parts were there.
During a previous installation a small spring had been lost when
it fell into a drain hole in the bottom of the seal gland. It came
loose after a later installation. This is a problem when several
people work on the same pump.
The bellows plates were breaking, but there was no
evidence of corrosion, excessive wear, physical contact, or
A discharge recirculation line was directing high velocity
abrasive particles at the thin metal section of a metal bellow
The inner seal of a dual, rotating "Back to back"
seal was showing excessive face wear in a short period of
The inner seal stationary face was not locked in the bottom of the
stuffing box and when the system pressure overcame the barrier
fluid pressure, the stationary face was pushed into the inside
rotating face. When the pump was stopped the spring pushed the
stationary face back to its normal position.
The carbon seal face showed massive damage in a
cryogenic (cold) application.
The carbon had been lubricated at assembly and the lubricating oil
froze in the cryogenic atmosphere.
The metal bellows plates showed massive
The seal was rotating in an abrasive
slurry. Metal bellows seals should be designed to rotate the fluid
inside of the stuffing box, instead of rotating through the
The pump had been recently overhauled and at start
up the pump was reading high amperage, but low flow.
One of the wear rings had been left off of the suction side of the
impeller and the fluid was recirculating to the pump
The pump made a terrific racket during start up.
It produced the proper head, but the capacity was less than
It was a two speed pump and the second speed had been wired
In an acid application, a stationary seal showed
localized corrosion only on the gland.
This was an older pump with a bolted on stuffing box that would
slip because the bolts were worn. This caused the shaft to run
against the gland causing it to overheat and, in an acid
application, the corrosion rate of the acid doubles with an
18° F. (10° C) rise in temperature. It doesn't make any
difference if the acid or the part gets hot, the affect is the
The dual seal convection tank was running
The seal was not centered in its gland, and as the shaft turned,
the close tolerance between the seal and the gland outlet
increased the velocity of the liquid enough to drop the pressure
and cause the tank to convect backwards.
The pump was converted from packing to a
mechanical seal and then started to break shafts.
The pump was operating way off of its best efficiency point,
causing major shaft deflection. The packing was acting as a
bearing and supported the shaft during this
The product was solidifying in the stuffing box.
Steam was being used to heat a jacket around the pump. The header
gauge showed adequate pressure.
The gage was located too far away from the
pump jacket. The line was not insulated and this allowed the steam
to experience a pressure drop between the header and the stuffing
box heating jacket. The result was that the steam cooled down
below the necessary heating temperature. The problem was only
visible when the pump was stopped for a period of
The nickel base tungsten carbide face shows
evidence of chemical attacked.
A galvanic action occurred between the
passivated stainless steel and the active nickel contained in the
tungsten carbide face.
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