Face the facts, seals fail.
They do not wear out. Most often something comes along to
disturb the smooth running of the pump and you are facing a steady leak which has
already destroyed your seal by damaging the seal faces. But there is another case. The
seal that leaks on startup after maintenance. A seal that lasts a week without letting go
is generally thought to be OK. By the way that's a ROT (Rule of Thumb).
Running mechanical seals is an art form. There is a lot of science in it but either you
have the knack or you do not (in which case you need this web site bad).
A seal that leaks after maintenance has been badly installed. It
is very unwise to ignore the basic checks listed here because without these checks there
is no certainty that your seal will perform at all, let alone give a reasonable running
life. I hate having to go over a job again after having fitted it all back together...
- You have the correct seal and all the parts needed for the replacement.
- You have the pump drawing to hand with installation dimensions or the seal
- The pump stuffing box is clean
- On split casing pumps the gasket does not extend into the stuffing box.
- The shaft is free of scratches and burrs, threads are taped, and keyways are
filled flush with the shaft surface to prevent seal elastomers from being cut on the
keyway edge (a dummy wooden key insert is ideal).
- All the seal parts are in their protective coatings at this stage.
Shafts get bent. The spinning impeller has unequal loading on in
causing the shaft to deflect away from the volute throat. Constant deflection causes
weakness and can lead to a permanent offset of the shaft leading to shaft run out. Shaft
run out is bad for seals. It causes them to flex twice on every revolution of the shaft.
At high enough speeds this can cause a vibration in the seal which allows the seal faces
to OPEN. BANG failed seal.
So, look into the dark recesses of your lockers and pull out the
Dial Test Indicator (DTI) or Clock Gauge that lurks there, unloved & unused and check
the shaft of your pump for any damaging shaft deflections.
Single stage overhung pumps should be checked near the seal
running position but multi stage pumps should be checked at suitable intervals along the
shaft as well as at the seal running position.
The run out should not exceed 0.002 inches or 0.05
Shaft Sleeve Concentricity.
You have checked the shaft for run-out and
because the seal elastomer has a tendency to wear a fret ring on the shaft a shaft sleeve
is fitted to protect the shaft. When a new shaft sleeve is fitted, and this should be with
every new seal, it is a good idea to re-run the shaft run-out check to ensure that the
sleeve is concentric with the shaft.
The run out should not exceed 0.002 inches or 0.05 m.
A note about shaft sleeves. It is a false economy to omit to change the shaft
sleeve when replacing a mechanical seal.
I was called out to a cooling water pump supplying a
100Mw Power station. The shaft size was 230mm and it took three men two days to strip and
rebuild the seal box. The shaft sleeve cost $4,000 and the seal cost $10,000. The new seal
had been fitted onto the old sleeve and leaked immediately on startup. The seal faces were
intact but having been run for 24 hours in that condition another new seal assembly was
required. On examination it was found that the o-ring contacting the shaft sleeve surface
had worn a groove
damage) and the new o-ring
was unable to seal against this damaged surface. The extent of the damage was not
immediately obvious to the eye but by carefully measuring the surface the fault was found.
Amount saved on first installation $4,000, total cost of seal change $25,500, and it
should have cost $15,500. Believe me, skimping on the job is not the same as saving hard
Axial Shaft Movement
Set up your DTI to measure the amount of axial movement
of the shaft. The amount will vary according to the type of pump, its bearing
configuration, and the type of thrust bearing in use.
Essentially there are four types of thrust bearings
- Deep groove ball bearings
- Roller bearings
- Michell, Kingsbury, or thrust pad bearings, usually made of white
metal bearing surfaces.
- Balance piston thrust absorbing arrangement. This type is often
found on high pressure multi-stage water pumps where the hydraulic forces are partially
balanced by the impellers and controlled leakage past a balance piston provides the final
stage of rotating unit positioning.
The basic principle is that the shaft should be
set to its running position before attempting to fit the seal. In the case of
cartridge seals, the seal cover plate should be fixed to the pump casing, the shaft
positioned, and then the seal locking screws tightened to the shaft. Non cartridge
types need to have a datum mark scribed onto the shaft relative to the seal plate position
and then the fitting dimension marked from this point.
A note about fitting position. It is not good practice to fit a new seal
by looking at the old set-screw marks and then lining up on them. If you want good
seal performance then start out right ... measure the distance required, don't take short
cuts. The last seal could have been fitted incorrectly, perhaps causing the rebuild
that is now necessary. You are storing up future trouble if you skimp.
Seal Housing Squareness
The seal stationary must be fitted at 90
degrees to the axis of the shaft. Failing to achieve this will cause the seal head
to move to take up any mis-alignment. This movement offers an opportunity for the
seal faces to open and for the ingress of dirt particles. If you are changing out
packing and up-grading your equipment to a mechanical seal you need to pay close attention
to setting the seal housing closing plate in the correct position. The basic check
is as shown in the diagram.
It is also wise to check the bore of the seal housing at this point for
concentricity with the shaft. Put the sensing tip of the Dial Indicator inside the
bore on the wall of the seal housing and rotate the shaft. A small amount of
misalignment is permitted but the important thing is to check that the seal body cannot
touch the seal housing wall at any point of its rotation.
While the pump unit is in the shop for maintenance take the
opportunity to ensure that the cooling water jacket is clear of debris, that any other
cooling water arrangement is cleared of any obstruction. Orifice plates controlling
the flow of water to a seal housing should be checked dimensionally correct. A seal
starved of its ration of cooling water will be very unforgiving and cause you lots of
grief in a short time. This kind of fault is very difficult to diagnose for the
average engineer. Even the best have trouble with this one, too! So check it
out now while the doing is easy.
Bearings need to be replaced if they have been running with any pump leakage
around. Moisture ingress into a bearing dramatically reduces a bearing's useful
life. If you are changing out soft packing for a mechanical seal replace the
bearings on the unit too. The leakage from the packing gland is more than enough to
damage the bearings.
Check the impeller for cavitation damage indicating a system problem that might
go un-noticed during normal running conditions. Cavitation can cause vibration in
the pump shaft which will affect the seal 's performance.
I know you will ensure that the impeller sealing rings are replaced or
re-bushed to keep the clearances within design limits. Allowing recirculation
within the pump volute is no way to keep the efficiency of your plant at the highest
level, and it can increase the pressure inside the seal housing which will cause your seal
to wear out faster!