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Education Tutorial
Mechanical Seal

Pre-installation checks

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 manufacturer's drawing.
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.

Installation Checks

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... don't you?

Pump Checks

Shaft Run-out

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.
Mechanical Seal Shaft Runout

The run out should not exceed 0.002 inches or 0.05 m.metres.

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.

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.
Mechanical Seal Housing Squareness

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.

General Checks

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!



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