SUBJECT : An overview of seal troubleshooting 3-1

Seal problems are almost always associated with face leakage, but as we will soon learn, there are other leak paths in addition to the obvious one between the lapped seal faces.

In the following paragraphs, we'll be looking at all of these leak paths. Keep in mind that seals are classified into many categories : stationary, rotary, balanced, unbalanced, inside, outside, metallic, non-metallic, single, dual, elastomer, metal bellows, rubber bellows, cartridge, split, solid, etc.. Try to keep these classifications in mind as we investigate the cause of seal failure.

I will be presenting the troubleshooting hints in an outline form. You should not find these terms confusing because I've assumed you have a pretty good knowledge of mechanical seals or otherwise you wouldn't be attempting to trouble shoot them.

In the event you do have trouble with some of the terms or techniques, you can look them up in the glossory

LEAKAGE AT THE SEAL FACES.

The seal face is not flat. (Flatness should be measured within three helium light bands, (0,000033" or 1 micron)

The face has been chemically attacked.

The plating or hard coating is coming off of the hard face.

The seal face is cracked, pitted or damaged.

The movable face is not free to follow whip, wobble or run out.

The product has plated, or formed on the face and a piece of it has broken off.

The set screws have come loose.

The face has lost its spring load.

The product is vaporizing and blowing the faces open. This happens in hot applications if there is water in the product.

The inner seal, of a dual seal application was not balanced in both directions and is opening up with reversing pressure. This is a common problem in unbalanced seals that are subject to both vacuum and pressure or if the barrier fluid pressure varies.

The single spring, found in some seal designs, was wound in the wrong direction for the shaft rotation.

The Bellows seal has lost cooling and the anti vibration lugs are engaging the shaft. Shaft movement will cause the faces to open.

LEAKAGE AT THE ELASTOMER LOCATION.

Compression set ( the elastomer has changed shape).

The elastomer is cracked.

The elastomer is cut or damaged.

The elastomer is not seated properly.

The elastomer has swollen or changed color.

The elastomer leaks when pressurized in the opposite direction.

OTHER LEAK PATHS TO CONSIDER

Between the carbon and its metal holder.

Between the shaft and the sleeve.

Stationary face gasket or elastomer leaking.

Gland gasket or gasket surface leakage.

Pipe flange leaking above the seal and dripping into the seal area.

At the weld location if a seal face holder is welded to the cartridge sleeve.

At the pipe connections, ancillary hardware, A.P.I. Gland fittings, and recirculation lines.

A scratch or nick in the o-ring groove. Remember that up to 100 p.s.i. (6 bar) o-rings seal on the O.D. and the I.D. not the sides.

Seal faces will not leak visibly if they are lapped flat and we keep them in total contact. Shaft movement is the main contributor to the opening of the seal faces and allowing solids to penetrate. Shaft movement is caused by many factors. In the following paragraphs we'll be looking at most of them:

CAUSES OF EXCESSIVE SHAFT MOVEMENT, INCLUDING VIBRATION.

Cavitation

The bearings are worn excessively.

The shaft is bent.

The impeller is out of balance.

An unbalanced rotating assembly.

Pressure surges or water hammer.

Worn coupling.

The pump is operating off of its best efficiency point.

Rubbing of a rotating component.

The stationary seal face is not perpendicular to the rotating shaft. This causes the spring loaded, rotating face to move back and forth twice per revolution.

VIBRATION AT THE SEAL FACES.

Harmonic vibration.

Slipstick (an alternating slipping and sticking of the seal faces,) caused by:

A discharge recirculation line aimed at the seal faces.

Vaporization of the product at the seal face.

EXCESSIVE AXIAL MOVEMENT OF THE SEAL

THE SHAFT IS NOT CONCENTRIC WITH THE STUFFING BOX, this will cause a wiping action in stationary seals.

Heat is always an indication of wasted energy, but it can also have a disastrous affect on seal life and performance. Let's take a look at what's causing this heat

CAUSES OF HIGH HEAT AT THE SEAL FACES.

Too much spring compression.

Problems with some seal designs.

Problems with face materials.

Problems with the pump operation that causes high heat at the faces.

Other causes of high heat.

® DuPont Dow elastomer

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