Párrafo 6.2

The failure mechanisms of mechanical seals are covered extensively by B J Woodley in "Mechanical Seal Practice for Improved Performance" (J D Summers-Smith (ed.) 2nd edition 1992). It is not proposed to cover the same ground here, though it is perhaps worth mentioning that failures of mechanical seals are frequently catastrophic with the resulting damage making it difficult to distinguish the original cause of failure. Most failures are the result of operating faults, e.g. failure of external cooling, flush or quench resulting in breakdown of the film between the seal faces. With the seal damaged, wear continues, even with the re-establishment of the coolant supply, so that it is hard to identify that the real problem was earlier, temporary loss of coolant.

In such circumstances the nature of the face materials plays an important role. With two hard faces (e.g. silicon carbide, tungsten carbide), failure is almost immediate; whereas the seal with one soft, self-lubricating face (e.g. carbon-graphite, ptfe) can continue to operate some time before failure while the soft face wears out. If failure does not manifest itself for some weeks after the event that prompted it, identification of the cause of failure is not straightforward.

OIL BARRIER SEALS

Floating-bush seals are commonly used on the shafts of process gas centrifugal compressors. Figure 1 shows the general

arrangement, with mineral oil, normally the system lubricating oil, fed between a pair of floating bushes to about 5 m head above the pressure of the sealed gas. Despite the pressure differential, some leakage occurs across the seal (the mechanism for this is not clear, but it seems likely to be precession of the seal ring about the shaft through slight departures from roundness in the seal ring bore or the shaft causing cavitation in the oil film.). High temperature occurs in the seal oil film because the high rate of shear and negligible cooling by the leaking oil. This can lead to chemical reactions in the seal that led to failure.

Figure 1: Schematic diagram of floating-bush seal system

Figure 2 shows a deposit of copper sulphide on the white metal lining of a seal ring. Figure 3 shows a failure when the deposit had taken up the clearance.

Figure 2: Black deposit of copper sulphide on white metal

lining of seal ring Figure 3: Failure of seal ring through loss of clearance caused by build up of copper sulphide deposit

This reaction occurs between the active sulphur present in ‘sour’ hydrocarbon gases and copper in the tin-rich white metal lining of the seal. Some relief can be obtained by using a copper-free lead white metal.

Figure 4 gives a Guidance Chart for white metal selection based on operating experience using shaft peripheral velocity as an index of temperature. In very severe conditions, high speeds and high sulphur contents, sulphur reacts with lead forming a deposit of lead sulphide on the seal ring and an alternative seal design is required.

Zone A Tin-rich white metal acceptable Zone B Copper-free lead-rich white metal acceptable Zone C Alternative seal design required

Figure 4: Guidance chart for selection of white metal lining for floating-bush seal rings for use with sulphur containing hydrocarbon process gas

Reaction can also occur with active sulphur additives in the oil. Figure 5 shows a failed seal ring caused by operating with a high duty hydraulic oil containing zinc dialkyldithiophosphate (zddp).

Figure 5: Failure of floating-bush seal ring caused by build up of copper sulphide on white metal lining caused by reaction with zddp additive in the oil

Similar failures can occur through loss of clearance caused by deposition of reaction products between the process gas and additives in the oil. Figure 6 shows a failure of a seal ring caused by reaction between ammonia in the gas and the succinic acid corrosion inhibitor in the oil. The ammonium succinate reaction product is insoluble in the oil and is deposited on the hot lining of the seal ring.

Figure 6: Failure of seal ring through deposit of ammonium succinate formed by reaction between ammonia in the gas and the corrosion inhibitor in the oil

Rings with low clearance are used to limit the inward leakage of the barrier oil. This results in a low oil film thickness between the rotor and the casing and this is the most likely place for discharge to occur in the event of electrostatic build up on the rotor. Figure 7 shows electrostatic erosion of the white metal lining of a seal ring giving increased clearance and excessive inward leakage.

Figure 7: Electrostatic erosion of white metal lining of seal ring causing increased inward leakage of the seal oil

Radial face seals are also used as oil barrier seals. Again, problems can occur because of high temperature in the seal. Figure 8 shows a seal ring that was removed because of excessive inward leakage of oil. This was caused by three equispaced deposits that separated the faces. Analysis showed the deposits to be oxidised oil. The ring was found to have a three-wave undulation, the thin film at the high spots causing excessive temperature leading to the deposition of oil oxidation products. Distortion had resulted from the relief of locked-in stresses during manufacture in a three-jaw chuck. The problem was solved by stress relieving before final machining.

Figure 8: Three equidistant deposits of oil oxidation roducts on a radial face seal

Similar failures have occurred through the breakdown of oil additives at the high temperature in the seal. Figure 9 shows the presence of additive breakdown products on a radial face seal removed because of excessive leakage. The oil used in the lubrication system contained e.p. additives that broke down in the seal.

Figure 9: Deposit of e.p. additive breakdown products on radial face seal

GLANDS ON RECIPROCATING RODS

Lubrication of the glands on reciprocating rods is essential. If adequate lubrication is not provided, severe scoring of the rod results even with packings of self-lubricating materials.

Figure 10 shows scoring on the rod of a 240 bar methanol ram pump at the point where it passed through the filled ptfe packing.

The local safety authority required that there should be no leakage of methanol. Scoring occurred when the gland was tightened up to stop the leakage. The problem was solved by fitting a lantern ring at the centre of the packing and supplying a low viscosity lubricating oil.

Figure 10: Scoring on chrome-plated rod of 240 bar methanol pump as the result of overtightening the gland

A similar failure occurred on a 3-row ram pump discharging light aldehydes at 250 bar using glass-fibre reinforced ptfe chevron rings to seal the rods. The pump had been specified to operate without leakage. Predictably the seals failed. Automatic packings require adequate viscosity in the pumped liquid and wetting of the rod to ensure lubrication of the rings. The viscosity of the light aldehydes was inadequate and failure was inevitable, even with self-lubricating ptfe rings.