Seal failure, premature

First, one pump running in a parallel system tends to suffer from cavitation because operation to the right of the BF.P indicates that the NPSHr of the pump rises drastically. To survive this condition, you should use dual mechanical seals on these pumps. Dual or double mechanical seals can withstand cavitation better than a single seal. There is a discussion on this in the mechanical seal chapter of this book. Many engineers perceive that parallel pumps are problematic becau.se they appear to suffer a lot of premature seal failure. Parallel pumps deserve double seals even if it s only a cold water system. 

Less heat Generated - Less force between the faces indicates less heat generated. Heat is the principal reason for premature seal failure for two reasons ... 

There are numerous reasons why seals fail prematurely. The origin of the failure can reside in the pumping system, in the pump operation, or the maintenance shop, the storeroom, or even before the seal arrived into the plant. The first sign of failure normally is lit]uid on the floor. 

The easiest way to prevent many unplanned premature seal failures is to use balanced o-ring cartridge designed seals. Most of the seal manufacturers make models incorporating these design features. 

Solid oxide fuel cells (SOFCs) involve structures composed of multiple ceramic layers, or interleaved layers of ceramic membranes and metal interconnects. To eliminate premature mixing of fuel and air gases or leaking of these gases from interior regions of the structure, the interfaces of adjacent layers are sealed with a glass or ceramic seal. These seals must withstand the high-temperature environment of the SOFC over its lifetime. Therefore these materials must be thermally matched with the adjacent layers to minimize transient stress risers and eliminate the potential for consequent seal failure. 

Such clamping pressure loss may bring about relative movement, induced by both tensile and torsional loads, which may lead to premature seal failure. The variable compression of traditional gaskets makes overall assembly tolerances difficult to achieve and maintain. 

Misapplication is another major cause of premature seal failure. Little attention is generally given to the selection of mechanical seals. Most plants rely on the vendor to provide a seal that is compatible with the application. Too often there is a serious breakdown in communications between the end user and the vendor on this subject. Either the procurement specification does not provide the vendor with appropriate information, or the vendor does not offer the option of custom ordering the seals. Regardless of the reason, mechanical seals are often improperly selected and used in inappropriate applications. 

Third, you would. suppose that parallel pumps are identical, that they were manufactured and assembled together. But it is possible that one pump of the pair is the dominant pump and the other is the runt pump. If you start the dominant pump first in the parallel system, and then decide to add the runt pump of the pair, the weaker pump may not be able to open the check valve. The pump operator perceives that the flow meter on the second pump is stuck or broken. This is because the second pump might be dead heading against a closed check valve, maintained that w ay by the dominant pump. If this situation exists, it may re.sult in premature failure of bearings and. seals, leading maintenance and operations personnel thinking that parallel pumps are problematic. 

When a seal is installed into a pump, and the motor started, an imaginary line is drawn, and the seal begins a journey toward the day when the seal will eome out of serviee, either from premature failure, or from obtaining its maximum serviee life. On one side of the imaginary line are the events in the manufaeture of the seal, its handling, storage, and installation. On the other side of the imaginary line are the events that oeeurred after the pump and seal were started the first time. 

Certainly, at the moment of starting a pump with a new seal installed, the events prior to the installation begin to disappear as a cause or origin of failure, and the factors of operation, process, and design in the system begin to appear as possible reasons for any premature failure. 

The anodes most suitable for burying in soil are cylindrical anodes of high-silicon iron of 1 to 80 kg and with diameters from 30 to 110 mm and lengths from 250 to 1500 mm. The anodes are slightly conical and have at the thicker end for the current lead an iron connector cast into the anode material, to which the cable connection is joined by brazing or wedging. This anode connection is usually sealed with cast resin and forms the anode head (see Fig. 7-2). Ninety percent of premature anode failures occur at the anode head, i.e., at the cable connection to the anode [29], Since installation and assembly costs are the main components of the total cost of an... 

While preventive maintenance is concerned with regularly testing, and reconditioning equipment to prevent failures in service and premature deterioration, it follows that predictive maintenance procedures are concerned with the ability to predict when the equipment will fail and then developing schedules to implement timely repairs. Predictive maintenance does not imply that with the use of these techniques, failure modes in equipment can be prevented rather, it suggests that the occurrence of failure can be predicted and thus planned for. An appropriate example would be the inspection and change of a major compressor face-type oil seal where random heat checking (FM) has been observed over the years. 

Locate conduit to avoid moisture. Provide breathers and drains if necessary. The use of conduit seals within 18 inches of the detector is required to prevent the passage of moisture through the conduit and into the detector enclosure. If moisture is allowed to accumulate in the detector housing, premature detector failure can occur. 

An overflow pipe (or downcomer), with an opening located below the top of the risers, is recommended for preventing liquid from overflowing the risers at high liquid levels (Fig. 4.10). The overflow pipe should be liquid-sealed at the bottom to avoid vapor rise through it. Two experiences have been described (57, 237) where failure to provide an overflow pipe caused liquid to overflow the risers and prematurely flood the column section above the chimney tray. In a third case (334), liquid overflowing the risers caused entrainment (Sec. 8.2). 

Glass reactors described by CampbelP each contain a hairpin-shaped filament, 24 in. long, attached to tungsten electrodes which are sealed into the glass. The filament is of pure titanium, not more than about 0 003 in. diameter, suspended vertically, and a second one is sealed into the reactor in a similar manner in case of premature failure in use. A small tungsten weight is attached to the lowest point of the filament to maintain it in a vertical position. 

Gap filling and joint sealing reduce the risk of fretting, corrosion and premature failure loads are more uniformly distributed and the easing of tolerances and substitution of turning for grinding reduces machining costs. 

Failure modes that affect shaft seals are normally limited to excessive leakage and premature failure of the mechanical seal or packing. Table 19-1 lists the common failure modes for both mechanical seals and packed boxes. As the table indicates, most of these failure modes can be directly attributed to misapplication, improper installation, or poor maintenance practices. 

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