Introduction of mechanical seal
Mechanical seal are being used increasingly on fluid pumpstoreplace packed glands and lip seals. Pumpswithmechanical seals perform more efficiently and generallyperformmore reliably for extended periods of time.
Mechanical seals are provided to prevent pumped fluids fromleakingout along the drive shafts. The controlledleakage pathis between two flat surfaces associated with therotating shaft andthe housing respectively. The leakagepath gap variesas the faces are subject to varying external loadswhich tend themove the faces relative to each other.
The mechanical seal requires a different shaft housingdesignarrangement compared to that for the other type of sealsbecausethe seal is a more complicated arrangement and themechanical sealdoes not provide any support to the shaft.
In order for the mechanical seal to perform over an extendedtimeperiod with low frictional the faces are generallyhydrodynamicallylubricated. The fluid film will need tocarrysubstantial load. If the load becomes to high forthefilm surface contact will take place with consequentbearingfailure. This lubricating film is generally of the order of3micrometres thick , or less. This thickness is critical totherequired sealing function. Mechanical seals oftenhaveone face of a suitable solid lubricant such that the seal canstilloperate for a period without the fluid film.
Design Features
The mechanical seal generally includes a three staticseals.
- The sleeve seal - this is usually an O-Ring
- The seal between the moving seal member and the shaft orsleeve.-This is often an o-ring but can be a wedge or vee seal.This sealmay not be used for bellows type mechanicalseals
- The housing seal is generally an o-ring of agasket.
All of these seal must be compatible with the fluid beingcontainedand the associated environment. These sealsmay limitthe design for high temperature applications. In this casethebellows type alternative may be the best option.
The sealing faces are generally pressed together using some formofspring loading. Several different spring loading systemsareavailable.
- Single spring
- Multiple springs distributed around seal body
- Disc Springs
- Disc Springs
- Bellows
- Magnetic
For conventional mechanical seals the single spring arrangementsisused. The other spring arrangements are used inthespace is restricted.
It is vitally important that the sealing surfaces perfectly flatandare parallel.
The seal faces are usually dissimilar materials with the softerfacebeing the narrower surface. For abrasive applications similarhardmaterials are used e.g tungsten carbide. Thesealsurfaces must have sufficient strength to withstand thehydrostaticfluid forces and must be able to remove the heatgenerated bysliding action. Carbon is often usedagainst bronze,cast iron, stainless steel etc.
The seal surface must be flat, smooth and square totheshaft. Both surfaces a normally lapped to a highqualityfinish. The harder surface is most importantbecausethe softer surface is designed to run-in over the initialoperatingperiod.
The shaft design is critical. It must be rigid enough to supporttheseal in the correct position and the shaft surface finish mustbesuitable to ensure good sealing on the static seals (0.4micrometersCLA or better). The shaft Total Indicated Runout (TIR)should notexceed 0.125mm. There should be minimumshaftvibration. The shaft may be subject tofrettingcorrosion as a result of micro-movements of the seal and isoftendesireable to have locally hardened surfaces or tousesleeves.
Assembly Options
There are a number of mechanical seal options
- External Seal.. This design is installed on the outside ofthestuffing box with the sealed pressure inside. This providesgoodaccess allowing the seal components to be cleaned.
- Internal Seal.. Generally mechanical seals are mounted insidethestuffing box with the sealed pressure outside theseal.
Double Seals.. Mechanical seals mounted in pairs are usedforsealing hazardous, toxic or abrasiv fluids and are oftenprovidedwith clean flushing fluid between the seals. Doubleseals also provide an additional degree of safety were thepressuredifferentials are likely to reverse and/or there is a highrisk ofthe sealing failing.
Additional Equipment
The use of mechanical seals generally involve the use ofadditionalequipment primarily for the flushing /coolantsystems. This includes pumps, coolers, strainers,filters etc.