Single Sealed
Mechanical Seals
In general a mechanical seal is a piece of equipment which enables you to connect the systems or mechanisms collectively to stop the outflow or the leakage in a plumbing structure which contains pressure.
In the fluid business, the meaning is much narrower. By a mechanical seal we mean a device which is used to seal shafts of pumps, mixers or agitators.
The older technology, in many cases still used now, is mechanical packing. Mechanical packing is compressed in a stuffing box by a gland so it effectively fills the space between the shaft or stem and the casing of the equipment. Certain contact pressure on the shaft or stem is needed to effect the sealing function. We could call it a radial sealing device. While mechanical packing is indispensable to seal valve stems where the speed is very low, it is not as good to seal pump shafts due to a number of problems associated with shaft rotation:
- Friction consumes power;
- Frictional heat requires cooling by allowing certain leakage of the sealed product, which may be expensive or unsafe;
- Friction leads to shaft sleeve wear;
- Friction leads to mechanical packing wear which calls for frequent packing adjustments.
While waste of power is an issue in every industry, in many industries use of mechanical packing is prohibited where even small, controlled amount of leakage from a pump is not acceptable due to environmental or fire safety reasons.
In mechanical seals main friction happens in the axial direction, between faces (seal rings) lapped to a high precision. Wear is compensated in axial direction by springs or bellows. Many mechanical seals do not wear shaft at all. Contact pressure and thus the friction force is kept to a minimum by a balanced design. A balanced seal consumes much less power and it does not require product leakage to cool the seal faces. Actually when a mechanical seal leaks (visually seen leakage) usually it means that the seal needs to be repaired. Sometimes a perfectly normal mechanical seal leak. This is true if the sealed pressure is high. Good seals do not have visual leakage up to 50 bar and above.
The technology of mechanical seals is by no means new. It has been accepted in industry for decades. In most cases it has proven to be effective and reliable. The range of designs and materials extends from home heating circulating pumps to 380C hot hydrocarbons pumps at oil refineries or to 100 bar crude oil transfer pumps, costing from 10 dollars to 20,000 dollars and even more, depending on the industry and country.
Construction:
Mechanical seals consist of the following components:
- Seal rings or faces, rotating and stationary;
- Secondary seals such as o-rings, PTFE V-Rings or wedges, rubber boots, PTFE or flexible graphite gaskets;
- Spring action elements such as springs, metal bellows;
- Drive elements such as pins, set screws, etc.;
- Metal parts such as shaft sleeves, glands, collars, set plates or clips, holders, etc.
- Additional elements such as safety bushings which, if a seal malfunctions, will direct leakage to a specialized drain.
Mechanical Seals can be classified by two ways.
1: Classification on the basis of Arrangement
Single – inside and outside;
Multiple (usually, dual) – tandem or double. Such a seal will require a seal support system to supply fluid or gas between the inboard and outboard faces. If the pressure of the supplied fluid is lower than the pressure in the seal chamber, it is called buffer fluid or gas, and the seal is called a tandem mechanical seal. And if the pressure of the supplied fluid is higher than the pressure in the seal chamber, it is called barrier fluid or gas, and the seal is called a double mechanical seal.
2: Classification on the basis of Design
Balanced and Unbalanced. Unbalanced seals are the cheapest and used only for low linear speed and pressure;
Pusher type – O-Ring, V-Ring and Wedge Ring, single spring or multiple spring;
Non-Pusher type such as a metal bellows seal.
There are competent companies that provide mechanical seals for process pumps with high dependability and good value for money. Some mechanical seal manufacturers are strong in providing seals in huge quantities for automotive industry or home heating at very low cost, others are best in pulp and paper industry, third ones are very strong in making mechanical seals for the oil and gas industry.
Seals for each industry will have certain commonalities driven from field experience. Some industries may even accept a standard which will specify minimum requirements for seal design, testing and documentation. The API 682 Standard for making seals for petroleum industry is a good example. End-users specifying such standards in their purchasing requirements greatly benefit from this as they are protected from low quality or inexperienced manufacturers.
About the Author
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