In general a mechanical seal is a piece of equipment which enables you to connect the systems or mechanisms to stop the leakage in a structure which contains pressure.
In the fluid handling business (most often read pumps), 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 is mechanical packing. The problem with this technology is that mechanical packing requires some leakage to avoid overheating and burning. Mechanical packing is still used very often with fluids which are not hazardous. Cooling water pumps is a good example where packing is still used because small controlled leakage of cold water does not present a hazard. Besides loss of fluid there are other problems associated with mechanical packing in pumps:
Due to these reasons mechanical packing is often replaced with mechanical seals even in applications which are not hazardous. But if we do consider hazardous 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. The answer is to use a mechanical seal, of course.
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.
The technology of mechanical seals is by no means new. It has been accepted in many industries 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.
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 poor design. In many industries there are so called vendor lists. A supplier has to be approved by a company to supply its products to this company.
Particularly, API 682 Standard mechanical seals offer many good features for the end user automatically. They have at least moderate pressure capability, long life, they 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 has a smaller chance of overheating or fluid flashing - boiling between faces).
Do mechanical seals leak at normal operation? Yes they do. Though good seals do not have visual leakage up to 50 bar and above. At lower pressures, when a mechanical seal leaks (visually seen leakage) it means that the seal needs to be repaired. At high pressures (at 50 bar or above) all perfectly normal mechanical seals leak. Users have to collect the leakage safely.
We should mention that dual seals, tandem or double, are often used in the oil and gas industry. Such a seal will require a seal 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. In a double mechanical seal the pumped fluid cannot leak outside. At high pressure barried fluid will leak a little out of the seal (such leakage is always collected). Some of the barrier fluid will get into the pipeline.
All in all our advice to a user would be to identify a competent seal manufacturer for the particular industry. It is a good idea to check if after-sales service will be required and what will be the prices for service and/or spare parts. Sometimes a user will find that spare parts cost more than the original seal.