There are many types of seals each having definite advantage as under.
When a seal is mounted inside the stuffing box of the pump, it is called an inside seal. Inside seals are more difficult to install and maintain. However, main advantage is that it is possible to control the seal environment inside the stuffing box.
An outside seal is located outboard of the pump stuffing box. Where stuffing boxes are shallow and it is not possible to install a seal inside the stuffing box, it is installed outside. It is also easy to install and maintain. Due to lake of heat dissipation from below the seal faces, outside seals are suitable for low temperature, low speed and low pressure (as in these seals, fluid pressure is exerted outward on seal face rather than inward) applications.
All seals are available in either unbalanced or balanced versions. A seal is unbalanced when fluid force to close the seal faces (due to the area of rotating seal face exposed to the pumped fluid in stuffing box) is greater than force acting on rotating seal face at the area of contact (pressure gradient between rotating and stationary seal faces). In simple terms, it has a seal closing force in excess of the actual pressure to be sealed. In a balance seal as seal face is subject to low force, less heat is generated and seal life is more. As a stepped shaft sleeve is required for balancing, coat of a balanced seal is higher than unbalanced seal.
To balance a seal, area of rotating seal face exposed to stuffing box pressure is reduced using a stepped shaft sleeve. In a standard 70 – 30 balanced seal design used by most mechanical seal manufacturing companies, only 70 % of rotating seal face area is exposed to stuffing box pressure as shown in above sketch.
Double mechanical seal arrangement is used to handle toxic, volatile, hazardous or abrasive fluids. In a double seal arrangement, there are two seals with a fluid circulating between them. The fluid that circulates between the seals is called a barrier fluid if its pressure is higher than stuffing box pressure and it is called a buffer fluid if its pressure is lower than stuffing box pressure. The two seal faces are installed in three different configurations as under.
Back to Back or facing in opposite directions
This configuration requires a higher barrier fluid pressure between the seals. In this arrangement an inner seal leak will cause a dilution of the product. In case of failure of the barrier fluid system, the inner seal can blow open dumping the pump contents to the environment.
Tandem or facing in the same direction
In this configuration two glands are required to house both seals and this adds to the cost as well as the axial space requirement. A low pressure buffer fluid is circulated between the seals, eliminating the possibility of product dilution. In this arrangement loss of buffer fluid will not cause the seal faces to open. This configuration is generally found in Oil Refinery applications.
Face to Face or facing towards each other
Face to face configuration is a compromise between the “back to back” and the tandem arrangements. Here half the seal is housed in the stuffing box and the other half outside it. In this arrangement a lower pressure buffer fluid is supplied between seal faces.
The catridge design changes none of the functional components of the basic seal. In a catridge seal, all items are containerized and preset to working dimensions. They eliminate need to scribe lines and make critical measurements during seal assembly. Such seal installation requires only tightening of the gland bolts.
Methods of Environment Control
The successful and reliable operation of a mechanical seal is dependant upon the conditions that are imposed on the seal assembly during running. The fluid being sealed fills the stuffing box in which the seal is mounted and thus the physical and chemical nature of this liquid will have direct effect on seal operation and life. Slurries and fluids carrying solid particles are especially dangerous as there is a tendency for solid particles to collect in the vicinity of the mating faces and finally even entering the fluid film gap between the mating faces. Hard particles entering this gap will cause premature seal face failure.
Improved seal operation is possible by controlling the environment surrounding the seal. The most commonly used methods for control are flushing and quenching.
In flushing, a fluid is injected (through connection F as shown in API Gland – Plan # 62) into the stuffing box such that it impinges or jets onto the mating faces. This fluid may be the same fluid that is being sealed, tapped from a point at a higher pressure than that existing in the stuffing box, or any other fluid, preferably at a lower temperature, that may be permitted to mix with the sealed fluid.
Flushing effectively aids cooling of the seal mating face area. In addition, the introduction of a pressurized clear fluid ensures that solid particles present in the sealed media do not collect near the sealing faces.
In quenching, a fluid is introduced (through connection Q as shown in API Gland – Plan # 62) on the atmospheric or outer side of the seal mating faces that either helps in cooling or in maintaining a require temperature at the mating faces. This also creates a barrier between the atmosphere and seal faces as the atmospheric air creates problem to seal faces in some cases. Few such applications are given below.
- When pumping cool media (say at – 40 deg. C), moisture in the atmosphere condenses and ice is formed below seal face hindering its operation.
- In case of high temperature oils when vapors keep on escaping in the atmosphere, they come in contact with oxygen and burn. These carbon particles cerate problem in seal area.
- Crystallizing media gets into crystals when solvent in it gets evaporated because of the atmospheric air present blow seal faces. These crystals create problems for seal faces.
The American Petroleum Institute (API) issues guide lines to help petroleum people select and then pipe various types of controls for mechanical sealing applications. These piping arrangements are described in API standard 610.
For satisfactory seal performance, equipment parameters shall be as under.
Radial movement of shaft (runout / deflection) shall be less than 0.08 mm.
Axial movement of shaft (end play) shall be less than 0.26 mm.
Stuffing box face squareness (face runout) shall be less than 0.05 mm.
Stuffing box bore concentricity (with respect to shaft) shall be less than 0.13 mm.
Stuffing box shall be free of burrs and sharp edges.
Shaft / sleeve shall be free of burrs and sharp edges.
Please refer to manufacturer’s drawing /instruction for above checks.