Seals are necessary to prevent leakage at the point where the shaft enters the pump case. The most inexpensive seal is made of packing material that acts as a pressure breakdown device. Packing must be flexible and capable of being compressed for proper operation. Packing material may be made of flexible metallic strands with graphite or oil lubricant impregnation. Asbestos is another material that was common in the past, and though its use has decreased, there are applications where asbestos is the only choice, especially in very-high-temperature service. Besides low cost, packing has the advantage of being easy to replace. However, it has the disadvantages of requiring both some small leakage rate for proper operation, and continuous adjustment as it wears. Packing is generally limited to water service where leakage can be tolerated.
Mechanical seals provide a continuous contact between two flat sealing surfaces located on a plane perpendicular to the shaft centerline as shown in Figure 11-13. They have the advantage of essentially eliminating the leakage rate under normal conditions, requiring less frequent adjustment and maintenance, and no run-in time. Besides higher initial and operating costs than packing, they have the disadvantage that when failure occurs it tends to be swift and with large leakage.
There are several types of mechanical seal arrangements. Unbalanced seals are used for pressures less than 150 psi. Balanced seals are used for pressures in excess of 50 psi.
A throttle bushing is a restrictive bushing or sealing device designed to limit flow out of the seal in the event of failure. Since a leaking seal will increase pressure between the seal and throttle bushing, throttle bushings are used whenever a seal failure alarm is required.
Tandem seals are used in critical service where leakage due to seal failure must be prohibited. They are constructed of two seal assemblies acting in series and separated by a buffer fluid at less pressure than the sealing pressure. Should the primary seal fail, the pressure or reservoir level in the buffer fluid system would increase, triggering an alarm. There is normally a throttle bushing and alarm downstream of the secondary seal to provide warning of secondary seal failure as well.
Double seals are used in toxic services where a pressurized clean seal fluid is designed to leak into the lower pressure process should there be a failure in the primary seal. A throttle bushing and alarm downstream of the seal between the clean fluid and the atmosphere is normally installed to warn of failure of this seal.
In order to clarify seal type descriptions with a concise and brief technique an API Seal Classification Code is used. It is a five-letter code described by:
First Letter B = Balanced
U = Unbalanced
Second Letter S = Single
D = Pressurized dual (double)
T = Unpressurized dual (tandem)
Third Letter P = Plain end plate seal gland
T = Throttle bushing seal gland
Fourth Letter Gasket material, see Table 11-2
Fifth Letter Face material, see Table 11-3
For example, a BTPFL seal is a balanced, tandem seal with plain end plate, fluoroelastomer “O” rings, a carbon steel seal ring, and a tungsten carbide-1 mating seal ring.
API 610 establishes standard piping systems for mechanical seals. These systems provide a flushing fluid across seal faces, establish flow paths for various seal configurations and establish location for components (e.g., coolers, reservoirs, pressure switches). Table 11-4 is a general guide to seal usage to provide a starting point for making a cost-benefit decision for any installation.