Mechanical seals for hot pump services

Use medium pressure steam in seal jackets for hot pump (above 300^oC) services to cool the seal fluid during operation and keep standby pump seal fluid warm. Most hop pump services (bottoms and gas oil refinery services) use bellows seals (to eliminate the dynamic secondary seal) and dead ended (no flush) configurations to minimize ingress of fluid particles into the seal chamber.

As a result, seal chamber jacket cooling is required during operation. Medium pressure steam in the seal chamber jacket has proven to be the best solution in this case, since it will provide adequate cooling during operation and keep the seal fluid viscosity in the standby pump (frequency on auto-start in these applications) at an acceptable level to prevent excessive seal wear during start-up and operation.

The part of the pump that is exposed to the atmosphere and the rotating shaft or reciprocating rod passes through is called the stuffing box. A properly sealed stuffing box prevents the escape of pumped liquid. Mechanical seals are commonly specified for centrifugal pump applications. The mechanical seal is comprised of two basic components:

• Stationary member fastened to the casing
• Rotating member fastened to shaft, either direct, or with shaft sleeve

The mating faces of each member perform the sealing. The mating surface of each component is highly polished, and they are held in contact with a spring or bellows which results in a net face loading closure force.

In order to prevent fluid escaping into the atmosphere, additional seals are required. These seals are either ‘O’ rings, gaskets or packing. For high temperature applications (above 200^oC [400^oF]) the secondary seal is usually ‘Graphoil’ or ‘Karlez’ material in a ‘U’ or chevron configuration. An attractive alternative is to eliminate the secondary seal entirely by using a bellows seal, since the bellows replaces the springs and forms a leak-tight element thus eliminating the requirement for a secondary seal.

Lubrication and cooling

To achieve satisfactory seal performance over extended periods of time, proper lubrication and cooling is required. The lubricant, usually the pumped product, is injected into the seal chamber and a small amount of leakage depends on the pressure drop across the faces. This performance can be considered to be flow through an equivalent orifice.

The amount of heat generated at the seal face is a function of the face loading and the friction coefficient, which is related to the materials and lubrication of the faces. As the lubricant flows across the interface, it is prone to vaporization. The initiation point of this vaporization is dependent upon the flush liquid pressure, and its relationship to the margin of liquid vapor pressure at the liquid temperature. The closer the liquid flush pressure is to the vapor pressure of the liquid at the temperature of the liquid, the sooner the vaporization will occur.

Low seal MTBFs have been experienced in hot services (bottoms and gas oil) caused by incompatible external flushes (too high a vapor pressure) or failure of the standby pump seal during start-up.

This best practice solves both issues by eliminating the need for an external flush as well as ensuring the seal fluid in the standby pump chamber will be at an acceptable viscosity under start-up conditions. This best practice has been used since the mid-1970s to optimize refinery bottoms and gas oil pump mechanical seal MTBFs (greater than 36 months).