A pump's mechanical seal is very susceptible to failure in the adverse operating conditions found in many chemical and petrochemical facilities. Failures not only result in high maintenance costs and downtime, but also create safety and environmental hazards. Therefore, plants must select a mechanical seal of the type, arrangement and material of construction (MOC) to suit its specific service conditions.
However, seal properties get less attention than pump operating parameters and properties. In other words, the ideal seal often is replaced by an alternative one if the pump's operating parameters satisfy the service conditions.
This article is based on a mechanical seal reliability assessment performed at Hindustan Petroleum Corp. Ltd.'s Mumbai Refinery in India. The Mumbai refinery has a seal population of approximately 600. These seals are used in a variety of service areas, including:
The performance of the seals in the Mumbai Refinery, like that of the seals in other similar facilities, is greatly influenced by factors such as service temperature, vapor pressure at operating pressure, solids contained, pump rotating speed and hazard control measures.
Lessons learned from this assessment can help plants determine the shortcomings of their existing seal systems, set up an appropriate preventive maintenance schedule based on system criticality, and take any other necessary actions to correct the shortcomings.
A seal's operation and longevity depend on maintaining the proper seal operating environment. The mating area of the seal ring and mating rings must be lubricated with a compatible liquid at the designed flow and temperature. The seal ring (face) keeps the mating area leakproof by virtue of axial pressure against the mating ring, which does not have any axial movement.
Despite the fact that the surface of the seal is lapped to precision flatness, each face develops a natural surface waviness. The effective operation of a seal depends on the lubricating film between the sliding faces of the seal. The surface waviness will be larger than the initial waviness of a new surface during stable operation.
The film allows operation of a seal with minimal wear and power consumption, and also helps prevent leakage. If the load on the seal face increases, the surface waviness will increase, and patches of the sealing plane will break through the hydrodynamic film. When this occurs, the seal is operating in a region of thermoelastic instability. Heavy leakage will occur as portions of the surface that have broken through the film increase in temperature, turning bright red. The liquid film will begin to carbonize, vaporize or flash, producing more leakage and seal part wear and making a spitting or sputtering sound.
The flow rate of flush depends on the seal size, operating temperature, speed and specific heat of the flushing media. The line/orifice size is calculated on the basis of differential pressure between the flushing media header and the stuffing box. The flush should not impinge directly on the mating area; it should enter toward the driven side and flow into the seal faces in a streamlined motion.
Table 1 details the seal flushing American Petroleum Institute (API) plans, equipment and preventive monitoring actions taken on the Mumbai Refinery seal systems.
To enhance seal life and avoid sudden failures, the refinery uses a number of flushing systems. The systems were selected according to the application in the particular refinery area.
Sealing faces are lubricated (flushed) with:
Service liquid from discharge (API Plan 11).
Service liquid from discharge through strainer, cooler, regulating orifice (RO), seal (API Plan 22 for elevated-temperature service).
Service liquid from casing to stuffing box, with stuffing box cooling and steam purging (suitable for high-temperature and congealing liquid service)(API Plans 02, 62).
Compatible fluid (external flushing) with vapor pressure lower than stuffing box pressure at operating temperature (API Plan 32).
Service liquid (first seal) and external circulating pot lubrication (second seal) with all monitoring instruments (Plan 52 for double-tandem seal applicable for light inflammable hydrocarbon service).
Self-flush (first seal) and no lubrication and with seal failure monitoring device (second seal) (API Plan 63 for emission containment seal).
No lubrication for dry-running seal with spiral-groove technology applicable for high-speed compressor.
Operation and maintenance
At the Mumbai Refinery, checklists were prepared and circulated to the operators identifying potential failure modes and immediate remedial actions to avoid failure. The checklists provide details for the operation crews about the parameters to be checked and the procedures to check them, as well as any required responses to such situations to avoid failures.
The refinery's preventive maintenance checklists for different types of pumps and their associated seals are provided on page 33.
An equipment "criticality classification" was performed at the Mumbai Refinery on the basis of a "failure effect analysis," which considered failure consequence, severity, redundancy, frequency and detection probability. A detailed preventive maintenance schedule then was prepared and executed, calling for:
Preventive maintenance every three months for critical equipment.
Preventive maintenance every six months for semicritical equipment.
Preventive maintenance on an "as-needed" basis for noncritical equipment, based on feedback from daily parameter monitoring.