Nico Schmaeling | John Crane

Chemical processing can expose rotating equipment to some of the most demanding conditions in industry. Pumps, mixers and other rotating assets routinely handle aggressive media ranging from acids and solvents to slurries, polymerizing fluids and liquefied gases. Many operate at extreme temperatures or pressures. Mixers and agitators may handle flammable gases or vapors that can create a potentially explosive atmosphere. Their seals and systems must therefore be correctly engineered, operated and maintained to control leakage and reduce safety risks. In these environments, a mechanical seal is more than a replaceable component. It is an engineered solution that is part of the plant’s process containment strategy.

Seal performance affects equipment availability and operating costs, with implications for worker safety and environmental compliance. An improperly selected or poorly supported seal can leak repeatedly, forcing unplanned maintenance and interrupting production. Conversely, a sealing system matched to the service can extend mean time between repair (MTBR), control fugitive emissions and reduce water and energy consumption. Achieving those outcomes requires operators to evaluate the complete sealing environment: the seal, its support system, the equipment and process conditions, monitoring and maintenance practices.

Controlling Fugitive Emissions

Fugitive emissions are unintended releases of process fluids, vapors or gases from equipment and components such as mechanical seals, valves and connectors. Chemical plants may contain thousands of potential emission sources handling volatile organic compounds (VOCs), hydrocarbons and other hazardous materials. Even small leaks can expose workers to harmful substances, create broader safety risks and increase a plant’s environmental impact. Operational consequences can include lost product, more frequent maintenance and regulatory penalties.

In the U.S., Environmental Protection Agency (EPA) requirements under the Clean Air Act include leak detection and repair programs for applicable processes and equipment. Requirements vary by industry, process and rule, but the operational objective is consistent: Maintain process integrity by identifying and addressing leakage before it leads to wider reliability, safety or compliance problems. Mechanical seals contribute to that objective by controlling the interface between stationary equipment and a rotating shaft.

Seal selection should start with the process fluid, operating envelope and consequences of leakage. Pressure, temperature, shaft speed, solids content, fluid lubricity, equipment condition and expected transients all influence the decision. Engineers must then consider the seal arrangement, dimensions, face materials, metallurgy and support system together. Cartridge construction can simplify installation and reduce assembly risk. Metal bellows can accommodate demanding temperature conditions, while gas-lubricated designs can support applications where emissions control and reduced friction are priorities. No design is universally superior. The appropriate solution is the one engineered around the application and its operating risks.

Building Containment Through Dual Seals & Support Systems

Where hazardous, abrasive, caustic fluids are present, or where the process fluid does not provide sufficient lubrication at the seal faces, a dual mechanical seal can provide an added containment level. In an unpressurized dual arrangement, a buffer fluid and outboard containment seal help manage leakage from the inboard seal and direct it to an appropriate collection system. In a dual pressurized arrangement, barrier fluid is maintained above seal-chamber pressure so it moves toward the process rather than allowing it to escape to the atmosphere. The correct arrangement depends on the fluid, operating conditions and consequences of leakage.

Noncontacting, gas-lubricated dual-cartridge seals provide another option for applications where the goal is to avoid contamination risks and reduce the energy requirements associated with some liquid-lubricated solutions. A controlled barrier-gas supply creates a film that separates and lubricates the seal faces, limiting friction and wear. Achieving this depends on selecting the appropriate technology for the application and ensuring it is correctly installed, operated, monitored and maintained. Together, these measures can reduce process-fluid emissions and, in suitable applications, help prevent them from reaching the atmosphere during normal operation.

Seal support systems remove heat from the seal faces and build a barrier against process fluids coming into contact with the atmosphere. Depending on their arrangement, they may include barrier-fluid reservoirs, circulation pumps, heat exchangers and various types of instrumentation. For example, a support system built according to American Petroleum Institute (API) Plan 53B supplies clean, pressurized barrier liquid from an accumulator to a dual pressurized seal, while an API Plan 74 provides a regulated barrier-gas supply to a dual pressurized pump gas seal.

Seal support systems should meet applicable requirements, including API 682, relevant American Society of Mechanical Engineers (ASME) codes and site-specific hazardous-area requirements. Standardization, however, should not replace application review. The system must reflect available utilities, ambient conditions, process transients, operator capabilities and maintenance access. Instrumentation should monitor the variables most likely to affect performance, such as pressure, temperature, level, flow and leakage. When support systems and monitoring are considered late in the design process, plants risk installing a capable seal without providing the operating environment it needs.