Why are mechanical seals still the preferred choice in the process industries?
This month’s “Sealing Sense” was prepared by FSA members Phil Peck & Andy Martin.
The challenges facing process industries have changed although they continue to pump fluids, some hazardous or toxic. Safety and reliability are still of prime importance. However, operators increase speeds, pressures, flow rates and even the severity of the fluid characteristics (temperature, concentration, viscosity, etc.) while processing many batch operations. For the operators of petroleum refineries, gas processing facilities and petrochemical and chemical plants, safety means controlling and preventing loss of, or exposure to, the pumped fluids. Reliability means pumps that operate efficiently and economically, with less required maintenance.
A properly designed mechanical seal assures a pump operator of long-lasting, safe and reliable pump performance with a proven technology. Among multiple pieces of rotating equipment and a myriad of components, mechanical seals are proven to perform dependably under most types of operating conditions.
Pumps & Seals—A Good Fit
It is hard to believe that almost 30 years have passed since the mass promotion of sealless pump technology into the process industry. The new technology was promoted as the solution to all the issues and perceived limitations of mechanical seals. Some suggested that this alternative would eliminate the use of mechanical seals entirely.
However, not long after this promotion, end users learned that mechanical seals could meet or exceed legislated leakage and containment requirements. Further, pump manufacturers supported the technology by providing updated seal chambers to replace the old compression packing “stuffing boxes.”
Today’s seal chambers are designed specifically for mechanical seals, allowing for more robust technology in a cartridge platform, providing easier installation and creating an environment that allows the seals to function to their full potential.
In the mid 1980s, new environmental regulations forced the industry not only to look at containment and emissions, but also at equipment reliability. The average mean time between repair (MTBR) for mechanical seals in a chemical plant was approximately 12 months. Today, the average MTBR is 30 months. Currently, the petroleum industry, subject to some of the most stringent emission levels, has an average MTBR of more than 60 months.
Mechanical seals maintained their reputation by demonstrating the ability to meet and even exceed the requirements of best available control technology (BACT). Further, they did so while remaining an economical and energy efficient technology available to meet emission and environmental regulations.
New computer programs allow seals to be modeled and prototyped prior to manufacturing to confirm how they will handle specific operating conditions before being installed in the field. Seal manufacturing design capabilities and the technology of seal face materials has progressed to the point that they can be developed for a one-to-one fit for a process application.
Today’s computer modeling programs and technology allow the use of 3-D design review, finite element analysis (FEA), computational fluid dynamics (CFD), rigid body analysis and thermal imaging diagnostic programs that were not readily available in the past or were too costly for frequent use with earlier 2-D drafting. These advancements in modeling techniques have added to the design reliability of mechanical seals.
These programs and technologies have led the way to the design of standard cartridge seals with much more robust components. These included the removal of springs and dynamic O-rings from the process fluid and made flexible stator technology the design of choice.
Custom Design Testing Ability
The introduction of standard cartridge seals has contributed significantly to greater sealing system reliability through their robustness and ease of installation. This robustness enables a wider range of application conditions with reliable performance.
In addition, more rapid design and fabrication of custom-designed sealing systems have enabled “fine tuning” for varying pump duty requirements. Customization can be introduced either through changes in the seal itself or, more readily, through auxiliary system components such as a piping plan. The ability to control the seal environment under varying operating conditions by way of a support system or piping plans is most critical to seal performance and reliability.
A natural progression also occurred that was more custom-designed pumps, with a corresponding customized mechanical seal. Today, a mechanical seal can be rapidly designed and tested for any type of process conditions or pump characteristics. The seal faces, dimensional parameters of the seal chamber and how the seal fits into the seal chamber can be designed and fabricated to a custom fit for a wide range of applications. Updating of standards such as American Petroleum Institute (API) Standard 682 also has driven greater seal reliability through requirements that validate seal design, materials and functionality.
A Custom Fit
The seal industry battles with the commoditization of seal technology daily. Too many buyers think that “a seal is a seal is a seal.” Standard pumps often can use the same basic seal. However, when installed and applied to specific process conditions, some type of customization in the sealing system is often implemented to obtain the required reliability under that specific set of operating conditions and chemical process.