New features to extend the life and capabilities of mechanical seals
Contributing editor Sheila Kennedy reports on new features that help mechanical seals last longer and handle tougher environments.
By Sheila Kennedy, contributing editor
Some say the most vulnerable item in a pump is its seal. Taking steps to minimize wear, extending equipment service life, and increasing the mean time between failures (MTBF) using better seals or better seal environments is cheap compared to the cost of an outage. The clean air and water acts, and emission suppression goals are further influencing developments in mechanical seal technology.
Improved seal environments: According to ITT Goulds Pumps, most seal failures aren’t typically the result of a bad seal design, but of a poor sealing environment - one that lacks proper lubrication, lacks cooling and is filled with suspended solids.
The ITT Goulds TaperBore Plus seal chamber allows the seal to run cooler with better face lubrication, thus reducing life cycle cost and ensuring longer life. Seal face heat dissipation is improved with an enlarged seal chamber, allowing increased radial clearance between the mechanical seal and the chamber. Clean liquid moves toward the mechanical seal and continuously flushes the face, while a vane particle ejector directs solids away from the seal and centrifugal force assists in forcing the solids out. Air and vapors also flow away from the seal and exit the seal chamber bore.
More recently, the company developed an alternative seal chamber design for submersible and end-suction centrifugal pumps. The Cyclone Seal Chamber also has a tapered bore design, and it has two cast helical grooves in the tapered walls of the seal chamber that modify the flow pattern in the chamber to keep solids away from the seal faces. ITT’s R&D testing revealed that the liquid’s rotational velocity in the seal chamber moves solids caught in the grooves along the helical path until they are transported out of the seal chamber.
Keeping the chamber environment free of solids prevents erosion along with clogging and wear of the mechanical seal hardware. The Cyclone Seal Chamber’s self-venting design prevents vapor buildup in the seal area, and its “flushless” operation reduces the operating costs, environmental liability and risk of product contamination and dilution associated with flush water.
Innovative seal coatings: Argonne National Laboratory has invented a new form of diamond that is being commercialized for mechanical seals by Advanced Diamond Technologies (ADT). The work was supported in part by the National Science Foundation and the U.S. Department of Energy.
Diamond, nature’s hardest substance, adds its low-wear and low-friction attributes to ADT’s UNCD (ultrananocrystalline diamond) family of mechanical seals. UNCD has nano-scale roughness, making it smooth enough to avoid degrading a soft counterface. For hard-on-hard sealing applications, test results show the application of the diamond-treated seal face is a major improvement.
UNCD Seals potentially provide energy savings through reduced friction, better tolerance to poor lubrication conditions, lower maintenance costs, higher MTBF and reduced face temperatures. The price of UNCD Seals is comparable to high-performance, silicon carbide-face seals. A line of component seals is currently available and a line of cartridge seals is expected in 2008.
For high-pressure applications in the turbomachinery industry, John Crane has developed a variant of its Type 28EXP dry gas compressor seal with a new face material. The hard face is made of silicon carbide, which provides a high modulus and limits pressure deflection, and is coated with a diamond-like coating (DLC) to minimize friction and increase wear performance. The new face material allows operation well beyond existing limits — up to 425 bar. This characteristic is increasingly necessary in the oil and gas industry. The need to exceed traditional pressure limits is being driven by rising requirements of re-injection compressor trains to improve oil extraction yields, and increasing demands on seals associated with the emerging carbon sequestration market.
Another company leveraging diamond-coating technology is Burgmann Industries. EagleBurgmann’s DiamondFaces mechanical seals have a microcrystalline coating that is applied to the seal face using a chemical vapor deposition (CVD) process, forming a chemical bond with the silicon carbide facing. The coating is described as exceptionally hard and long-wearing, with excellent heat conductivity, chemical resistance and low friction. Suited for the oil and gas, chemical and pharmaceutical industries, it protects against wear during dry running, including mixed friction and abrasive media applications.
E-mail Contributing Editor Sheila Kennedy, managing director of Additive Communications, at Sheila@addcomm.com.
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