Taking a look at the impact of variable speed drives on mechanical seals
The use of variable speed drives has become more prevalent in industry in an effort to increase the efficiency of pumping systems. The ability to adjust the rotational speed of a rotodynamic pump has been a major factor in the ability to match a pump’s hydraulic characteristics to those of the system in which it operates.
Whether the mismatch was due to the variation in required pump output or due to incorrect sizing of the pump in the first place, there is no question that the ability to easily vary the pump speed has been a major advance in the overall performance of the pumping system.
Click here to read this article on pages 41 and 42 in the September/October 2014 issue of Fluid Handling magazine.
The fourth edition of the American Petroleum Institute (API) Standard 682 was released in May 2014. The May edition includes several updates to reflect the changing design and application needs of mechanical seals. Annex G defines several new piping plans and associated auxiliary hardware. Piping Plan 03 is defined as as dead-ended seal chamber with a tapered bore and no throat bushing. Tapered bore seal chambers are well-established in many industries. These designs have significant performance differences from traditional, closed-throat cylindrical bore seal chambers, which are defined separately in Piping Plan 02.
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Piping plans for mechanical seals are used to improve the environmental conditions around a seal, extend its capabilities and allow operators to monitor seal performance. American Petroleum Institute (API) 682 contains the most widely referenced specifications for mechanical seal piping plans. This standard includes existing piping plans used in the field and introduces new piping plans that follow advancements in technology and respond to improvements required by the industry.
Click here to read a “Sealing Sense” article that discusses this issue further.
Dual gas-lubricated seals use a pressurized, nitrogen gas barrier – usually at a higher pressure than the pumped fluid – combined with lift features to separate the seal faces to a small, controlled gap. Lift features – such as spiral grooves, waves, T-slots and hydropad grooves – create aerodynamic lift with the barrier gas pressure and shaft rotation, which generate a high film stiffness. This film keeps the seal faces separated, even during challenging conditions, such as vibration and cavitation.
This combination of features and design elements provides advantages not offered by other solutions. “Sealing Sense” articles in Pumps & Systems, November and December 2009, presented an overview of the basic principles of design and application of these seals.
Click here to read a “Sealing Sense” article that discusses the advantages that these unique seal designs offer when compared to other sealing solutions.
An agreement has been reached between the FSA and the Hydraulic Institute (HI) to sponsor the American Council for Energy Efficient Economy (ACEEE) effort to promote an extended product approach for electric motors, pumps, fans and compressors. This would include sealing systems that are components of both pumps and compressors. This initiative began October 2013 and will continue for 18 months with direct access to Utilities that offer rebate programs and the DOE through the FSA Government Affairs Committee.
Discussions on the DOE standards process for electric motors, pumps, fans and compressors, with ACEEE and manufacturers of these products recognized that opportunities for motor system energy savings are much greater than savings from individual components. This led to a suggestion that industry develop a voluntary label for efficiency of an extended product that includes the driven equipment (e.g., fan, pump or compressor), the motor and associated controls. Sealing systems are an important consideration for energy efficiency of both pumps and compressors. This extended product label would provide the relative efficiency of the equipment as it is installed into a motor system application. Testing and labeling specifications for these extended products are being considered as a complement to minimum efficiency performance standards that are established through the DOE rulemaking process.
The energy efficiency community has long been aware of the large opportunity that exists from optimizing motor systems, but the ability of programs to realize these savings have been largely restricted to smaller systems that have very wide application and are easily measured. Larger systems where the savings are of a much larger magnitude (e.g., denominated in 100s of horsepower) require a large investment in analysis and monitoring that is required for a custom rebate program. Prescriptive rebates have been restricted to efficient products, such as NEMA Premium motors, which have modest savings opportunity relative to the system opportunity.
In the cover article for the September 2013 issue of Pumps & Systems, “A Big Picture Evaluation Can Produce Big Savings,” the author advocated a systems approach to pumping system efficiency improvements. The author noted that the most common source of wasted energy in these systems are pumps that are not properly sized for their current applications, resulting in excessive throttling of the pump flow and operation of the pump outside its best efficiency point (BEP). This type operation can produce high levels of vibration that culminate in damage and the abbreviated functional lives of the bearings and mechanical seals. In these applications, the mechanical seals can serve as a harbinger of other maintenance issues resulting from the off-design operation of the pump equipment.
WHAT INITIATES SEAL FAILURE?
Average mechanical seal life has increased significantly during the last 20 years, with improvements in mean time between failure of 50 percent during the last 10 years alone. However, seal service life can fall short of expectations if the seal is exposed to conditions outside its intended operating environment. In most cases, these conditions result form the pump being operated outside its designed application range.
A key takeaway from the September cover article is that a properly designed, selected and installed mechanical seals can fall well short of its optimal service life if the overall system is not selected and operated properly. In taking a systems approach to evaluate and optimize the operation of an existing pumping system, it is important to look beyond the failure modes that can result from improper system design and operation and drill down to the root causes of the failure.
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