The Moppet Show

Polymer seal ring aids success of field-repairable Moppet valve

Looking for ways to reduce compressor downtime in the petrochemical and natural gas industries, Cook Manley of Stafford, TX, developed the patented field-repairable Moppet compressor valve, using PEEK polymer from Victrex, Greenville, SC, for the seal ring component. “We’ve used PEEK polymer for years on our radiused-disc valves with great success,” says Charles Parks, plant manager at Cook Manley. “So, when we began developing the Moppet valve — which is actually a series of individual radiused-disc valves encapsulated in modular cartridges — we knew that PEEK polymer would give us the reliability we needed for the seal ring.” With diameters ranging from 1-½ to 12 inches, the valves are designed to go into a reciprocating gas compressor. Depending on the bore of the cylinder, the stroke of the compressor and the rotating speed, the volumes can vary anything from a few scfm to tens of thousands of scfm. In large gas transmission stations, where they are pumping gas down a pipeline, the volumes are tremendous.

“Because Moppet cartridges are interchangeable from valve to valve and from compressor to compres-sor,” says Parks, “they can significantly reduce the end-user’s inventory. Damaged or worn cartridges can be replaced by pressing out the old cartridge and pressing in a new one. This reduces the compressor down-time and eliminates the need to outsource parts for repair or wait for replacements.”

Since these compressors operate in harsh environments in petrochemical and natural gas industries, it is essential that the valves maintain their dimensional stability. “When compared to other materials, such as nylon, PEEK polymer is more dimensionally stable and will hold tolerances while in the end use cus-tomer’s inventory,” says Parks. “Nylon will absorb moisture from the atmosphere or from within the com-pressor itself if it is subjected to any kind of moisture. PEEK polymer, being a semi-crystaline material, has extremely low moisture absorption.”

PEEK polymer also handles a much wider temperature range than nylon. “For example,” says Parks, “Compressors can be used in a vacuum refrigeration-type service or in a very high-temperature, high-pressure process service in a refinery. It can handle temperatures of 380°-390°F — and under any pressure — whereas nylon can only handle up to 280°-290°F, depending upon the type of pressure being experi-enced.” The Moppet valve is designed to operate in temperatures from as low as -150°F (-120° is more common for “vacuum refrigeration”) and in pressures from 0 to 7000/8000 psi, with a maximum differen-tial of 3000-4000 psi.

Another reason why Cook Manley chose PEEK polymer is its fatigue and flexural properties. “When a compressor is running, the process can be very contaminated with dirt and liquids,” says Parks. “Unless the ring is flexible, it can snap under these conditions. While there are many materials that are flexible in na-ture, very few have the fatigue strength of PEEK. This is an essential property as a seal ring has to bend and flex millions of times. For example, most of the large compressors run at about 300-330 rpm — which is relatively slow compared to a car engine — but they must do it 24 hours a day 7 days a week, 365 days a year. That adds up to millions of cycles per year. Other compressors can operate up to 2000 rpm.”

The fact that worn parts can be replaced incrementally on-site and within a few minutes is a major factor in the success of this valve, opines Parks. He reports that a major offshore oil and gas producer has been able to significantly reduce inventory by replacing forty-eight Clark plate compressor valves with the Mop-pet valves. “With the Moppet valve, the people on the platform can do all the repair work themselves. Pre-viously, if an emergency arose, the company had to dispatch a helicopter to the platform, pick up the valves and bring them back to the shore to have them fixed. The repaired valves then had to be helicoptered back to the platform. It was a very costly operation.” Parks also cites the example of the Hanover Company, a large compressor rental company, who substantially decreased valve repair costs by replacing OEM plate valves with the Moppet valves. According to Bill Peterson, area supervisor for Hanover, “At one station, we were replacing OEM plate valves every week. We installed a set of Moppet valves over a year ago — and they’re still going strong.”

—SG