Innovations For Rotating Equipmentactive Magnetic Bearings And Mechanical Dry Seal Systems
Authors: Pragnell D.A. , Holzner D.O. , Uptigrove S.O. 
Published: March 2010

In an effort to improve the reliability of high pressure centrifugal compression equipment, NOVA Corporation of Alberta began a program in the early 1970s to find alternatives to lube and seal oil systems. This program was successful, resulting in the application, testing and utilization of mechanical dry gas seals and active magnetic bearings in NOVA 's gas transmission system.

This paper covers the development history of dry seals and magnetic bearings for centrifugal compressors in high pressure natural gas service. The operating principles, experiences and advantages of mechanical dry seals and active magnetic bearings are also discussed.

History

NOVA Corporation of Alberta has over 30 years of experience with pipeline and compressor equipment associated with the planning, design, construction, operation and maintenance of a 14 000 km natural gas transportation system. This system includes 36 compressor stations totalling 388 mW, 825 major receipt and delivery stations and carries over 75% of marketed Canadian natural gas production.

Centrifugal compressors inherently present operating problems associated with maintenance, reliability, and safety. As an operator of 63 gas turbine compressor units, NOVA has always been concerned with these operating problems. Internal studies conducted in the early 1970s indicated that over 80% of the downtime on NOVA's centrifugal compressors was related to problems with the seal or lube oil systems. This prompted NOVA to initiate a program aimed at the elimination of these oil systems. In 1976, NOVA entered into a joint development program for a mechanical dry 1$ seal with John Crane Inc. By 1978, a dry seal was available witch reliably replaced the conventional seal oil system in centrifugal compressors.

Dry seal development and enhancement continues. NOVA is also working with Pacific Wietz, a German seal manufacturer, on the application of an alternative design of a mechanical dry seal.

In 1982, NOVA became aware of a new device developed by a French manufacturer (Societe de Mecanique Magnetique -S2M) which could be the key to completely eliminating the oil system in centrifugal compressors. This device was an active magnetic bearing system. A prototype was installed in one of NOVA's operating compressors which had already been retrofitted with mechanical dry seals. NOVA was pleased with the results on the prototype installation, and has been engaged in a product enhancement and retrofit program since that time.

In 1984, Magnetic Bearings Inc. (MBI), a subsidiary of Kollmorgan Corporation was formed and licenced by S2M tomanufacture magnetic bearings in the Western Hemisphere.

The first magnetic bearing installation took place in October, 1985. It yielded the world's first oil free mainline compressor unit. This unit is an Ingersoll-Rand CDP 230, 10 444 kW ISO compressor with a 1590 kg shaft and a 5440 kg thrust load. There are currently over 10 000 operating hours on this combined mechanical seal/magnetic bearing installation (Fig. la).

Mechanical Dry Gas Seal

Operating Principle

The mechanical dry seal design is based on technology originally developed for air bearings.