Soft-Landing Semi-Active Valve For Reciprocating Compressors Inspires Confidence During Field Tests

Ongoing field testing of a semi-active valve (SAV) concept on a BP natural gas gathering facility reciprocating compressor through Oct. 3, 2008, demonstrated that the concept is practical and can significantly extend compressor valve life. Specifically, the field tests showed:

• In all tested operating conditions, reduction of the plate impact kinetic energy above 70% was achieved.
• Without any operator input, the valve automatically self-regulated to any compressor speed and operating conditions.
• The valve can safely operate in natural gas compression facility meeting Class 1, Division 2 (or Division 1) requirements.
• The SAV is inherently retrofitable to existing compressor installations.
• The valve reverts to passive operation and continues to function when the control mechanism fails or is disabled.
• The SAV is capable of providing flow capacity control.

The success of the field testing is important because the single largest maintenance cost for a reciprocating compressor is compressor valves. Plate valve failures can primarily be attributed to high-cycle fatigue and sticking of the valve plate resulting in excessive plate impact velocities. Thus, controlling the valve plate motion and impact velocities can greatly increase the life of a plate valve while also allowing for improved capacity control of the compressor.

A new valve concept, developed under a GMRC program by Southwest Research Institute® (SwRI®) and co-funded by BP, uses electromagnetic actuators to control the valve plate motion and create a soft landing at both the valve seat and guard. This concept is referred to as a “semi-active valve” (SAV) since, although the valve still relies on gas forces for the plate to move, the device senses and then controls the plate motion using electromagnetic coils.

The SAV development program has evolved and matured through three prototype devices, each of which has been tested at Southwest Research Institute. The latest version of the SAV, manufactured by Southwest Research Institute and Cook Compression, first underwent functional testing at Southwest Research Institute and is currently operating in a long-term field test at a BP production site in the U.S.

Cost Savings

The operation of a reciprocating compressor is closely linked to the performance of its cylinder valves. These compressors have traditionally used passive valves to control the suction and discharge flow process of the compressor cylinder. However, valve failures are generally cited as the most common cause of scheduled and unscheduled compressor outages, and the single largest maintenance cost items on reciprocating compressors are valve replacements and repairs.

With the emergence of larger machines operating over wider speed and pressure ratio ranges over the last 15-20 years, this trend has worsened. Consequently, the industry has to consider improvements in valve technologies to be able to compete with alternative compression technologies.