Controls and Instrumentation

Each user has a favorite list of vendors that conform to plant standards for the various control and instrument items. These include gauges, transmitters, control valves, and the host of various hardware used. In the past, the compressor vendor was involved with supplying vibration and bearing temperature sensors and monitors. Larger users now prefer the purchase of monitors to be a user function, allowing the compressor vendor to supply the sensors in most instances. It is probably best to allow the compressor vendor to furnish the control loop used locally or the lube and seal system, primarily to give overall responsibility on vendor furnished items. On those items interfacing to user control loops, the user should probably maintain responsibility. These thoughts are offered in the form of suggestions rather than recommendations. It certainly is in order for a user to include a list of preferred vendors with the specification. However, it is good to keep an open mind, particularly where the compressor vendor, who is being held responsible, has a strong feeling for a given instrument vendor.

For centrifugal and axial compressors, some form of override control is recommended for constant speed motor drivers to sense motor overload and override the process control until the cause of overload has passed. The override can take the form of closing, moveable inlet guide vanes, when these are purchased, or a suction throttle valve. The compressor vendor frequently furnishes them.

Control panels usually become quite a complex item because of the great amount of coordination necessary in having them conform to plant panel standards, including non-compressor vendor items that usually end up there, and generally can be quite a problem. It is recommended that a great deal of planning take place prior to specifying a panel in the compressor specification. If it can be fully specified early, it may not be a total nightmare. Generally, a user is better off to take on these items himself. This does not include the highly standard panels that are offered with plant air packaged compressors.

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