Most motor-driven compressors are equipped with suction valve or plug-type unloaders on both ends of all cylinders on the frame to permit loadless starting. Loadless starting is not mandatory, but facilitates startup, and reduces disturbance in the electrical system. It is also a convenient and less punishing feature for all types of drivers.
Alarms and Shutdowns
Figure 300-63 is a typical list of alarm and shutdown functions for the compressor frame and cylinders. Local preferences may call for additions to or deletions from this list.
The data sheets in API 618 list many other functions. These data sheets also serve as a good checklist.
Refer to the Driver Manual for recommended alarm and shutdown parameters for drivers.
If a vibration detection device is used on the frame, an accelerometer type detector is recommended to provide continuous measurement. Ball-and-seat or magnetictype switches are unreliable. Consult the vendor regarding the best location for the device. Sometimes it is necessary to install two devices (one for transverse vibration, and one for axial vibration).
Note that whatever alarms and shutdowns are chosen, it is important to make sure they are installed with facilities to allow testing.
Location of the gages is generally a matter of plant preference. They can be mounted locally on the compressor or piping, or mounted in a local panel. Gages mounted on the compressor or attached piping may be subject to vibration. Check with plant operating and maintenance personnel regarding the best location for these gages. (Figure 300-64 is a typical list of gages.)
Monitoring systems of the continuous type are not widely used on reciprocating compressors. However:
Eutectic bearing temperature safety devices are available for main and connecting rod bearings. These devices are spring-loaded and they vent control air to alarm or shut down the unit when high temperature melts the eutectic material. At one large Company installation, such devices prevented a major failure. Main bearing thermocouples or RTD’s are also available.
One device that is gaining more acceptance is a rod-drop monitor. This device can be a eutectic sensor that melts when rubbed by the rod allowing a signal to be transmitted for alarm or shutdown. This function can also be accomplished with a proximity probe.
Various lubricator flow and drive failure sentries are also available for cylinder and packing lubricator systems.