Location

The location as well as the quantity of the instruments is important. Four are preferred, two can be used, but voting is not possible. If space for three can be found, then there is probably room for four. Figure 10-9 is a sketch taken from a field test where only two sets of instruments could be installed.

Thermowells must be used for safety. The wells slow things down. The wells should be filled with a conducting oil like kerosene, or if not filiable, with conducting grease. No one wants to measure thermowell air ambient temperature. See Figure 10-10 for a thermowell detail.

1" NPT 3000# Coupling

Drill 1" Dia. Hole

1" NPT 3000# Coupling

Process Line Pipe Wall

Drill 1" Dia. Hole

Figure 10-10. Typical temperature tap detail.

Field testing should make use of any redundant method available. However, while a somewhat personal preference, shaft torque is preferred. Couplings instrumented for torque measurement are becoming more common. The reliability of the torque measurement coupling has improved considerably. Possible detractors are cost and spacer length. If the spacer is not long enough to provide for the necessary deflection, the torque output readings are not reliable.

If not available and the driver is a motor, the local electrical engineers may be coerced into procuring accurate current (CT) and potential (PT) transformers to permit the use of watt meters. The everyday plant CTs and PTs aren't suitable for a quantitative field test. When having to solve a compressor's performance, one will use what is available, but only because it is expedient.

One of the most important parameters to measure is speed, yet it is often overlooked. The fan law in Chapter 5 states that for a centrifugal compressor, the head is proportional to speed squared. For an axial, the sensitivity to speed is as great or greater. Capacity is directly proportional to speed on all compressors, both the positive displacement and the

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