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The effects of the size and configuration of the connectedsystem， as wellas different operating conditions， on the intensity of surge can be astonish-ing. For example， a compressor system in a test set-up at the factory mayexhibit only a mild reaction to surge. At the installation， however， the same compressor with a different connected system may react in a tumultuousmanner. Surge can often be recognized by check valve hammering， pipingvibration，noise， wriggling of pressure gauges or an ammeter on thedriver， or lateral and/or axial vibration of the compressor shaft. Mild cases of surge sometimes are difficult to discern.
Surge Detection and Control
Surge-detection devices may be divided into two groups: (1) staticdevices， and (2) dynamic devices. Todate， static surge-detection devices have been widely used; more research work needs to be done before dynamic detection devices are generally used. A dynamic device will prob-ably meet the requirements and hopes of many engineers for a controldevice that can anticipate stall andsurge， and prevent it. Obviously， detec-tion devices must be linked to a control device that can prevent the unstable operation of a compressor.
Static surge-detection devices attempt to avoid stall and surge by the measurement of compressor conditions and ensure that a predecided valueis not exceeded. When conditions meet or exceed thelimit， control action is taken. A typical pressure-oriented anti-surge control system is shown in Figure 6-41. The pressure transmitter monitors the pressure and controls adevice， which opens a blowoff valve. A temperature-sensing device corrects the readings of flow and speed for the effect of temperature. A typical flow-oriented device is also shown in Figure 6-42.

In all static surge-detectiondevices， the actual phenonemon of flow rever-sal (surge) is not directly monitored. What is monitored are other conditions related to surge. Control limits are set from past experience and a study of compressor characteristics.
Dynamic surge-detection and control methods are under study. They attempt to detect the start of a reversal of flow before it reaches the critical situation of surge. This procedure uses a boundary-layer probe.
The author has a patent for a dynamic surge-detection system， using aboundary-layer probe， presently undergoing field tests. This system consists of specially mounted probes in the compressor to detect boundary-layer flow

Figure 6-4.. Boundary-layer surge prediction techni.ue.
reversal， as shown in Figure 6-43. The concept assumes that the boundary layer will reverse before the entire unit is in surge. Since the system ismeasuring the actual onset of surge by monitoring the flow reversal， it is not dependent on the molecular weight of the gas and is not affected by the movement of the surge line.
The use of pressure transducers and casing accelerometers in the exit piping has been instrumental in detecting compressor surge. It has beenfound that as the unit approachessurge， the blade passing frequency (num-ber of blades times rpm) and its second and third harmonic become excited. In a limited number of tests it has been noted that when the second harmonic of the blade passing frequency reaches the same order of magnitude as theblade passing frequency， the unit is very close to surge.
Process CentrifugalCompressors
These compressors have impellers with a very low pressure ratio (1.1.1.3) and thus large surge-to-choke margins. Figure 6-44 shows a cross section of a typical multistage centrifugal compressor used in the process industries.
The common method of classifying process-type centrifugal compressors driven by gas turbines is based on the number of impellers and the casing design. Table 6-2 shows three types of centrifugal compressors. For eachtype of compressor， approximate maximum ratings of pressure，capacity， and brake horsepower are also shown. Sectionalized casing types haveimpellers， which are usually mounted onthe extended motorshaft， and similar sections are bolted together to obtain the desired number of stages. Casing material is either steel or cast iron. These machines require minimum supervisionand maintenance， and are quite economic in their operating range. The sectionalized casing design is used extensively in supplying air for combustion in ovens and furnaces.
　
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