Compressors Helical-Lobe

The dry helical lobe rotary compressors nonlubricated cylinder types of compressors are used for injecting of the fuel in gas turbines at the high pressure needed. The gas turbine application requires that the compressor be dry. This standard is primarily intended for compressors that are in special purpose application and covers the minimum requirements for dry helical lobe rotary compressors used for vacuum, pressure, or both in petroleum, chemical, and gas industry services. This edition also includes a new inspector s checklist and new schematics for general purpose and typical oil systems. 

The helical-lobe compressor is further divided into a dry and a flooded form. The dry fonn uses timing gears to hold a prescribed timing to tlie relative motion of the rotors the flooded form uses a liquid media to keep the rotors from touching. The helical-lobe compressor is the most sophisticated and versatile of the rotary compressor group and operates at the highest rotor tip Mach number of any of the compressors in the rotary family. This compressor is usually referred to as the screw compressor or the SRM compressor. ... 

Figure 1-6. Cutaway of an oil-free helical-lobe rotary compressor. (Courtesy of A-C Compressor Corporattori ...

For most of the rotary compressors in process service, the driver is an electric motor. Compressors in portable service, however, particularly the helical-lobe compressor, use internal combustion engines. Many of the rotary compressors require the high speed that can be obtained from a direct-connected motor. The dry type helical-lobe compressor is probably the main exception as the smaller units operate above motor speed and require a speed increasing gear which may be either internal or external (see Figure 4-1). The helical-lobe compressor is the most likely candidate for a driver other than the electric motor. Aside from the portables already mentioned, engines are used extensively as drivers for rotaries located in the field in gas-gathering service. Steam turbines, while not common, probably comprise most of process service alternate drive applications. 

Figure 4-2. The compression cycle of a helical-lobe compressor.

The limits given are for the centrifugal compressor and for the steam turbine. API 619 should be consulted for the limits of vibration of the helical lobe compressor. 

Mechanical testing of the non-lubricated helical-lobe compressors is modified from the previously described lest. For example, API 619 only requires a two-hour mechanical run. The procedures and monitoring requirements are generally the same as previously described. A run comparable to the overspeed run is the heat run. The compressor is run on air at the maximum allowable speed, and the discharge temperature is allowed to stabilize at a value 20°F higher than the rated discharge temperature. The compressor is then run for 30 minutes. 

Gases that tend to polymerize with temperature must be recognized and, as mentioned for corrosion, temperature limits must be imposed at the application phase to prevent problems later in operation. The oniv compressor, of the types covered, that is not as sensitive to the presence of polymers is the helical-lobe compressor. It should be recognized ihat even this machine does have limits. 

Reciprocating compressor pulsations were covered in Chapter 3, but neeti to be mentioned with the discussions on reliability. Problems with reciprocating compressor pulsations and the potential for acoustic and mechanical resonances are very similar to those experienced with helical-lobe compressors. The significant difference is the frequencies are much lower and the number of discrete frequencies per compressor aie much less. However, piping vibrations can occur and there is always a... 

Where current practice seemed to dictate I updated curves, and added a table in Chapter 4 to help with the sizing of the oil-free helical lobe-compressors. Instrumentation was updated to take rod-drop monitoring of reciprocating compressors into consideration. Improvements in torque monitoring are also included. 

Rotary compressors are classified into three general groups sliding vane, helical lobe, and liquid-seal ring. 

The helical lobe, or screw, compressor is shown in Figure 36.7. It has two or more mating sets of lobe-type rotors mounted in a common housing. The male lobe, or rotor, is usually direct-driven by an electric motor. The female lobe, or mating rotor, is driven by a helical gear set that is mounted on the outboard end of the rotor shafts. The gears provide both motive power for the female rotor and absolute timing between the rotors. 

Helical-lobe compressors are best suited for base-load applications where they can provide a constant volume and pressure of discharge gas. The only recommended method of volume control is the use of variable-speed motors. With variable-speed drives, capacity variations can be obtained with a proportionate reduction in speed. A 50 per cent speed reduction is the maximum permissible control range. 

Always install a relief valve that is capable of bypassing the full-load capacity of the compressor between its discharge port and the first isolation valve. Since helical-lobe compressors are less tolerant to over-pressure operation, safety valves are usually set within 10 per cent of absolute discharge pressure, or 5 psi, whichever is lower. 

Figure 8,7 Helical lobe, or screw, rotary air compressor...

A blower uses mating helical lobes or screws and is used for the same purpose as a fan. They are normally moderate- to high-pressure devices. Blowers are almost identical both physically and functionally to positive-displacement compressors. 

The helical and spiral-lobe compressors are generally similar and use two imermeshing helical or spiral lobes to compress gas between the lobes and the rotor chamber of the casing. The compression cycle begins... 

The spiral-lobe and helical-screw compressors are rotary positive-displacement machines and quite adaptable to a wide assortment of process and refrigeration gases. This class of equipment is usually built to comply with the American Petroleum Institute Standard 619. These units oper-... 

Screw compressors have two rotors with interlocking lobes and act as positive-displacement compressors (see Figure 44.11). This type of compressor is designed for baseload, or steady state, operation and is subject to extreme instability should either the inlet or discharge conditions change. Two helical gears mounted on the outboard ends of the male and female shafts synchronize the two rotor lobes. 

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