Cooling lubricating systems, compressor

The lubrication system for the turbine is designed to provide both lubrication and cooling. It is not unusual that in the case of many gas turbines the maximum temperatures reached in the bearing section is about 10-15 minutes after the unit has been shutdown. This means that the lubrication system should continue to operate for a minimum of 20 minutes after the turbine has been shutdown. This system closely follows the outline in API Standard 614, which is discussed in detail in Chapter 15. Separate lubrication systems for various sections of the turbine and driven equipment may be supplied. Many vendors and some manufacturers provide two separate lubrication systems One for hot bearings in the gas turbines and another for the cool bearings of the driven compressor. These and other lubrication systems should be detailed in the specifications. 

A V-belt-driven, force-fed oil lubrication system is used on water-cooled compressors. Oil goes to both bearings and to several points in the cylinder. Ten times as much oil is recommended to lubricate the rotary cylinder as is required for the cylinder of a corresponding reciprocating compressor. The oil carried over with the gas to the line may be reduced 50 per cent with an oil separator on the discharge. Use of an aftercooler ahead of the separator permits removal of 85 to 90 per cent of the entrained oil. 

The lubrication system has two basic functions to lubricate the compressor s moving components and to cool the system by removing heat from the compressor s moving parts. While all compressors must have a lubrication system, the actual design and function of these systems will vary depending on compressor type. 

Ammonia has low miscibility in mineral oils, alkylbenzenes, and polyol ester lubricants, particularly at low temperatures. A typical ammonia system uses a coalescing separator that removes all oil in droplet or aerosol form and drains it back to the compressor. Sometimes separators are equipped with some means of cooling the discharge gas to condense any oil that is discharged as a vapor. 

The contact seal can be used under 1,000 psig. It is more complex, but has the advantage of not leaking while shut down. The contact seal is used extensively in refrigeration service where the compressor is part of a closed loop, and the shutdown feature is desirable. As mentioned, the seals must have a source of cooling and buffer fluid. In many cases, this fluid is lubricating oil. If contamination is not a problem, a combined lube and seal system can be used. 

Lubricating oil is pumped from a reservoir to the compressor and then runs down from the machine to the tank. The tank should have level and temperature indication and a low-level alarm. The pumped oil is cooled and filtered before going to the compressors. Dual filters rated at about 10 p.m are used, and a standby pump is provided. The oil must be dry, and so its reservoir is frequently blanketed with nitrogen. Some of the alarms on the oil system will be supplemented by shutdown switches in case operator action fails to prevent further departure from the control point. 

Lubrication oil system is used to lubricate and cool the bearings at compressor, its driver, gear, and coupling. Seal oil system is used to provide seal oil for lubrication and cooling at seals. For heavily contaminated gas service, a separate lubricaiioD and seal oil system is required. 

Screw compressors require very little maintenance because the rotors turn at conservative speeds and are well lubricated with coolant oil. Screw compressors generally run cool, minimizing the need for both oil changes and intercoolers. Another result of injecting so much oil into the compression system is that the process is considered to be isothermal. An exploded view of a screw compressor is shown in Fig. 5.31. A capacity-control valve provides a capacity variation of between 10 and 100%. This is accomplished by allowing some of the gas in the rotors to escape through a valve located in the stator. 

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