Compressors/compressor drivers

A. Pikiilik and H. E. Diaz [Chem. Eng., 84, 106-122 (Oct. 10, 1977)] presented a graphical method for estimating the fabricated cost of distillation cohimns and pressnre vessels, storage tanks, fired heaters, pnmps and drivers, compressors and drivers, and vacnnm eqnipment. 

Details Exchangers Vessels Pump aud driver Compressor aud driver Tanks... 

This chapter will describe all common compressor drivers, but as a practical consideration, details on selection or sizing, hazardous area... 

The condensing steam turbine has a relatively low thermal efficiency because about two-thirds of the steam enthalpy is lost to cooling water in the condenser. Expensive boiler feedwater treatment is required to remove chlorides, salts, and silicates, which can be deposited on the blades causing premature failure. The blades are already under erosion conditions because of water drops present in the condensing steam. Even with these disadvantages, the condensing turbine is still selected, especially in a process that requires very large compressor drivers and relatively low amounts of process steam. 

Whenever there is an inherent speed mismatch between a compressor and driver, several solutions are available. For small sizes, V-belts offer advantages in flexibility. For compressors where the power levels approach 100 hp, a more positive drive should be considered. Depending on the application, this value may move up or down by 25 hp. 

Ibis chapter will discuss various reliability issues. The discussion will kept at a philosophical level rather than getting into a statistical analy The statistical analysis is best left to others equipped with the training, toi and the data. Hopefully, this material will give the reader some comn sense insight into the various considerations involved in the selection applicadon of reliable compressors, their drivers, and auxiliary systems. 

Pumps, compressors, turbines, drivers, and auxiliary machinery should be designed to provide reliable, rugged performance. Pump selection and performance depend on the capacity required and tlie nature of Uie fluids involved. Remotely controlled power switches and shutoff valves are necessary to control fluid flow during an emergency. The inlets for air compressors should be strategically located to prevent the intake of hazardous materials. 

Compressor, Driver ond Speed Gears See Specification Sheet. 

In establishing specifications, the first important items to identify from the plant process material balance are normal, maximum, and minimum intake or suction flow rates together with corresponding conditions of temperature and pressure. The required discharge pressure must be established. If it is necessary or important to be able to operate at reduced or over-normal flow rates, these should be identified for the manufacturer, together with the length of time of such expected condition e.g., full time at one-half rate, 20 minutes out of every hour at 10% over normal, etc. These operating requirements may separate the types of equipment. Because it is uneconomical to purchase horsepower that cannot be used by the fluid system, ask that the manufacturer state the maximum load and/or conditions that will fully load the available horsepower of the compressor-driver unit. 

Many compressors are designed and operated at a fixed condition in a process or refrigeration cycle. However, at least an equal number are designed and operated over a varying, or at the initial selection unknown, set of conditions of suction or discharge pressures. This situation is a reality and an economical necessity if the full horsepower of the compressor and driver combination is to be realized. 

Oil coolers for bearing oil (and often sized for turbine oil requirements when turbine is used) must be a part of the system and are usually mounted on or in the compressor-driver base plate (s). This makes a compact arrangement but is sometimes so congested as to give poor maintenance conditions. The cooler, oil filter, and regulator can also be mounted adjacent to the unit. For some sizes, the compres-... 

Usually, dual oil pumps are included, so that one pump failure will not shut down the compressor-driver unit. The first or main pump may be driven by electric motor, and the standby steam or gas may be driven by turbine. Any combination is acceptable as long as the selection takes into account the specific local conditions and service reliability. Figures 12-50A-C show an overall assembly, including accessories. 

Pressure lubricating system for compressor and driver bearings. 

Steam, oil and water piping directly associated with connecting the compressor-driver unit. 

One set special tools and wrenches for dismantling compressor and driver. 

All gases deviate from the perfect or ideal gas laws to some degree. In some cases the deviation is rather extreme. It is necessary that these deviations be taken into account in many compressor calculations to prevent compressor and driver sizes being greatly in error. 

Compressors with drivers over 150 kW that handle flammable gases (the compressor driver should be shutdown through the interlock system before the valves are closed). 

Control Devices In many installations the use of gas is intermittent, and some means of controlling the output of the compressor is therefore necessary. In other cases constant output is required despite variations in discharge pressure, and the control device must operate to maintain a constant compressor speed. Compressor capacity, speed, or pressure may be varied in accordance with requirements. The nature of the control device will depend on the function to be regulated. Regulation of pressure, volume, temperature, or some other factor determines the type of regulation required and the type of the compressor driver. 

An obvious method of controlling the capacity of a compressor is to vary the speed. This method is applicable to units driven by variable-speed drivers such as steam pistons, steam turbines, gas engines, diesel engines, etc. In these cases the regulator actuates the steam-admission or fuel-admission valve on the compressor driver and thus controls the speed. 

Overhung rotors present an easier alignment problem because shaft-end alignment is necessary only at the coupling between the compressor and driver. 

Lowering the compressor discharge pressure can be achieved by changing to cooler water to the refrigerant condenser. But how does this affect the horsepower load or electrical power demand on the compressor s driver Well, it depends. It depends on how much the refrigerant vapor flow increases, as the compressor discharge pressure is reduced. 

On the other hand, suppose we are working with a centrifugal compressor. This particular machine has a very flat operating curve. This means that a small decrease in discharge pressure will result in a large increase in vapor flow. Therefore, the load on this compressor s driver will increase. 

A very large percentage of refrigeration systems are not limited by either the compressor driver horsepower, the condenser, or the evaporator. Many refrigeration systems are limited by either ... 

For example, adding lighter propane to heavier isobutane in a refinery alkylation unit is a common practice to increase refrigerant circulation. But this will work only when the compressor driver has spare... 

Compressors also have vibration trips. These trips measure the amplitude of the vibrations—which, if they become excessive, will shut off the fuel, steam, or electricity to the compressor s driver. 

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