Compressors process considerations

In selec ting the machines of choice, the use of specific speed and diameter best describe the flow. Figure 10-67 shows the characteristics of the three types of compressors. Other considerations in chemical plant service such as problems with gases which may be corrosive or have abrasive solids in suspension must be dealt with. Gases at elevated temperatures may create a potential explosion hazard, while air at the same temperatures may be handled qmte normally minute amounts of lubricating oil or water may contaminate the process gas and so may not be permissible, and for continuous-process use, a high degree of equipment rehability is required, since frequent shutdowns for inspec tion or maintenance cannot be tolerated. 

Compressors are normally one part of an overall process train. When a compressor is selected for use in a process, consideration must be given to the required operating range. It will become quickly apparent when a compressor with a fixed capacity is installed in a process with a variable capacity requirement. Modifying capacity of the individual compressors was discussed in the earlier chapters. 

Th chapter presents an overview of the types of compresL . ,. eratic i for selecting a type of compressor, a procedure for estimating hors iwer and number of stages, and some process considerations for both ciprocating and centrifugal compressors. Chapter 11 discusses... 

In preliminary process design, the primary consideration is contact by inhalation. This happens either through accidental release of toxic material to the atmosphere or the fugitive emissions caused by slow leakage from pipe flanges, valve glands, and pump and compressor seals. Tank filling causes emissions when the rise in liquid level causes vapor in the tank to be released to the atmosphere. 

Turboexpanders with Dry Gas Seals. As mentioned above, the application of dry gas seals in centrifugal compressors dates back to the early 1980s. They are useful in process compressors because seal gas leakage is considerably reduced. Also, the contact of process gas and other constituents that may dilute lubricating oil is minimized. 

A methane gas stream taken off the demethanizer process, and still at 350 psig, is compressed via byproduct energy from the turboexpanders and raised to 410 psig. The gas is then introduced into a 15,000 hp compressor and raised to 850 psig for delivery back to the El Paso Natural Gas Company. The 60 psig boost by each turboexpander represents a 15% reduction in required horsepower. This amounts to considerable energy saved and is yet another reason why the turboexpander is useful in a cryogenic process of this type. 

Flooded compressors use the asymmetric profile rotor extensively because the rotor s efficiency is most apparent in this size range. Flooded compressor size has, over the more recent times, been increased. The upper range is in the 7000 cfm range. While most applications are in air and refrigeration, certain modifications can make it applicable for process ga.s service. One of the considerations is the liquid used for the flooding. 

For chemical service, some lubrication qualities may be sacrificed in order to obtain a fluid compatible with the process gas. In these applications, alternate bearing materials such as graphite or silver have been required. While the requirements may make the operation somewhat special and require considerable care, the life of the compressor and service can be greatly improved. 

Continuous monitoring is necessary on critical machines where problems can develop rapidly and have severe financial consequences. Typical machines in this category are unspared process compressors. Remotely located machinery such as pipeline gas compressors also require continuous monitoring. Also, continuous monitoring may be dictated by safety considerations. Even though the cost of a failure is small, machines should be continuously monitored if a failure will result in hazards to personnel. Figure 8-38 depicts a typical continuous monitoring system. 

A few basics on manual and automatic control will be covered to familiarize the user with some of the considerations necessary to acr -mate a compressor to the process environment. 

The previous section discussed the various types of compressors, their selection, and process flow. This chapter presents greater detail concerning the major components, performance, operational and installation considerations, and standard specifications for reciprocating compressors. 

The following guidelines should be used for the selection process. While the criteria listed are not all inclusive, they will provide definition of the major considerations that should be used to select the best compressor for a specific application. 

Noise can cause a serious nuisance in the neighbourhood of a process plant. Care needs to be taken when selecting and specifying equipment such as compressors, air-cooler fans, induced and forced draught fans for furnaces, and other noisy plant. Excessive noise can also be generated when venting through steam and other relief valves, and from flare stacks. Such equipment should be fitted with silencers. Vendors specifications should be checked to ensure that equipment complies with statutory noise levels both for the protection of employees (see Chapter 9), as well as for noise pollution considerations. Noisy equipment should, as far as practicable, be sited well away from the site boundary. Earth banks and screens of trees can be used to reduce the noise level perceived outside the site. 

Used in a rinse system, agitation can improve rinsing efficiencies dramatically.[6] This can result in lower rinse water volumes and open the door to further source reduction opportunities. The agitation is introduced at the bottom of the rinse tank through an H-shaped set of pipes (sometimes called a sparger) drilled with sufficient holes to create considerable turbulence. It is this turbulence which enhances process solution removal and enables less water to do more work. Care should be taken to use only clean air, but small diaphragm compressors (< 60) are often sufficient when equipped with a ball valve for throttling the flow. 

The efficiency factors of gas compressors are based on ideal comparative processes such as the isothermal or isentropic ones, and therefore show considerable differences according to the method used for evaluation. 

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