S. Kanikar, Technip Energies, Mumbai, India
This article discusses the best practices for selecting compressors for different applications in refining, petrochemical, chemical and carbon capture plants. It details the different types of compressors commonly used, the advantages and disadvantages of each type, the important parameters required for selecting a compressor type, the selection procedure and the application of each type. Valuable insights are provided for engineers and technicians involved in the selection of compressors for various industrial applications. These guidelines should be referred to when specifying and selecting compressors in projects, and when reviewing selections already made by the licensor and/or processor.
Commonly used compressor types. When selecting compressors, the objective should be to minimize the use of different types of compressors. Users should select the most cost-effective compressor for the specified operating condition.
Compressors may be classified based on the principle by which energy is added to the fluid. Under this system, all compressors may be divided into two major categories:
Dynamic: In these compressors, energy is continuously added to increase the gas velocities within the machine to values greater than those occurring at the discharge, so subsequent velocity reduction within or beyond the compressor produces a pressure increase.
Positive displacement: In these compressors, energy is periodically added by the application of force to one or more movable boundaries of any desired number of enclosed volumes, resulting in a direct increase in pressure up to the value required to move the gas through valves or ports into the discharge line.
FIG. 1 shows the various types of compressors that fall into these two categories (i.e, dynamic and positive displacement).
Considering the variety of compressors available, a concise comparison has been completed among the various types. FIG. 2 details different types of compressors according to basic criteria such as pressure and capacity.
The lines plotted in FIG. 2 for each compressor type represent the upper limits of pressure and capacity generally available commercially throughout the world. At or close to the limits shown, only a few sources may be available. At lower pressures and capacities, well within the envelopes of coverage, compressors may be available from various sources as pre-engineered or standard products.
The selection of the specific compressor type should be based on how the machine will be employed in operations and on various specifications, such as capacity and head limitations. These factors play a vital role in the selection of the type of compressor. TABLE 1 provides limitations for various compressors.
Note: The limitations provided are not intended to encompass the full range of compressors available in the market. The advantages and disadvantages of different compressors are summarized in TABLE 2. This listing will help in selecting the correct compressor for service.
TABLE 3 is a typical comparison of the summarized guidelines to help select the best compressor, based on specific criteria.
SELECTION GUIDELINES
Best practices in selecting various compressor types are discussed in the following sections.
General. To determine the best compressor for service, the following process parameters are required:
The mass flowrate
The range of operating pressures and temperatures
Gas data (composition, molecular weight, ratio of specific heat, compressibility)
Any limitations on discharge temperature
The presence of any impurities, such as hydrogen sulfide (H2S), ethylene oxide, polymerizing compounds, etc.
The application for the compressor (e.g., make-up, recycle, transfer, pipeline).
In addition, several other operating conditions can affect compressor selection:
Number of installed units
Any requirement to operate in parallel or series
Different operating conditions
Operation at turndown condition
Stringent startup requirements
Any future operating scenarios
The presence of liquid in the gas.
With the above information, the rated duty case is selected in conjunction with the process.
Selection procedure. Centrifugal compressors are preferred for all applications where operating conditions permit, notwithstanding the relatively high initial cost of centrifugal compressors compared with reciprocating compressors. Under certain operating conditions, reciprocating compressors will offer specific advantages and are preferred for such applications.
Step 1: Determine the number of required compressors. Determining the number of compressors is greatly influenced by the following factors:
The required process turndown
The required availability of the plant
The availability of a suitable compressor
The availability of a driver
The availability of power for the driver
The requirement of future expansion (capacity enhancement)
Criticality of the service
Lifecycle cost analysis
Project life expectancy.
Step 2: Type selection for process gas. The range chart shown in TABLE 1 indicates the operating limits of various compressor types. The following points provide the initial screening for determining the compressor type.
Application of centrifugal compressors. For most applications, centrifugal compressors (single or multi-stage) are the preferred compressor type, if available within limits.
For specific low-pressure ratio applications, if a single-stage centrifugal compressor can be selected, the first preference is an end-suction compressor. End-suction compressors provide specific advantages in terms of simplicity and low cost compared to conventional process-type units. However, their general application is limited to low-head pressure (a pressure ratio of < 2).
Process applications that demand high availability, high reliability and integrity should use API 617-compliant compressors.
The use of centrifugal compressors for inlet flows below 170 m3/hr should be confirmed with the compressor vendor. In such very low flow applications, a limited number of vendors are available.
Compressor stages may be combined in one casing or in different casings to increase the pressure ratio capability. However, the use of more than three casings in a single drive train should be avoided.
As per API guidelines, unless otherwise specified, casings should be radially split when the partial pressure of hydrogen (H2)—at maximum allowable working pressure (MAWP)—exceeds 13.8 bar. However, horizontally-split casings can be employed for operating pressures below 60 bar. The choice of the casing split also depends upon the ease of maintenance required. The typical vertical-split casing design incorporates many features to ease the installation and removal of the internal assembly without affecting the connected piping. This layout reduces machine downtime.
Application of positive displacement compressors (reciprocating type). Reciprocating compressors have great flexibility. As positive displacement compressors, reciprocating units can easily compress a wide range of gas densities, from H2 (with a molecular weight of 2) to gases such as chlorine (with a molecular weight of 70). Reciprocating compressors can quickly adjust to varying pressure conditions with stage compression ratios ranging from 1.1 on recycle services to > 5 on gases with low K values or low ratios of specific heat. Therefore, they are deployed when process conditions are likely to change widely.
Typical compression ratios are about three per stage to limit discharge temperatures to 150°C–175°C and are employed to achieve a very high-pressure ratio. Some reciprocating compressors have as many as six stages to provide a total compression ratio of more than 300. However, they are limited to relatively low-flow applications. Configurations are available wherein the capacity can be increased by connecting the cylinders in parallel.
As per API guidelines, unless otherwise specified, the maximum predicted discharge temperature should not exceed 150°C. This limit applies to all specified operating and load conditions. Special considerations should be given to services—such as high-pressure H2 or applications requiring non-lubricated cylinders—where temperature limitations should be lower. Predicted discharge temperatures should not exceed 135°C for H2-rich services (molar mass ≤ 12).
Conservative rotative and piston speeds are used for process compressors, as most units operate continuously for many years with only occasional shutdowns for maintenance. With many applications, gases containing entrained liquid and/or foreign abrasive particles can cause corrosion. For these reasons, low- to medium-speed compressors are used, which have rotative speeds from 275 rpm–600 rpm, with piston speeds varying from 3 m/sec–5 m/sec and compressor strokes from 150 mm–460 mm. Normally, for higher-kilowatt (kW)-rated units, longer strokes and slower speeds are used. For non-lubricated applications, lower rotative and lower piston speeds are recommended to obtain improved piston and packing ring life.
Application of positive displacement compressors (screw type). Positive displacement screw compressors should be used primarily for clean gas service. However, dry screw compressors may tolerate some contamination. These machines are normally used in limited services like refrigeration, tail gas applications and fuel gas compression, among others.
Step 3: Compressor type selection for plant and instrument air. For plant and instrument air, both positive displacement and centrifugal-type compressors are commonly used. In case some oil contamination is allowed for air, a wet screw compressor is the preferred choice up to 3,800 Nm3/hr with downstream oil removal filters.
In the case that oil-free air is required, then dry screw compressors can be employed until 3,500 Nm3/hr. Beyond this capacity, integrally geared compressors should be employed. Note: Some dry screw compressors might be available until 6,500 Nm3/hr. However, the compressor model’s availability greatly depends upon ambient conditions, the availability of the cooling water, etc.
The choice of compressor depends upon the required turndown based on the normal/peak air requirement. Screw compressors have a very high turndown ratio, whereas integrally-geared compressors need a blowoff below certain capacities (approximately 70%–80%). This may cause a lot of power wastage in turndown operation. HP
Suyash Kanikar is the Head of the Mechanical Department at Technip Energies in Mumbai, India. He has 24 yr of experience in refining and petrochemical/chemical projects. Prior to joining Technip Energies, Kanikar worked in various consultancies such as Toyo Engineering, Jacobs H&G and Reliance Industries. He has been recognized as a subject matter expert for rotating equipment, pumps and compressors, and has worked on some of the largest critical service compressors, pumps and other rotary equipment and packages in India and internationally. He earned a Bch degree in mechanical engineering from the University of Mumbai and has earned an Executive Diploma in project management.