Apply Wet Screw Compressors Wisely

The machines suit many applications but pose a number of specific issues.

By Amin Almasi, rotating equipment consultant

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Its compact design, relatively low cost, pressure ratio capability per stage, high efficiency and good reliability make a wet screw compressor the best choice for numerous small- and medium-size applications. Such machines, which also are called "oil-flooded" or "oil-injected," offer the same performance advantages as reciprocating compressors, mainly a constant (adjusted) capacity under varying pressures and a high efficiency. In addition, screw compressors boast the same advantages as centrifugal compressors with respect to reliability, availability and small footprint. Wet screw compressors don't have a surge limitation (which is the main restriction of centrifugal and axial compressors). Also, they don't present high pulsation amplitudes and cylinder valve issues (which can pose major problems in reciprocating machines).


A wet screw compressor contains male and female screw rotors, with the 4/6 combination (4 male lobes and 6 female flutes) traditionally popular at process plants. The compressor uses a slide-valve capacity control system to regulate the volume flow; this system can offer step-less control (usually in a 20–100% range) and excellent energy efficiency.

Because of the high volume ratio, the internal pressure ratio can be high, which is a great advantage for medium-pressure applications. For best efficiency, the volume ratio should be set so the machine's internal compression ratio matches the system compression ratio. For optimum efficiency, internal clearances within the machine should be kept as small as possible. The presence of a large quantity of oil during the compression process lessens the chance of contact between the screw rotors.

THE OIL SYSTEM
Oil injection in the compressor results in simpler, less-expensive and more-reliable operation. The oil enables control of the compressed gas temperature, which permits relatively large pressure rises across the compressor. It serves as sealant, partially filling the clearance between rotors, and the rotor and casing. The oil, which covers all the metal surfaces in the compression chamber, also acts as a significant barrier to corrosion and thus allows the compressor to handle difficult gases. (Selecting the proper oil is critical for tough services.) Another important feature is that the oil dampens noise.

The oil is injected in the region where gas compression is taking place (the oil can also enter from the bearing). It absorbs around 70–85% of the heat generated by gas compression. Commonly, a pumped oil circuit serves the bearing and seal while a pump-less circuit supplies the suction injection. For a pump-less oil-injected machine, gas discharge temperature could remain relatively constant over a wide range of operation (for example, within 10–15°C at varying pressure ratios). With a pumped system (forced-oil injection), gas outlet temperature usually can be maintained more tightly. The forced-feed oil-injection system could be a reliable option for large (critical) screw compressors.

Always check the compatibility of the injected oil with the process gas — to avoid risks of process system deterioration or oil degradation. Examine the entire downstream system; the oil shouldn't cause problems in exchangers, reactors, etc.

Recovery of oil from the discharge gas is an important consideration. The oil separator usually uses coalescing filter technology. If vapor-phase oil carryover is a significant concern, put in two oil separators and install an after-cooler (to condense vaporized oil) between the primary and secondary oil-separator vessels. Usually, oil content (including vapor, aerosols, etc.) ranges from 1 to 5 ppm. Services requiring lower residual oil content (for example, 0.1–0.5 ppm or even less) may need a three-level oil-separator system; these systems require more-than-usual attention.

For special applications, a liquid other than oil may be injected. For instance, water-injected two-stage screw compressors are popular where gas tends to polymerize. Other types of compressors such as centrifugal ones wouldn't work in these difficult services because of rapid polymerization.

COMPRESSOR COMPONENTS
The screw rotor length-to-diameter (L/D) ratio usually falls in range of 1.1–2.2/1, with 1.5–1.9/1 most common. Using larger ratios can increase the capacity at a given speed but might reduce the permissible differential pressure — with long, slender rotors, this may be as low as 4 bar. Short (stubby) rotors can accept differential pressures of 24 bar. A single stage of compression can handle most chemical processing duties. However, for pressure ratios between 15 and 24, a two-stage screw compressor will afford better efficiency.

Screws commonly are made of carbon steel (Figure 1) but other materials can be used whenever carbon steel isn't compatible with process conditions.

The compressor usually is driven by direct coupling to an electric motor (Figure 2). Sometimes a gas or diesel engine serves as the driver (Figure 3); it usually requires an intermediate gear unit.

Large and medium (above 500 kW) screw compressors generally use sleeve bearings. Small screw compressors employ rolling-element (anti-friction) bearings. The compression of gas in screw compressors can result in considerable axial force; this force should be resisted by an axial (thrust) bearing. The thrust bearing is a critical component in a screw compressor.

Mechanical oil seals are most common. Shaft seals should be accessible for inspection and replacement without opening the casing.

SELECTION FACTORS
The wet screw compressor originally fit the area between centrifugal and reciprocating compressors. However, its application areas have expanded. Large wet-screw compressors (say, above 3.5 MW) now cross into the centrifugal compressor area. The smaller ones completely overlap reciprocating compressors in capacity and power.

Advantages. Potential benefits of a wet screw compressor compared to a centrifugal machine include:
• considerably reduced sensitivity to gas molecular weight changes;
• higher efficiency;
• for medium sizes (up to 4 MW), lower cost and better performance;
• high ratio pressure capability; and
• direct connection to the electric motor driver, eliminating speed-increasing gear units.
Compared to a reciprocating machine, a wet screw compressor boasts:
• capability to accept more liquid and entrainment;
• considerably less maintenance;
• higher reliability and availability;
• smaller size (for a compact design);
• lower cost; and
• the possibility of doing without a spare machine.

Wet screw compressors commonly are used in range of 100 kW to 3.5 MW. Capacities typically range from around 400 m3/h to approximately 18,000 m3/h. The compressors usually are applied for discharge pressures up to around 50 barg (for vertically split casing designs). Some very large screw compressors (usually with horizontally split casings) are available for capacities as high as around 50,000 m3/h — but they have limited pressure capabilities (say, below 17 barg).

Operational flexibility is another advantage. Starting-up a wet screw compressor with nitrogen and then gradually bringing in the process gas mixture doesn't change the performance (if the selected oil is suitable for all the compressed gases). This is a great advantage compared to dynamic compressors such as centrifugal ones.

Disadvantages. Although horizontally split casing screw compressors can provide high capacity, they only are made by a few companies and rarely are used. So, realistically, the maximum capacity of a wet screw compressor is 10,000–20,000 m3/h, depending upon the application. (Above these limits, use a centrifugal machine.) The biggest frame that many screw compressor manufacturers can offer is limited to 2.5–4.5 MW, depending upon the service. In other words, a wet screw compressor only can cover small and medium ranges. There also are some pressure capability limits, say, 25–55 bar, depending upon the application.

Oil carryover is the biggest problem. It has afflicted a wide variety of services. If an application requires a dry gas, don't consider a wet screw compressor.

It also might not make sense for handling complex process gases that may change composition over time — unless at least two successful references exist for the application. For example, process gases containing a considerable amount of relatively heavy hydrocarbons that can dissolve into the oil can cause breakage of the oil film at the bearings and could lead to wear and other problems. Advanced synthetic oils can lessen this issue but are expensive. (A reciprocating compressor probably is the best option.) Flare gas recovery applications also pose challenges in selecting an oil that can operate successfully in the wide range of gas compositions encountered. (A liquid ring compressor is a better choice.) Gases containing corrosive or sour traces, particularly gases that can contaminate the oil, can create problems for the compressors.

Many processors still prefer low-speed, long-stroke reciprocating compressors to wet screw compressors for various small- and medium-size applications — feeling the proven track record of reciprocating machines outweighs the first cost advantage of wet screw compressors. The reality is that reciprocating compressor technology is more mature than that of wet screw compressors. There's lots of accumulated knowledge and experience in the design, fabrication, operation and troubleshooting of reciprocating machines. (Reciprocating compressors are the best choice for some specific applications — such as ones involving small flows, light gases and high-pressures.)

Always consider the capacity, the pressure and the service (particularly successful references for it) in compressor selection. Carefully evaluate all plant-specific requirements and all aspects of offered compressors.

PERFORMANCE
Because of their high availability and reliability, screw compressors built by reputable manufacturers (particularly machines manufactured according to API 619) are popular for single stream installation, i.e., without a spare. However, straying far in operating pressure or — in particular — temperature from design conditions can lead to deformations that generally exceed the specified manufacturing tolerances. In other words, off-design operations could result in screw rotor rubbing, permanent material loss and efficiency reduction.

To more realistically evaluate the behavior of a wet screw compressor package, the shop performance test should use the application's gas and oil. If using these isn't feasible, perform the test with a gas (usually a mixture of inert gases) of molecular weight and isentropic index value (known as the "k") equivalent to the job gas across the specified operating envelope, and carefully note the characteristics of the oil.

Critical issues with the performance test are:
• the guaranteed power (usually a 3–4% allowance is acceptable); and
• covering the full operating envelope, particularly the low and high suction pressures.

The oil flush prior to start-up is critical. A wet screw compressor is sensitive to particulates in the oil. Monitoring of oil pump discharge pressure is important; it should be set to around 3 bar above the compressor gas discharge pressure.



AMIN ALMASI is a rotating equipment consultant based in Brisbane, Australia. E-mail him at amin.almasi@ymail.com.

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