ARNOLD, K. (1999). Design of Gas-Handling Systems and Facilities (2nd ed.) Episode 2 Part 2 pps

1. Fully-enclosed camshaft is built in sections for easy removal. frame top is bored, and the precision-machined cylinder extension is 2. Crosshead guides are cast integrally with engine frame. provided with O-rings. 3. He toil compressor valves for any service selected from the 14, water jackets are provided with removable cover plates for unmatched Dresser-Rand line, including famous gas-cushioned inspection. Channel Valves. r 1 ). Pistons are precision-ground for a perfect fit in a honed cylinder 4. Clearance pockets and other types of capacity control devices are liner bore. Long-skirt, lightweight piston reduces wear. available to suit any application. 1o, For sustained low oil consumption, narrow, deep-groove piston rings 5. Compressor cylinders (dry or water-cooled) of cast iron, nodular conform easily to liner walls. Top compression ring is chrome-plated iron, or forged steel are engineered to suit required pressures and to condition liners during break-in. capacities. 17, Fuel gas headers, one for each bank of power cylinders, are 6. Full-floating packing adjusts itself in operation and assures best seal controlled by common automatic valve and safer/ devices. Orifice with minimum wear. Packing is pressure-lubricated and vented. plates equalize the distribution of gas to each of the cylinders. 7. Oil wiper rings remove excess oil from piston rod and seal the Individual adjustments are eliminated. frame. 13. Simple fuel injection valves are operated from single camshaft. 8. Crossheads, running in bored guides, have shim-adjusted babbitted 19. Long-life special alloy valves have chromium-plated stems, hardened shoes at top and bottom and either full-floating or fixed crosshead shrink-f it valve seats, and replaceable guides. pins. Suitable for addition of balance weights. 20. Common air inlet manifold conducts air from turbochargers to each 9. Simple, low-cost foundation, made possible by the smaller size, power cylinder. lighter weight, and smooth running blance of the KVSR. 21. Large covers give easy access to valve gear, exclude dust and dirt, 10. Large frame openings give ample and unrestricted access to 22. Water-jacketed exhaust eliminate expansion strains and crankcase. keep engine room temperature down. 11. Flywheel-mounted ring gear for starting motors permits cranking 23. Fitted with a reliable Altronic II CPU solid slate tow tension with either air or gas between 150 psi and 225 psi supply pressure. breakerless ignition system, 12. alloy-iron frame and top are well reinforced with cast-in ribs, 24. The engine can be fitted with either hydraulic or electronic governor Integrally cast bulkheads hold main bearings on both sides of every systems to control engine speed crankthrow. Keys, double-bolting, ami Me rods are used to secure 25. in recent years, each power head is fitted with a bolt-in pre- tfie frame ami frame top together as a solid structure. combustion chamber, which allows the engine to burn a very lean 13. To assure oil-tight joints between power cylinders and frame top, the mixture, resulting in very bw exhaust emissions 262 Design of GAS-HANDLING Systems and Facilities Figure 10-5. Integral engine compressor. (Courtesy of Cooper Industries Energy Services Group.) sidered low-speed units. They tend to operate at 400-600 rpm, although some operate as low as 200 rpm. Figure 10-5 shows a very large integral compressor. This would be typical of compressors in the 2,000 hp to 13,000 hp size. The size of this unit can be estimated by the height of the handrails above the compressor cylinder on the walkway that provides access to the power cylinders, This particular unit has sixteen power cylinders (eight on each side) and four compressor cylinders. It should be obvious that one of these large integrals would require a very large and expensive foundation and would have to be field erected. Often, even the compressor cylinders must be shipped separate from the frame due to weight and size limitations. Large integrals are also much more expensive than either high-speeds or centrifugals. For this reason, even though they are the most fuel efficient choice for large horsepower needs, large integrals are not often installed in oil and gas fields. They are more common in plants and pipeline booster service where their fuel efficiency, long life, and steady performance outweigh their much higher cost. There are some low horsepower (140 to 360) integrals that are normal- ly skid mounted as shown in Figure 10-6 and used extensively in small oil fields for flash gas or gas-lift compressor service. In these units the power cylinders and compressor cylinders are both mounted horizontally Comp res so t~s 263 Figure 10*6. Small-horsepower skid-mounted integrals. (Courtesy of Cooper Industries,} and opposed to each other. There may be one or two compressor cylin- ders and one to four power cylinders. They operate at very slow speed. Their cost and weight are more than similar sized high-speed separable units, but they have lower maintenance cost, greater fuel efficiency, and longer life than the high speeds. 264 Design of GAS-HANDLING Systems and Facilities The major characteristics of low-speed reciprocating compressors are: Size • Some one and two power cylinder field gas compressors rated for L40 hp to 360 hp. »Numerous sizes from 2,000 hp to 4,000 hp. • Large sizes 2,000 hp increments to 12,000 hp. • 2 to 10 compressor cylinders common. Ac^antages • High fuel efficiency (6-8,000 Btu/bhp-hr). • High efficiency compression over a wide range of conditions. • Long operating life. • Low operation and maintenance cost when compared to high speeds. Disadvantages • Usually must be field erected except for very small sizes. • Requires heavy foundation. • High installation cost. • Slow speed requires high degree of vibration and pulsation suppression. Vane-Type Rotary Compressors Rotary compressors are positive-displacement machines. Figure 10-7 shows a typical vane compressor. The operation is similar to that of a vane pump shown schematically in Figure 10-10 of Volume 1, 2nd Edi- tion (Figure 10-9 in 1st Edition). A number of vanes, typically from 8 to 20, fit into slots in a rotating shaft. The vanes slide into and out of the slots as the shaft rotates and the volume contained between two adjacent vanes and the wall of the compressor cylinder decreases. Vanes can be cloth impregnated with a phenolic resin, bronze, or aluminum. The more vanes the compressor has, the smaller the pressure differential across the vanes. Thus, high-ratio vane compressors tend to have more vanes than low-ratio compressors. A relatively large quantity of oil is injected into the flow stream to lubricate the vanes. This is normally captured by a discharge cooler and after-scrubber and recycled to the inlet. Compressors 265 Figure 10-7. Vane-type rotary compressor. [Courtesy of Dresser-Rand Company,} Vane compressors tend to be limited to low pressure service, generally less than 100 to 200 psi discharge. They are used extensively as vapor recovery compressors and vacuum pumps. Single-stage vane compressors can develop 27 in. Hg vacuums, two-stage compressors can develop 29,9 in. Hg, and three-stage compressors can develop even higher vacuums. The major characteristics of vane compressors are: Size • Common sizes up to 250 bhp, but mostly used for applications under 125bhp. • Available in sizes to 500 bhp. • Discharge pressures to 400 psig. • Single- or two-stage in tandem on same shaft. Advantages • Good in vacuum service. • No pulsating flow. • Less space. • Inexpensive for low hp vapor recovery or vacuum service. 266 Design of GAS-HANDLING Systems and Facilities Disadvantages • Must have clean air or gas. «Takes 5 to 20% more horsepower than reciprocating. * Uses ten times the oil of a reciprocating. Usually install after-cooler and separator to recycle oil. Helical-Lobe (Screw) Rotary Compressors Screw compressors are rotary positive displacement machines. Two helical rotors are rotated by a series of timing gears as shown in Figure 10-8 so that gas trapped in the space between them is transported from the suction to the discharge piping. In low-pressure air service, non-lubri- cated screw compressors can deliver a clean, oil-free air. In hydrocarbon service most screw compressors require that liquid be injected to help provide a seal. After-coolers and separators are required to separate the seal oil and recirculate it to suction. Screw compressors can handle moderate amounts of liquid. They can also handle dirty gases because there is no metallic contact within the casing. Figure 10-8. Screw-type rotary compressor. {Courtesy of Dresser-Rand Company.) Compressors 267 It tends to be limited to 250 psig discharge pressures and a maximum of 400 hp in hydrocarbon service, although machines up to 6,000 hp are avail- able in other service. Screw compressors are not as good as vane compres- sors in developing a vacuum, although they are used in vacuum service, Non-lubricated screw compressors have very close clearances and thus they are designed for limited ranges of discharge temperature, tempera- ture rise, compression ratio, etc., all of which can cause changes in these clearances. Lubricated compressors have a somewhat broader tolerance to changes in operating conditions, but they are still more limited than reciprocating compressors. The major characteristics of screw compressors are: Size « Up to 6,000 hp in air service, but more common below 800 hp, • Up to 400 hp in hydrocarbon service, • Discharge pressures to 250 psig. « Single- or two-stage in tandern on same shaft. Advantages «Available as non-lubricated especially for air service. • Can handle dirty gas. • Can handle moderate amounts of liquids, but no slugs. • No pulsating flow. • At low discharge pressure (<50 psig) can be more efficient than reci- procating. Disadvantages • In hydrocarbon service needs seal oil with after-cooler and separator to recycle oil. • At discharge pressure over 50 psig takes 10 to 20% more horsepower than reciprocating. • Low tolerance to change in operating conditions of temperature, pressure, and ratio. Centrifugal Compressors Similar to multistage centrifugal pumps, centrifugal compressors, as shown in Figure 10-9, use a series of rotating impellers to impart velocity 268 Design of GAS-HANDLING Systems and Facilities Figure TO-9. Centrifugal compressor. {Courtesy of Dresser-Rand Company,! head to the gas. This is then converted to pressure head as the gas is slowed in the compressor case. They are either turbine or electric motor driven and range in size from 1,000 hp to over 20,000 hp. Most larger compressors (greater than 4,000 hp) tend to be turbine-driven centrifugal compressors because there is such a first cost advantage in that size range over integrals. Centrifugal compressors have high ratios of horsepower per unit of space and weight, which makes them very popular for off- shore applications. As shown in Figure 10-10 they can be either horizontally split case or vertically split case (barrel). To develop the required gas velocities and head they must rotate at very high speeds (20,000 to 30,000 rprn), making the design of driver, gear, and compressor extremely important. Turbine drives are also high speed and a natural match for centrifugal compressors. There is a disadvantage in centrifugal machines in that they are low efficiency. This means it requires more brake horsepower (bhp) to com- press the same flow rate than would be required for a reciprocating com- pressor. If the compressor is driven with a turbine, there is even a greater disadvantage because the turbines are low in fuel efficiency. The net result is that turbine-driven centrifugal machines do not use fuel very Comp res so rs 269 Figure 10-10. Horizontally split centrifugal compressor (top) and vertically split centrifugal compressor, barrel (bottom). {Courtesy of Dresser-Rand Company.) 270 Design of GAS-HANDLING Systems and Facilities efficiently. This fuel penalty can be overcome if process heat is needed. Waste heat can be recovered from the turbine exhaust, decreasing or eliminating the need to burn gas to create process heat. As with electric motor and engine-driven high-speeds, turbine and elec- tric motor-driven centrifugals can be easily packaged for use in oil and gas fields. They are very common in booster compressor service (high volume, low ratio) and for very high flow rate gas-lift service. Centrifugal compressors cannot be used for high ratio, low-volume applications. The major characteristics of centrifugal compressors are: Size « Starts about 500 hp. * 1,000 hp increments to 20,000 hp. Advantages * High horsepower per unit of space and weight. * Turbine drive easily adapted to waste-heat recovery for high fuel efficiency. * Easily automated for remote operations. * Can be skid mounted, self-contained. * Low initial cost. » Low maintenance and operating cost. * High availability factor. » Large capacity available per unit. Disadvantages * Lower compressor efficiency. * Limited flexibility for capacity. * Turbine drives have higher fuel rate than reciprocating units. * Large horsepower units mean that outage has large effect on process or pipeline capabilities. SPECIFYING A COMPRESSOR In specifying a compressor it is necessary to choose the basic type, the number of stages of compression, and the horsepower required. In order [...]... consideration of the advantages and disadvantages listed earlier In addition, local foundation conditions, type of drivers available, cost of fuel, availability of spare 27 2 Design of GAS-HANDLING Systems and Facilities Figure 10-11 Curve for estimating compression horsepower (Reprinted wirfi permission from GPSA Engineering Data Book, Wth Ed.) parts and personnel familiar with operating and maintenance, waste...Compressors 27 1 to do this the volume of gas, suction and discharge pressure, suction temperature, and gas specific gravity must be known The detailed calculation of horsepower and number of stages depends upon the choice of type of compressor, and the type of compressor depends in part upon horsepower and number of stages A first approximation of the number of stages can be made by assuming... Centrifugal 100 2 2 4 2. 7 2. 0 2. 0 2. 0 3 1 2 2 19,6 02 88 190 380 Centrifugal Screw High Speed High Speed High Speed Screw 4.0 3.0 3.0 1 2 2 9 143 28 6 Vane Screw High Speed Screw Vane Screw 0.1 1.0 2. 0 Integral (onshore only) Figure 10- 12 shows the pressure-volume curve for both single stage compression and two stage compression (neglecting interstage losses) By adding the second stage and cooling the... pressure, psia 27 4 Design of GAS-HANDLING Systems and Facilities Figure 10- 12 Horsepower reduction by multistaging (neglects interstage losses) k = ratio of gas specific heats, Cp/Cv T| = polytropic efficiency = 1.0 for reciprocating, 0.8 for centrifugal It is desirable to limit discharge temperatures to below 25 0°F to 27 5°F to ensure adequate packing life for reciprocating compressors and to avoid lube... temperature to each stage cannot be decreased, increase the number of stages by one and recalculate the discharge temperature • Once the discharge temperature is acceptable, calculate the horsepower required, and calculate suction pressure, discharge temperature, and horsepower for each succeeding stage 27 6 Design of GAS-HANDLING Systems and Facilities »If R > 3, recalculate, adding an additional stage... Determination of Horsepower and Number of Stages There are economic and operational reasons for considering an additional stage of compression The addition of a stage of compression requires an additional scrubber, additional cylinder or case, and more complex piping and controls In addition, there are some horsepower losses due to additional mechanical friction of the cylinder or rotating element and the... This horsepower loss and additional equipment cost may be more than offset by the increased efficiency of compression Comp res so rs 2 73 Table 10-1 Example Compressor Type Selections Service Flow Rate MMscfd Booster Gas Lift Flash Gas Vapor Recovery R n Approx bhp Most Likely Selection Alternate 100 2. 0 1 4,400 Centrifugal Integral (onshore only) 10 5 20 2. 0 2. 7 2, 7 ! 3 3 440 980 3, 920 High Speed High... almost always beneficial to continue to produce the liquids while the Figure 10-13 Example process flow diagram of reciprocating compressor 27 8 Design of GAS-HANDUNG Systems and Facilities cause of the compressor shutdown is investigated The flare valve must always be installed upstream of the suction shutdown valve, Suction Pressure Throttle Valve A suction pressure throttling valve can also be installed... the performance curve For a constant compressor speed: 28 2 Design of GAS-HANDLING Systems and Facilities Figure 10-15 Graphic illustration of a "stonewall/' or a choked flow condition «If the flow rate to the compressor decreases, the compressor approaches the surge point and a recycle valve is needed • If the suction pressure decreases, and discharge pressure remains constant, the compressor head must... and the flow rate will increase A flare valve will avoid stonewalling or overranging driver horsepower Suction Pressure Throttle Valve A throttling device can also be placed in the suction piping to protect against overpressure or to limit the horsepower demand to the maximum available from the driver Figure 10-16 Example process flow diagram of centrifugal compressor 28 4 Design of GAS-HANDLING Systems . Gas Vapor Recovery Flow Rate MMscfd 100 10 5 20 100 2 2 4 0.1 1.0 2. 0 R 2. 0 2. 0 2. 7 2, 7 2. 7 2. 0 2. 0 2. 0 4.0 3.0 3.0 n 1 ! 3 3 .3 1 2 2 1 2 2 Approx. bhp 4,400 440 980 3, 920 19,6 02 88 190 380 9 143 28 6 Most Likely Centrifugal High Speed High. suction pressure, psia 27 4 Design of GAS-HANDLING Systems and Facilities Figure 10- 12. Horsepower reduction by multistaging (neglects interstage losses). k = ratio of gas specific heats, . greater fuel efficiency, and longer life than the high speeds. 26 4 Design of GAS-HANDLING Systems and Facilities The major characteristics of low-speed reciprocating compressors