Build Your Working Knowledge of Process Compressors H ow goo good d are
you at sel ecting cti ng medium-power reciprocating
compressors?
Edward H. Livingston, Howden Compressors Incorporated
he role of compressors in the chemical process industries (CPI) is critical since they are used to circulate gas through a process, enhance conditions for chemical reactions, provide inert gas for safety or control systems, recover and recompress process gas, and maintain correct pressure levels by either adding and removing gas or vapors from a process system. The chemical engineer involved with process proc ess design desi gn and equipment equi pment selec tion must have a working knowledge of com pressors, pressors, since they are the most mechanically complex machinery used in the CPI. This working knowledge must not only be related to the thermodynamics of the gas being compressed, compressed, but also to the type of compressor to be used for a particular process. The latter has become more important as the result of the passage of the Clean Air Act Amendments of 1990, legislation which places limitations on emissions from process equipment. equipment. Fugitive emissions from such sources as compressors, pumps, valves, and piping systems and connections must be reduced over the coming years in order to comply with EPA Equipment Leak Regulations.
T
work done enhances the pressure and density. Flow through the cylinder is controlled by valve actions. Examples of reciprocating machines include piston compressors, compressors, lubricated and nonlubricated, and metal diaphragm compressors. compressors. 2. Turbomachinery, or dynamic com pressors, pressors, are those in which a dynamic head is imparted to the gas by means of high speed impellers rotating in a confining case. This category includes axial-flow, radial, centrifugal and fan-blower compressors. 3. Rotary machines are those in which gas is moved by the positive displacement of two rotating lobes or by oscillating vanes confined in an eccentric cylinder. 4. Ejector machines are those in which gas is moved by kinetic energy induced through high-velocity nozzles. This article will primarily deal with reciprocating, positive displacement compressors compressors with emphasis placed on applications and machines having installed power of 200 kW or kW or less. However, some comments with respect to larger compressors compressors will be made due to their importance in process applications. Before we discuss specific compressor types in detail, let’s look at typical applications of the units.
Brief background
Oil refinery processes
The principal types of compressors found in the CPI are reciprocating, turbo (centrifugal and axial), and rotary flow designs. Within some of the types are variations. Nevertheless, in all cases, compressors are used to convert energy from one form to another. 1. Reciprocating machines are those in which gas is moved by the linear motion of a piston within a confining cylinder. The
Both positive displacement and dynamic compressors are used in the refining of crude oil. Crude oil feed-stock contains polluting compounds such as sulfur, chlorides and salts. Refining processes extract these pollutants and convert them into needed byproducts, thus reducing emissions into the atmosphere. Hydrogen sulfide and carbon dioxide are being treated by chemical absorption systems such as
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methyldiethanolamine (MDEA) with a high absorption efficiency. Hydrogenation processes occur in the initial distillation column, as well as hydrotreating, hydrocracking, and catalytic reforming units.
Hydrotreating Hydrotreating removes objectionable elements such as sulfur, nitrogen, oxygen, and halides from feedstock by reacting them catalytically with hydrogen. In this application, three compressors are frequently used: the recycle compressor which takes hydrogen-rich gas from the hydrogen separator and recycles it to the process front end, the make-up com pressor that adds hydrogen to the process at the front end, and the vent gas compressor which handles hydrocarbon gas mixtures of molecular weights from 20 to 40. The size of the recycle and makeup compressors depends on the hydrotreater capacity. In general, recycle compressor flow rates vary from 25,500 to 127,500 m³/h and reciprocating or centrifugal compressors may be used since compression ratios are low. The make-up compressor flow rates vary from 5,950 to 29,750 m³/h, and while the flow rates can support centrifugal design compressors, the compression ratios are too high for centrifugal design and thus reciprocating designs are used.
Hydrocracking Hydrocracking produces gasoline from heavy feedstocks. In many cases, hydrocracking and cat reforming units work in unison. Hydrocracking takes place at higher pressures, 6.9 MPa to 13.8 MPa and has a high concentration of hydrogen with a fixed bed catalyst. Two compressors are used for this application: the first is the recycle compressor which takes hydrogen-rich gas from the separator and recycles that gas to the process front end where it mixes with the liquid feedstock, and the second is the make-up compressor which adds hydrogen to the process after the separation and before the recycle com pressor. The recycle compressor is
usually a barrel-type centrifugal com pressor since the compression ratio is low, while the make-up compressor is a multistage balanced opposed reciprocating compressor because the compressor must boost the gas from 1.4 MPa to 13.8 MPa. To reduce emissions within the refinery, hydrocarbon gases are collected, recompressed and used elsewhere. Hydrogen collected from the reciprocating compressor packing group and from the barrel-type com pressor mechanical seal are often recycled back into the process by small, positive displacement, reciprocating compressors.
Polymerization Compressors are required to feed gases at elevated pressures into reactors or compress gases to a pressure that will permit liquefaction after which the liquid is pumped directly into the reactor. Typical gases com pressed include ethylene, hydrogen, hydrogen chloride, methyl chloride, pho sge ne, propa ne, and butane. Depending on the polymer and the process used, pressures can range from 0.1 MPa to 380 MP a
H igher flow rate appli cations r equir e centr if ugal or axial f low machines. The original low density polyethylene process required gas pressures of 200 MPa to 320 MP a Three positive displacement compressors were used in series to compress ethylene from 0.5 Pa to 240 MP a The combined power requirements of these three compressors exceeded 7,500 kW. The operating pressures of the linear linear low density process have been reduced to 1 to 2.1 MPa, however, compression equipment is required to keep the gas circulating in the reactor and fluidize the polymer particles. Some specialized co-polymers still use pressures of 200 to 320 MPa. The
flow rates are small in comparison to standard, full-size plants. Feed compressors for additives operate to 320 MPa, and power is in the range of 75 k W . High density polyethylene is often co-produced with polypropylene in a pressure range of 1 to 3.5 MPa. The catalyst is fed to the reactor with the gas is flashed, separated, and recycled.
Electronics and semiconductors Gases are produced for the manufacture of electronic components and semiconductors. Purity of the gases is vital and ultrapure systems are used in all phases of manufacturing. Adding to the purity requirements is the handling problem associated with strong oxidizers, flammable, pyrophoric, and highly toxic compounds. Oxidation processes are used for the formation of protective silicon dioxide coating on wafer surfaces. This is accomplished in a diffusion furnace in an oxygen atmosphere. Protective atmosphere doping uses nitrogen in the purity range of of 99.9999% for the manufacture of microelectronic components to protect the material as well as being a carrier gas for dopants. Chip manufacturing involves ultrahigh-purity argon for silicon crystal growing, oxide removal (etching) and doping of wafers for desired chemical composition. Manufacturing of integrated circuits and semiconductors use ultrahigh purity gaseous chemicals for dopi n g etching, epitaxy, and ion implantation. Gaseous chemicals such as arsine, phosphine, silane or chloroslanes, diborane, halocarbons, hydrogen selenide, hydrogen sulfide, and sulfur hexafluoride are used in a mixture of diluent gases like argon, helium, nitrogen, and hydrogen.
Hydrogen recovery Even though the cost of hydrogen is relatively low, hydrogen is recovered for safety reasons. The hydrogen vapor from liquid storage tanks is recovered by a compressor rather than venting it to the atmasphere. This reduces the possibilities of auto-ignition, a situation that could occur when
vapor is vented by a relief device. High-pressure piston com pressors use packing vents to dispose of effluent leaks. Small compressors recover, recompress, and inject the gas into the process stream.
10 0
10
Reactor gas feeds H y d r o g e n . Hydrogen is
required for full or partial 1 hydrogenation of fats and oils by convert ing unsaturat ed radicals of fatty glycerides to highly or completely saturated glycerides, pharmaceutical 0.1 intermediates, and final products. Compressors are used Inlet Flow, Actual m³ /hour for initial hydrogen feed and Figure 1, Range of CPI Compressors recycle of hydrogen in the process. , Intercooler , Pulsation Hydrogen from either gas plants or tube trailers is boosted by positive displacement piston and diaphragm compressors to the reaction feed pressures required. C a r b o n d i o x i d e is a solvent at supercritical conditions. High purity carbon dioxide is compressed to the critical pressure conditions and is then recycled after separation. Nitrogen is sparged into reactors to reduce dissolved oxygen, blankets sensitize compounds against oxidation and contamination, and purges inder process reactors and piping systems at Crankcase (Frame) shutdown and startup. Refri gerant gases such as chloron Figur e 2. Pi ston design. difluoromethane are fed to reactors for required by the plant. These comfrom storage tanks or tank cars. manufacture of fluoropolymers such as presso rs are si zed to meet peak Typically, bone dry nitrogen is intro polytetralluoroethylene (PTFE) demands for the gas (gases) and are duced under pressure to induce flow. frequently specified with a standby Chlorine tank cars frequently use Gas separation unit of 50 or 100% capacity. this method during cold weather and Membrane separation can produce the importance of using an oil-free, Cryogenic separation of atmosphernonlubricated compressor is apparic gases results in the highest purity higher purity gas streams and are used ent. Hydrocarbons from lubricants in upgrading hydrocarbon gases. levels in comparison to membrane sepProcessing pressure drops may could carryover and react with the aration or Pressure Swing Adsorption require the treated gas to be comchlorine, while water vapor (150 (PSA). If the final product can tolerate pressed for recycle. In most cases cen ppm or more) would cause a highly lower gas purity levels, PSA produccorrosive condition. trifugal compressors are applicable tion of nitrogen and oxygen can result because of the low compression ratio. Nitrogen also acts as a blanketing in cost reductions of 20-60%. gas to prevent fire or explosion condiConventional air compressors feed tions and creates an oxygen free envithe PSA unit. Depending on the presNitrogen boosting ronment to enhance long-term storage, sure requirements, reciprocating pisUnder certain conditions, nitrogen especially for perishable products. ton compressor can be installed to is required to maintain flow of liquid Nitrogen is used to balance pres boost the PSA outlet pressure to that .
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First Stage Diaphragm Cylinder
sures in high-speed mechanical seals to prevent leakage of gas to the environment. The system also permits monitoring of seal performance by pressure decay in the seal. Small, positive displacement, reciprocating compressors are used to pressurize the seal and maintain purge flow.
Second Stage
Compressor types Compressor selection is based on the process operating variables and how those variables fit the design ranges of the types of available com pressors. Figure 1 illustrates the range of compressors used in the CPI. Positive displacement piston compressors are normally selected for applications where the inlet flow rate is no greater that 6,800 m³/h. Design discharge pressures can range from 0.5 MPa to 380 MPa. The latter for small displacement machines assure low density polyethylene process. Diaphragm com pressors, a specialized version of positive displacement piston com pressors, are limited to single cylinder inlet flows up to 204 m³/h. Centrifugal compressors range from inlet flows of 850 m³/h to 340,000 m³/h with case pressures to 70 MPa for small centrifugal units and considerably lower pressure for large units, and axial flow compressors range from 34,000 m³/h to 1 , 0 2 0 , 0 0 0 m³/h with case pressures generally limited to 1 to 2 MPa for all sizes.
Drive Motor n
Crankcase (Frame)
Baseplate
Fi gure 3. Diaphragm design. Inlet Pulsation Dampener
\ Last Stage Crankcase (Frame) Diaphragm Cylinder 1 First Stage _ I Baseplate Piston Cylinder n
\ Second Stage Piston Cylinder
Fi gure 4. H ybri d design.
Crosshead
\
Packing Case
I
Suction Valve Pockets w/Valves
/
I
Engineering considerations and economics For the higher flow rate applications, the process engineer has few alternatives, with selections being limited to either centrifugal or axial flow machines. However, in the lower flow range, several choices are available: positive displacement piston and diaphragm compressors, or hybrid machines that combine piston and diaphragm cylinders on one crankcase. General arrangements of these com pressors are shown by Figures 2 through 4.
Piston compressors
Crankcase n
Compression Cylinder
Figure 5. A simpli fi cation of compressor cr ankcase elements.
crankcase that converts rotary motion to linear motion: a crosshead for guiding the motion of the piston, a piston fitted with seal rings, a cylinder in which gas compression takes place, and one or more suction valves and one or more discharge valves that regulate the flow of gas into and out of the compression cylinder. A simpli-
fied diagram of these elements is shown in Figure 5. Air compressors should not be considered as process gas compressors. There are significant design differences inherent in the air compressors and the improper use of an air compressor in a process gas application could have severe consequences,
such as excessive leakage, fire, or explosion. For example, the air com pressor piston is sealed from the crankcase by piston rings, usually of cast iron. Since gas can leak into the crankcase, the simple air compressor should be limited to air or nitrogen even if the crankcase is pressurized. Process-gas piston compressors are designed with a distance piece to eliminate gas leakage into the atmos phere and crankcase. The piston is mounted on a piston rod which is connected to a crosshead. The piston rod is relatively small in diameter, and therefore can be sealed within a packing case with one or more sets of packing. Figure 6 illustrates a typical packing case. Special purged packing cases and distances pieces are fitted on these compressors to further minimize gas leakage. In the lubricated process-gas com pressor fluid is injected into the com pression cylinder to provide lubrication for the piston rings and the compressor valves. Packing is both lubricated and cooled by injecting the same lubricant into the packing set(s). The lubrication is provided by either a separate, crankshaft driven lubrication pump, or an auxiliary motor driven lubricator. Nonlubricated compressor operation prevents the entrainment of oil in
Distance
Vent to Flare
.
Cylinder
’ Purge Connection n
F igu re 6. A typical packin g case.
the gas being compressed. Types of service for nonlubricated compressors include oxygen compression, food processing, container manufacturing, breweries, chemical, and specialty gas plants where oil contamination of the end product cannot be tolerated. Nonlubricated reciprocating compressors vary in design, but fundamentally the piston is driven through a crosshead from the crankshaft. The piston rod is sealed by a stuffing box with packing rings. The compression cylinders are mounted to distance pieces which iso-
Valve Flange Cylinder
Packing Rings
Suction Valve &Retainer
Valve Hold-down
late them from the crankcase thus preventing oil carryover. The cylinders may be mounted horizontally, vertically, at an angle, or in combinations of these on multistage, multithrow crankcases. Nonlubricated cylinders are normally limited to pressures of 25 to 41 MPa. A typical non-lubricated piston cylinder is shown in Figure 7. For the successful operation of nonlubricated piston compressors, nonmetallic piston rings are required. Materials such as PTFE with fillers have proved to be the most efficient. Discharge Valve & Retainer
Gas Plate Contour Diaphragm Group O-Ring Seals Head Integrity O-Ring Head Integrity Detection Port
Piston Seals
Piston Rider (Guide)
Piston Ring
F igur e 7. A typical l ubri cated piston cylin der.
Crankcase
Discharge Valve nF igu re 8. Moti on of th e displacing element causes the diaph ragm to move into th e compression chamber to r educe vol ume and th ereby incr ease gas pressur e.
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Head Integrit y Detection O-Ring
Not only are the wear rates extremely low on the rings made of these materials, but cylinder wear is also reduced. It is vital that these materials be uniform throughout the entire cross section so that wear rates and sealing properties are continuous for the life of the ring. Another type of oil-free piston compressor is one in which a labyrinth profile is used. Normally, the leakage volume past the labyrinth does not exceed 5% of the rated displacement. A two-compartment distance piece for oil wiping and for holding piston alignment is required.
Diaphragm compressors
, Gas Plate
been developed by the EPA for all equipment, including compressors. For compressor seals, the Diaphragm emission factor is 0.228 kg/h/s Group Using standard design and Hydraulic construction methods, leak rates O-Ring from a diaphragm compressor are in the order of 1 x 1 0 - 7 standard Head Integrity Detection Port Oil Plate cc/s. When extremely low leak F igur e 9. Typical l eak detection ar rangement f or a rates are required, in the order of 1 n diaphragm compressor. x 10e8 standard cc/s or less, the diaphragm can be sealed by metallic “0” rings or it can be sealwelded to the gas head. In the event the integrity of the diaphragm or seal is breached, effluent process gas is retained in the head assembly detection system. With a relatively inexpensive monitoring system, an anomaly can be detected and corrected before there is a discharge to the atmosphere. Figure 9 illustrates a typical arrangement for diaphragm compressors and Figure 10 illustrates a monitoring system Process O-Ring
Piston compressors and diaphragm compressors share many of the same components: a crankcase, crankshaft, connecting rod(s), and piston. The main difference between piston and diaphragm compressors is how the gas is comVent to Flare pressed. Unlike other types of Vent to Flare reciprocating piston compressors in which the primary disnF igu re 10. Typical l eakage moni tor in g system. placing element, a piston, contacts the gas, the metal diaphragm ture diaphragm failure is minimal. As compressor completely isolates the a result, this equipment has become gas from the displacing element durwidely accepted for all types of conting the entire work cycle. The motion amination-free applications in laboraof the displacing element is transmittory, pilot, and plant operations. ted to a hydraulic fluid, and the Corrosive gases can be handled in hydraulic fluid transmits its motion to one or more thin, flexible metal discs these compressors because the com pression cylinder can be manufaccalled “diaphragms.” This motion tured from virtually any machinable causes the diaphragm to move into the compression chamber, reducing the material. Certain limitations do apply volume and thereby increasing the and these limitations are related to the diaphragm material. Materials of congas pressure. See Figure 8. struction commonly used for gas conBecause the diaphragms isolate the tacting parts include 17-4ph, 17-7ph, gas from the compressor lubricants, the discharged gas is as pure as the 304SS, 316SS, 400SS, 20Cb, nickel, gas entering the compression head. carbon and low alloy steels. Designs The gas only contacts clean, dry to handle H2S and conforming to National Association of Corrosion metallic surfaces and static elastomer Engineers (NACE) MR-01-75 can be or metallic seals. With improved produced. diaphragm materials and contour configurations, reliabi lity of these The benefit of a no-leakage design is evident and is of greater importance machines has been demonstrated by with the advent of the Clean Air Act. years of service in critical applications. With proper installation and Synthetic Organic Chemical Manufacmaintenance, the likelihood of prema turing Industry (SOCMI) factors have
Hybrid compressors Hybrid compressors are unique since they combine nonlu bricated piston technology with diaphragm technology on one reciprocating frame (crankcase). These compressors find application when inlet pressures are low, the gas flow rates are relatively high, and the gas must be compressed to high pressure. Depending on the gas flow rate, multiple two or three stage diaphragm compressors, or a five stage piston compressor may otherwise be required. To avoid such a situation, two or three stages of nonlubricated piston cylinders are used with a final stage diaphragm cylinder. In a single machine, the large capacity of a piston compressor is combined with the high pressure and leak-tight performance of a diaphragm cylinder.
Piston and diaphragm compressor efficiency The volumetric efficiency of a positive displacement compressor is the ratio of the gas handled to the compressor displacement including
gas compressibility. Several factors influence volumetric efficiency: • compression ratio; • compressibility factors of the gas at suction and discharge conditions; • cylinder clearance volume; • valve action (losses); • piston ring leakage (piston compressors); • adiabatic or polytropic exponent; and • water vapor. Compressor manufacturers can control clearance volume, valve action (losses), piston ring leakage, and the compression ratio (by multistaging). Of great concern to the compressor manufacturer is the control of clearance volumes at high-compression ratios and when gases have a low specific heat ratio. Piston compressor clearance volumes can range from 7% to 22% with fixed valve pockets; diaphragm compressors are normally designed with clearance volumes between 4% and 7% depending on the size of the diaphragm cylinder. For pressure applications to 300 MPa diaphragm compressor clearance volumes may be as high as 10% to 12%, due to practical manufacturing tolerance limits, particularly in the valve pocket area. The effect of clearance volume is illustrated in Figure 11. Clearance volumes of 5%, 10%, and 15% are given for illustrative purposes. The 5% compression slope ABC will attain P2 quicker than the compression slopes of 10%. Likewise, upon re-expansion at the end of the compression stroke DEF, the slope is steeper and therefore allows the gas to enter the cylinder sooner during the suction cycle. Compression efficiency is controlled by the valves and valve pocket design. For example, the compressor
Figure 11. Compressor efficiency (valve design effect).
designer may concentrate on the clearance volume reduction to improve volumetric efficiency and valve losses could be high, thus affecting compression efficiency. A decrease in compression efficiency leads to increased power requirements. Compression efficiency (valve design effect) is also illustrated in Figure 11. The pressure increases along curve ABC, and when the cylinder pressure exceeds the line discharge pressure P2 at point B, system energy unseats the discharge valve. Pressure spikes to point C, and the gas is released into the discharge piping. The discharge event is a series of pressure waves that will degenerate until the piston reaches the end of its stroke at point D. From top dead center, unexpelled gas pushes on the piston and expands along curve DEF. Slightly below point E at P1, the pressure is further reduced to point F. Here a sufficient differential pressure exists to unseat the suction valve. Gas is drawn in by the piston for the remainder of the stroke until the piston reaches bottom dead center.
Table 1. Relative Cost Factors. Installed Power to 30 kW 31 to 50 kW 51 to 100 kW 101 to 200 kW
Lubricated 1.0 1.0 1.0 1.0
Nonlubricated
Diaphragm
1.3 1.3 1.4 1.4
1.5 1.9 1.7 1.6
Volumetic efficiency increases with a decrease in clearance volume and a decrease in compression ratio. While the other factors do influence volumetric efficiency, clearance volume has the most pronounced effect. In the end, a compromise position must be taken to balance volumetric and compression efficiency. In general, the design compromises are related to compression ratio. For high compression ratios (6 to 15), clearance volume is the principal factor and valves are secondary. For the intermediate compression ratios (3 to 6), clearance volume and valve design should be balanced. For low compression ratios (less than 3), valve design is primary. For a given operating condition where either a piston or diaphragm compressor can be considered, diaphragm compressors will have a higher volumetric efficiency since clearance volumes are less and ring leakage is not a factor. This will also permit compression ratios of 15: 1 across one cylinder. Most piston compressors limit compression ratios to 5: 1 or less in nonlubricated designs due to the inherent problem of heat removal and the effect of high temperatures on piston rings. Isothermal efficiency is usually higher in the diaphragm compressor. This is due in part to the large, flat surface area of the cylinder, the proximity of the cooling passages to the compressor valves, and the recirculation of the hydraulic fluid on the back side
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provide low temperature imp act strength and thus avoid brittle failure of critical component\. Compressor Type Process and Operating Requirements Liners are recommended for cylinders since they can be replaced easily Lube or NL piston Contamination during compression in comparison to complete cylinder. Limited contamination during compression NL piston or diaphragm Two types are used--a wet liner or a Medium corrosive gas A ll Highly corrosive gas Diaphragm dry liner. Wet liners form the inside Oiaphragm or NL piston Hazardous gas (toxic, flammable) pressure boundary of the cylinder and Leaktight Diaphragm also the inside of the water cooling Lube or NL piston Low suction pressure, low compression ratio jacket. Because it is a pressure boundHybrid Low suction pressure, high compression ratio ary, the liner wall must be designed to High discharge pressure (to 25 MPa) NL piston or diaphragm withstand the internal gas pressure High discharge pressure (> 25 MPa) Lube piston or diaphragm and the external cooling water pressure. The liners arc either pressed or of the diaphragm group. However, machines combine the high flow of shrunk into place. Dry liners are not isothermal efficiency of any positive the nonlubricated piston compressor pressure boundaries. They arc thinner displacement compressor is less than with the leaktight, high-pressure in section and are pressed into the a dynamic compressor. Hence, a capabilities of diaphragm comprescylinder. Liner materials include comparison of isothermal efficiency steel, stainless steel. nodular iron or sors. The benefit to the engineer is between compressor types is a false that the total package is on one comgray iron. The liner material selection comparison. Overall efficiency. volu pressor and driver, not multiple com- should be reviewed with the manufacmetric and compression will normal pressors and drivers. A limitation of turer since there may be limitations ly favor diaphragm types. such equipment is the diaphragm relating to pressure and piston ring combinations. cylinder operating speed which must be in the range of 400 to 450 rpm. Table 4 lists typical materials of Engineering economics construction for diaphragm compresFor comparison purposes, capital sor cylinders. Process and cost factors for lubricated, non-lubriDiaphragm compressor gas plates equipment considerations cated and diaphragm compressors arc can be produced from any machinable given in Table 1. These values are relProcess c o n t a c t i n g materials of material. Some limitations do apply ative and are based on manufacturers’ construction. Table 3 lists typical because of the special requirements standard materials of construction placed on diaphragm material by the materials of construction for lubricatand exclude drivers. ed and non-lubricated piston comcompressor manufacturers. Certain Performance cost factors are usu pressor cylinders. alloys are not readily available. or are ally based on such ratios as cost/kW, For temperatures of -2 5ºC to available only with a tremendous cost cost/m³/h, and cost/Nm³/h. Whatever -200ºC, type 304SS low carbon, impact. Such materials include high measurement is used, the piston comalloyed cast iron, or nickel alloy steel nickel alloys, and so forth. pressor has the most favorable ratio should be used. These materials will The diaphragm compressor cylinwithin the context of its application. However, relative cost factors should Table 3. Typical piston compressor materials of construction. not be the sole criteria for compressor selection assuming that any of the Component Material types discussed have the pressure and Remarks displacement capabilities. Process or Cylinders and Heads Pressures to 10 MPa Gray Iron environmental constraints can raise Pressure to 16 MPa Ductile Cast Iron (DCI) the cost factors of lubricated piston Steel Pressur e to 400 MPa compressors above other types. Stainless Steel Pressure to 25 MPa Cleanup and disposal equipment and monitoring instruments for hazardous Piston Al um in um Lar ge cy li nd ers , l ow in ert ia gases will rapidly escalate the ratios. Cast Iron, Steel High pressures, chlor ides This points out the need to match Corrosive conditions Stainless Steel application. In Table 2, some typical Liners selection guidelines are given. Gray Iron, DCI Most common Low temperature, corrosion Ni-resist Applications requiring compresCorrosive conditions Stainless Steel sion equipment to boost pressure in the range of 101 kPa to 21 MPa and as Packing PTFE-Fitted Pressures to 28 MPa high as 40 MPa should consider the Metal Pressures > 28 MPa use of hybrid compressors. These
Table 2. Typical selection guidelines.
34 . FEBRUARY 1993 . CHEMICAL ENGINEERING PROGRESS
Table 4. Typical diaphragm compressor process materials of construction.
Table 5. Typical compressor valve materials of construction.
Component
Materi al
Remarks
Component
Material
Remarks
Gas Plate
Carbon Steel ‘Low Alloy Steel 304 SS, 316 SS 17-4 PH, A296 High Nickel Alloys
Valve Seat/Guards
Steel 400 ss 17-4 PH
Standard, medium pressure Standard, all pressures Optional, corrosion resistance
Valve Discs
20C b-3
Pressure to 63 MPa Pressure to 200 MPa Pressure to 63 MPa Pressure to 200 M Pa Pressure to 63 MPa Pressure to 63 MPa
301 SS, 316 SS
Standard for most service
400 ss 17-4 PH/17-7 PH 316 SS Plastics
Standard Corrosion r esistant Corrosion r esistant Corrosion resistant/valve action
Ni-Cu alloy
Oxidizer service Spri ngs
17-7 PH Nickel superalloy 302 SS, 316 SS
Standard Corrosion r esistant Corrosion r esistant
Diaphragms
der can be designed for structural tem peratures from -150°C to 317° C. Special construction methods and hydraulic fluids with acceptable viscosity at these temperature extremes are required for successful operation of the compressor at low or high temperatures. Occasionally, the diaphragm cylinder is located off the crankcase with an extended distance piece and piston rod so that the temperature extremes do not affect the reciprocating frame. Separate hydraulic systems, one for frame lubrication and the other for diaphragm pulsing can be employed. Valves are common to all reciprocating compressors. Table 5 lists typical materials of construction for com pressor valves. Certain gases or gas mixtures are corrosive, flammable, or explosive, When pressures and temperatures are increased, or when water vapor is present, the gas can be more difficult to handle. Process contacting materials of construction should be specified by the user because the user is most familiar with the process. However, the experience of the compressor manufacturer permits solutions to be offered, including material of construction choices, In Table 6, guidelines are given for some of the common gases used in the CPI.
Energy use In today’s energy conscious environment, users require the correct solution for gas compression applications. Energy costs can amount to 80% of a compressor’s operating cost. It is important to use only the energy required for the process.
For constant speed compressors, several methods can be used to control capacity. They are suction control, bypass control, and suction valve clearance control. All of these methods can be applied to piston compressors. Diaphragm compressors are limited to suction control and bypass control. Suction control limits the gas pressure at the suction valve. Because piston and diaphragm compressors are positive displacement, the inlet flow varies directly with the suction pressure and provides an infinite number of steps between fully opened and fully closed. For example, a 10% decrease in suction pressure will result in a minimum flow decrease of 10% since both the gas density and the volumetric efficiency are decreased. Some power savings can be expected, but compression ratios and discharge temperatures will be higher affecting compressor performance. Bypass control (capacity bypass) requires additional external piping, a control valve, and instrumentation. This provides an infinite number of steps, but the compressor operates at full discharge pressure and capacity all the time. Compressed gas is recycled back to suction but it must be cooled to the normal suction pressure or else higher discharge temperatures will result. Power savings are non-existent. Suction valve unloading controls capacity by maintaining one or more suction check valves in a partially or fully opened position. This is a step control and capacities of 0% , 25%, 50%, 75%, a n d 1 0 0 % c a n b e achieved. A variation of this control
method is the progressive capacity control. The action of the suction valve is delayed and the compression diagram is altered. This permits capacity fine tuning from 40% to 100%. No-load power is 15% to 20% of the full-load power Clearance control can be either fixed or variable, the latter preferred for line control. This control method permits the capacity to vary by changing clearance volumes which, in turn, affects the volumetric efficiency. Like suction valve unloading, it is efficient and results in power savings almost proportional to the capacity with load and full-load power. Power savings can be realized by using variable-speed control. Capacity and power vary linearly with speed. A speed decrease of 10% will result in a power decrease of 10%. However, variable-speed drives have a high initial cost and they must be carefully sized to meet the torque requirements of the compressor which generally limits the practical turndown to 50% to 80%.
Multistage compressors Multistage compressors are selected when the single stage compressor design limitations are reached. Generally, these limitations are: compression ratio (clearance volume); pressure differential; discharge temperature; and power savings. Practical, maximum single-stage l
l
l
l
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E.H. Livingston is President and CEO of Howden Compressors Incorporated, Langhorne, PA 19047 Tel: 215/702-7777, Fax: 215/702-7787
Table 6. Material and construction guidelines Type
Remarks
Ac ety len e
Explosive
Temperature limit, 55 C, low gas velocity, no copper/copper alloys
pressors he has spent 30 years associated
Ammonia
Corrosive
No copper/copper alloys
wi th chemical processing equipment, such as compressors, pumps, and high pressure
Carbon Dioxide
Corrosive
Corrosive when wet, use 316 SS
Carbon Monoxide
Toxic
Temperature limit 150ºC, carbon steel or low nickel alloys
Chlorine
Toxic
Temperature limit 125ºC, no hydrocarbon greases/oils corrosive when wet
Chlorofluorocarbons
Environmental hazard
Temperature limit 110ºC, leaktight construction
Fluorine
Corrosive
Fluorinated hydraulic fluid s, 316 SS or high nickel alloys, degrease all components
Hydrogen
Explosive
Leaktight construction, potential embrittlement above 30 MPa
Hydrogen Sulfide
Corrosive
Leaktightness, toxic gas, material hardness limitations, 21 Rc per NACE public ations
Oxygen
Flammable
Fluorinated hydraulic fluids , 316 SS or high nickel alloys, degrease all components, low gas velocity, limit compression ratios
Ga s
His responsibiIities include
management of overall operations, application and design engineering of piston and diaphragm type reciprocating com-
vessels and piping. The author of several articles on diaphragm compressors and high pressure equipment, Mr. Livingston has also lectured in courses on compressors and high pressure equipment for applications in the chemical and petrochemical industries. He received his engineering degree in chemical engineering from Drexel University. He is an active member of AIChE, American Society for Testing and Materials (ASTM), ASM International, and National Association of Corrosion Engineers (NACE).
compression ratios for piston diaphragm compressors are 5: 1, and for diaphragm compressors 8: 1 to 10: 1. These limitations are based on the clearance volumes and the heat characteristics of each compressor type. High-pressure differentials can create higher loads (stresses) on the mechanical parts of the compressor. In most cases, this can be solved by multistaging. High discharge temperatures are to be avoided to prevent the deterioration of piston rings, packing, and cylinders. Gas discharge temperatures should be l imited to 200°C. Diaphragm compressors can tolerate higher gas discharge temperatures because of the limited use of nonmetallics and a higher rate of heat rejection. Power savings from multistaging can range from 5% to 25% depending on the number of stages. Actual savings will be a function of the compressor load factor and com pressor size.
In conclusion The compressor types available to the process engineer are many and the proper selection can be quite difficult. More users are relying on the corn
36
l
.
Vinyl Chloride
Flammable
pressor manufacturers to develop the complete gas handling system for a particular process. Still, the process engineer must evaluate and select the type of compression equipment to be used. During this phase of the pro ject, several important points must be considered: 1. Are the process conditions accurately stated and are there any contingencies relating to flow rate and pressure identified? 2. Will the process be scaled-up in the future and, if so, should the com pressor specifications reflect immediate needs, future needs or both? 3. Are standard specifications such as API 618 relevant to the type of equipment to be specified? Should a stand-alone specification be written?
FEBRUARY 1993 . CHEMICAL ENGINEERING PROGRESS
Temperature limit 90ºC, will polymerize or liquify
4. Can lubrication be tolerated during compression and if so, must lubricants be removed from the gas? 5. Can leakage be tolerated? 6. Can neither lubrication or leakage be tolerated? 7. Is there an actual or potential corrosion problem? 8. Can the manufacturer service the equipment? The responses to these questions are very important and they will help in the selection process. If there are questions relating to the type of com pressor, controls, process changes, and the like, discuss these issues with a manufacturer or another user. If these steps are not taken, the engineer is faced with difficult equipment and cost comparisons.
