21st century reciprocating compressors for downstream applications Greg Phillippi Ariel Corporation Corporation
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echnology, like time, marches on. Think about the advancements in technology that have taken place in the last 50 years. Advancements have taken us from punch cards to medical computers that can be swallowed. Telecommunication has gone from rotary dial to cell phones. Most advancements led to smaller, less expensive means to do the same thing. It begs the question, why do some industry standards and the people who write them, refuse to acknowledge proven technologies that are as reliable but less expensive than those used 50 years ago? Take the reciprocating compressor industry as an example. For nearly 100 years reciprocating compressors used in downstream oil and gas industry (process) applications have utilised long strokes and low rotating speeds. Virtually all have been block mounted and equipped with compressor cylinder liners and provision for cooling supported since 1964 by American Petroleum Institute Standard 618 “Reciprocating Compressors for Petroleum, Chemical, and Gas Industry Services”. Today alternate designs are available offering the industry equal reliability at lower capital cost. Several manufacturers offer designs with shorter strokes, in the 76 to 229 mm (3 to 9 inch) range, with rotating speeds varying from 600 to 1000 rpm. These modern designs are packaged into complete compression system modules contributing to reduced capital cost through reduced installation time and therefore cost. Many of these modern designs omit liners and provision for cooling as required by API Standard 618 further reducing cost. Very early reciprocating compressors had strokes in the range of 914 mm (36 inch) or longer, had rotating speeds that today would be considered to be very low, in the range of 100 rpm, and were derived from steam engine tech-
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nology. Over time the rotating and average piston speed increased as the sealing technology for piston rod packing and piston rings improved. For example, very early piston rod packing was called a “stufng box” because it was a cylindrical cavity surrounding the piston rod lled (stuffed) with coiled rope to create a seal. This “stufng box” was derived from steam engine technology. Over time the packing became much more sophisticated developing into the segmented packing ring sets made of various metallic and non-metallic materials so common today. There is end-user interest to increase rotating and piston speed because increases in both lead to reduced capital cost. Increases in rotating and piston speed reduce the physical size of the machine resulting in less mass and therefore less cost. Today, the vast majority of downstream process reciprocating compressors are driven by electric motors, which have their cost decrease as their rotating speed increases. So an increase in rotating speed decreases the cost of both of the compressor and the driver.
Block Mount versus Packaged The physical size of the typical long stroke (305 to 508 mm, 12 to 20 inch) low speed (250 to 500 rpm) compressor, as shown in Figures 1 and 2, lends itself to being stick-built at site (block mounted) rather than packaged into a module in a fabrication facility. Most of the reciprocating compressors that exist today in reneries and petrochemical facilities are block mounted meaning bare compressors are built in a factory, shipped to the installation site assembled or disassembled (depending on size and shipping restrictions), and installed on a large concrete foundation (the “block”, see Figure 1). After which all the supporting systems, such as pulsation bottles, separators, process and utility
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Figure 1 Large block mount low speed long stroke compressor during construction at site
Figure 2 Large block mount low speed long stroke compressor in operation
piping, lubrication systems, driver, coupling, instrumentation and control system, are installed. Long stroke low speed compressors lend themselves to this manner of installation because they are typically very large and heavy. Modern short stroke medium speed compressors lend themselves to being packaged (as shown in Figure 3) as they are smaller and lighter for the same capacity. A compressor package is a complete gas compression system module having the compressor with its driver, coupling, all process gas and utility piping, lubrication systems, and all the instrumentation and control systems mounted on a structural steel skid. This skid serves as a platform on which to mount all the previously mentioned equipment, but also, in many instances, the compressor’s foundation.
Lined versus Unlined
API Standard 618 requires compressor cylinders to have liners. Fundamentally, liners are included only for commercial reasons. A liner is not a part required for a compressor cylinder to be able to compress gas. The reasons a liner may be used include: • A liner can be a lower cost replaceable wear element in a compressor cylinder assembly where a bare replacement cylinder body might be very expensive with a long lead time relative to the cost and lead time of a replacement liner. • A liner can be made of a suitable wear material, such as grey iron, when the cylinder body is made of an unsuitable wear material. An example of such an unsuitable material is ASTM A395 “Standard Specication for Ferritic Ductile Iron Pressure-Retaining Castings for Use at Elevated Temperatures”, the material API 618 requires be used for cast ductile iron cylinder bodies. ASTM A395 happens to be a very poor material for use in an application that subjects the material to rubbing wear as is the case in a cylinder bore where the piston and piston rings, or the wear bands and piston rings, are rubbing against it. Consideration must be given to protect A395 ductile iron material and a liner is only one way to accomplish that. • Utilisation of a liner allows the cylinder bore diameter to be changed rather easily. One of the major reasons why liners are not utilised in short stroke cylinders is the liner’s effect on the cylinder’s capacity capability. This Figure 3 Packaged medium speed short stroke compressor is a technical issue that leads to signicant
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commercial harm. The following chart, Figure 4, will help to explain. A liner adds xed clearance which reduces capacity. This capacity reduction can be substantial in shorter stroke cylinders. The addition of the liner also reduces piston displacement, which when combined with the effect of the additional xed clearance, results in a signicant reduction in a cylinder’s capacity capability. The Figure 4 chart takes both into account. For example consider a 300 mm liner bore diameter (11.8 inch, “300” on the horizontal axis). This is actually a 325 mm (12.8 inch) diameter bore cylinder with a 12.5 mm (0.5 Figure 4 Chart showing change in capacity capability due to the inch) thick liner installed, so the addition of a liner capacity capability has been reduced from that of a 325 mm bore cylinder to that of a Rockwell C and provides substantial case depth 300 mm. Also, the addition of the liner has with a hardness of approximately 30 Rockwell C increased the xed clearance volume, which at a depth of 0.15 mm (0.006 inch). reduces the volumetric efciency further reducing the capacity capability. For the 300 mm liner Cooled versus Non-cooled bore example this capacity capability reduction API Standard 618 also requires compressor is on the order of 35 percent for 76 mm (3 inch) cylinders to have provision for cooling. Figure 5 stroke, and is still signicant for a 457 mm (18 is a cylinder with a liner and a cooling jacket and inch) stroke at about a 17 percent reduction. The Figure 6 is a cylinder without a liner or a cooling chart assumes a compression ratio of 2.5 and a jacket. Like liners, cooling is not required for a gas adiabatic exponent of 1.4. Assumed liner compressor cylinder to compress gas. API Standard 618 mentions one instance where thickness is 9.5 mm (0.375 inch) up to 254 mm liner bore (10 inch) and 12.5 mm (0.500 inch) provision for cooling may be benecial. It 254 mm bore (10 inch) and larger following API mentions “when cylinders are operated while Standard 618 guidelines. unloaded for extended periods of time”. This For a given required compressor capacity, the refers to the situation when it may be required or addition of a liner requires a larger compressor benecial to operate a double-acting cylinder with by the percentage shown in the chart. Again both the head and crank ends deactivated at the referring to the 300 mm example, a 76 mm same time. The standard requires that a forced stroke compressor would have to be about 35 liquid coolant system be used when cylinders may percent larger if equipped with a liner. So an be required to operate fully unloaded. And it is end-user is buying a compressor about 35 correct that in some situations the cooling system percent larger just to have it equipped with lined may have the capacity to remove enough of the parasitic heat generated when the gas washes in cylinders. While a liner provides one method of protect- and out of the head and crank end compression ing A395 ductile iron from wear, other methods chambers to allow “operation fully unloaded for exist. Another possibility is to harden the extended periods of time”. It is not a given that unlined cylinder bore to improve the wear char- every compressor cylinder incorporating forced acteristics. One proven hardening method uses liquid coolant can operate fully unloaded for an the ion-nitride heat treat process. A full explana- extended period as the cooling system may not be tion of the process is beyond the scope of this capable of removing enough parasitic heat, potenarticle but it results in a hardness at the surface tially causing piston ring, wearband and of A395 ductile iron of approximately 55 compressor valve premature wear or failures.
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Figure 5 Drawing of a cylinder body with a liner and cooling jacket
Figure 6 Drawing of a cylinder body without a liner or cooling jacket
When discussing the cooling requirement, means there is reduced chance of a future strucend-users and manufacturers will often mention tural failure. the need to use the cooling jackets as heaters. The cooling jackets will be used to heat the cylinders Conclusion prior to startup to avoid liquid condensing out of Short stroke medium speed reciprocating the gas stream (warm saturated gas contacting compressors utilising unlined and non-cooled cold metal) and causing damage to the compres- compressor cylinder technology represent the sor valves or other components when the newest technology and are the “21st century compressor starts. While this certainly works and reciprocating compressors for downstream can be a valid reason for using cylinders with applications”. These compressors dominate the cooling (heating?) jackets, not every application upstream and midstream oil and gas industries encounters this issue and it’s not the only way to and are becoming more common and gaining start a compressor to avoid this condensation. wider acceptance downstream. Non-cooled cylinders have been used with However, many downstream end-users have success in the upstream natural gas industry for been reluctant to consider short stroke higher 50 years and the downstream industry for 20 speed compressors primarily because of concerns years. There can be no question that the technol- about reliability. Many nd it difcult to underogy works, especially considering that almost stand how a higher speed reciprocating every manufacturer offering reciprocating compressor can have equal reliability to a low compressors to the upstream and midstream speed, but it is possible and being proven every markets has models utilising non-cooled cylin- day. An example is two 4.1 MW (5500 horsepower) 146 mm (5.75 inch) stroke 713 rpm ders with one having shipped over 150 000. The benets to end-users of non-cooled cylin- packaged compressors operating in a hydrogen product application in a hydrogen plant in the der technology include: • Less capital cost as there is no cylinder jacket United States Gulf Coast area. Both have achieved over 24 000 hours of uninterrupted operation water system to buy. • Less operation and maintenance cost. There is and are scheduled for an overhaul at 32 000 no cylinder jacket water system to operate or hours. This is exactly the type of success being maintain. achieved with this 21st century technology. Although the basic design of a reciprocating • Higher quality compressor cylinder bodies. Non-cooled cylinder body castings are of signi- compressor has not changed much over the cantly higher quality due to the fact there is no years, manufacturing, design capabilities, cooling jacket complicating the casting. This also non-metallic materials and performance model-
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ling have. Modern reciprocating compressors are capable of higher rotating and piston speeds without compromising reliability. To paraphrase an ad from a United States automobile manufacturer, “these are not your father’s reciprocating compressors”.
LINKS More articles from: Ariel Corporation More articles from the following category: Rotating Equipment
Originally sourced from the August 2015 issue of Hydrocarbon Engineering.
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