Cutting edge compression
Market for short stroke moderate speed reciprocating compressors in downstream process gas compression
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The market for reciprocating process gas compressors in North America is not as robust as it was in the first decade of the 21st century, primarily because the demand for hydrogen in refineries has not seen any growth. This is a result of the ‘shale revolution’. Crude oil produced from shale wells is light and sweet, requiring less hydrogen to process it into low sulphur gasoline and low sulphur diesel fuel. In the first decade, refiners worked projects to make ultra low sulphur diesel (ULSD) fuel (15 ppm sulphur) and Tier 3 low sulphur gasoline (10 ppm sulphur), causing demand for hydrogen and, therefore, hydrogen compression to increase substantially. Many smaller refiners have yet to configure their facilities to make Tier 3 gasoline, having delayed the capital expenditure through the purchase and use of Tier 3 gasoline credits issued by the US Environmental Protection Agency (EPA). Eventually, the manufacture of Tier 3 gasoline will create additional demand for hydrogen and hydrogen compression. Furthermore, during the first decade, many refineries were being reconfigured in order to process additional heavy sour crude oil, as it sold at a discount. Now, refiners are spending capital to reconfigure their facilities to process more light sweet crude as it is local, readily available and may sell at a discount to the global price.
Currently, significant investments are being made in the downstream petrochemical market in the US, again as a result of the shale revolution, but with natural gas rather than crude oil. There is an overabundance of natural gas and natural gas liquids (NGLs) in the US, therefore their price is low, and should remain as such for quite some time. Demand for natural gas will have to grow substantially in order to catch up to supply and achieve a price rise. This creates an attractive environment for petrochemical companies, as they use natural gas for fuel and both natural gas and NGLs as feedstock. However, the majority of the process gas compression applications in these facilities will be filled by centrifugal, rather than reciprocating, compressors as the applications tend to be higher power and compress heavier mole weight gases. High mole weight gases are ideal for centrifugal compressors, while low mole weight gases, such as hydrogen, are not.
Given the low price environment the oil and gas industry is experiencing today, with the focus on reducing capital expenditure, packaged short stroke moderate speed reciprocating compressors have become much more prevalent in North American process gas applications. Where long stroke low speed reciprocating compressors have dominated the market, today the concept of using a compression module (package) that has been built in a shop rather than onsite has gained acceptance with increased market share. The short stroke moderate speed reciprocating compressor lends itself quite well to the package concept due to its smaller size for an equivalent power.
These compressors offer the same reliability and features offered by the traditional long stroke low speed machines, at a lower installed cost and with reduced project cycle times.
Reciprocating compressor designs
Several manufacturers offer reciprocating compressor designs with short strokes, typically in the 76-229 mm (3-9 in) range, with rotating speeds varying from 750-1800 rpm (known as ‘high’ speed compressors, the smaller machines have the higher speeds). Rated rod loads of these machines range from 22 kN to over 445 kN (5000 lbf to over 100 000 lbf ). For the upstream and midstream natural gas markets, they are typically driven by a natural gas-fuelled engine and furnished as a gas compression package. What these manufacturers offer for the downstream process market is a ‘moderate’ speed version of these same high speed compressors. Rotating speed is reduced to provide the reliability required by the downstream user. Rotating speed is reduced to 50-60% of the compressor’s rated speed, or typically in the range of 600-1000 rpm. This reduction accomplishes two things: first, it reduces the number of compressor valve opening and closing cycles benefiting compressor valve life, and second, it reduces the piston speed, thus increasing the life of the piston rod packing, piston rings and wearbands.
This does not imply that a short stroke, reciprocating compressor running at its rated speed suffers poor reliability. The reliability demands of the upstream and midstream markets are different from downstream. The natural gas engine driver typically used in the up stream and midstream markets requires very regular preventative maintenance that requires the engine to be shutdown. Roughly every three months, or every 2200 hours (this varies with the manufacturer), the engine lubricating oil and spark plugs must be replaced. This offers an opportunity to replace leaking packing or a failing compressor valve, for example. So, every 2200 hours there is an opportunity to carry out some minor maintenance to the compressor while the engine is off. There is not always something that requires repair or replacing on the compressor at every engine shutdown, just that the opportunity exists, thus allowing the compressor design to be more ‘aggressive’.
‘Aggressive’ has been used in the sense that both the rotating and piston speed can be pushed higher, thus reducing the size and cost of the machine and the package.
Having the compressor shut down on a regular basis is not typical for the process user, nor is it desired. The driver is always an electric motor, which requires no maintenance, and therefore no regular shutdowns. The typical refinery desires a reciprocating compressor to run for three years (approximately 26 000 hours) completely uninterrupted. Of course, this is impossible with a natural gas-fuelled engine as the driver. So, the ‘aggressive’ upstream compressor design must be reconfigured to make it appropriate for the downstream process application, and able to provide the desired reliability. The primary modification is to apply the compressor at the previously mentioned reduced rotating speed.
State of the art maximum piston speeds for the high speed compressors used today in the up stream and midstream markets are in the range of 5-6 m/s (1000-1200 fpm). At these piston speeds, the piston rings and wearbands will routinely achieve a life of 12 000 hours or more, with the piston rod packing typically having to be replaced first. Many factors enter into the achievable life of reciprocating compressor wear components, being most influenced by the cleanliness of the gas stream (entrained solids and liquids) and temperature. Any reciprocating compressor running at virtually any rotating or piston speed will be more reliable if the gas is very clean and the temperature is conservative.
Piston speeds of reciprocating compressors, typically applied in the downstream market, are in the range of 3.6-4.3 m/s (700-850 fpm), no matter the stroke length. This more conservative piston speed provides the possibility that the piston rings, wearbands and piston rod packing will last the desired minimum of 26 000 hours, or three years.
Figure 1 shows the average piston speed of four arbitrary short stroke lengths, 76, 127, 178 and 229 mm (3, 5, 7 and 9 in) over a range of 50 Hz and 60 Hz electric motor speeds. Circles mark what might be the rated rotating and piston speed, while triangles mark the combinations of stroke and rotating speed, which result in a piston speed that would be acceptable for a process application.
Table 1 lists the specific combinations of stroke and rotating speeds acceptable from a piston speed perspective. Of course, these are arbitrary stroke lengths, and not specific for any manufacturer. Figure 1 and Table 1 demonstrate how reducing the rotating speed results in an acceptable piston speed that meets the process end user’s requirement for reliability.
A question commonly raised by process rotating equipment engineers considering a moderate speed reciprocating compressor concerns how, hypothetically, a compressor valve operating at 720 rpm could last as long as one operating at 327 rpm. Both can have the same piston speed – the 3 27 rpm compressor may have a stroke of 381 mm (15 in) with a piston speed of 4.2 m/s (818 fpm), and the 720 rpm machine a stroke of 17 1 mm (6.75 in). The answer is that modern valve technology and materials provide the opportunity.
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