Cutting Edge Compression

By: Greg Phillippi, Director of Process Compressor Marketing and Sales, Ariel Corporation

The market for reciprocating process gas compressors in North America is not as robust as it was in the first decade of this century, primarily because refinery demand for hydrogen has not grown. This is a result of the “shale revolution.” Crude oil produced from shale wells is light and sweet and requires less hydrogen to process into low sulfur gasoline and low sulfur diesel fuel. In the first decade, refiners worked on projects to make ultra-low sulfur diesel fuel (ULSD, 15 ppm sulfur) and Tier 3 low sulfur gasoline (10 ppm sulfur), 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 EPA. Eventually, the manufacture of Tier 3 gasoline will create additional demand for hydrogen and hydrogen compression. Also, in the first decade, many refineries were being reconfigured to process additional heavy sour crude oil because it sold at a discount. Now refiners are spending capital to reconfigure their facilities to process more light sweet crude because it is local, readily available, and may sell at a discount to the world price.

Currently, there is significant investment being made in the downstream petrochemical market in the United States, again the result of the shale revolution but with natural gas rather than crude oil. There is an overabundance of natural gas and natural gas liquids in the U.S. therefore, their price is low and should remain low for quite some time. Demand for natural gas will have to grow substantially to catch up to supply and have the price rise. This creates a very attractive environment for petrochemical companies because they use natural gas as fuel and natural gas and natural gas liquids as feedstock. However, the majority of the process gas compression applications in these facilities will be filled by centrifugal rather than reciprocating compressors because the applications tend to be a higher power and compress heavier mole weight gases. High mole weight gases are ideal for centrifugal compressors, while low mole weight gases (hydrogen) are not.

Given the low price environment the oil and gas industry finds itself in today, with the focus on reducing capital expenditure, packaged short-stroke moderate speed reciprocating compressors have become much more prevalent in North America process gas applications. Where long-stroke low-speed recips have dominated the market, today, the concept of using a compression module (package) that’s built-in a shop rather than at site 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 lower installed cost and reduced project cycle times.

Some definition is in order. Several manufacturers offer reciprocating compressor designs with short strokes, typically in the 76 to 229 mm (3 to 9 inch) range, with rotating speeds varying from 750 to 1800 rpm (known as “high” speed compressors, the smaller machines having the higher speeds). Rated rod loads of these machines range from 22 kN to over 445 kN (5,000 lbf to over 100,000 lbf). For the upstream and midstream, natural gas markets, they are typically driven by a natural gas-fueled 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% to 60% of the compressor’s rated speed, or typically in the range of 600 to 1000 rpm. This reduction accomplishes two things: 1) reduces the number of compressor valve opening and closing cycles benefiting compressor valve life, and 2) 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 upstream and midstream markets requires very regular preventative maintenance that requires the engine to be shut down. Roughly every three months (about every 2200 hours and 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 do some minor maintenance to the compressor while the engine is off. There is not always something that needs 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 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 that requires no maintenance, thus no regular shutdowns. The typical refinery desires a recip compressor to run for three years (about 26,000 hours) completely uninterrupted. Of course, this is impossible with a natural gas-fueled engine as the driver. So the “aggressive” upstream compressor design must be reconfigured to make it appropriate for the downstream process application and 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 upstream and midstream markets are in the range of 5.0 to 6.0 m/s (1000 to 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. It is a good reminder to note that 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 recip compressor running at virtually any rotating or piston speed will be more reliable if the gas is very clean and the temperatures conservative.

Piston speeds of recips typically applied in the downstream market are in the range of 3.6 to 4.3 m/s (700 to 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 (three years).

Figure 1 is a chart that shows the average piston speed of four arbitrary short stroke lengths, 76, 127, 178 and 229 mm (3, 5, 7 and 9 inches) over a range of 0 Hz and 60 Hz electric motor speeds. Circles mark what might be the rated rotating and piston speed and triangles the combinations of stroke and rotating speed that result in a piston speed that would be acceptable for a process applicaA process end user will find the following (Figure 2) specific combinations of stroke and rotating speed acceptable from a piston speed perspective.tion.

A process end user will find the following (Figure 2) specific combinations of stroke and rotating speed acceptable from a piston speed perspective.

Of course, these are arbitrary stroke lengths and not specific for any manufacturer. Figures 1 and 2 show how reducing the rotating speed results in an acceptable piston speed that meets the process end-users requirement for reliability.

A question commonly raised by process rotating equipment engineers considering a moderate speed recip concerns how a compressor valve operating at 720 rpm (for example) could last as long as one operating at 327 rpm (again, for example). Both can have the same piston speed - the 327 rpm compressor might have a stroke of 381 mm (15 inch) with a piston speed of 4.2 m/s (818 fpm) and the 720 rpm machine a stroke of 171 mm (6.75 inch). The answer is that modern valve technology and materials provide the opportunity. Many of the compressor valves applied today in short-stroke high-speed compressors that achieve 12,000 hours or more of life use a lift (the distance the seal element travels from close to open) of 2.6 mm (0.102 inch). This relatively high lift is required to achieve reasonable energy efficiency. When the compressor operates at speed 40% to 50% slower than rated, and the lift is lowered (to maybe 1.5 mm, 0.060 inch), or even lower in the case of hydrogen, the valve can last well into the 26,000-hour range. Lower lift generally increases valve life but at the cost of efficiency. Note that efficiency used in this context is relative to the energy required to compress a certain volume of gas.

The disadvantage to the compressor manufacturer of slowing the rotating speed is that the compressor cannot be loaded to its maximum power, so the offering may not be quite as commercially attractive as if the full piston speed and power were being utilized. The full rated rod load (torque) can be utilized but not the power. In a sense, the end-user is buying a compressor that potentially could compress 50% to 100% more gas (depending on the speed reduction) for just about the same cost. But it will provide the desired reliability.

Even with this commercial disadvantage, the packaged moderate speed recip offers significant capital cost reduction. First, the bare compressor is smaller for the same power, thus costing less - a result of the higher rotating speed. Second, the compressor package is built in a shop rather than a site, as is typical of a long stroke low-speed block mounted machine. The shop is clean and organized, labor is less expensive, fewer labor hours are required, and inclement weather causes no fabrication delay. The savings in total installed cost can be as much as 50%.

The packaged short-stroke moderate speed reciprocating compressor has proven successful in this market. An independent refiner in the USA recently purchased two 1250 horsepower two throw 720 rpm short-stroke hydrogen recycle compressors to be used in a low sulfur gasoline project. These two compressors will be in addition to two 2500 horsepower four throw 720 rpm hydrogen compressors installed in 2007 as part of an ultra-low sulfur diesel fuel project. The savings in total installed cost cannot be ignored in this low price oil and gas environment, especially with no compromise in reliability.