Reciprocating compressors play an essential role in many industries. Commonly applied in the oil & gas industry, they are used to compress and transport gases efficiently, for example. Another example is using them for the high-pressure hydrogen required to remove the sulfur contained in crude oil.
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They are also known as piston compressors, which gives an idea of the working principle. The machine compresses or pumps gas through the reciprocating motion of pistons powered by the rotational motion of a crankshaft.
Japanese industrial group Kobe Steel — operating globally as Kobelco — offers a wide range of products and services. Back in , the company produced Japan’s first reciprocating compressor.
Hitoshi Takagi, Manager of the company’s Reciprocating Section in the Compressor Division, says, “Reciprocating compressors are fundamental machines that form the basis of an oil refinery.”
Kobe Steel also supplies reciprocating compressors for tankers transporting liquefied natural gas (LNG). Most conventional ships use diesel engines fueled by heavy oil. However, in recent years, dual-fuel engines that run on both heavy oil and natural gas have been put into practical use.
As air pollutants from ships are increasingly scrutinized, such dual-fuel engines can significantly reduce emissions of sulfur oxides (SOx), nitrogen oxides (NOx), and greenhouse gases to comply with stricter regulations.
Natural gas is a clean energy source that contains almost no impurities. It is transported in significant quantities by LNG tankers, utilizing the fact that it liquefies when cooled to 162 degrees Celsius below zero, resulting in a volume 1/600th that of gas.
However, it is challenging to prevent vaporization caused by external heat during transportation due to the extremely low temperature. As a result, the vaporized content ends up being discarded as waste. But reciprocating compressors increase the pressure of the vaporized LNG to 300 bar, so
it can be used as fuel for such dual-fuel engines, providing an environment-friendly fuel supply system.
Although offering rugged and efficient performance, reciprocating compressors generally have a large footprint and can cause many vibrations during operation. Kobe Steel uses Nord-Lock washers as well as Superbolt and Boltight series products to tackle these issues.
Using Nord-Lock wedge-locking washers improves safety and reliability, reducing the risk that vibrations loosen bolts. In addition, unlike many other companies, Kobe Steel performs vibration analysis and acoustic analysis in-house, resulting in high analysis accuracy thanks to the accumulated on-site data.
Kobe Steel’s reciprocating compressors are notably quiet. Takayuki Tomochika, Section Head of the Reciprocating Section for Kobe Steel’s Compressor Division, says: “In the 15 years I have been involved in the compressor business, I have never had a customer complain about the noise.”
Apart from the compressor, Kobe Steel's packaging services also include the drum, gas cooler, and other equipment, as well as piping and valves mounted on a base. This enables a compact design — a significant advantage given the limited space in LNG tankers.
These cramped conditions complicate the installation and maintenance of reciprocating compressors. Kobe Steel has found that using Nord-Lock Group tensioning solutions improves work efficiency. They use either Superbolt tensioners or Boltight hydraulic tensioning, depending on the workspace and efficiency of the fastening process.
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The Superbolt series is used to fasten the piston rod to the crosshead on the other side of the piston. Previously, a geared wrench with a long handle was used, but this proved problematic in confined spaces. With Superbolt, a regular torque wrench can fasten large diameter bolts, eliminating the need for space or heavy tools.
Kobe Steel uses Nord-Lock Group's products in machines that must keep running. “We can never stop production because it would cause serious losses,” says Kazuo Kusaba at Kobe Steel’s Machinery Business.
As compressors continuously get more reliable, the maintenance intervals have lengthened. “But if you can only access the inside of the equipment once every few years, you will not be able to build up the maintenance experience,” Kusaba explains. “This makes it difficult for experienced technicians to pass on their skills to the next generation.
However, with Superbolt and Boltight, we are now able to quantitatively manage the torque of bolt fastening, which is very encouraging.”
Kusaba adds that Nord-Lock Japan is different from other companies because they provide prompt technical support, including engineers, for all kinds of issues. He is thankful that Kobe Steel can rely on them.
Across all industries, environmentally friendly solutions are becoming the norm. The United Nation’s Sustainable Development Goals (SDGs) are crucial for the future and driving this change.
Kobe Steel is shifting its focus from oil refining to hydrogen energy for the next generation. Their hydrogen compressor units (HyAC® series) are already in use at hydrogen refueling stations, and product development is in progress with an eye to the future.
A reciprocating compressor is a mechanical device for increasing the pressure of a natural gas stream. Increased pressure is needed for the treatment or processing of natural gas and for promoting the movement of natural gas from production, gathering and boosting, and processing sites to customer distribution systems. Reciprocating compressors vent natural gas from piston rod packing systems during normal operation. The rod packing systems are designed to have a tight enough fit around the piston rod to reduce leakage; however, the systems cannot entirely eliminate leakage without binding the rod. During extended operation, the rod packing wears, and emissions increase over time.
Note that other components connected to the compressor, such as flanges, valves, and fittings, tend to leak more due to thermal and mechanical stresses associated with the compressor operation. For information about these emissions see the Equipment Leaks page. This page focuses on emissions from rod packing systems.
A reciprocating compressor, or positive displacement compressor, uses a plunger or piston to compress gas in the compressor cylinder as the engine drives the piston back-and-forth inside the cylinder. The engine turns the crankshaft, which connects to the piston rod via a “crosshead” that converts the engine rotating motion to the piston reciprocating motion. Piston rod packing systems are mounted where the rod passes through the inboard cylinder head and are used to maintain a relatively tight seal around the piston rod, preventing the compressed gas in the compressor cylinder from leaking, while allowing the rod to move freely. Reciprocating compressors often use multiple cylinders with gas moving from the inlet low pressure to intermediate pressure, and then to high pressure. Figure 1 shows a typical rod packing for reciprocating compressors.
Piston rod packing consists of a series of flexible rings that fit around the piston rod to create a seal against gas leakage. The packing rings are lubricated with circulating oil to reduce wear, help seal the unit, and draw off heat. Packing rings are held in place by a set of packing cups, normally one for each pair of rings, and kept tight against the rod by gas pressure. The number of cups and rings will vary depending on the compression chamber pressures. A “nose gasket” on the end of the packing case prevents leaks around the packing cups.
Reciprocating compressors vent natural gas from piston rod packing systems during normal operation. The rod packing systems are designed to have a tight enough fit around the piston rod to reduce leakage, but not so tight as to bind the rod and cause faster wear. Thus, the packing cannot entirely eliminate leakage from the inboard side of the cylinder. Under the best conditions, new packing systems properly installed on a smooth, well-aligned piston rod can be expected to vent a minimal amount of gas. Higher vent rates from rod packing are a consequence of fit, alignment of the packing parts, and wear. Rod packing emissions typically occur from four areas:
Gases are vented to the atmosphere either through an open “distance piece” connecting the cylinder to the “cross head” and “crank shaft,” or through a vent stack connected to the packing flange.
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