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The Importance of the Crankcase in Reciprocating Compressors

The crankcase, also known as the frame, is an essential component of a reciprocating compressor that houses all its rotating parts, including the connecting rod and crosshead. The frame’s rated horsepower is determined by the maximum horsepower that can be efficiently transferred through the crankshaft to the cylinders.

The Importance of the Crankcase in Reciprocating Compressors - KB Delta

Reciprocating compressors are among the most widely used compression machines available in various types and sizes. Also known as piston compressors or constant-volume variable pressure equipment are positive-displacement tools that efficiently compress gases via a piston and push the gases at high pressure, following reciprocating rules.

Reciprocating compressors are composed of several components, without which they will never function as designed. One of the most crucial components of the reciprocating compressor is the frame, otherwise known as the crankcase.

This article focuses on the frame and its importance to the overall structure of the reciprocating compressor.


The Reciprocating Compressor Frame A.K.A. Crankcase

Manufacturers of reciprocating compressors rate frames for a maximum frame and continuous horsepower, rod load, and speed. The frame – also called the crankcase – of a reciprocating compressor is a rugged and heavy casting that houses every rotating component of the machine. The crosshead guide and cylinder are mounted on the crankcase.

The compressor frame is shaped like a rectangle or square. The primary purpose of a reciprocating compressor crankcase is to provide adequate support to the crankshaft.

The rated horsepower of the crankcase is determined by the maximum horsepower that can efficiently be transmitted via the crankshaft to the cylinders of the reciprocating compressor. The rod load refers to the force enforced on the piston rod by the pressure differential between 2 ends of the piston.

Every reciprocating compressor frame is designed explicitly for a maximum number of cylinders. However, the machine’s frame doesn’t indicate the compressor’s duty or number of stages.

Some reciprocating compressors lubricate their frame running gear with integral, shaft-driven pumps, while others come with a more extensive, skid-mounted lubrication system. The running gear is housed in the compressor crankcase. It consists of the connecting rod and crosshead that connect the piston rod to the crankshaft and convert all its rotary motion into a reciprocating linear motion.


Different Categorizations of the Crankcase

An independent frame can be used for a wide range of sizes of compressor cylinders as well as for different applications. Compressor crankcases are typically categorized as integral-type or balanced-opposed (separable).

  • Integral-Type Crankcase

Integral-type crankcases are distinguished by having the engine-power cylinders and compressor cylinders brilliantly mounted on the same crankcase and driven by the same crankshaft.

  • Balanced-Opposed (Separable) Crankcase

Separable (balanced-opposed) crankcases are distinguishable by an adjacent pair of crank throws 180 degrees out of phase. The cranks are arranged so that an opposing piston’s motion effectively balances every piston’s motion.

Other Importance Components of the Reciprocating Compressor

Here are some of the most vital components of a reciprocating compressor:


The Crankshaft

The crankshaft for a large reciprocating compressor – typically above 200 horsepower (150kW) – is forged steel. In comparison, the crankshaft of compressors less than or equal to 200 horsepower (150kW) is made of ductile iron.

Crankshafts of reciprocating compressors are directly connected to the drive to transfer the rotary motion. When force lubrication is utilized, drilled oil passage is provided within the crankshaft.

Crankshafts come with removable balance weights to adequately compensate for the rotary unbalance and reciprocating unbalances. The crankshaft should be balanced dynamically above 800 revolutions per minute (rpm).


Distance Piece

The distance piece is a severable housing connecting the cylinder to the frame. The distance piece may be available as a single, double, or extra-long type. This compressor component may be closed or open and have multiple compartments.

The primary purpose of longer-distance pieces is to efficiently prevent lubricants from entering the cylinder from the crankcase. Without this action, the lubricant that enters the machine via the cylinder will contaminate the gas to be compressed.

But for lubricated cylinders, the distance piece keeps the synthetic lubricant from getting corrupted by the crankcase lubricant.

The compartments within the distance piece collect and efficiently control packing leakage, especially when the gas is flammable or toxic. The leakage can be an inert buffer gas used to prevent toxic gas leakage into the atmosphere.

Distance pieces are categorized, as per API-618, into types A, B, C, and D.


The Cylinder

The cylinder is the pressure-retaining aspect of a reciprocating compressor. This vital component accommodates the discharge and valves. Cylinders in small capacity reciprocating compressors – i.e., up to 100kW – are directly connected to the crankcase, i.e., the compressor frame.

But the cylinder of a larger capacity reciprocating compressor – i.e., above 100kW – is connected to the compressor crankcase via the distance piece.

Most reciprocating compressor cylinders in the processing industries are often equipped with standardized liners. The primary purpose of the replaceable liner is to offer a renewable surface to the worn-out portion of the cylinder.

This will save users lots of money instead of replacing a complete cylinder as soon as the bore has already been worn out or metal scoring.

Large capacity compressor cylinders come with replaceable linear. But in smaller capacity reciprocating compressors, especially for single-stage models, all replaceable liners are not so economical and, therefore, may not be readily available.

The cylinders are outfitted for the cooling arrangement to minimize the temperature rise during the compression cycle. The cooling arrangement is typically through water jackets.

If water jacketing within the cylinder is not executable, fans are provided in the cylinders for air cooling. However, in most cases, the procedure is severely limited to special-purpose or small machines.


Cylinder Materials

The cylinder in a reciprocating compressor, especially the low-pressure, larger versions, is usually made of cast iron. The high-pressure but smaller cylinders are constructed of steel. But sometimes, ductile or nodular iron can be employed instead of cast iron.

But for hydrocarbon services, steel is the most desirable material for constructing the reciprocating compressor cylinder. However, steel is not globally available.

Clearance pockets are included in the cylinders for capacity control. But the clearance pocket is employed in the first stage cylinder, especially in the case of multistage reciprocating compressors.



A reciprocating compressor is a powerful machine used in the processing and the oil and gas industry. It is practically mounted on a frame, otherwise known as the crankcase. The crankcase houses most of the components of this positive displacement machine.


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