In this article, we will discuss the four major types of Reciprocating Compressor distance pieces based on API 618.
What is distance Piece?
The distance piece provides separation between the reciprocating compressor cylinder and the compressor frame. Distance pieces can be either a single- or double-compartment arrangement, the selection is based on the property of compressed gas.
If oil contamination is a concern, an oil slinger can be provided to prevent packing lube oil from entering the compressor frame. For toxic service, a two-compartment design may be used. No part of the piston rod enters both the crankcase and the compartment adjacent to the gas cylinder.
Four Major Types of Reciprocating Compressor Distance Pieces
As per API 618, the distance pieces are classified as four types, they are
- Type A
- Type B
- Type C
- Type D
Type A
It is a short, single-compartment distance piece. This type of distance piece is mainly used for lubricating service where oil carryover is acceptable with the compressed gas. Usually, the oil carry-over is at the wiper packing and cylinder pressure packing.
In a reciprocating compressor, part of the piston rod may alternately enter the crankcase (crosshead housing) and the gas cylinder pressure packing. This arrangement shall not be used when cylinders are lubricated with synthetic oils.
Distance pieces are used for
- Nonflammable or non-hazardous gases application
- Oil carry-over by the gases is acceptable in the process application.
Type B
The Type B distance piece is Long, single-compartment distance piece. This type is used for non-lubricated service or lubricated service where oil carryover is not acceptable in the compressed gas. No part of the piston rod shall alternately enter the crankcase (crosshead housing) and the gas cylinder pressure packing.
Normally the piston rod is fitted with an oil slinger of spark resistant material. The oil slinger is preferably a split design for easy access to the piston packing.
Type C
The Type C distance piece consisting of two compartments. Both the compartments are the Long/long type. This type of distance piece designed for the application of flammable, hazardous, or toxic gases.
No part of the piston rod shall alternately enter the wiper packing, intermediate partition packing, and the cylinder pressure packing. (Refer figure)
In Type C distance piece, segmental packing shall be provided between two compartments. Depend upon the application, the distance piece has a provision for lubricating the segmental packing.
For flammable or hazardous gas application, buffer gas injection provision provided. It is to prevent the compressed gas leak in the atmosphere.
The Type C distance piece normally consists of two oil slingers. One in each compartment is not normally used in process compressors. This type of distance piece is used only for special services such as Hydrocarbon and oxygen service.
Disadvantages:
- This distance piece design causes the overall length of the gas end assembly to become excessively large. This causing the overall width of the reciprocating compressors to become large, and therefore increasing foundation requirements.
- Distance Piece can cause piston rod diameters to increase because of the column effect of excessively long piston rods.
Type D
The type D distance piece consists of two compartments, similar to type C. The only difference is one of the compartments is short and another one is long. Type D distance piece used for compressor handling flammable, hazardous, or toxic compressed gases.
No part of the piston rod shall alternately enter the wiper packing and the intermediate partition packing. Segmental packing is provided between the two compartments for easy installation and maintenance. Depend upon the application, the distance piece has a provision for lubricating the segmental packing.
Also, buffer gas provision is provided in the distance piece to contain the compressed gas leak into the atmosphere.
Normally the provision for buffer gas requirement and type of buffer gas is specified to compressor manufacturer by the user.