Equipment Design

Reciprocating Compressor Construction

Understanding the major components of reciprocating compressors: frame design, power train, compression components, and sealing systems that enable reliable gas compression.

Rod Load Calculator View cylinder types

Frame types

Integral & Separable

Engine-driven integral or motor/engine-driven separable designs.

Cylinder action

Single or Double

Compression on one or both sides of the piston.

Rod load limits

Frame-Specific

Critical limit that sizes frame and determines capacity.

Contents

This guide covers:

  • Major compressor components
  • Frame and cylinder configurations
  • Sealing systems and materials
  • Design considerations

1. Overview

A reciprocating compressor consists of several major systems working together to compress gas reliably and efficiently. Understanding these components is essential for proper sizing, operation, and maintenance.

Component Groups: The major systems are the frame (structural support), power train (crankshaft, connecting rods), crosshead assembly, distance piece (containment), cylinders, and sealing systems (packing, rider bands).

Compressor Types

Type Description Application
Integral Compressor and engine share common frame/crankcase Pipeline, large installations
Separable Compressor driven by separate driver via coupling General purpose, packages
High-speed 900-1800 RPM, compact design Gas gathering, packages
Low-speed 200-450 RPM, large bore/stroke Pipeline, process

2. Frame & Crankcase

The frame is the structural foundation of the compressor, housing the crankshaft, supporting the cylinders, and containing the lubrication system. Frame design determines the rod load capacity and overall machine capability.

Frame Construction

  • Material: Cast iron or fabricated steel
  • Design: Ribbed structure for rigidity and vibration resistance
  • Mounting: Machined pads for cylinder and driver attachment
  • Lubrication: Integral oil galleries and sump

Frame Configurations

Layout

Horizontal Balanced-Opposed

Cylinders on both sides of crankshaft. Excellent balance, most common design.

Layout

Vertical

Cylinders above crankcase. Small footprint, used for small units.

Layout

L or V Configuration

Angled cylinders. Compact design for multi-throw machines.

Frame Rating

Each frame size has a maximum allowable rod load, which is the primary factor limiting cylinder size and pressure differential:

Rod Load Limits: The frame rod load rating must never be exceeded. This limit is typically expressed in pounds (tension and compression) and determines the maximum pressure differential and cylinder bore size that can be used.

3. Power Train

The power train converts rotary motion from the driver to reciprocating motion of the pistons. It consists of the crankshaft, connecting rods, and associated bearings.

Crankshaft

  • Forged or cast steel construction
  • Multiple throws for multi-cylinder arrangements
  • Counterweights for balance
  • Integral flywheel effect for speed regulation

Connecting Rods

Connect the crankshaft throws to the crossheads, transmitting power while accommodating the angular motion:

Component Function Material
Rod body Transmits force between bearings Forged steel
Big end bearing Connects to crankshaft throw Babbitt-lined steel
Small end bearing Connects to crosshead pin Bronze bushing

4. Crosshead & Guide

The crosshead converts the angular motion of the connecting rod into pure linear motion for the piston rod. This is a critical design feature that distinguishes industrial reciprocating compressors from simple single-acting designs.

Purpose: The crosshead absorbs the side thrust from the connecting rod angle, allowing the piston rod to move in a straight line. This protects the cylinder and packing from side loads.

Crosshead Components

  • Crosshead body: Cast or forged, carries the crosshead pin
  • Crosshead pin: Connects to connecting rod small end
  • Shoes/slippers: Slide along the crosshead guide
  • Guide: Machined ways in frame that constrain crosshead motion

Lubrication

Crosshead components require continuous lubrication:

  • Pressurized oil to crosshead pin bearing
  • Splash or forced lubrication to shoes
  • Oil scraper rings prevent migration to distance piece

5. Distance Piece

The distance piece is the compartment between the cylinder and the frame that provides separation for safety and contamination control. It's a critical component for hazardous gas service.

Distance Piece Types

Type Compartments Application
Standard Single open Non-hazardous gases
Type A (API 618) Single, may be open Mild service
Type B (API 618) Two, outer open Prevent process gas in crankcase
Type C (API 618) Two, both closed Toxic/hazardous gases
Type D (API 618) Three compartments Extreme hazard service
Hazardous Service: For toxic gases (H2S, CO, etc.), Type C or D distance pieces are typically required to prevent process gas from reaching the crankcase area where it could create safety hazards.

6. Cylinders

Cylinders contain the compression process and house the valves, packing, and piston. Cylinder design significantly affects capacity, efficiency, and reliability.

Cylinder Action

Configuration

Single-Acting

Compression on one side of piston only. Simpler, lower capacity.

Configuration

Double-Acting

Compression on both sides. Standard for industrial use, twice the capacity.

Cylinder Construction

  • Body: Cast iron, nodular iron, or steel
  • Liner: Replaceable wear surface (optional)
  • Heads: Inner (frame end) and outer (head end)
  • Valve ports: Machined pockets for valve assemblies
  • Clearance pockets: Variable volume for capacity control

Cylinder Types by Service

Type Pressure Features
Standard Up to 1500 psig Cast construction, standard clearance
Pipeline Up to 1500 psig High clearance (40-100%), low ratio
High pressure 1500-10,000 psig Forged steel, plunger type
Non-lubricated Varies PTFE piston rings, special materials

7. Pistons & Rods

The piston and rod assembly transmits motion from the crosshead to compress gas in the cylinder. These components must withstand high loads and temperatures while maintaining sealing.

Piston Construction

  • Body: Cast iron, aluminum, or steel depending on service
  • Rings: Metallic or non-metallic for sealing against cylinder wall
  • Rider bands: Support piston weight, prevent metal-to-metal contact

Piston Rod

Connects piston to crosshead, passing through the packing:

Feature Specification
Material Alloy steel, hardened and ground
Surface finish 16 microinch or better in packing area
Coating Chrome, tungsten carbide, or ceramic for wear
Connection Threaded to piston, bolted to crosshead
Rod Reversal: The rod must experience both tension and compression each revolution to ensure proper lubrication of the crosshead pin bearing. This requirement can limit operating conditions.

8. Valves

Compressor valves are automatic check valves that control gas flow into and out of the cylinder. They are among the most critical and maintenance-intensive components.

Valve Types

Type

Plate Valves

Single or multiple concentric plates. Simple, reliable, best for low speeds.

Type

Ring Valves

Concentric sealing rings. Good flow, moderate speed applications.

Type

Poppet Valves

Individual disc elements. Excellent for high speed, high pressure.

Valve Components

  • Seat: Stationary ring with flow ports
  • Sealing element: Plate, ring, or poppet that opens/closes
  • Guard/stop plate: Limits lift travel
  • Springs: Return element to closed position
Valve Life: Valves are wear items that require regular inspection and replacement. Valve life depends on gas cleanliness, operating conditions, and speed. Dirty gas, liquids, or high speeds dramatically reduce valve life.

9. Packing & Seals

Packing seals the piston rod where it exits the cylinder, preventing gas leakage to atmosphere. Proper packing design and maintenance is essential for safety and emissions compliance.

Packing Case Design

A typical packing case contains multiple ring sets:

  • Pressure breaker rings: Reduce pressure in stages
  • Sealing rings: Primary gas containment
  • Oil wiper rings: Prevent oil migration to gas side
  • Backup rings: Support primary rings at high differential

Packing Materials

Material Temperature Application
PTFE (Teflon) Up to 500°F Non-lubricated service
Bronze-filled PTFE Up to 500°F Improved wear resistance
Carbon-filled PTFE Up to 500°F Low friction, dry service
Metallic (bronze, iron) Up to 600°F Lubricated service, high pressure

Packing Vents and Purges

  • Vent: Routes leakage to safe location or flare
  • Purge: Inert gas buffer for hazardous service
  • Coolant: Circulating oil or water for high temperature

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