Reciprocating Compressor Performance Analysis

1. Overview

Performance analysis of reciprocating compressors involves measuring and calculating key parameters to ensure the machine is operating efficiently and within design limits. Regular analysis helps identify developing problems before they cause failures.

Performance Metrics: The key performance indicators are volumetric efficiency (capacity), adiabatic efficiency (power), discharge temperature, and rod loading (mechanical limits).

Analysis Methods

Method	Measures	Application
P-V diagram	Cylinder pressure vs. volume	Valve condition, leakage, work
Flow measurement	Actual throughput	Volumetric efficiency
Power measurement	Driver input power	Overall efficiency
Temperature monitoring	Discharge, valve temps	Valve condition, ratio
Vibration analysis	Mechanical condition	Bearing, foundation issues

2. P-V Diagrams (Indicator Cards)

The pressure-volume diagram is the most powerful tool for analyzing reciprocating compressor performance. It shows cylinder pressure plotted against piston position through a complete cycle.

Theoretical P-V Diagram

An ideal cycle consists of four phases forming a closed loop:

1→2

Compression

Polytropic curve, pressure rises as volume decreases.

2→3

Discharge

Horizontal line at discharge pressure, gas expelled.

3→4

Expansion

Clearance gas re-expands, mirror of compression.

4→1

Suction

Horizontal line at suction pressure, cylinder fills.

Interpreting Real P-V Cards

Actual P-V diagrams deviate from ideal due to valve losses, pulsation, and leakage:

Observation	Cause	Action
Rounded corners at valve events	Normal valve losses	None if within spec
Suction line slopes down	Suction valve restriction	Inspect/clean valves
Discharge line slopes up	Discharge valve restriction	Inspect/replace valves
Compression curve shifted left	Ring or valve leakage	Inspect rings, valves
Oscillations on suction/discharge	Pulsation	Check dampeners, piping

Area = Work: The area enclosed by the P-V curve represents the indicated work per cycle. Comparing actual to theoretical area gives indicated efficiency.

3. Volumetric Efficiency

Volumetric efficiency (VE) is the ratio of actual gas volume delivered to the theoretical piston displacement. It's the primary indicator of compressor capacity and health.

Volumetric Efficiency: VE = 1 - C × [(Pd/Ps)^(1/k) - 1] Where: C = Clearance volume / swept volume (fraction) Pd = Discharge pressure (absolute) Ps = Suction pressure (absolute) k = Specific heat ratio Typical values: 70-95% depending on ratio

Factors Affecting Volumetric Efficiency

Clearance volume: Higher clearance = lower VE
Compression ratio: Higher ratio = lower VE
Valve losses: Restriction reduces effective VE
Ring leakage: Blow-by reduces delivered volume
Gas heating: Suction heating reduces density
Compressibility: Z-factor variation affects VE

Clearance Effects

Compression Ratio	VE at 10% Clr	VE at 20% Clr	VE at 40% Clr
2:1	92%	84%	68%
3:1	86%	72%	44%
4:1	80%	60%	20%
5:1	74%	48%	-4%*

*Negative VE means cylinder cannot compress at this ratio with this clearance

Pipeline Cylinders: Pipeline service cylinders have high clearance (40-100%) by design for low ratio service. They cannot achieve high compression ratios without unacceptable VE loss.

4. Rod Load Analysis

Rod load is the force transmitted through the piston rod to the crosshead. It consists of gas load (from pressure differential) and inertia load (from reciprocating mass acceleration). Total rod load must stay within frame limits.

Gas Load Calculation

Double-Acting Cylinder Rod Loads: Compression Load = Pd × A_HE - Ps × A_CE Tension Load = Pd × A_CE - Ps × A_HE Where: Pd = Discharge pressure (psia) Ps = Suction pressure (psia) A_HE = Head end area (in²) A_CE = Crank end area = A_HE - A_rod (in²)

Inertia Load

The reciprocating mass creates additional load that varies with crank position:

Inertia Load: F_inertia = M × R × ω² × (cos θ + R/L × cos 2θ) Where: M = Reciprocating mass (piston, rod, etc.) R = Crank radius (half stroke) ω = Angular velocity (rad/sec) L = Connecting rod length θ = Crank angle

Rod Reversal

The rod must experience both tension and compression each revolution to ensure proper lubrication of the crosshead pin bearing:

Reversal Requirement: If the rod stays in compression (or tension) for the entire cycle, the crosshead pin bearing oil film cannot be renewed. API 618 requires minimum reversal for reliable operation.

Condition	Cause	Effect
No reversal	Low suction P, high discharge P	Crosshead bearing damage
Marginal reversal	Operating near limits	Reduced bearing life
Good reversal	Balanced loading	Normal bearing life

5. Power Calculation

Compressor power can be calculated theoretically or measured. Understanding both methods helps identify inefficiencies and validate performance.

Indicated Horsepower

Power calculated from P-V diagram (work per cycle × cycles per unit time):

Indicated HP (from P-V card): IHP = (Area × L × N) / 33,000 Where: Area = P-V diagram area (psi × in³) L = Stroke (ft) N = Cycles per minute (RPM for single-acting) 33,000 = ft-lbf/min per HP

Theoretical Adiabatic Power

Adiabatic Horsepower: HP = (k/(k-1)) × (Ps × Qs / 229) × [(Pd/Ps)^((k-1)/k) - 1] Where: Ps = Suction pressure (psia) Qs = Suction volume flow (acfm) k = Specific heat ratio

Brake Horsepower

BHP is the power delivered to the compressor shaft, accounting for all losses:

Brake HP: BHP = IHP / η_mechanical Where: η_mechanical = Mechanical efficiency (typically 0.90-0.95) Or from adiabatic: BHP = HP_adiabatic / η_overall η_overall = η_volumetric × η_adiabatic × η_mechanical

6. Troubleshooting Guide

Common performance problems and their diagnostic indicators:

Low Capacity

Symptom	Possible Cause	Check
Low flow, normal pressures	Suction valve leakage	P-V card shows compression curve shift
Low flow, high discharge temp	Discharge valve leakage	Valve temperature high
Low flow, normal temps	Ring blow-by	P-V card, ring inspection
Gradual capacity loss	Packing leakage	Vent flow measurement

High Power Consumption

Symptom	Possible Cause	Check
High power, normal capacity	Valve pressure drops	P-V card rounded corners
High power, low capacity	Internal recirculation	Valve leakage test
Erratic power readings	Liquid carryover	Separator operation

Temperature Monitoring: Discharge and valve temperatures are excellent early warning indicators. A valve running 20-30°F hotter than its pair often indicates leakage or damage before capacity loss is noticeable.