Compressor Integrity:
Reciprocating and Centrifugal Systems

Why Compressor Integrity Fails

Most compressor failures are not sudden. They accumulate. A valve running with reduced flow efficiency for months. A piston rod with a slow-developing misalignment absorbing load asymmetrically. A lube oil system running slightly off-temperature, marginalising film thickness on every cycle. The machine runs until it doesn't — and the post-failure investigation finds evidence of a condition that was present and detectable well before the event.

The distinction between reciprocating and centrifugal compressors is not just mechanical — it changes the failure mode profile, the monitoring strategy, and the intervention criteria. Both types demand structured integrity programs. Neither type is forgiving of deferred maintenance in high-cycle or high-consequence service.

Reciprocating Compressors — API 618

API 618 governs the design, materials, fabrication, inspection, and testing of reciprocating compressors for petroleum, chemical, and gas industry services. It establishes the basis for pulsation control, mechanical design, and pressure boundary integrity. RISL applies API 618 as the primary reference for all reciprocating compressor integrity assessments.

Critical Components and Failure Modes

Reciprocating compressors are positive-displacement machines. Each component in the compression cycle — valves, piston, piston rod, crosshead, connecting rod, crankshaft — carries a defined load in a defined direction at a defined frequency. When any of these components degrades, the load redistribution is measurable.

• Suction and discharge valves: The highest-frequency wear item in any reciprocating compressor. Valve failure is the leading cause of unplanned shutdown. Leak-by increases cylinder temperature, reduces volumetric efficiency, and drives thermal load into adjacent components. Valve condition monitoring via cylinder pressure analysis detects valve leakage before it progresses to failure.
• Piston and piston rings: Ring wear causes blow-by, reducing compression efficiency and contaminating the crankcase. API 618 establishes clearance limits and ring replacement criteria.
• Piston rod and rod packing: Piston rod runout is a direct indicator of crosshead alignment and bearing condition. Rod drop monitoring — measuring vertical displacement of the rod relative to the packer — is a standard predictive indicator. Packing leakage in sour gas or toxic service triggers regulatory notification requirements.
• Crosshead and crosshead pin: Crosshead wear increases side loading on the piston rod, accelerating rod and packing degradation. Clearances must be maintained within manufacturer limits and API 618 guidance.
• Crankshaft and main bearings: Bearing condition is monitored through lube oil analysis and vibration. Crankshaft deflection measurements detect misalignment and bearing wear before catastrophic failure.
• Connecting rod and wrist pin: Fatigue-critical components. Dimensional inspection at major overhaul and NDT on connecting rods are standard integrity requirements.

Pulsation and Vibration — API 618 Design Approach

API 618 defines three design approaches (DA1, DA2, DA3) with increasing analytical rigor. DA3 requires full acoustic and mechanical simulation. Uncontrolled pulsation causes pipe fatigue, flange leakage, and instrument damage. RISL reviews pulsation control design as part of any reciprocating compressor integrity assessment involving piping modifications or compressor rerates.

Lube Oil System — API 614

API 614 governs lubrication, shaft-sealing, and control oil systems. For reciprocating compressors in critical service, the lube oil system is a primary integrity system. Oil pressure, temperature, cleanliness, and viscosity are all monitored parameters. RISL treats lube oil analysis as a mandatory condition monitoring input, not a discretionary PM task. Metal content trending in lube oil provides early detection of bearing and cylinder wear well ahead of any vibration or performance indicator.

Centrifugal Compressors — API 617

API 617 covers axial and centrifugal compressors and expanders for petroleum, chemical, and gas industry services. Centrifugal compressors are dynamic machines — they impart velocity to the gas through impeller rotation and convert that velocity to pressure in the diffuser and volute. Their failure modes are distinct from reciprocating machines but equally systematic.

Critical Components and Failure Modes

• Impellers: Impeller integrity is the highest-consequence item in a centrifugal compressor. Fatigue cracking from high-cycle loading, stress corrosion cracking in wet or sour service, and erosion from liquid carryover or particulate contamination are the primary degradation mechanisms. Any indication of cracking is a mandatory removal trigger.
• Rotor and balance: Rotor balance condition is monitored continuously through vibration. A shift in the spectrum at 1× running speed indicates mass redistribution from erosion, deposit buildup, or corrosion. Balance must be verified after any impeller repair or replacement.
• Radial and thrust bearings: Thrust bearing condition is monitored through axial position probes (API 670). Thrust bearing failure is one of the fastest-developing catastrophic failure modes — undetected axial migration leads to impeller-to-casing contact within seconds.
• Dry gas seals: DGS condition is monitored through seal gas flow and vent flow rates. Elevated primary vent flow indicates seal face degradation. API 682 seal systems are addressed in the companion article on Mechanical Seal Systems.
• Casing and diaphragms: Casing distortion from pipe strain and diaphragm cracking in high-pressure stages have both been initiating events in documented centrifugal compressor failures in refinery service.
• Inlet guide vanes and diffusers: IGV condition affects surge margin and efficiency. Diffuser fouling reduces surge margin, increasing risk during process upsets.

Surge — The Critical Operating Boundary

Surge is the most destructive normal operating event a centrifugal compressor can experience. It occurs when flow falls below the minimum stable flow for a given speed and pressure ratio — gas flow reverses, the impeller unloads, and the machine cycles through forward and reverse flow at high frequency. A single surge event can cause impeller damage, seal damage, bearing overload, and coupling failure.

Standards Framework — Applied

Condition Monitoring — Minimum Program

Reciprocating Compressors

• Cylinder pressure analysis — periodic or continuous depending on criticality and valve history
• Rod drop monitoring — continuous preferred; periodic with defined frequency in lower-criticality service
• Frame vibration at bearing housings and crosshead guides
• Lube oil analysis — metals, viscosity, water content; minimum quarterly in critical service
• Valve temperature monitoring — thermocouple or IR on valve covers to detect early leak-by
• Packing leak detection — in sour or toxic service, mandatory and continuous

Centrifugal Compressors

• Online vibration monitoring — continuous, API 670-compliant, alarm and trip setpoints reviewed annually
• Axial position monitoring — continuous, thrust bearing protection
• Bearing temperature — radial and thrust, continuous
• Dry gas seal monitoring — primary and secondary vent flow, seal gas differential pressure
• Process performance monitoring — head, flow, efficiency trending to detect fouling, erosion, or internal recirculation
• Lube oil analysis — metals, viscosity, contamination

Alignment — The Underweighted Factor

Shaft alignment between the compressor and its driver is one of the most consequential and most commonly underweighted variables in compressor integrity. Misalignment imposes radial loads on bearings outside the design envelope. It generates vibration at 1× and 2× running speed that is frequently misattributed to balance. It accelerates seal wear. In reciprocating compressors, crankshaft deflection measurements will show the signature of misalignment as bearing wear progresses.

RISL requires laser alignment verification at every major reassembly and at any time a compressor is disconnected from its driver for any reason. Thermal growth offsets must be applied — cold alignment to hot running targets, not to zero cold offsets.

Documentation and the Forensic Record

Every compressor intervention produces a forensic record: what was found, what was measured, what was replaced, what was adjusted, and what was the condition of components removed from service. This record has operational value — it builds the machine's history, enables trend analysis, and provides the evidence base for interval optimisation. It also has legal and regulatory value in the event of a future incident.