A Practical Guide to Vibration Analysis for Reciprocating Compressors

The High Cost of a Misunderstood Machine

In any plant environment, an unexpected shutdown of a mission-critical reciprocating compressor is not just a maintenance event—it's a direct impact on production and profitability. These machines are the workhorses of industries from petrochemicals to manufacturing. However, their inherent design, characterized by powerful, intermittent forces, creates complex vibration signatures that are frequently misinterpreted, leading to a significant vibration problem. The operation of reciprocating compressors generates a unique vibration behavior that requires specialized analysis.

Applying generic vibration analysis techniques suitable for reciprocating and rotating machinery like turbines or centrifugal compressors to a reciprocating machine often leads to incorrect diagnoses. A missed vibration signal or a wrong conclusion can be the difference between a scheduled component replacement and a catastrophic failure of the crankshaft, connecting rod, or frame. Vibration poses significant challenges in these assets.

This guide moves beyond surface-level analysis. It provides a systematic, field-tested methodology for diagnosing reciprocating compressor faults through vibration data. We will explore the unique characteristics of these machines, the diagnostic process that works, and the specific data signatures that point to real-world mechanical and operational problems within the entire compressor system. Our focus is on providing an understanding of vibration in reciprocating machines.

Foundational Understanding: Why Reciprocating Compressor Vibration is Unique

To accurately diagnose a reciprocating compressor, one must first respect its fundamental mechanical differences from purely rotating equipment. The mechanical vibration profile is not smooth; it is a direct reflection of the machine's powerful, cyclical, and often violent internal events. This is why vibration analysis on reciprocating machines is a specialized field.

Rotational vs. Reciprocating Forces: The Core Difference

A centrifugal compressor generates relatively steady pressure through smooth, continuous rotation. Its vibration profile is dominated by rotating mass dynamics.

A reciprocating compressor, by contrast, creates pressure through discrete, forceful events within each revolution of the crankshaft. These are primary vibration sources:

• Gas Compression: The intense, cyclical loading and unloading of gas pressure on the piston creates significant pressure pulsation.

• Valve Actuation: The sharp opening and closing of suction and discharge valves.

• Piston Reversal: The immense change in momentum as the reciprocating piston assembly stops and reverses direction at Top Dead Center (TDC) and Bottom Dead Center (BDC).

• Crosshead Reversal: The shifting of the crosshead, which absorbs the side-load forces from the connecting rod, against its upper and lower guides, causing crosshead vibration.

These events generate high-impact forces that transmit throughout the machine structure, creating a complex vibration signal that must be analyzed in the context of the machine's own cycle. In addition to linear forces, torsional vibration, a twisting force on the crankshaft, is a critical concern that requires separate analysis and simulation. The effects of torsional vibration can lead to crankshaft failure.

The Critical Role of Time Waveform Analysis

For most rotating equipment, analysts often jump straight to the frequency spectrum (FFT). This is a critical error in reciprocating compressor analysis.

The time waveform is paramount. It is a direct, unfiltered plot of vibration amplitude versus time, and it allows a skilled analyst to visualize the impacts from valve events and piston slap as they occur. This signal analysis captures the short-duration, high-energy events that get averaged out or are difficult to isolate in a vibration spectrum alone.

Understanding Orders vs. Frequency (Hz)

Analyzing vibration in terms of "orders" is essential. An order is a multiple of the compressor's running speed (RPM).

• 1X: The primary running speed.

• 2X: Twice the running speed (common in a 2x compressor analysis).

• 0.5X: Half the running speed (a sub-harmonic).

Using orders instead of fixed vibration frequencies (Hz) is critical because it ensures the analysis remains accurate even if the compressor's speed changes. A fault tied to reciprocating components will always appear at the same order, regardless of the machine's RPM.

Early Warning Signs & Symptoms: What to Look and Listen For

Effective condition monitoring begins with knowing which indicators, both from vibration data and physical observation, signal a developing problem. An effective monitoring system is crucial.

Interpreting Vibration Data: Key Indicators

• Increased Overall Vibration: This is the most basic alarm. A sustained increase in overall vibration amplitude is a clear sign that the machine's dynamic behavior has changed. For guidance, API Standard 618, "Reciprocating Compressors for Petroleum, Chemical, and Gas Industry Services," provides charts for acceptable frame vibration levels, though these are primarily for new machine acceptance testing and serve as a useful, conservative reference. When vibration is high, it demands immediate attention.

• Sharp Impacts in the Time Waveform: The appearance of sharp, repeating "spikes" in the time waveform is a classic indicator of impact-related events. This could be a compressor valve slamming shut improperly, a loose internal component, or the onset of liquid slugging. These spikes are a form of abnormal vibration.

• Changes in the Frequency Spectrum: A clean baseline spectrum is a valuable asset. The emergence of new peaks or a significant increase in the amplitude of existing peaks at specific orders (e.g., 1X, 2X, or their harmonics) points directly to specific types of faults. You may see energy at higher frequencies that indicates specific issues.

Physical and Audible Symptoms

Data analysis tells only part of the story. Correlate your vibration readings with physical observations:

• Audible Changes: A healthy air compressor has a consistent, rhythmic sound. The development of sharp "knocking" noises often points to mechanical looseness, bearing issues, or piston slap. Hissing can indicate a valve or gas leak.

• Visible Signs: Look for excessive shaking or pipe vibration on attached small-bore piping, which can lead to fatigue failures in the reciprocating compressor piping system. Check for "fretting corrosion" (a fine, reddish-brown dust) around fasteners or foundation interfaces, indicating movement from looseness.

• Process Changes: A drop in flow or pressure for a given stage, or an increase in discharge temperature, can be a direct result of a malfunctioning valve—a fault that will also have a distinct vibration signature.

The Step-by-Step Diagnostic Process

A successful vibration program is systematic. Follow a structured process to ensure data is accurate, repeatable, and, most importantly, actionable for the compressor system.

Step 1: Establish a Baseline and Define Operating Conditions

You cannot identify an abnormal condition without first defining "normal." This is the core of any vibration monitoring program. Collect baseline vibration data when the compressor is known to be healthy and operating under typical compressor load conditions.

For every reading, you must document the corresponding operational parameters:

• Compressor RPM (Speed of the compressor)

• Suction and Discharge Pressures (for all stages)

• Suction and Discharge Temperatures (for all stages)

Without this context, comparing vibration readings over time is meaningless. A compressor will vibrate differently when fully loaded versus unloaded, affecting the measured vibration levels.

Step 2: Strategic Sensor Placement

Where you measure is as important as what you measure. Proper placement of vibration sensors is key to effective vibration measurement.

• Frame Vibration: Mount accelerometers on the compressor frame at each main bearing housing location. Collect data in the horizontal, vertical, and axial directions. The horizontal direction is often the most sensitive to unbalance and misalignment forces.

• Cylinder & Crosshead Guide Vibration: This is critical for isolating piston and valve-related faults. Place accelerometers directly on the crosshead guide or cylinder flange. Measurements should be taken perpendicular to the piston rod's direction of travel to best detect crosshead slap and in the axial direction to detect rod-related issues. This helps isolate casing vibration.

Step 3: Analyze the Time Waveform First

Before looking at any frequency spectrum, analyze the time waveform. A detailed performance analysis starts here.

• Look for repeating patterns of sharp impacts.

• Measure the time between these impacts to see if they correlate with the running speed or twice the running speed.

• If a Keyphasor® or other phase reference sensor is installed, you can correlate the exact moment of a vibration spike to the crankshaft's angular position. This allows you to definitively determine if an impact is occurring, for example, at the end of the compression stroke, pointing directly to a discharge valve issue.

Step 4: Analyze the Frequency Spectrum (FFT)

With insights from the time waveform, move to the frequency spectrum analysis to categorize the fault. Look for energy concentrated at specific orders, which act as fingerprints for mechanical problems. This spectrum analysis is a cornerstone of vibration testing.

Common Causes & Prevention Strategies

The following chart links common causes of compressor vibration to their most probable causes. This is the core of reciprocating compressor vibration diagnostics. Understanding these links is crucial for vibration elimination.

Reciprocating Compressor Vibration Fault Chart

The Power of Integrating Pressure Data (P-V Curves)

While vibration analysis is a powerful tool for detecting a fault, Pressure-Volume (P-V) curve analysis is the ultimate tool for confirming it. Vibration and pressure pulsation are often linked. For example, if vibration data suggests a faulty discharge valve on cylinder #2, an analysis of that cylinder's P-V card can definitively show the pressure drop indicative of a leak, confirming the diagnosis. This integrated response analysis demonstrates a mastery of machine health and compressor performance.

Key Takeaways

• Prioritize Time Waveform Analysis: For reciprocating compressors, the time waveform provides more immediate diagnostic clarity for impact events than the frequency spectrum. This is key to an effective vibration analysis.

• Context is Everything: Always analyze vibration data alongside operating parameters like RPM, pressures, and temperatures. Changes in vibration are only meaningful when compared to a known baseline under the same conditions as the compressor operates.

• Think in Orders, Not Hz: Using orders (multiples of running speed) ensures your vibration analysis is valid even if the compressor speed varies.

• Specific Signatures Point to Specific Faults: Mechanical looseness, unbalance, misalignment, and valve failures each have distinct fingerprints in the vibration data.

• Confirm with Process Data: Use P-V curves, temperature readings, and pressure data to validate diagnoses made through vibration analysis.

The Turbo Airtech Advantage

This guide provides the foundational knowledge to systematically analyze and diagnose the most common failure modes in reciprocating air compressors. However, when multiple faults exist simultaneously or when vibration signals are ambiguous, a deeper level of expertise is required. A dynamic analysis may be needed to understand the complex interaction of forces.

Interpreting the complex interplay of forces inside these machines is a specialized skill. If your team has implemented corrective actions based on vibration data and the problem persists, or if the data points to a critical but unclear fault, it is time to consult an expert.

Contact the Turbo Airtech Experts. Our team can analyze your raw vibration and operational data, providing a definitive diagnosis and a clear action plan to restore your machine's reliability and prevent a costly failure.

References

• API Standard 618, 5th Edition, "Reciprocating Compressors for Petroleum, Chemical, and Gas Industry Services."

• The Vibration Institute, technical papers on reciprocating machinery analysis.

Disclaimer

Turbo Airtech is an independent provider of parts and services for centrifugal and reciprocating compressors. It is not an authorized distributor for any OEMs mentioned. All brand names are property of their respective owners and are used for reference purposes only. Our content is for informational purposes to showcase our technical expertise in compressor maintenance.