Understanding Surging in Compressors

Devices

Portable balancer & Vibration analyzer Balanset-1A

€1,975.00 + VAT (if applicable)

Parts

Vibration sensor

€90.00 + VAT (if applicable)

Parts

Optical Sensor (Laser Tachometer)

€124.00 + VAT (if applicable)

Devices

Balanset-4

€6,803.00 + VAT (if applicable)

Parts

Magnetic Stand Insize-60-kgf

€46.00 + VAT (if applicable)

Parts

Reflective tape

€10.00 + VAT (if applicable)

Devices

Dynamic balancer “Balanset-1A” OEM

€1,735.00 + VAT (if applicable)

Surging — often simply called compressor surge — is a violent aerodynamic instability in centrifugal and axial compressors in which the entire flow through the machine periodically reverses direction. The result is oscillating pressure and flow, usually at frequencies of 0.5–10 Hz. In each surge cycle the flow momentarily stalls or reverses, discharge pressure collapses, then forward flow resumes and pressure rebuilds, and the cycle repeats. These reversals impose enormous fluctuating forces on the rotor, producing severe vibration — especially in the axial direction — a loud booming noise, and the capacity to destroy a compressor within minutes if it is not stopped at once.

Surge is fundamentally a system instability involving the compressor together with its connected piping and volume, not a property of the compressor alone. It arises when the machine is pushed beyond its pressure-rise capability at low flow, and preventing it requires anti-surge control that keeps flow safely above the surge line.

1. The Surge Mechanism

The surge cycle

A typical surge cycle progresses through a repeating sequence:

1. Flow reduction: system demand falls, so flow through the compressor drops.
2. Stall onset: at very low flow the blading stalls and the flow separates from the blade surfaces.
3. Pressure collapse: the stalled compressor can no longer sustain its discharge pressure.
4. Flow reversal: high-pressure gas trapped in the discharge piping or plenum drives backward through the compressor.
5. Pressure equalisation: discharge pressure falls as gas escapes backward.
6. Forward flow resumes: once pressure has dropped, the compressor can again move gas forward.
7. Pressure rises: renewed forward flow rebuilds the discharge pressure.
8. Cycle repeats: the high pressure stalls the machine again, and the loop continues.

Surge frequency

• Set by the system volume (piping, plenums, vessels) together with the compressor’s characteristics.
• Larger volumes give a lower surge frequency.
• Typical range: 0.5–10 Hz.
• Small systems: roughly 5–10 Hz.
• Large systems: roughly 0.5–2 Hz.
• The frequency stays relatively constant for a given system.

This low, system-fixed frequency sits well within the working range of a portable analyser. It is worth noting that the Balanset-1A measures vibration from 5 Hz upward, so the higher-frequency surge cycles of small systems fall inside its band; the key diagnostic, however, is less the exact frequency than the unmistakable pattern of large, unstable, predominantly axial low-frequency pulsation that appears suddenly.

2. Conditions That Lead to Surge

Operating beyond the surge line

Every compressor’s performance map carries a surge line that defines its stable limit:

• Surge line: the leftmost stable operating boundary on the map.
• Safe operation: to the right of the line, at higher flows.
• Surge zone: to the left of the line — unstable and forbidden territory.
• Margin: machines are normally run with a 10–20% flow margin to the right of the surge line.

Triggering events

• Demand reduction: the process draws less, so flow falls toward the surge line.
• Discharge restriction: a valve closing or a blockage downstream.
• Speed reduction: the compressor slowing without a proportional drop in required flow.
• Density changes: shifts in gas molecular weight or temperature that move the compressor characteristic.
• Fouling: blade deposits that erode the machine’s capacity over time.

3. Effects and Consequences

Vibration

• Amplitude: can reach 25–50 mm/s (1–2 in/s) or more.
• Axial component: particularly severe along the shaft axis.
• Low frequency: 0.5–10 Hz pulsations.
• Whole-machine motion: the entire compressor assembly rocks and shakes.

Mechanical damage

• Bearing failure: shock loads can destroy bearings in hours.
• Seal damage: axial motion and pressure reversals wreck seals.
• Shaft damage: bending and torsional stress from the flow reversals.
• Blade damage: alternating aerodynamic loads cause fatigue and can lead to blade liberation.
• Coupling damage: torsional shock damages couplings.
• Thrust bearing: rapidly alternating axial thrust can destroy the thrust bearing — frequently the first casualty of a surge.

Process consequences

• Pressure and flow oscillations propagate into the downstream process.
• Temperature excursions arise from the repeated compression and expansion.
• Process upsets or safety-system trips may follow.
• Product quality can suffer from the unstable conditions.

4. Detection

Vibration signature

• Sudden onset of large-amplitude, low-frequency pulsation.
• Frequency in the 0.5–10 Hz range.
• Severe axial vibration.
• Unstable, continually varying amplitude.

That signature is distinctive on a vibration spectrum and on a time waveform: a sudden burst of energy far below running speed, dominant in the axial channel, that grows and shrinks rather than holding steady. Vibration monitoring on the compressor’s thrust bearing is among the quickest ways to catch surge in progress, since the axial pulsation registers there most strongly.

Acoustic signature

• A loud booming or whooshing sound.
• Rhythmic pulsation audible at the surge frequency.
• Distinctive and, to anyone who has heard it, unmistakable.

Process indicators

• Oscillating discharge pressure.
• Oscillating flow, which may actually reverse.
• Temperature fluctuations.
• Motor current fluctuations.

5. Prevention: Anti-Surge Control

System components

Recycle valve. A fast-acting valve that bypasses discharge gas back to suction. It opens to add flow as the operating point approaches the surge line and is sized for the full compressor flow if required.

Flow and pressure measurement. Continuous monitoring of flow rate and pressure rise plots the live operating point on the compressor map and detects any approach toward the surge line.

Controller. The controller computes the distance to the surge line, opens the recycle valve with a safety margin as surge is approached, and — in modern installations — uses adaptive algorithms. Response time is critical, typically requiring action in under one second.

Operating procedures

• Never operate to the left of the surge line.
• Maintain a 10–20% flow margin from surge.
• Make load changes gradually and avoid abrupt demand drops.
• Verify the anti-surge system is functional before every startup.
• Test the anti-surge system periodically.

6. Emergency Response

If surge occurs

1. Immediate action: open the recycle valve manually if the automatic system has failed.
2. Increase flow: open the discharge, reduce resistance, or start parallel units.
3. Reduce pressure rise: slow the compressor if it is variable speed.
4. Emergency shutdown: if surge cannot be stopped within 10–30 seconds, trip the machine.
5. Do not restart: until the cause has been identified and corrected.

Post-surge inspection

• Inspect for blade damage.
• Check the bearing condition.
• Verify seal integrity.
• Examine the thrust bearing.
• Perform vibration analysis before returning the machine to service — a baseline spectrum will reveal any new unbalance, misalignment or bearing damage left behind by the event.

7. Surge vs. Other Instabilities

Surge vs. rotating stall

• Surge: a system-wide flow oscillation at very low frequency (0.5–10 Hz).
• Rotating stall: localised stall cells that rotate around the annulus at a higher frequency, typically 0.2–0.8× rotor speed, placing it among the sub-synchronous phenomena.
• Severity: surge is the more destructive of the two; rotating stall can be a precursor to surge.

Surge vs. recirculation

• Surge: a compressor-specific, system-wide flow reversal.
• Recirculation: can occur in pumps or compressors, is a localised flow reversal, and is generally less severe.
• Relationship: recirculation can develop into full surge in compressors.

Surging is the single most dangerous operating condition for centrifugal and axial compressors, capable of destroying equipment in minutes. Understanding the surge mechanism, recognising the surge-line boundary, implementing effective anti-surge control, and holding proper operating margins are absolutely critical for safe compressor operation in industrial gas-compression service.

---

← Back to Main Index