Pump and Fan Surging: 3 Conditions, Fixes, vs. Cavitation
A trouble report arrives from a Japanese plant: "サージング発生" (sājingu hassei, "surging occurred"). Or the O&M manual for a Japanese pump or blower warns against operating in the "surging range." On your own site, the discharge gauge on a lightly loaded pump swings back and forth at a steady rhythm, and someone asks whether it is cavitation.
Surging and cavitation have entirely different causes. Surging is a low-flow instability of the whole system, while cavitation is liquid boiling at low pressure inside the pump. This guide explains the three conditions that produce surging in pumps, how fans and blowers differ, the fixes, how to tell surging from cavitation, and how the Japanese word "surging" maps to English terms that sound similar but mean something else.
- Surging is a periodic swing in pressure and flow when a pump or fan runs at low flow. Pipes or ducts shake with it. Japan's pump terminology standard, JIS B 0131, defines the term.
- In a pump, it takes three conditions together: (1) operation on the rising part of the head curve, (2) a volume that can store pressure (a tank or an air pocket) between the pump and the throttling valve, and (3) throttling downstream of that volume.
- Air compresses, so a fan or blower can surge even without a tank.
- The fix is more flow through the machine: a bypass or blow-off valve to keep a minimum flow, throttling close to the pump, and venting air pockets.
- Cavitation is a different problem. Surging comes from discharge-side operating conditions and involves the whole system. Cavitation is liquid boiling at low pressure on the suction side.
Contents
What Is Surging?
Surging is a sustained, periodic swing in pressure and flow that occurs when a pump, fan, or compressor runs at low flow. JIS B 0131, the Japanese standard for turbopump terminology, describes it as a periodic oscillation of pipe pressure and discharge flow that occurs when the flow is throttled below the normal rate.
Nothing has to break inside the machine for surging to start. It is a self-excited oscillation of the whole system: the pump plus the connected piping, tanks, and valves. Put simply, the balance between the pump pushing fluid out and the system pushing back is lost, and the flow rocks back and forth.
"Surging," "Surge," and "Hunting": Japanese vs. English Terms
In Japanese, サージング (sājingu) is a katakana loanword. Japanese engineers use the same word for at least five different things, and only two of them (pumps and fans) share the same mechanism.
Pumps and fans are the two that matter in plant equipment work.
The jet engine and compressor fixes are examples of preventive measures listed by Oyama (1984).
English uses related words, but they do not line up one-to-one with the Japanese term. Watch these three when you read or write reports:
- Surge (compressors and fans): "Compressor surge" and "fan surge" are the standard English terms for the same low-flow oscillation that Japanese sājingu describes in those machines.
- Surge (piping): In pipeline and water-system work, "pressure surge" and "surge analysis" usually refer to water hammer, the pressure transient caused by a pump trip or a fast-closing valve. That is a different event. If a translated report says "surge," check which one is meant.
- Hunting: Some people use "hunting" for any fluctuating flow. Fan engineering guidance reserves it for an under-damped control loop that keeps overcorrecting a damper, vane, or speed setting. Pump surging can look similar on a trend chart, but the cause is hydraulic, not the controller.
For pumps, English sources discuss the same problem as "pump surging" or as unstable operation on an unstable (drooping) head curve. If you send a question to Japan, writing "サージング (surging: periodic pressure and flow swing at low flow)" removes the ambiguity.
The non-plant meanings in brief. In engines, "valve spring surge" is spring resonance at high rpm that keeps the valves from opening and closing correctly. It causes abnormal noise and, in severe cases, engine failure.
Engine makers counter it with stiffer springs, variable-pitch springs, or nested springs with different natural frequencies. Air (pneumatic) springs avoid it too, but they need a compressed-air supply, so they are used mainly in racing engines such as F1.
In jet engines and gas turbines, compressor surge is repeated flow reversal and pulsation, with rising exhaust temperature and fluctuating speed. If it continues, the engine can shut down or be damaged, which in an aircraft can lead to a serious accident.
With stout beer, "the surge" is the cascading of fine nitrogen bubbles after the pour. The rest of this guide covers pumps and fans only.
Pump Surging: How It Starts
Pump surging is violent vibration of the pump and piping at low flow, bad enough that the pump cannot keep running. The head-flow curve below shows why it happens.
Region (1) is left of the peak, where head rises as flow increases. Region (2) is the normal, stable range.
Schematic. Read the actual curve and minimum flow from the pump manufacturer's data.
A pump curve plots head, flow, shaft power, and current. This one shows head and flow only.
Over most of the range, head falls as flow rises, and head rises as flow drops.
Head is how high the pump can lift the liquid, in meters. For water, 1 m of head is about 9.8 kPa (about 1.4 psi), often rounded to 10 kPa, and the exact value depends on the liquid's density.
In region (2), if downstream demand changes and flow dips for a moment, head goes up (from C toward B). The pump pushes harder, flow recovers, and the pump settles back to its operating point on its own. That is normal operation.
In region (1), a dip in flow also lowers the head. Running there lets flow and head swing periodically, and the pump and piping vibrate. The loop below shows the sequence.
A small change in flow does not die out. It repeats as a cycle.
Oyama's (1984) analysis shows the oscillation dies out where the curve slope is negative (falling) and grows into surging where the slope is positive (rising).
A pump whose head curve has a hump, like region (1), risks surging when it runs at flows below the hump. Pump type alone does not tell you. Check the rising section on the actual curve, and stay above the manufacturer's minimum flow.
If you are writing a purchase specification, note that API 610 (petroleum, petrochemical, and natural gas pumps) is reported, in trade-press quotations of the standard, to prefer a stable curve with a continuous rise to shutoff. For parallel operation, the same sources cite a head rise of at least 10% of rated head.
Confirm the exact wording in the edition your project uses.
The Three Conditions for Pump Surging
Pump surging occurs when all three of these are true:
- The pump runs on the rising section of its head curve.
- Between the pump and the throttling valve, there is a volume whose size changes with pressure, such as a tank or an air pocket.
- The flow is throttled at a valve downstream of that volume.
With all three in place, flow and pressure keep swinging. Remove any one and surging stops, so the fixes start from these three points.
Break one condition and surging cannot occur.
Oyama (1984) notes that throttling resistance works better the closer it is to the pump, and that self-excited oscillation does not occur when the pump discharges into a tank with a constant water level.
Fan and Blower Surge
Fans differ from pumps in one key way: air compresses. The air in the ductwork and any connected vessel plays the role of the pump's tank.
In other words, condition 2 is already built into a fan system. A fan can surge without any tank, which is why surge comes up more often with fans and compressors than with liquid pumps. One US fan manufacturer's technical paper (Aerovent) lists three criteria for surge:
- A relatively large volume of pressurized air, such as a large plenum or vessel
- A section of ductwork with relatively high velocity
- An operating point to the left of the peak pressure, where the fan curve has a positive slope
Like a pump, a fan has a performance curve that links pressure rise (outlet minus inlet) to airflow. The figure shows a typical shape.
Left of the peak, pressure rises as airflow increases. Operating there invites surge.
Schematic. Curve shapes vary by fan type; use the manufacturer's curve.
When a fan runs in the low-airflow region where pressure rises with airflow, pressure and airflow swing widely and the ductwork vibrates. That is fan surge. The same paper says observers commonly describe the sound as a "whoosh" or "whoomp."
Some fan types, such as axial fans and multi-blade (sirocco) fans, have a deep dip in the stall region at low airflow, so low-flow operation needs care. Surge can damage the fan and pull duct joints apart.
How to Fix Surging
Sustained surging can damage equipment and displace piping, so it needs a fix. The options fall into three groups, from operating changes you can make today to changing the machine itself.
Each approach acts at a different point, from operation to equipment design.
Based on Oyama's (1984) summary of preventive measures, ordered by how easy they are to apply on site.
Pump Surging Fixes
The basic fix for pump surging is to force more flow through the pump, so it leaves the rising section and runs in the stable range. The most reliable way is a bypass line that returns part of the discharge to the suction side.
Top: a bypass returns flow to the suction side. Bottom: flow is throttled at a valve right after the pump.
Schematic. Pressure gauges, check valves, and other fittings are omitted.
With the bypass valve set to pass enough flow, the pump should stay out of surge even when downstream demand drops almost to zero. The trade-off is that the maximum flow you can deliver downstream drops by the bypassed amount, so check it against demand.
A bypass also protects the pump from overheating during long periods at low flow. That is why manufacturers specify a minimum flow.
Sometimes a review of the pump's outlet valve setting is enough; discuss any change with the pump manufacturer and follow your site's operating procedures. The closer the throttling valve is to the pump, the better it suppresses surging. Throttling upstream of the tank or air pocket breaks condition 3, so if you are throttling at a distant valve, switch to the valve right after the pump.
An air pocket at a high point in the piping also counts as condition 2. Venting it through an air release valve, following your site's procedures, removes one of the three conditions.
Speed control with a variable frequency drive (VFD) shifts the whole pump curve down, so the operating point moves differently than it does with valve throttling. If you plan to run at low flow on a VFD, confirm the allowable operating range with the pump manufacturer.
Fan and Blower Surge Fixes
The principle is the same as for pumps: keep a minimum airflow through the fan. Two piping arrangements do this.
Top: part of the outlet air returns to the inlet. Bottom: a blow-off valve vents part of the air to atmosphere.
Schematic. The blow-off valve opens when airflow falls toward the surge line.
- Recirculation bypass: A line connects the fan outlet to the inlet and returns part of the outlet air, which keeps a minimum airflow through the fan.
- Blow-off line: A relief line on the outlet side vents part of the air to atmosphere. The valve that opens as airflow drops toward surge is called a blow-off valve (放風弁, hōfū-ben).
Blow-off wastes the energy in the vented air, but it prevents surge.
On the machine side, adjustable inlet guide vanes widen the range where the fan runs stably at low airflow. Multistage axial compressors also bleed air from intermediate stages.
Pump Surging vs. Cavitation
Surging and cavitation are often mentioned together, but their mechanisms are completely different. Here is the comparison first.
Surging is an oscillation of flow in the whole system. Cavitation is liquid boiling inside the pump.
Surging is driven mainly by discharge-side operating conditions; cavitation by suction-side pressure and liquid temperature.
For a quick field diagnosis, the table below condenses the same points.
| Check | Surging | Cavitation |
|---|---|---|
| What it is | Periodic oscillation of flow in the whole system | Liquid partly vaporizes; bubbles collapse |
| Which side causes it | Discharge-side operating conditions | Suction-side pressure and liquid temperature |
| Operating condition to suspect | Low flow with a valve throttled | Low suction level, high liquid temperature |
| Occurs with gases? | Yes (fans, compressors) | No; liquid-handling equipment only |
| Most visible symptom | Pressure gauge and motor current swing at a regular period | Head loss, impeller erosion |
| Direction of the fix | More flow; throttle near the pump | Raise NPSH available above NPSH required |
Cavitation starts when the liquid pressure inside the pump drops below its vapor pressure at the operating temperature, and part of the liquid vaporizes. The sequence runs like this:
- Liquid pressure inside the pump falls below the vapor pressure at that temperature.
- Part of the liquid vaporizes and forms vapor bubbles.
- The bubbles move into a higher-pressure zone, condense, and collapse.
- The collapse shocks make the pump and piping vibrate.
- Over time, the pump can be damaged and fail.
The usual spot is near the impeller inlet. As the liquid speeds up around the impeller, its pressure drops, as Bernoulli's principle predicts, and it vaporizes more easily.
On the system side, the two factors that matter most are the liquid temperature and the suction pressure feeding the pump. A hotter liquid has a higher vapor pressure, and lower suction pressure makes vaporization easier.
When you select a pump, calculate the net positive suction head available (NPSHA), set by the system, and make sure it exceeds the NPSH required (NPSHR) for that pump. The Hydraulic Institute's ANSI/HI 9.6.1 gives guidance on how much margin to keep; agree on the basis and margin with the supplier.
In practice, many details, such as the blade surface shape, change how cavitation develops. Cavitation is also distinct from air entrainment, where air gets mixed into the liquid.
Field Checklist: Surging or Cavitation?
Use this list before you report a problem or call the supplier. It helps you collect the facts that point to one cause or the other.
- Flow rate at the time: Was the pump or fan running well below its rated flow? Compare with the minimum flow on the data sheet.
- Valve positions: Which valve controls the flow, and how far is it from the pump? Is it downstream of a tank, receiver, or high point?
- Rhythm of the swing: Does the discharge gauge or motor current swing at a regular period? Record roughly how many cycles per minute.
- Suction side: What were the suction tank level, suction pressure, and liquid temperature? Were they lower or higher than usual?
- Sound: Does the noise rise and fall at a steady, countable rhythm (typical of surge), or is it continuous? Record a short video if you can.
- Air in the system: Are there high points without air release valves? Was the line recently drained and refilled?
- Control loop: Does the swing stop when you put the flow or pressure controller in manual? If it does, the cause may be controller hunting rather than surging.
- Equipment curve: Does the manufacturer's curve show a rising section or a dip at low flow? Where was the operating point on that curve?
Questions to Ask the Japanese Supplier
Japanese manuals and data sheets may use the Japanese terms below. These questions get you the numbers you need. Include the Japanese term in parentheses so the supplier's engineer reads it the same way.
- What is the minimum continuous flow (最小流量, saishō ryūryō) for this pump at our operating speed, and is it set by surging, temperature rise, or both?
- Does the head curve (揚程曲線, yōtei kyokusen) have a rising section or hump at low flow? Please send the full curve down to shutoff.
- Where is the surging range (サージング域, sājingu iki) for this fan or blower, and at what airflow should the blow-off or bypass valve open?
- Is a minimum-flow bypass line (バイパスライン) included in your scope, or should we provide it? What size and set point do you recommend?
- If we run on a VFD, what is the allowable speed and flow range?
- What NPSHR value and basis does your curve use, and what NPSH margin do you recommend for our liquid and temperature?
- Is the pump suitable for parallel operation, and does the head rise continuously to shutoff?
Where This Guide Does Not Apply
This guide explains the general mechanism. Actual limits depend on the specific machine and system, so treat it as background, not a design or operating rule.
- Minimum flow and surge limits come from each manufacturer's data. Use their numbers, not rules of thumb.
- Large process compressors and gas turbines are a specialist area beyond this guide.
- Water hammer (pressure surge from pump trips or fast-closing valves) is a separate problem with its own transient analysis methods.
- Purchase specifications such as API 610 have their own requirements for curve stability and minimum flow. Check the edition named in your contract.
FAQ
How Do You Explain Surging in Simple Terms?
It is what happens when a pump or fan is throttled too far: the flow rocks back and forth, and the piping shakes with it. Figure 3 shows the cycle step by step.
How Can I Tell Surging from Cavitation in the Field?
Start with the operating conditions. Low flow with the discharge throttled points to surging; a low suction level or hot liquid points to cavitation. Rather than deciding on symptoms alone, check the operating point and the suction conditions together, and consult the manufacturer if it is still unclear.
Can Pressure Gauge or Motor Current Swings Reveal Surging?
They are a useful clue. Because pressure and flow swing periodically during surging, the discharge gauge needle and motor current may swing widely at a regular period.
A swing alone does not confirm surging, though, because motor current changes for other reasons too. Vibration measurement or continuous condition monitoring helps narrow it down.
Is Surging the Same as Water Hammer?
No. Water hammer is a sudden pressure spike that occurs when flowing water is stopped abruptly, for example by a fast-closing valve. It can also happen when a pump stops and a check valve slams shut against reverse flow.
Water hammer is triggered by a sudden action, such as a valve closing or a pump stopping. Surging keeps swinging on its own during low-flow operation, so treat the two separately. Note that English piping engineers often call water hammer "surge," which Japanese sājingu does not mean.
Sources
- Surging mechanism, stability condition, and preventive measures: Koichi Oyama, "Surging in Turbomachinery," Turbomachinery, Vol. 12, No. 9 (1984) (Japanese)
- Rising section of the head curve and surging: Shigeaki Kobayashi, "Study on Stable Characteristic Curves of Centrifugal Pumps," Hitachi Hyoron (October 1968) (Japanese)
- Air pockets in discharge piping and surging: Ministry of Land, Infrastructure, Transport and Tourism, technical standards for drainage and irrigation pump equipment (March 2014) (Japanese)
- Fan surge criteria, sound, and the meaning of "hunting": Aerovent, "Surge, Stall, and Instabilities in Fans" (FE-600)
- Definition of surging per JIS B 0131: Teral, glossary entry "Surging" (Japanese)
- API 610 stable curve and 10% head rise for parallel operation: Water & Wastes Digest, "Unstable Curves"
- "Pressure surge" as water hammer in English usage: KSB, "Transient flow analysis helps avoid risks of surge pressure"
- Overheating at low flow and manufacturer-specified minimum flow: Schroedahl, SUL automatic recirculation valve catalog (Japanese)
- Cavitation, vapor pressure, and NPSH: Motohiko Nohmi, "Basics of Pump Cavitation, Part 1," Ebara Engineering Review No. 245 (2014) (Japanese)
- NPSH available and required: AxFlow, "What is NPSH?"
- NPSH margin guideline: Hydraulic Institute, "Understanding the 2024 Updates to ANSI/HI 9.6.1"
- Cavitation symptoms (head loss, erosion, noise and vibration): Strojniški vestnik, "Noise as an Indicator of Cavitation and Instability in Centrifugal Pumps"
- Compressor stall and surge and their effects: FAA, Pilot's Handbook of Aeronautical Knowledge, Glossary ("Compressor stall," "Compressor surge")
- Valve spring surge and variable-pitch springs: Tomei Powered, valve spring product information (Japanese)
- Air springs in F1 engines: Yamaha Motor Technical Review No. 19 (1995), pneumatic valve system for F1 racing (Japanese)
- Guinness bubble patterns: Kirin Holdings news release (June 22, 2021) (Japanese)
- Water hammer: Japan Water Research Center, Q&A on causes and countermeasures for water hammer (Japanese)
Original article in JapaneseThis guide is adapted from a MONOSCOPE article written for readers in Japan: サージング（サージング現象）とは？ポンプ・送風機で起こる3つの条件と対策、キャビテーションとの違い
About the editor
MONOSCOPE EditorFOUNDER & EDITORSpent about ten years at a plant equipment maker and a plant engineering company, proposing, designing, and commissioning equipment, then moved into web marketing. Now a web consultant for B2B manufacturers, helping them explain their products and technology. Licensed in Japan as an Energy Manager and a Third-Class Chief Electrical Engineer.
Has run an engineering blog since 2017 (500+ articles) and a Japanese YouTube channel with about 20,000 subscribers. Each English guide is written from the Japanese shop-floor point of view and checked against the cited standards.
