Diagnostic Guide: Water Hammer of Check Valves – Analysis, Diagnosis and Resolution

1. Problem Description and Scope

Water hammer is a transient hydraulic phenomenon characterized by a rapid and intense pressure shock wave, often audible, resulting from an abrupt change in the flow speed of a fluid in a pipe. In the context of check valves, this phenomenon is typically caused by the sudden closing of the valve when a flow reversal occurs or is imminent. This rapid closure, called “slam”, generates pressure peaks which can seriously damage pipes, supports, valves, instruments and rotating equipment (pumps).

Affected Equipment and Systems:

• Aeronautical sector: Aircraft hydraulic systems, fuel lines, cooling circuits of on-board equipment. Reliability is essential due to weight and space constraints.
• Energy Sector: Cooling circuits of power plants (nuclear, thermal), steam or hot water distribution networks, fuel supply systems, oil and gas pipelines. The installations are often large-scale and under high pressure.
• Other Industries: Pumping networks, water treatment stations, firefighting systems, chemical processes.

Severity Classification:

• Critical: Structural damage to pipes (cracks, ruptures), pump or compressor failure, immediate production stoppage, risk for staff safety. Requires immediate intervention.
• Major: Excessive noise, significant vibrations, leaks at joints or fittings, accelerated wear of components, unplanned shutdowns. Requires rapid corrective maintenance planning.
• Minor: Audible noise, slight vibrations. Indicates an incipient problem that, if left untreated, will progress to major or critical severity.

2. Safety Precautions

WARNING: Diagnostic and maintenance operations on systems under pressure and potentially subject to water hammer involve significant risks. Failure to follow safety procedures can result in serious injury or death, as well as significant property damage.

Stored Energy: Pressurized fluids can release energy explosively. Hydraulic accumulators or spring systems can also store considerable energy even after the pumps have stopped.

Mandatory Personal Protective Equipment (PPE):

• Hearing Protection: Earplugs or noise-cancelling headphones (in case of audible water hammer).
• Eye Protection: Safety glasses or face shield.
• Hand Protection: Protective gloves adapted to the fluid (chemical/thermal resistance).
• Foot Protection: Safety shoes with protective toe cap.
• Body Protection: Resistant work clothing, flame retardant if applicable.

Essential Procedures:

• Lockout and Lockout (Lockout/Tagout - LOTO): Before any physical intervention on the system, ensure that all energy sources (electrical, hydraulic, pneumatic) are cut off, isolated and logged in accordance with standard NF C18-510 and the company's internal procedures.
• Depressurization and Draining: Check and confirm complete depressurization of the affected pipe section before opening a fitting or component. Use appropriate drain and purge valves.
• Residual Energy Check: Confirm the absence of residual energy. For hydraulic or pneumatic systems, operate the relief valves several times.
• Section Isolation: Use heavy-duty isolation valves to separate the work section from the rest of the system.
• Piping Support: Before dismantling sections, ensure that adjacent pipes are properly supported to avoid any unforeseen mechanical stress.
• Risk Analysis: Carry out a risk analysis (RDA) specific to the intervention and the system.

3. Required Diagnostic Tools

An accurate diagnosis of water hammer requires suitable instrumentation to capture rapid transient phenomena.

4. Initial Assessment Checklist

Before initiating a thorough diagnosis, it is essential to collect preliminary information about the system and the water hammer phenomenon.

5. Systematic Diagnostic Flowchart

Follow this flowchart to identify the likely cause of water hammer.

1. Symptom: Intense clicking noise and/or loud vibration at (or near) a check valve when stopping a pump or rapidly changing flow rate.
   1. First Check: Is this a standard check valve (swing, ball)?
      • IF YES : Go to step 2.
      • IF NO (axial disc valve, guided lift, double leaf, etc.): Continue the analysis, but the problem could be linked to an internal malfunction of the valve or to unsuitable sizing for the conditions.
   2. Check 2: Visual and Sound Assessment.
      • Observe the immediate surroundings of the damper. Are there any signs of movement, loose brackets, leaks?
      • Listen to the noise. Is it a single snap or a series of shocks?
        1. IF Single, dry click: Probable “slam” of the valve, closing too quickly.
           → Go to step 3.
        2. SI Series of shocks or “hammering”: May indicate valve instability or cavitation. To investigate further (see Probable Cause: Unstable Valve).
           → Go to step 3.
   3. Check 3: Analysis of Pressure Peaks (with dynamic transducer).
      • Install a dynamic pressure transducer upstream and downstream of the valve.
      • Record pressure profiles during a water hammer event.
      • IF Pressure peaks > 1.5x Nominal Pressure, and waveform with rapid oscillation (frequency > 50 Hz): Confirms a water hammer.
      • IF NO: The noise can be of mechanical origin (excessive play, loose support) or cavitation without major shock wave.
        1. Check for signs of cavitation: “Gravel” or “creosote” noise, erosion or pitting on the internal surfaces of the valve or piping.
           → Probable Cause: Cavitation / Excessive flow velocity.
        2. Check for mechanical anomalies: Excessive play, warped leaf pin, broken or tired spring.
   4. Check 4: Diagnosis of the Valve Closing Speed.
      • For valves equipped with a shock absorber:
      • IF Shock Absorber Present and Adjustable:
        1. Check the adjustment of the shock absorber (adjustment screw, oil pressure).
        2. Check the oil level or spring integrity.
        3. IF Damper faulty or incorrectly adjusted (closing too quickly):
           → Probable Cause: Shock absorber faulty or incorrect adjustment.
      • IF Valve Without Specific Damper or Passive Damper (with weighted leaf):
        1. Closing is too rapid for the installation conditions.
        2. Evaluate the flow reversal time: If the time is very short (< 0.5s), a quick-closing valve is imperative.
           → Probable Cause: Inappropriate valve selection.
   5. Check 5: Analysis of the System in Transit Condition.
      • Measure the fluid flow speed before stopping the pump and the reversal speed.
      • IF High initial flow speed (> 3 m/s) and/or Very rapid flow reversal:
        1. The kinetics of the system are too aggressive for the current valve.
        2. Consider transient modeling: Use software like “AFT Impulse” to simulate and validate solutions.
           → Probable Cause: Inherent system dynamics.

6. Cause-Fault Matrix

This matrix relates observed symptoms to probable causes and specific diagnostic tests.

7. Root Cause Analysis for Each Defect

7.1. Closing the Valve Too Fast (Slam)

• Why this happens: When a pump stops, the downstream pressure drops and the fluid begins to slow down, then reverse direction. A check valve is designed to prevent this backflow. If the valve is not fast enough to close before the fluid reaches a significant reflux velocity, the flapper is "slammed" into its seat by the force of the reversed flow, generating a major shock wave. Conventional swing valves are particularly vulnerable to this phenomenon in systems with large fluid columns or rapid flow reversals.
• How to confirm: Analysis of the dynamic pressure recording will show a very high pressure peak (often triangular or square) followed by damped oscillations. A visual inspection may reveal impact marks on the leaf or seat. Transient modeling can predict this behavior.
• Damage if not resolved: Breakage of pipe welds, damage to supports, premature failure of seals and seals, destruction of the valve itself, failure of the pump (shaft, bearings).

7.2. Faulty or Poorly Adjusted Valve Damper

• Why this happens: Many flappers are equipped with hydraulic or pneumatic dampers to slow the closing of the flapper just before the seat. If the shock absorber is leaking, dirty, poorly adjusted (too “soft” or too “hard”), or if the damping fluid is degraded, it can no longer fulfill its role. The valve then closes without control, like an undamped valve.
• How to confirm it: Visual inspection of the shock absorber (oil leaks, degradation of the seals, dirt), check of the oil levels, functional test of the closing speed of the valve (manual if possible). The pressure recording will show a slam similar to an undamped collapse.
• Damage if not resolved: Same as closing too quickly, because the shock absorber does not work. Accelerated wear of the internal components of the valve.

7.3. Unsuitable Valve (Type or Size)

• Why this happens: The choice of check valve must precisely match the hydraulic characteristics of the system. A heavy flapper in a low flow velocity or fast flow reversal system will never close fast enough. A valve that is too large may not open completely or generate instabilities at low flow (flutter). A valve that is poorly sized for the induced pressure losses can force the pump to work outside its optimal operating point. The NF EN 12095 standard gives guidelines for valves.
• How to confirm: Comparison of the valve performance curves with the system operating conditions. Calculation of the flow reversal time and the required critical closing speed. Hydraulic simulation.
• Damage if not resolved: Recurrent water hammer, cavitation, high energy losses, structural failure of the pipes, premature wear of the valve.

7.4. Flapper Instability (Flutter)

• Why this happens: At low or fluctuating flow rates, a valve flap may not be fully open and oscillate rapidly (flutter). These oscillations can cause small repetitive water hammer, cavitation, and very rapid mechanical wear of the internal components (hinge, leaf, seat).
• How to confirm: Distinctive “hammering” sound. High frequency vibrations. Internal inspection of the valve revealing uneven wear, rubbing and signs of fatigue.
• Damage if not resolved: Rapid destruction of the valve, deterioration of the pump bearings, increased noise.

7.5. Severe Cavitation

• Why it happens: Cavitation occurs when the local pressure of the fluid drops below its vapor pressure, forming vapor bubbles that violently implode upon reaching areas of higher pressure. Although distinct from slam water hammer, cavitation can generate localized shock waves and similar noises, and can be aggravated by pressure transients. It is often linked to excessive flow velocities, obstructions or poor hydraulic profiling.
• How to confirm: Characteristic “gravel” or “creosote” sound. Internal inspection showing pitting or erosion on valve or piping surfaces. Analysis of pressure peaks that are more localized and of higher frequency than the classic water hammer.
• Damage if not resolved: Material erosion, degradation of hydraulic performance, excessive noise, vibrations, component failure.

8. Step-by-Step Resolution Procedures

8.1. Resolution of the Valve Closing Too Fast (Slam)

1. SECURITY: Apply complete LOTO procedures (section 2).
2. Replace with a Quick Close Valve:
   • Action: Install a low inertia axial disc check valve (for example, type UNITEC-D AXIAL-PRO) or a double leaf valve. These flaps have a very short leaf stroke and close quickly.
   • Specification: Choose a valve with a suitable flow resistance coefficient (Kv) and which has a closing time less than the flow reversal time (generally < 0.1 second).
   • Verification: After installation and gradual return to service, perform pump stop/start cycles with dynamic pressure recording. Pressure peaks must be reduced by at least 50% compared to the initial state, ideally < 1.2x P_nominal.
3. Add or Repair a Shock Absorber:
   • Action: For swing or lift valves, install an external or internal hydraulic shock absorber. If a shock absorber exists, disassemble it, inspect (oil level, seals, cleanliness), repair or replace.
   • Adjustment: Adjust the shock absorber to obtain a smooth closing without excessive float. This often involves a period of trial and error. Start with a firm setting and gradually loosen.
   • Verification: Pressure peaks must be significantly attenuated. The clicking noise should disappear or be greatly reduced.

8.2. Fixing Inappropriate Flap Selection

1. SECURITY: Apply LOTO procedures.
2. Reevaluate Sizing:
   • Action: Perform a thorough hydraulic sizing analysis taking into account fluid characteristics, nominal flow rate, pressure, fluid column length, and flow reversal times. Use the manufacturer's charts and simulation software if available.
   • Specification: Select a valve which remains completely open at nominal flow to minimize pressure losses and flutter, and which ensures rapid and gentle closing when reversing flow.
   • Verification: Monitoring of pressure losses across the valve (ΔP must comply with specifications).

8.3. Mitigation of Instability (Flutter)

1. SECURITY: Apply LOTO procedures.
2. Increase Minimum Flow:
   • Action: If flutter occurs at low flow, ensure that the minimum flow through the valve is sufficient to keep it fully open. This may require modifications to the pumping system or the addition of a bypass line with regulated flow.
   • Check: The valve must remain in the fully open position under normal operating conditions.
3. Replace with a More Stable Valve:
   • Action: Opt for a valve whose design minimizes flutter, such as an axial disc valve which offers better stability at varying flow rates.
   • Verification: Vibration analysis after replacement. High frequency vibrations should be reduced.

8.4. Treatment of Severe Cavitation

1. SECURITY: Apply LOTO procedures.
2. Reduce Flow Velocity:
   • Action: Resize the piping for larger diameters, thereby reducing the fluid velocity. Recommended speeds are generally 1.5 to 3 m/s for water.
   • Verification: Line speed measurement with an ultrasonic flow meter. Cavitation noise eliminated.
3. Modify Hydraulic Profile:
   • Action: Remove obstructions, tight bends or abrupt reductions just before or after the valve. Installing a flow straightener can help.
   • Verification: Internal visual inspection after modification.

9. Preventive Measures

10. Spare Parts and Components

Maintaining an adequate stock of critical spare parts is essential for rapid problem resolution and minimizing downtime. Visit our online e-catalog to order.

Find all our references and request a quote on our E-catalog UNITEC-D.

11. References

• NF EN 12095: “Industrial fittings. Non-return valves. Prescriptions for examinations”.
• NF EN 13441: “Measurement of mechanical vibrations on rotating machines”.
• NF C18-510: “Operations on electrical works and installations or in their vicinity – Prevention of electrical risks”.
• Equipment Manufacturers (OEM) Operation and Maintenance Manuals.
• ASME B31.1 / B31.3: Pressure Piping Codes.
• “Fluid Transients in Pipe Networks” by E. Benjamin Wylie and Victor L. Streeter.