Diagnosing and correcting water hammer on check valves

1. Problem description & scope of application

This guide addresses the phenomenon of water hammer (pressure surge) at check valves in piping systems. Water hammer typically occurs during rapid changes in fluid flow, particularly when pumps abruptly stop or during rapid valve actuation. This results in pressure spikes that can significantly exceed the system's rated operating pressures. The primary symptoms are loud, banging noises, strong vibrations on pipes and fittings and, in extreme cases, material fatigue, leaks and even bursting of pipes or damage to pumps and valves.

Affected facilities primarily include pumping stations, long water supply pipelines, wastewater systems, chemical processing plants and heating/cooling systems where fluids are transported under pressure. The type of water hammer on check valves can be primarily divided into two categories:

• Shutter speed too slow: The valve does not close quickly enough to prevent backflow before the flow changes direction. The backflow that then begins is only stopped by the closing valve, which leads to a pressure surge.
• Shutter speed too fast (Slam): The valve closes extremely quickly, before or exactly when the flow stops or reverses. The kinetic energy of the flowing medium is abruptly slowed down, causing high pressure peaks.

Severity classification:

• Critical: Pipe burst, pump damage, valve failure, uncontrolled media leakage, production stoppage. Immediate action is required.
• Major: Material fatigue, seal leaks, frequent noise, increased maintenance requirements, short-term production impairment. Planning and implementing corrective actions within a short period of time.
• Minor: Occasional noises and vibrations without visible damage, minor impairment of operational safety. Monitoring and preventive planning recommended.

2. Safety precautions

ATTENTION! Working on pressure-carrying systems involves considerable dangers. Strict compliance with safety regulations is essential.

• Lockout/Tagout (LOTO): Before any work on the system begins, pumps must be switched off and secured against being switched on again. All energy sources must be isolated and labeled.
• Pressure relief: The affected pipeline segment must be completely depressurized and drained before opening any fittings or pipeline sections. Residual pressure can result in a dangerous leak of fluids or components.
• Personal protective equipment (PPE): Wearing safety glasses (DIN EN 166), hearing protection (DIN EN 352), protective gloves (DIN EN 388), safety shoes (DIN EN ISO 20345) and, if necessary, protective clothing is mandatory.
• Temperature: There is a risk of scalding with hot media. Sicherstellen, dass das System abgekühlt ist, oder entsprechende Hitzeschutzkleidung tragen.
• Hazardous substances: When conveying dangerous or aggressive media, additional PPE and emergency measures (e.g. emergency showers) must be observed in accordance with the media's safety data sheets.
• Electrical safety: When diagnosing pump drives or electrical actuators, VDE 0105-100 (operation of electrical systems) must be observed.

Always make sure that there is no stored energy (pressure, kinetic energy in the fluid) before intervening in the system.

3. Required diagnostic tools

Specialized measuring devices are required for precise diagnosis of water hammer events.

4. Checklist for initial assessment

Before the detailed diagnosis begins, a comprehensive recording of the operating conditions and the system status is essential. This information helps understand the context of the problem.

5. Systematic diagnostic flowchart

This decision tree guides the technician through diagnosing the water hammer problem. Start at SYMPTOM.

1. SYMPTOM: Water hammer detected (loud knocks, vibrations, pressure peaks).
   1. CHECK: When does the water hammer occur?
      1. IF: When the pump stops.
         1. CHECK: Is the pump stop abrupt or controlled?
            1. IF: Abrupt (e.g. power failure, emergency stop). → Continue with step 1.a.i.I.A.
            2. IF: Controlled standstill (e.g. shut down frequency converter). → Continue with step 1.a.i.I.B.
         2. CHECK: Visual inspection of the check valve. (ATTENTION: Only with unpressurized system, see 2. Safety precautions)
            1. IF: Valve flap blocked, dirty or damage visible. → Probable cause: Mechanical defect/contamination. Continue to Section 7.A.
            2. IF: No visible abnormalities. → Continue with step 1.a.ii.
      2. IF: When starting the pump. (Rather rare with check valves, usually suction difficulties or filling level problems, but possible with extremely rapid opening in an air-filled line). → Probable Cause: Suction of air/cavitation or uncontrolled filling of an empty line. Check system venting and pump suction behavior.
      3. IF: During continuous operation. → Probable Cause: Cavitation on other components, pump problems (e.g. bearing damage), or pulsations in the system. This is probably not a primary check valve water hammer.
   2. CHECK: Measurement of the pressure curve with high-frequency pressure transducers (position: directly before and after the valve).
      1. IF: Pressure peaks occur AFTER the valve is closed or show a "ringing" characteristic. (e.g. pressure increase > 2 * P_Operation). → Probable Cause: The valve flap closes too quickly (slam effect) against the reversing flow. Continue to Section 7.B.
      2. IF: Pressure peaks occur DURING the closing process and are correlated with strong backflow. → Probable Cause: The valve flap closes too slowly, causing significant backflow, which is then abruptly stopped. Continue to Section 7.A.
      3. IF: No significant pressure peaks (> 1.5 * P_Operation), but strong vibrations. → Probable Cause: Resonance or flow separation without direct strong water hammer. Proceed to Section 7.C.
   3. CHECK: Measurement of flow reversal with ultrasonic flow meter.
      1. IF: Flow reversal > 0.5 m/s before the valve is completely closed. → Probable Cause: Valve closes too slowly. Continue to Section 7.A.
      2. IF: Flow reversal < 0.5 m/s, aber immer noch starke Schläge. → Probable Cause: Valve closes too quickly (slam). Continue to Section 7.B.
   4. CHECK: Analysis of the flap movement with a high-speed camera (if accessible).
      1. IF: Valve flap remains open for > 0.5 seconds after the pump has stopped while flow reversal is detected. → Probable Cause: Valve closes too slowly. Continue to Section 7.A.
      2. IF: Valve flap hits the seat abruptly shortly after the flow reversal begins. → Probable Cause: Valve closes too quickly (slam). Continue to Section 7.B.

6. Error-cause matrix

This table lists common symptoms and their likely causes, ordered by how likely they are to occur. The recommended diagnostic tests will help verify the cause.

7. Root cause analysis for each error

7.A. Insufficient valve response time / Closing too slow

Why it happens: A check valve is designed to close quickly when flow is reversed. If this is not the case, the media will flow backwards through the valve before the flapper reaches its seat. Reasons for this could be:

• Contamination and deposits: Particles in the medium can deposit on hinges, bearings or the valve seat, which impairs the smooth movement of the valve.
• Corrosion and wear: Over time, corrosion (particularly with unsuitable materials) can limit the mobility of the components or lead to increased play.
• Increased friction: Deformation of the valve housing (e.g. due to external forces or incorrect assembly), bearings or seals that are too narrow can increase the friction of the valve movement.
• Incorrect or fatigued valve spring: For spring-loaded check valves, a weak or fatigued spring cannot provide sufficient force to close the valve quickly enough.
• Low density of the flap: With gravity check valves, a flap that is too light cannot close quickly enough at certain flow speeds.

How to confirm: In addition to pressure and flow measurement, manual operation (with no pressure system) can check the ease of movement of the flap. An endoscopy can reveal deposits or corrosion. Disassembling the valve to visually inspect the internal components provides definitive evidence.

Damage if not corrected: Slow closing will result in continued backflow which can cause erosion of the valve seat and flapper. The periodic impacts cause material fatigue and can cause seal failure or cracks in the valve body and adjacent piping. This also increases the risk of cavitation downstream.

7.B. Valve closing too quickly (slam)

Why it happens: The slam effect occurs when the valve flap closes extremely quickly, often at a low flow reversal speed. The kinetic energy of the fluid that is still flowing forward or reversing is abruptly converted into pressure energy. Causes:

• Low inertia of the flap: A light flap closes very quickly, which can be problematic in systems with rapid pump shutdowns.
• Incorrect spring configuration: A spring that is too strong and closes the flap too aggressively increases the slam effect.
• Valve incorrectly sized or design unsuitable: Certain types of check valves, such as globe valves, are more susceptible to slam at high flow velocities or rapid flow reversals. Axial check valves are often superior here because they have a shorter stroke distance and therefore shorter closing times.
• No or insufficient damping: The system has no or insufficient measures to dampen the valve closure (e.g. oil, air dampers).

How to confirm: High-frequency pressure transient analysis is the key to confirming the slam effect. The analysis of the closing time of the valve in relation to the pressure wave velocity (according to Allievi) also provides information. A high-speed camera can visualize the abrupt flap impact.

Damage if not corrected: The slam effect causes the highest pressure peaks, which can lead to cracks, fractures and fatigue failure in pipes, flanges and other system components. Seals often fail. The pump can also be damaged by the reflected pressure waves. The noise pollution can exceed the applicable workplace guidelines (e.g. noise protection according to DIN EN ISO 9613).

7.C. Resonance and pipeline vibrations

Why it happens: When the frequency of water hammer events matches the natural frequency of the pipeline or the entire system, resonance occurs. Even moderate pressure surges can then cause increased vibrations. Causes are:

• Inadequate pipe support: Insufficient or incorrectly placed pipe supports allow the pipe to oscillate freely.
• Incorrect pipe diameters: Non-optimized pipe diameters can lead to flow separation or turbulence, which induces vibrations.
• System design: The geometric arrangement of the pipes can promote unfavorable vibration modes.

How to confirm: A vibration analysis with frequency spectrum analysis indicates dominant frequencies. If these correlate with the frequency of water hammer events (measured by pressure transducers), resonance is the cause. A visual inspection can reveal loose or damaged pipe supports.

Damage if not repaired: Sustained resonance vibrations lead to fatigue fractures in weld seams, flanges and fastening points. This can lead to leaks, corrosion under insulation (CUI), and ultimately failure of the piping system. The VDI 3842 (vibration isolation for pipelines) provides guidelines for this.

8. Step-by-step fix procedure

8.A. For closing too slowly (insufficient valve response time)

1. Ensure safety: Switch off, lock and completely empty the system safely in accordance with the LOTO protocol. Carry out residual pressure test. Wear PPE!
2. Provide valve access: Remove pipe insulation and, if necessary, dismantle protective covers.
3. Dismantle the valve: Loosen the flange screws (evenly, crosswise), carefully remove the valve from the line. Disassembly should be carried out in accordance with the manufacturer's instructions.
4. Cleaning and inspection:
   • Thoroughly clean the flap, hinges, bearings and valve seat from deposits, dirt and corrosion (e.g. with suitable cleaning agents and brushes).
   • Visually inspect for wear, cracks, deformation or fatigue on all components, particularly the flapper, hinge pin and valve seat.
5. Component replacement:
   • Replace worn or damaged parts (e.g. flap, hinge bolts, seals, spring) with original spare parts or equivalent components certified according to DIN EN ISO 9001.
   • For spring-loaded valves: Check spring strength and length and replace if necessary according to manufacturer's specifications.
6. Assembly:
   • Assemble the valve carefully according to the manufacturer's assembly instructions.
   • Install the valve in the pipeline taking into account the direction of flow.
   • Fit flange connections with new seals (e.g. according to DIN EN 1514-1) and tighten flange screws step by step and crosswise with the correct torque (according to DIN EN 1515-1 or manufacturer's specifications).
7. Functional test (offline): Manual operation of the flap to check smooth movement and complete closing.
8. Commissioning and verification:
   • Slowly fill and vent the system.
   • Increase pressure gradually to operating pressure and check for leaks.
   • Put the pump into operation and drive the system while observing the measuring instruments.
   • Perform pressure transient and flow measurements again to verify successful resolution of the water hammer. The pressure peaks should now be within the permissible limits (e.g. < 1.2 * P_Operation).

8.B. For closing too quickly (slam)

1. Ensure safety: Switch off, lock and completely empty the system safely in accordance with the LOTO protocol. Wear PPE!
2. Valve testing and selection:
   • Check whether the installed valve type (e.g. lifting, flap, axial check valve) is fundamentally suitable for the system dynamics. For applications requiring rapid flow reversals, axial check valves or gravity-controlled damper valves are often a better choice.
   • If necessary, check the dimensioning of the check valve according to VDI 2067 or comparable hydraulic analysis tools.
3. Install or adjust damping devices:
   • Dampers: Installation of external oil or air dampers that control and slow down the closing process of the valve. The damping setting is critical and must be optimized on site to avoid slow closing (see 8.A).
   • Damped check valves: Replacing the existing valve with a damped check valve that has an integrated damping mechanism (e.g. oil piston or spring-oil damper).
   • Spring Adjustment: For spring-loaded valves, replacing an overly strong spring with a weaker spring or adjusting the preload (if possible) can slow the closing process. However, this requires precise calculation to prevent closing too slowly.
4. Alternative valve types: If problems recur, replacing the valve with a type that is designed for lower flow reversal speeds or has lower inertia (e.g. double flapper check valve, spherical check valve) may make sense.
5. Commissioning and verification:
   • Slowly fill and vent the system.
   • Increase pressure gradually to operating pressure.
   • Put the pump into operation and drive the system while observing the measuring instruments.
   • Carry out pressure transient and flow measurements again. The aim is to reduce the pressure peaks to an acceptable level (< 1.5 * P_Operation, ideally < 1.2 * P_Operation).

8.C. For resonance and pipeline vibrations

1. Ensure safety: Switch off, lock and completely empty the system safely in accordance with the LOTO protocol. Wear PPE!
2. Perform Vibration Analysis:Perform a detailed frequency analysis using the Vibration Analyzer (Section 3) to identify the dominant vibration frequencies.
3. Optimize pipe supports:
   • Check all pipe supports, clamps and supports for tight fit, damage and correct alignment.
   • Retrofit or reinforce missing or inadequate supports to change the natural frequency of the pipeline or increase damping. If necessary, adjust the distance between the supports according to VDI 3842.
   • Installation of vibration dampers or spring hangers at critical points to minimize the transmission of vibrations.
4. Pulsation dampeners or accumulators:
   • For system-related pulsations, pulsation dampeners (silencers for liquids) or membrane/bubble accumulators can be installed to absorb pressure waves and modulate the frequency of water hammer events.
   • The sizing of these components must be done by a professional based on the system hydraulics.
5. Review system design: In complex cases, a review of the overall piping layout and component placement may be required to identify flow non-uniformities or potential resonance points.
6. Commissioning and verification:
   • Slowly fill and vent the system.
   • Increase pressure gradually to operating pressure.
   • Put the pump into operation and drive the system while observing the measuring instruments.
   • Re-perform vibration analysis to ensure vibration amplitudes are within acceptable limits (e.g. < 3 mm/s RMS at operating frequency).

9. Preventive measures

Prevention is crucial to extending the life of assets and minimizing unplanned downtime.

10. Spare Parts & Components

Availability of spare parts is crucial for quick resolution and minimizes downtime. Ensure critical components are in stock.

For detailed information and to order suitable components, please visit our e-catalogue at www.unitecd.com/e-catalog/.

11. References

• DIN EN 1515-1: Flanges and their connections - screw connections - Part 1: Selection of screws and nuts.
• VDI 2067: Economic efficiency of building technology systems – basics and calculation methods. (Relevant for hydraulic dimensioning)
• VDI 3842: Vibration isolation of pipelines – guidelines for planning and execution.
• DIN EN ISO 9613: Acoustics – attenuation of sound when it spreads outdoors. (Relevant for noise protection)
• VDE 0105-100: Operation of electrical systems - Part 100: General requirements for operation.
• OEM manuals and technical documentation of the installed pumps and valves.
• UNITEC-D maintenance guides: "Analysis of pump cavitation", "Sizing of piping systems for optimal flow".