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Diagnostics and elimination of hydraulic shock on non-return valves: analysis of closure, speed of operation and transient processes in systems

Technical analysis: Troubleshooting check valve water hammer: slam analysis, closing speed diagnosis, damper selection,

Diagnostics and elimination of hydraulic shock on non-return valves: analysis of closure, speed of operation and transient processes in systems

1. Description of the problem and scope

Hydraulic shock (hydraulic shock) on non-return valves occurs during a sudden change in the fluid flow rate in the pressure pipelines of industrial enterprises (energy, water supply, chemical and food industries). The phenomenon is characterized by a critical pressure jump, which many times exceeds the working pressure of the system ($P_{slave}$). The main symptoms include metal knocking in the area of ​​installation of fittings, vibration of pipelines with an amplitude exceeding the permissible norms, leakage at flange connections, deformation or destruction of the valve body, as well as damage to supporting structures.

According to the classification of UNITEC-D GmbH, this type of fault belongs to the category Critical (Severity Level 1). It directly threatens the integrity of technological equipment, pumping units and causes an emergency stop of production lines. The scope of this manual covers centrifugal pumping stations, circulating water supply systems, pressure collectors and technological pipelines with operating parameters: pressure up to 64 bar, ambient temperature from -20°C to +250°C.

2. Precautions (Safety)

ATTENTION! HIGH PRESSURE AND ENERGY HAZARD OF COMPRESSED ENVIRONMENT

Before starting any diagnostic or repair work, perform the following mandatory safety requirements:

  • Lockout/Tagout (LOTO): Completely stop the pumping units, disconnect the supply electrical circuit breakers and install padlocks according to the LOTO procedure.
  • From pressure relief: Ensure complete pressure relief in the piping section before and after the non-return valve. Open the drain valves. Control the immersion of the manometer needle on the zero mark.
  • PPE: Use safety glasses with side protection, a safety helmet, heat-resistant gloves (if the ambient temperature is above +50°C), and special shoes with a metal toe (according to EN ISO 20345).
  • Load lifting works: The weight of dismantled heavy check valves (DN100-DN500) can exceed 50 kg. Use certified lifting mechanisms and rigging.

3. Necessary diagnostic tools

Name of the tool Specification / Model Measuring range Purpose
Digital manometer / Pressure sensor with registration Pewatron / Keller Leo Series 0 – 100 bar (accuracy 0.1%) Measurement of the amplitude of pressure peaks and transient processes when the pump is stopped
Portable vibration analyzer Fluke 810 or equivalent 10 Hz – 10 kHz, 0.1 – 50 mm/s Measurement of the vibration level on the valve body and pipe
High-speed acoustic recorder / Hydrophone Bruel & Kjaer Field Acoustic Kit 20 Hz - 20 kHz Fixation of the closing moment of the shutter and the delay time of the return flow
Laser tachometer / RPM sensor Testo 465 1 - 99,999 rpm Determination of the coast-down time of the pump impeller
Digital multimeter Fluke 87V 0 - 1000 V, 0 - 10 A, 0 - 50 MΩ Checking control circuits of frequency converters and relay protection

4. Initial condition assessment checklist

The observation parameter Normal value Limit value (Alarm) Actual status / Record
Frequency of water hammer Missing (0 events/launch) 1 or more beats per start/stop cycle [ ] Fixed / [ ] No
Vibration level of the valve body < 2.8 mm/s (ISO 10816-3) > 7.1 mm/s (critical danger zone)
Maximum peak pressure ($P_{max}$) $P_{max} \le 1.2 \cdot P_{slave}$ $P_{max} > 1.5 \cdot P_{rab}$ (or approaching the strength limit)
Pump run-in time before stopping According to the pump data sheet (> 3 sec) Momentary stop (< 0.5 sec) in case of power failure
Condition of external surfaces and fasteners Dry flanges, no microcracks Tearing leaks, weakening of pins

5. Systematic diagnostic block diagram

  • Step 1. Identifying the moment of occurrence of the shock
    • IF the water hammer occurs INSTANTLY after an emergency shutdown of the pump power supply:
      • Go to Step 2 (Analysis of the closing speed of the check valve).
    • IF water hammer occurs WHILE starting the pump or opening the valve:
      • Go to Step 5 (Air Traps / Quick Fill Lines).
  • Step 2. Check the valve gate dynamics
    • Connect the speed pressure sensor and acoustic recorder just before and after the check valve.
      • IF the reverse flow time ($t_{rev}$) is less than the valve closing time ($t_{cl}$), a slam occurs → Go to Step 3.
      • IF shutter remains stuck due to hinge wear → Go to Step 4.
  • Step 3. Analysis of System Damping and Inertia Characteristics
    • Check for a hydraulic or pneumatic damper on the check valve.
      • IF damper missing or incorrectly adjusted → Requires installation of a hydraulic damper or conversion to a two-leaf valve with torsion springs.
  • Step 4. Mechanical inspection of the internals
    • Perform valve cover removal (per LOTO).
      • IF axle wear, plate jamming or spring damage is detected → Replace internal repair kit.
  • Step 5. Check for trapped air pockets
    • Inspect the higher points of the pipeline for air pockets.
      • IF pistons missing or clogged → Installation and cleaning of air exhaust valves (DSTU EN 1074-4).

6. Fault-Cause Matrix

Symptom Probable causes (in order of probability) Diagnostic test Expected result when confirming the cause
A strong metallic impact immediately after the pump stops 1. Too fast closing of the heavy rotary disc.
2. Lack of spring or hydraulic adjustment.
3. Excessive backflow rate.		 Recording of the pressure oscillogram when the pump is stopped with high sampling frequency sensors (>1 kHz). Fixation of a steep front of pressure increase ($\Delta P > 15 ext{ bar}$) synchronously with the sound of closing the valve.
Hum, vibration and pressure cycling during pump operation 1. Insufficient flow of medium for full opening of the plate (valve flutter).
2. Wear of the lifting valve guide bushings.		 Measurement of vibration frequency (spectral analysis) and verification of the operating point of the pump on the Q-H curve. Detection of discrete peaks at vibration frequencies corresponding to the oscillation frequency of the unfixed shutter.
Shocks during the start-up of the pumping unit 1. Filling an empty or partially emptied pipeline with air.
2. Quick opening of the shut-off valve after the valve.		 Pressure control at the lower and upper points of the track when starting the engine. A sharp drop in pressure to zero followed by an extreme jump when the flow meets an air plug.
Slow closing with reverse leakage 1. Contamination of sealing surfaces with solid particles.
2. Wear or destruction of the elastomeric/metal seat.		 Pressure drop testing and backflow testing (tightness according to ISO 5208). Fixation of reverse flow of medium through a closed valve and reduction of holding pressure.

7. Root cause analysis for each type of malfunction

7.1. Instant closing of the rotary shutter (Disc Slam)

Why it occurs: When the pump motor is turned off, the liquid under the influence of gravity and drop begins to move back with acceleration. If the check valve has a large mass and large gate free travel without spring or hydraulic damping, the disc is picked up by the backflow and hits the seat at high speed until the mass of fluid in the entire system is completely braked.

How to confirm: Comparison of the liquid reversal time $t_{rev}$ (calculated from the length of the pipeline and the speed of sound in the medium) with the closing time of the valve $t_{cl}$ according to the acoustic sensor data. If $t_{cl} < t_{rev}$, the closure occurs at the moment of the maximum speed of the reverse flow.

Consequences: Destruction of the axis of the valve hinge, chipping off pieces of the plate, damage to the discharge nozzles of the pump, destruction of the foundation bolts.

7.2. Shutter flutter (Flutter / Oscillation)

Why it occurs: Occurs when the pump is operating in a mode that does not correspond to the nominal flow (for example, when throttling at the outlet). The flow rate is not enough to hold the valve fully open, so the plate constantly oscillates ("dances") near the seat.

How to confirm: Spectral vibration analysis detects high frequency harmonics in the range of 50-300 Hz on the valve body. External inspection shows intensive wear of the trunnions and hinged joints.

Consequences: Rapid abrasive wear of moving elements, formation of burrs, complete jamming or loss of tightness.

8. Step-by-step troubleshooting procedures (Resolution Procedures)

Procedure 1: Replacing a standard rotary valve with a two-leaf spring or hydraulic damper valve

  1. Follow the LOTO procedure (Chapter 2). Drain the medium from the piping section.
  2. Dismantle the faulty non-return valve. Check the parallelism and condition of the flanges on the pipeline (the deviation should not exceed 0.5 mm).
  3. Clean the seats from the remains of the old gasket.
  4. Install a new check valve (eg UNITEC-D Dual-Plate or Swing series with adjustable hydraulic damper). Note the flow direction arrow on the housing.
  5. Install new paronite or graphite gaskets (according to DIN EN 1514-1).
  6. Tighten the bolts/studs of the crosswise flange connection with a torque wrench in three stages (30% - 70% - 100% of the nominal torque, for example, for M20 strength class 8.8 - tightening torque 390 N·m).
  7. Carry out a test run of the system in manual control mode at minimum revolutions.
  8. Perform pressure and vibration control measurements.

Procedure 2: Adjusting the external hydraulic damper (for valves DN200 and above)

  1. Check the level and condition of the working fluid in the damper hydraulic cylinder (use a synthetic lubricant according to OEM recommendations).
  2. Open the damper control throttle by 50%.
  3. Carry out a control stop of the pump unit, fixing the closing speed of the shutter with the help of a manometer.
  4. Gradually reduce the throttle cross-section to increase the closing time of the last 15° of the plate stroke (recommended smoothing time: 1.5 – 3.0 seconds).
  5. Fasten the adjusting screw of the damper with a special nut and seal.

9. Preventive Measures

The root cause Prevention strategy Monitoring method Recommended interval
Wear of moving elements and hinges Planned replacement of internal repair kits and bushings. Periodic vibration control (ISO 10816). Every 6 months
Accumulation of air in the collector Installation and maintenance of automatic pistons at upper points. Visual inspection of the operation of the air release valves. Monthly
Reverse water hammer due to mismatch of characteristics Carrying out hydraulic modeling of the network when changing pump operating modes. Analysis of SCADA logs for the presence of pressure peaks. Annually / after modernization

10. Spare parts and components

Description of the part Specification When to replace Category UNITEC
Dual plate check valve DN50 – DN600, PN16-PN40, body GG25/GGG40, discs SS304 In case of mechanical damage to the discs or loss of seat tightness Industrial shut-off fittings
Repair kit for saddle seals EPDM / NBR / PTFE (according to the temperature of the environment) When leakage occurs in the closed position (> class A according to ISO 5208) Complementing fittings
Hydraulic damper of the check valve For DN250-DN800, adjustable damping time 1-5 s In case of cylinder failure or loss of tightness of the rod Water hammer protection systems
Torsion / compression spring (variable) Stainless steel AISI 316, high fatigue strength When the closing force is reduced or the turns are cracked Spare parts of mechanisms

To select and order compatible components, go to the UNITEC-D Electronic Catalog.

11. References and regulations

  • DSTU EN 1074-4: Fittings for water supply. Requirements for fitness for purpose and relevant tests. Part 4. Air release valves.
  • ISO 5208: Industrial fittings. Leakage testing of metal valves.
  • ISO 10816-3: Mechanical vibration. Evaluation of machine vibration by measurements on non-rotating parts.
  • UNITEC Engineering Guidelines: UG-VALV-04 "Methods of water hammer prevention in pressure networks of industrial enterprises".

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