1. Problem Description and Area of Application
Hydraulic shock (water hammer) on check valves is a critical dynamic failure, which occurs due to a sudden change in the flow velocity of the medium (water, process fluids, condensate) during closure of the valve closure element. This process is accompanied by a sharp local pressure increase, which exceeds the system operating pressure several times, and is characterized by a typical metallic sound ("shock"), strong pipeline vibrations, and a risk of catastrophic damage to the equipment, pump installations, and connections.
Involved equipment:
- Centrifugal pump stations (water supply, energy, chemical industries).
- Long main pipelines (L > 50 m) with high geodetic height difference.
- Frequency-controlled (VFD) pump unit lines.
Severity classification: CRITICAL. Ignoring the symptoms leads to fatigue failure of the check valve housing, shearing off of pump anchor bolts, failure of support brackets and leakage from flange connections with ejection of the working medium.
2. Precautionary Measures
ПОПЕРЕДЖЕННЯ БЕЗПЕКИ: Перед початком діагностики та будь-яких робіт на трубопроводі виконайте процедуру блокування/маркування (LOTO). Переконайтеся, що система повністю знеструмлена, а тиск у ділянці монтажу зворотного клапана знижено до 0 бар. Використовуйте засоби індивідуального захисту (ЗІЗ): захисні окуляри, захисне взуття, каску та рукавички для захисту від механічних пошкоджень та гарячої рідини/хімікатів. Пам'ятайте про накопичену потенційну енергію стиснутого повітря або пружин у демпфіруючих пристроях клапанів.
3. Required diagnostic tools
| Tool Name | Specification / Model | Measurement range | Purpose |
|---|---|---|---|
| Fast-acting pressure recorder (pressure gauge with peak memory) | WIKA / Keller with sampling frequency > 1000 Hz | 0 – 60 bar | Pressure peak fixation during pump stop and valve closure |
| Vibroanalyzer | SKF / Fluke with a simple accelerometer | 10 – 1000 Hz, 0 – 50 mm/s | Measurement of vibration level of the valve body and pipeline |
| Ultrasonic flow meter | Fuji Electric / Flexim (Sheet) | 0.01 – 25 m/s | Definition of flow velocity before closure |
| Digital multimeter | Fluke 87V | 0 – 1000 V / 4-20 mA | Signal feedback check and pump control |
4. Initial Assessment Checklist
| Parameter for checking | Normal value | Value in case of failure | Actual result (Fill in) |
|---|---|---|---|
| Full stop time of the pump (in case of emergency shutdown) | 3 – 5 seconds (with flywheel/VFD) | < 0.5 seconds (instantaneous power failure) | |
| Vibration level on the valve body (ISO 10816 / ISO 20816) | < 2.8 mm/s (Class A/B) | 11.2 mm/s (Class D, emergency) | |
| Maximum pressure peak at closure ($P_{max}$) | $P_{max} \le 1.15 \times P_{work}$ | $P_{max} \ge 2.0 \times P_{work}$ | |
| Condition of the travel limiter and damper | Smooth operation, no play | Sticking, rod wear, leakage from hydraulic cylinder |
5. Systematic Diagnostic Block Diagram
- STEP 1: Locating the source and nature of the sound
- IF the impact occurs at the moment of pump startup:
- Check for air in the system. Perform air venting.
- If the impact occurs at the moment of pump shutdown (emergency or scheduled):
- Proceed to STEP 2 (Valve Closure Speed Analysis).
- IF the impact occurs at the moment of pump startup:
- STEP 2: Measuring Flow Reversal Time and Closure Speed
- Connect the ultrasonic flow meter and pressure sensor.
- IF flow reversal occurs with a speed above 0.3 m/s to full closure of the disc:
- PROCEED TO STEP 3 (Checking the Mass and Inertia of the Disc/Spring).
- The IF plate clicks immediately when the pump is stopped:
- Proceed to Step 4 (Hydraulic/Pneumatic Damper Check).
- IF flow reversal occurs with a speed above 0.3 m/s to full closure of the disc:
- Connect the ultrasonic flow meter and pressure sensor.
- КРОК 3: Оцінка геометрії системи та часу вибігу насоса
- Розрахуйте фазу хвилі гідроудару: $T = \frac{2L}{c}$ (де $L$ — довжина трубопроводу, $c$ — швидкість звуку в середовищі).
- ІФ час вибігу насоса менший за час фази хвилі ($t_{coast} < T$):
- ПРИЧИНА ПІДТВЕРДЖЕНА: Недостатній інерційний момент агрегату або відсутність регульованого часу зупинки VFD.
- ІФ час вибігу насоса менший за час фази хвилі ($t_{coast} < T$):
- Розрахуйте фазу хвилі гідроудару: $T = \frac{2L}{c}$ (де $L$ — довжина трубопроводу, $c$ — швидкість звуку в середовищі).
6. Fault Matrix and Causes
| Symptom | Likely causes (by probability) | Diagnostic test | Expected result upon confirmation |
|---|---|---|---|
| Reported metallic impact at the moment of pump motor stoppage | 1. Too fast closure of the check valve disc. 2. Absence or malfunction of the hydraulic damper. 3. Excessive reverse flow velocity. |
Pressure recording with a fast-acting manometer and visual inspection of the valve stem. | Rapid pressure peak (pressure surge > 15 bar per millisecond), play in the valve hinge joints. |
| Constant vibration of the pipeline during pump operation at stable flow rate | 1. Incomplete opening of the check valve disc (flutter). 2. Mismatch of the valve size (DN) to the actual flow. |
Pressure drop measurement on the valve and vibration spectrum analysis. | High pressure drop ($\Delta P > 0.5$ bar) and high-frequency vibration at the blade passing frequency. |
| Water hammer at pump startup | 1. Presence of air accumulations at the upper points of the pipeline. 2. Empty pipeline in the absence of a filling valve. |
Testing of the operation of the vacuum pumps and the filling level of the line. | Low pressure on the manometer at start-up, sharp jump upon encountering a water column with an air cushion. |
7. Root Cause Analysis for each defect
7.1. Protected closing speed of the check valve
Why it occurs: Standard rotary or double-seat valves without dampers close in fractions of a second under the influence of springs or gravity when subjected to a reverse flow wave. When the mass of the liquid column moving backward strikes the stationary disc, the kinetic energy is instantly converted into potential energy of elastic deformation of the liquid and pipe walls, generating a pressure wave (according to Joukowsky's formula: $\Delta P = \rho \cdot c \cdot \Delta v$).
Як підтвердити: Графік швидкого датчика тиску показує вертикальний фронт зростання тиску одночасно з фіксацією кінцевого вимикача закриття клапана.
Наслідки: Руйнування шарнірних вузлів тарілки, зрізання осі клапана, поява тріщин у чавунному або сталевому корпусі.
7.2. Flap (vibration) of the disc due to insufficient flow
Why it occurs: If the check valve is selected based on the pipe diameter rather than the actual flow rate, its operating point is below the minimum speed of full opening. The disc balances in the half-open position, constantly striking against the seat.
Як підтвердити: Швидкість потоку нижче мінімальної експлуатаційної швидкості для даного типорозміру (для стандартних клапанів < 1.5 м/с). Знос ущільнювальних поверхонь сідла та тарілки носить рівномірний кільцевий характер.
Наслідки: Інтенсивний абразивний знос контактних поверхонь, втрата герметичності (зворотні протікання), руйнування пружини через циклічну втому.
8. Step-by-step troubleshooting procedures
- Valve replacement/modernization procedure with a hydraulic damper model:
- De-energize the pump unit, close the shut-off valve before and after the valve, perform LOTO.
- Disassembly of the old valve. Clean the flange surfaces to a metallic shine, replace the gaskets (according to EN 1514-1).
- Mount a new check valve with an external hydraulic or pneumatic damper (for example, series UNITEC-HC), following the direction of the flow arrow on the body.
- Set the damping cylinder throttling time: turn the conical valve of the damper to the stop, then turn it back 1.5 - 2 turns to ensure smooth closure of the last 15% of the piston stroke.
- Perform a test run and stop of the pump under the control of the pressure gauge. Achieve the absence of pressure spikes above 1.25 times the working pressure.
- Frequency converter (VFD) setup for water hammer mitigation:
- Enter the pump drive parameter menu.
- Change the deceleration time (Deceleration Time / Ramp Down) from 0.5 s to a minimum of 3.5 – 6.0 s depending on the length of the pressure pipeline.
- Program a special S-shaped deceleration curve (S-Curve) for smooth reduction of pump speed before complete engine stop.
9. Preventive Measures
| Primary Cause | Preventive strategy | Monitoring method | Recommended interval |
|---|---|---|---|
| Shaft seal wear and damper hinge wear | Planned inspection and replacement of bearings and working fluid of the hydraulic damper | Visual inspection of leaks, check of closing time | Every 6 months |
| Change of network operating modes (reduction of consumption) | Hydraulic system audit, selection of valve according to nominal flow rate (not pipe diameter) | SCADA Data Analysis by Flow and Pressure | Annually |
| Valve spring breakage | Preventive replacement of return spring before reaching the cycle life | Measurement of wear force on the test stand | Every 20 000 hours or 3 years |
10. Spare Parts and Components
| Description of the part | Specification | When to replace | Category UNITEC |
|---|---|---|---|
| Hydraulic Damper Check Valve | Du 50 – Du 600, Pu 16 / Pu 40, Steel/Stainless Steel | In case of mechanical damage to the body or constant hydraulic impact | Industrial valves -> Check valves |
| Hydraulic Damper Repair Kit | Sealing rings NBR/FKM, gaskets, flanged valve | In case of oil leaks or loss of damping effect | Hydraulic system components -> Seals |
| Spring of the check valve | Stainless steel AISI 316, strengthened | When the reverse force is reduced or during replacement during scheduled maintenance | Springs and mechanical parts |
| Fast-acting pressure sensor | 4-20 mA, output signal with peak detector | When zero drift or no signal is present | Test and measurement instruments -> Pressure sensors |
Full range of certified valve and components for water hammer protection is available in our catalog: https://www.unitecd.com/e-catalog/
11. References
- DSTU EN 12334:2017 Industrial Equipment. Metal Back Pressure Relief Valves for Water Supply Systems.
- ISO 5208 Промислова арматура. Випробування клапанів під тиском.
- ISO 20816 Vibration condition monitoring of machines.
- Technical documentation and installation instructions from OEM manufacturers of industrial valves UNITEC-D.
- UNITEC Maintenance Manual for Pumping Stations and Transition Processes in Hydraulic Networks.
