1. Scope and Objective
This technical guide provides a rigorous methodology for the inspection and preventive maintenance of check valves, critical elements in industrial fluid systems in the aerospace and energy sectors. It specifically covers flapper and tilt disc valves, focusing on assessing flapper (leaflet) wear, hinge pin condition, and seat sealing surface integrity. The goal is to provide the essential function of preventing backflow, protecting upstream equipment from potential damage, and maintaining the operational integrity of the system.
This procedure is intended to be applied during planned plant shutdowns, in response to detected flow anomalies, or as part of a predictive maintenance program based on condition monitoring. Failure to comply with the operating specifications and integrity of these components can result in major malfunctions, unplanned production downtime, and safety hazards.
2. Safety Precautions
CRITICAL WARNING: Any intervention on a non-return valve must be preceded by a complete lockout and delockout procedure (LOTO), in accordance with standard NF C 18-510 or equivalent for fluids under pressure. The absence of LOTO can result in serious or fatal injuries due to the release of residual energy.
Mandatory Personal Protective Equipment (PPE):
- Eye Protection: Safety glasses conforming to EN 166 or face shield.
- Hearing Protection: Earplugs or noise-canceling headphones if the ambient sound level exceeds 80 dB(A).
- Manual Protection: Protective gloves adapted to the risks (mechanical EN 388, chemical EN 374, thermal EN 407).
- Personal Protection: Resistant work clothing, flame retardant if there is a risk of fire, EN safety shoes ISO 20345.
- Respiratory Protection: Respiratory mask with appropriate filter if toxic vapors or fine dust are present.
Specific Risks Related to Hazardous Energy:
- Residual Pressure: Always check that there is no pressure before opening the circuit. Use certified pressure gauges (e.g. EN 837-1).
- High Temperature: Allow hot fluids to cool to a safe ambient temperature (< 40°C) or use thermal PPE.
- Chemical Fluids: Refer to the fluid's Safety Data Sheet (SDS) for specific precautions. Provide a fluid recovery system and emergency response kit.
- Stored Kinetic Energy: The flaps of the valves may be under stress and release energy during disassembly. Proceed with caution.
3. Tools and Materials Required
The table below lists the tools and materials essential for a compliant and effective intervention.
| Tool/Material | Specification / Reference | Quantity | Comments |
|---|---|---|---|
| Open-end / eye wrenches | Metric, 10mm to 36mm | 1 game | Alloy steel, compliance ISO 10102/10104 |
| Torque wrench | Range 20 Nm to 300 Nm | 1 | Calibrated, accuracy +/- 3% (EN ISO 6789) |
| Metric sockets | From 10 mm to 36 mm, 1/2'' or 3/4'' square | 1 game | Chrome-Vanadium Steel |
| Feeler gauges | Range 0.05mm to 1.0mm | 1 game | Stainless steel, precision 0.01 mm |
| Vernier caliper / Micrometer | Vernier caliper 0-150 mm (accuracy 0.02 mm); Micrometer 0-25 mm (accuracy 0.001 mm) | 1 of each | Calibrated, compliant ISO 3611 |
| Industrial endoscope | Flexible, diameter 6 mm, length 1 m | 1 | High resolution, LED lighting |
| Cleaning product / Degreaser | Industrial, non-flammable, low residue | 1 aerosol/can | REACH compliant, compatible with valve materials |
| Lint-free cloths | Microfiber or cotton, industrial quality | 1 packet | For cleaning critical surfaces |
| Soft wire brush / Scraper | Brass or fine stainless steel | 1 of each | For removing deposits without damaging surfaces |
| New flange gaskets | Complies with manufacturer's specifications (e.g. EN 1514-1), DN and PN of the valve | 1 game | Material adapted to the fluid and conditions (Graphite, PTFE, EPDM) |
| New studs and nuts | Resistance class 8.8 (EN ISO 898-1) or ASTM A193 B7 for high temperature | 1 game | Dimensions according to valve specifications |
| Anti-seize compound | High temperature (eg: Copper, Nickel) | 1 tube | For bolts and studs |
| Pressure Test Kit | Hydrostatic pump (0-200 bar), Calibrated pressure gauge (0-100 bar, accuracy +/- 0.5%) | 1 | Compliant EN 837-1 |
| Inspection lamp | LED, powerful, explosion-proof if ATEX zone | 1 | For dark areas |
| Collection tray | Chemical resistant, sufficient volume | 1 | To drain fluids |
4. Pre-Maintenance Checklist
Before initiating any dismantling procedure, a systematic check is mandatory to ensure adequate preparation and safety.
| Item | Verification | Acceptance/Rejection Criteria | Notes |
|---|---|---|---|
| Consignment (LOTO) | Visual and mechanical confirmation of energy separation points. | All energy sources (electric, pneumatic, hydraulic, thermal) are isolated, locked and labeled. LOTO procedure signed. | Critical. Do not proceed without validated LOTO. |
| System pressure | Checking the absence of residual pressure upstream and downstream of the valve. | Zero bar pressure gauges. No noise of circulating fluid. | Use a calibrated test gauge if the permanent gauge is doubtful. |
| Fluid temperature | Measurement of the surface temperature of the valve and adjacent pipes. | Surface temperature below 40°C for safe handling without specialized thermal PPE. | If the temperature is high, provide sufficient cooling time or appropriate PPE. |
| Draining the circuit | Visual confirmation of circuit drainage. | No visible fluid leaking from low points or traps. | Anticipate the presence of residual fluid and provide a recovery tank. |
| Maintenance history | Consultation of previous intervention reports on the valve concerned. | Information available on previous repairs, parts replaced, dates. | Allows you to anticipate recurring problems or premature wear. |
| Work environment | Checking the cleanliness, lighting and accessibility of the area. | Clear, well-lit work area, without obstacles. Water points or spill kit nearby if there is a chemical risk. | Provide an anti-fatigue mat for long procedures. |
| Specific risk assessment | Identification of risks not covered by the standard procedure. | All risks are identified and mitigation measures are in place. | Ex: Proximity to operating equipment, access to heights, confined spaces. |
| Availability of spare parts | Physical verification of gaskets, studs, nuts and other necessary spare parts. | All required parts are present, within specifications and undamaged. | Do not begin disassembly without having all the parts. |
5. Detailed Step-by-Step Procedure
System Isolation and Security
Action: Check that the complete LOTO procedure has been applied and validated by the operations manager. Confirm the absence of pressure upstream and downstream of the valve using a calibrated portable pressure gauge.
Specific Values: Pressure read on the pressure gauges must be 0 bar. Surface temperature of the valve and pipes below 40°C.
Good Execution Indicators: Pressure gauges at zero, absence of fluid noise, visual confirmation of isolation valves closed and padlocked, lockout labels in place.
Common errors: Do not independently check the pressure, relying only on fixed indicators. Ignore surface temperature.
Draining and Purging the Valve
Action: Open the purge valves located as close as possible to the valve to completely drain the residual fluid. Use a suitable drain pan and environmental protection if the fluid is dangerous.
Specific Values: Volume of fluid drained in accordance with estimates for the pipe section concerned.
Good Execution Indicators: Total stop of fluid flow, absence of drip. Drainage equipment is clean.
Common errors: Do not completely purge the circuit, underestimate the residual volume.
Removal of the Check Valve
Action: Using open-end wrenches or sockets, gradually loosen the flange studs in a criss-cross pattern. Do not remove the studs completely in one go. Support the weight of the valve.
Specific Values: Uniform loosening to avoid deformation of the flanges. No noticeable residual torque after the first quarter turn.
Good Execution Indicators: Valve separates from the flanges evenly. Flange surfaces undamaged. The studs are removed one by one once the flapper is ready to be removed.
WARNING: The flap may be heavy. Use appropriate lifting means (winch, hoist) if the weight exceeds the manual handling limits (25 kg according to NF X35-109).
Common mistakes: Remove all studs on one side, which can bind the flapper or damage the flanges. Drop the flap.
Detailed Inspection of the Flap (Leaf/Disc)
Action: Visually examine the leaf or disc for erosion, corrosion, pitting, cracking or deformation. Measure leaf/disc thickness at several points and compare to OEM specifications.
Specific Values: The measured thickness must not differ by more than 5% from the manufacturer's nominal value. No visible cracks, even under magnifying glass (x10).
Good Execution Indicators: Smooth surface, without significant degradation. Dimensional conformity. The leaf is removed from the hinge pin for a complete inspection of the sealing and mating surfaces.
Common errors: Ignoring superficial bites which may progress. Do not measure the leaf thickness at critical points.
Checking the Hinge Axis and its Supports
Action: Inspect the hinge pin (and flapper arm if applicable) for wear, excessive play, binding, deformation or corrosion. Measure the axle diameter at several locations and assess the clearance in the bracket bores.
Specific Values: The diameter of the axle must not show wear greater than 0.1 mm compared to the nominal value. The radial clearance between the axle and its support must not exceed 0.2 mm.
Good Execution Indicators: Free movement without hard points of the axis in its supports. Absence of abnormal friction marks. The axle retaining pins or rings are in perfect condition.
Common errors: Failure to check radial clearance, which can lead to vibration and accelerated wear. Do not inspect the axle retaining systems.
Headquarters Integrity Assessment
Action: Carefully clean the seat sealing surface. Examine visually with an industrial endoscope if necessary. Look for signs of erosion, gouges, pits, or embedded foreign bodies. Carry out a contact test with Prussian blue.
Specific Values: The sealing surface must be free of defects (> 0.05 mm in depth or width). The Prussian blue test should reveal a continuous, uniform contact line at least 2mm wide around the entire perimeter of the seat.
Good Workmanship Indicators: Shiny seat surface, without irregularities to the touch. Homogeneous contact line after test.
Common mistakes: Cleaning the seat with an abrasive tool. Ignore small defects that can compromise waterproofing.
Cleaning of Components and Preparation
Action: Clean all valve parts with a suitable degreaser and lint-free cloths. Remove any old gasket residue from the valve and pipe flanges. Inspect bolt holes for damaged threads.
Good Execution Indicators: Clean and dry metal surfaces. Bolt holes unobstructed, threads intact.
Common errors: Leaving gasket residue on sealing surfaces, leading to leaks.
Reassembling the Check Valve
Action: Install the new flange gaskets. Position the non-return valve respecting the direction of flow indicated on the body. Insert the new studs and nuts, then apply anti-seize compound to the threads. Hand tighten the studs, then apply the specified torque using the torque wrench, following a crisscross pattern.
Specific Values: Use of compliant joints EN 1514-1. Tightening torque for M20 Class 8.8 studs: 270 Nm (according to NF E 25-030). For M16 bolts, approximately 160 Nm. Check the valve manufacturer's exact specifications.
Good Execution Indicators: Uniform tightening of the studs, absence of space between the flanges and the valve. Torque wrench mark on each nut to confirm tightness.
WARNING: Over- or under-tightening of flange studs can result in permanent flange deformation or catastrophic leakage. It is imperative to respect the specified torque values.
Common Mistakes: Forgetting anti-seize compound, tightening studs unbalanced.
Waterproof and Functional Test
Action: Repressurize the system very gradually, purging residual air if necessary. Visually check for external leaks at the flanges and valve body at operating pressure. For critical applications, perform a complete hydrostatic test according to EN 12266-1 standard, maintaining a pressure of 1.5 times the design pressure for 15 minutes.
Specific Values: Test pressure in accordance with system specifications (e.g.: Nominal operating pressure = 40 bar; Hydrostatic pressure = 60 bar). No pressure drop during hydrostatic test.
Good Execution Indicators: Stable pressure gauges. Total absence of leak visible or detected by a leak detection product. The valve must close automatically in the absence of flow.
Common errors: Repressurize too quickly. Do not bleed air from the circuit before testing. Omit hydrostatic testing for critical applications.
6. Post-Maintenance Checklist
Once the intervention is completed, this list guarantees the safe and compliant return to service of the system.
| Test / Verification | Expected Result | Current Result | Pass/Fail |
|---|---|---|---|
| Absence of external leaks | No leaks visible or detected at the flanges, valve body, or connections. | ||
| Rated working pressure achieved | The system has reached and maintains its stable operating pressure. | ||
| Direction of flow confirmed | The non-return valve is installed in the correct direction of flow and operates correctly (blocking backflow). | ||
| Valve operation | The valve flap/disc opens fully under operating pressure and closes tightly in the absence of flow or in the event of reflux. | ||
| Cleaning the work area | The work area is clean, free of tools, debris or fluid residue. | ||
| Storage of tools and PPE | All tools are cleaned, inspected and stored. PPE is decontaminated and stored. | ||
| Updating LOTO records | The deconsignment procedure has been carried out and signed by the parties concerned. | ||
| Maintenance documentation | The intervention report is completed, including measurements, parts replaced, observations. |
7. Troubleshooting Guide
This table provides assistance in diagnosing and correcting common problems after inspection or maintenance.
| Symptom | Probable Cause | Corrective Action |
|---|---|---|
| External leak at flanges | Insufficient stud tightening; Damaged or incorrectly positioned flange gasket; Deformed or scratched flange surface. | Tighten the studs to the specified torque following the crisscross pattern. If the leak persists, replace the flange gasket. Inspect and grind flange surfaces if necessary. |
| Significant fluid reflux | Flap (flap/disc) stuck in open position; Excessive wear of the sealing surface of the seat or valve; Foreign body lodged on the seat. | Disassemble the valve. Clean the seat and the leaf. Replace the leaf or seat if wear exceeds tolerances. Check the hinge pin for free movement. |
| Clicking or vibration of the flap | Pulsating or excessive flow; Oversized flapper; Excessive wear or play in the hinge pin or leaf. | Check system flow and pressure conditions. Consider installing a check valve with hydraulic damper. Inspect and replace worn internal components. |
| Unexplained drop in pressure | Internal leak from the valve (non-sealing); External leak difficult to see; Sealing problem elsewhere in the circuit. | Perform a detailed pressure test with leak detection. Carefully inspect the internal sealing of the valve (dye or air test). |
| Flap does not open fully | Seizing of the hinge pin; Internal obstruction (foreign body); Deformed clapper. | Disassemble and inspect the hinge pin and leaf. Clear any obstructions. Replace damaged parts. |
8. Recommended Maintenance Schedule
This table provides planning for preventive maintenance of check valves.
| Task | Frequency | Estimated Duration | Skill Level |
|---|---|---|---|
| External visual inspection and leak detection | Quarterly | 0.5 hour | Operator / Technician L1 |
| Functional check (opening/closing) if possible | Semi-annual | 0.5 hour | Technician L1 |
| Detailed internal inspection (disassembly, examination of the leaf, axis, seat), replacement of seals | Every 2 years or 5000 operating hours (whichever comes first) | 4 to 8 hours | Technician L2 |
| Replacement of the hinge pin and leaf (if worn) | Every 5 years or according to the results of the detailed inspection | 6 to 10 hours | Technician L3 |
| Hydrostatic requalification of the valve (according to standard EN 12266-1) | Every 5 years or according to regulatory requirements | 2 to 4 hours | Technician L2 |
9. Spare Parts Reference
For the reliability of your systems, use quality spare parts. Find a wide assortment in the UNITEC electronic catalog.
| Part Description | Typical Specification | UNITEC category |
|---|---|---|
| Flange gasket set | Flexible graphite reinforced stainless steel, PN16/25/40, conformity EN 1514-1, DN dimensions of the valve | Waterproofing |
| hinge pin | 316/316L stainless steel, ground, specific valve model dimensions | Internal Components |
| Flap / Flap disc | Stainless steel (304, 316, Duplex), with or without Stellite coating on the sealing surface, specific dimensions of the valve model | Internal Components |
| Studs and nuts | Alloy Steel 8.8 (EN ISO 898-1) or ASTM A193 B7 / A194 2H Chromoly Steel (for high temperature/pressure) | Bolts and Fixings |
| Wear rings (if present) | PTFE, Bronze, or composite materials, specific dimensions of the valve model | Internal Components |
| Spring (if spring loaded valve) | Stainless steel, stiffness rate and specific dimensions of the valve model | Internal Components |
To order certified spare parts adapted to your needs, visit our UNITEC electronic catalog.
10. References
- EN 12266-1: Testing of valves – Part 1: Pressure testing, test procedures and acceptance criteria – Mandatory requirements.
- NF E 25-030: Fastening elements – Tightening torques.
- API 594: Check Valves: Flanged, Lug, Wafer and Butt-welding Ends.
- EN ISO 6789: Manual torque wrenches – Requirements and test methods for design conformity and quality – Tightening torque.
- NF C 18-510: Operations on electrical works and installations or in their vicinity – Prevention of electrical risks.
- Original Manufacturer's (OEM) Operation and Maintenance Manual: Always consult the specific documentation for the installed valve model for exact dimensional tolerances and precise tightening torques.
