Industrial facilities rely on different types of valves to control fluid flow, which is necessary for the continuity of several manufacturing processes. The design and manufacture of industrial valves follow diverse codes and standards that ensure they are durable, efficient and maintainable.
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These valves undergo repetitive stress and strain due to the pressure and temperature variations of manufacturing processes. The valves can also undergo corrosion due to service fluids' physical and chemical properties. External stress and fluid corrosiveness increase the valves’ susceptibility to failure. Valve failures cause unexpected process downtimes and increase safety risks.
Manufacturing facilities use various valve inspection standards to identify early signs of fatigue and failure – which may result in lower efficiency, leakages and fugitive emissions. These inspections are critical during initial installations and throughout the lifetime of industrial valves. In most facilities, valve inspection is a part of routine maintenance activities and happens at predetermined intervals.
Regular valve inspections following industry-approved standards – such as API 598 and ISO 5208 – enable maintenance supervisors to identify and replace valves with high leakage rates to enhance their efficiency and thus improve the overall effectiveness of the production processes. What are the most critical valve inspection points in a manufacturing facility?
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1) Valve body
The valve body houses internal components, and is the primary pressure boundary with maximum exposure to fluid pressure. As fluids change flow and velocity at the valve sections, the resultant pressure acts against the valve body.
Valve bodies are made from materials possessing unique physical properties to protect them from premature damage or rupture due to fluid pressure. Valve bodies also resist corrosion from the chemical composition and viscosity of the service fluid. The shell undergoes repetitive expansion and contraction due to the variation of process temperatures. Although the contraction or expansion is minute, their long-term impacts are noticeable.
Inspecting valve bodies provides insight into the resilience and potential durability of the valve. Fatigue due to fluid temperature and pressure is likely to cause cracks and crevices on the valve surface; these cracks may be visible to the naked eye. Other surface defects require the use of advanced detection tools. There could be noticeable corrosion on the surface of the valve due to a chemical attack from the service fluid or due to a gradual loss of protective coating.
Technicians can also leverage non-destructive testing techniques to check for tiny surface irregularities and cracks and verify wall thicknesses. The most popular defect detection method is ultrasonic testing; other inspection methods include radiographic testing and eddy current testing.
2) Flanges, Gaskets and Fasteners
Valve flanges connect adjacent pipes. They have unique designs to suit particular fluid applications. The flanges are connected to pipes using detachable connections like threaded, or screwed joints, and permanent methods like welding. Fasteners used for connecting pipes and valves are adjusted to a specific torque. It ensures valves are well secured, and prevents valve misalignment – which may impede fluid flow or cause vibrations at specific pipe sections.
When conducting valve inspection in a manufacturing facility, it is vital to prioritize flanges, gaskets and fasteners. Loose connections, worn-out gaskets or defective flanges can be the primary source of leakages and fugitive emissions. Flanges may develop cracks due to variations in fluid flow conditions. Some inspection methods for these components include:
- Visual inspection of flange ends, gaskets and fasteners. Technicians can identify cracks, rusty, broken or missing fasteners.
- Ultrasonic testing of welded flanges to evaluate thicknesses and identify tiny cracks
- Using torque wrenches to check the tension of screwed connections
It is vital to replace worn-out gaskets immediately. Damaged gaskets are unsuitable for high-pressure fluid applications as they increase safety risks. When changing damaged valves, technicians should also inspect the surface finish, end-to-end flange dimensions, fastener hole diameters and flange thickness.
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3) Valve Discs, Seats and Seals
Internal valve components come into direct contact with fast-moving service fluids. Internal valve components are optimized for media compatibility and durability. Despite adhering to stringent manufacturing standards, valve discs, seats, and seals undergo fluid corrosion – including wire drawing, which causes valves to lose sealing efficiency.
Internal valve damage may be difficult to notice. Inspecting discs, seals and seats is possible during a scheduled plant-wide shutdown. Automated processes utilize condition-monitoring devices to monitor valve performances. Changes in valve performance indicate underlying defects, and the production teams can plan a temporary shutdown for maintenance technicians to conduct further inspection and rectify the problem.
Internal valve components require comprehensive inspections at least once a year. Severe service valves and gas ball valves should be inspected in shorter intervals to ensure the seats and seals are in perfect working conditions. The valve trim contains detachable and replaceable mechanical components. Components that exhibit signs of fatigue require immediate replacement. Confirm the quality of disc connection points, return springs and the thickness of the flow control mechanism (ball, disc or gate).
When non-metallic seals age, they become brittle, while some begin rotting. The ease of replacing these seals depends on the design of the valves. It takes less time and effort to replace seals in 3-piece ball valves than in 2-piece ball valves. 3-piece ball valves have two detachable end caps and a central section; removing the end caps provides easy access to seals. Often, during the maintenance of a 2-piece ball valve, technicians need to remove it from the line, while a 3-piece construction allows the end caps to remain installed in-line, for the valve body can be removed or rotated to allow access to the seals in the center body section.
4) Valve Stem and Actuation Mechanisms
The stem, which connects internal valve components to the actuator, also undergoes corrosion due to the chemical action of fluids. It undergoes surface wear and tear due to friction between the stem and the packing as it moves to open or close the fluid flow path. The stem also experiences torsional and compressive stresses caused by actuator movements. As such, valve stems may fail if not inspected regularly. Actuators provide the force to open and close valves and require routine inspections. It is important to note that an actuator that is not properly mounted to a valve will wear out the valve stem exponentially faster than a properly mounted valve. Therefore, ensure your products are being assembled and tested by a reputable source prior to installation.
Inspection points for valve stems include:
- A build-up of rust or ground metal (or plastic) on the surface
- Loose connection points (including bolts and nuts)
- Level of play or tightness of the stem
The level of inspection for actuation mechanisms depends on the type of actuator used. When inspecting electric actuators, check if electrical terminals, connectors, cables and casings are well insulated. Check the power ratings of the external power supply, cleanliness of motors and lubrication on moveable parts like bearings. When handling hydraulic and pneumatic actuators, inspect cylinders, fluid supply systems and filters. Also, check the quality of electrical connections serving the actuators.
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Conclusion
Valve inspection forms a part of routine maintenance activities in a manufacturing facility. Companies must establish comprehensive asset management plans that ensure adequate and timely maintenance of process valves. Timely valve inspection allows companies to identify and rectify underlying defects. It extends valve durability, facilitates uninterrupted continuity of processes and guarantees optimum operational safety. Valve inspection combines manual and automated inspection techniques depending on the number of valves, accessibility of valve chambers and the intensity of inspection tasks.
