Problems with Valves?
Some valves have been installed in process plants and have operated for many years without any problems being encountered. Other valves have been troublesome or have failed in a short period of time – why is this?
The construction of some valve types is not always fully understood by the users; gate, globe check, plug, ball, butterfly, needle valves to name but a few, and valves with flanged ends, butt weld ends, socket weld ends, screwed ends and flangeless valves. Manually operated, gear operated or pneumatic, hydraulic or electrically actuated valves add to the variables available to users. Knowledge of valve types and their particular construction is vital to efficient and safe use in service.
Many valves were originally designed more than 50 years ago and have been manufactured and used continuously since that time. The introduction or revision of international valve design standards over the years has resulted in some design improvements being made, which in some cases eliminated potential safety hazards associated with some valve types. However many valves already in service have given no operational safety problems and remain unchanged.
Some examples of design changes that have removed potential safety hazards are the inclusion of anti blow out stems on ball and butterfly valves, inclusion of retention pin and screwed seat retainer on ball valves, removal of directly screwed bonnet retention on globe valves, threaded or pinned connection between stem and wedge in gate valves and the introduction of cavity relief provision in ball valves. All valves manufactured by reputable manufacturers now include these design improvements as standard.
There are, however, other factors that influence operational safety of valves in process plants. Among these are lack of knowledge of valve construction and lack of valve maintenance training among users of valves. Design and operation of all valve types should be an essential factor in the training of maintenance personnel.
In recent times, this situation may have improved, due to the PED statutory requirement that valves be provided with comprehensive operating instructions, which has ensured that the necessary technical information is available to users (in the European Economic Area). However, some examples of the creation of potential safety hazards during process plant modifications or maintenance include:-
Removal or loosening of flange bolting on Ball valve split body connection
Incorrect fitting of gland packing flange with operating stop on plug valve
Use of welded in line valves that do not permit access to internal sealing components
Removal of bonnet bolts instead of actuator retention bolts when removing pneumatic actuator from plug valve
Use of ‘cheater’ pipe extensions on handle or gear operated valves.
Valve design, valve manufacturer quality assurance and on site valve maintenance are important factors in operational safety on process plants. However, in recent years there has emerged another factor that is influencing this important application area. This is the emergence of low cost valves from the far east. Some of these valve manufacturers are producing valves that are of acceptable quality, but many are unproven and may be of suspect quality.
The reliability of valves on process plants is a vital issue and it is known to be false economy to purchase valves on price alone. BVAA member companies manufacture valves of all types, sizes, materials and pressure ratings suitable for all process requirements with guaranteed quality – why risk going elsewhere?
Published in Valve User Magazine Issue 7
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