I recently read an engineer’s confession on LinkedIn
about his OVD problem, otherwise known as “Obsessive Valve Disorder”. Judging by the comments section he had many sympathisers. Apparently OVD symptoms include “involuntary thoughts, impulses or images which invade the consciousness causing heightened anxiety or discomfort”. This reminded me of a recent interview done by one of our editors, in which an end user described what it takes to manage plants shutdowns, to the point there was a “sort of urgency if everything I do, which keeps me focused”. It seems being “bitten by the valve bug”
is commonplace if you’re in the industry long enough.
This got me wondering - do these people lie awake at night, dreaming of a perfect valve? In fact, does the perfect valve actually exist?
Of course we know there is no such thing - and in fact any expert will stress the importance of selecting the right valve for the right application. But let’s take a selection of valves for argument’s sake (as well as my notes from a recent course
) to have a look at the relative merits and shorfalls of some of the main valve types:
Perfect valve candidate # 1: ”the “through conduit” valve
The “through conduit” valve is a peculiar member of the gate valve family. Most gate valves have a solid component which moves into the pipeline to acheive shut-off. The through conduit valve has a closing rectangular-shaped element with a solid top half and a large aperture in the bottom half. When the valve is opened the aperture is fully aligned with the pipeline to allow flow. Such valves are very well suited to longer pipelines, allowing “pigs” to clean and monitor pipelines. Perfect? No. These valves often have spring loaded soft seats which means they are less suited to elevated temperatures.
Perfect valve candidate # 2: ”the “trunnion-mounted ball” valve
This candidate has a pivot or ‘trunnion’ mounted under the ball to provide support and keep the ball in place. It can be manufactured in a number of different ways, with either top-entry (allowing engineers to access and repair the internals without the expensive proposition of moving this heavy valve) or can be fully welded for buried pipelines where leaks to the surrounding areas cannot be risked. But what about the accessing these valves for maintenance, such as administering sealant or grease if they start to stick? Well, it seems these can be fitted out with surface-accessible injection lines to inject these substances. Solved!
The upstream seat moves towards the ball to create a seal on closing. Even on larger size the trunnion helps keep the required operating torque quite low, so that relatively low power actuators can be used. This design is well suited for high pressure applications, and can also be used for larger pipelines - sizes over 60” are indeed possible.
But what makes this valve imperfect is perhaps it’s relatively high cost of manufacture. What’s more, the valve cannot be opened or closed against a differential pressure if the correct seating arrangement has not been selected. Another possible issue I found was that impurities in the media can cause a malfunction of the seating construction. And finally, the valve cannot be used for throttling service.
Perfect valve candidate # 3: the globe valve
A tried and tested valve, and hasn’t changed much apparently in over 200 years. Robust in design, widely used for on/off use in refineries and chemical plants and where leak tightness is a concern. It can handle high differential pressures when opening or closing. Various disk shapes are available such as a flat-faced disk for fast opening or a conical disk design for throttling and regulating flow. It has the advantage of offering tight shut-off in on/off duty, and yet can be designed to regulate flow. The globe valve is faster to operate than the gate valve thanks to the shorter stem travel, which is a plus point if a valve is needed that has to be frequently cycled. It can also be relatively straightforward to manufacture in a wide variety of materials.
However, the relative size and weight and the large pressure drop across the valve due to the shape of the flow path. Nor is the design suited for viscous and polluted fluids, as fluids with contamination/debris should be avoided as this may block or limit the operation of the disk and spindle. Care is also required not to turn the valve shaft too far because that might damage the seating surface.
The perfect valve? Haven’t found it yet. But my search continues….