My last post
discussed some of the advantages and disadvantages of through conduit valves, trunnion-mounted ball valves and globe valves. Here follows a few more ‘perfect’ valve candidates, and why they might not be so ‘perfect’ after all.
The ball valve
The first thing to say about ball valves is that they offer a tight shut-off, meaning there is almost no leakage in the shut-off position. These valves can also be operated at low torques, meaning they can be opened with a minimum of effort. This is made easier by the fact that they are quarter-turn valves – just a quarter turn is needed to bring the ball valve to the closed position, meaning they are fast acting. They are generally compact, easy to manufacture and delivered in a wide range of sizes from ½” up to 60” or even larger.
So what are the disadvantages? Well, these valves prefer a clean media running through them, one without too much sediment that will scratch their soft seats. Because the seats in these valves are invariably soft, it also means the temperature range for these valves is limited, to say for example, valves with metal seats. It are these same soft seats that render the ball valve to be less suitable for throttling.
The butterfly valve
This valve type is considerably thinner than the heavier ball valve – and with this brings advantages of weight – it is lighter! So expect the installation of butterfly valves into piping systems to be a little easier. The reduced weight of the butterfly valve means it is cheaper, since it is using less material. Expect to also see a variety of linings available to isolate the valve body from especially corrosive fluids. This reduces the need for expensive body materials, which helps keep the cost of the butterfly valve within budget when compared to other valve types.
Butterfly valves can be used in a number of industries but are particularly useful in low pressure applications. You can expect to see them in large pipelines with large flows at relatively low pressures. Similar to the ball valve they are quarter-turn, so they’re fast acting. As well as this, the butterfly valve has no “dead spaces” as well as a low-pressure drop.
On the downside the disk of the butterfly valve does remain in the flow path, which reduces the Cv value. It also means the valve cannot be pigged. There are some cavitation problems for the butterfly as well, albeit under certain conditions. Cavitation is the formation of vapour cavities (or bubbles) in a liquid which can be caused when a medium passes an obstruction and undergoes a rapid change in pressure. The bubbles can then implode and cause shock waves, destroying metal surfaces.
Calcified butterfly valve
An old calcified butterfly valve I came across last year when visiting a water plant.
Butterfly valves have the disadvantage that the disc remains in the flow path, reducing the Cv value.
The plug valve
Many claim the plug valve, used by the Romans, to be the first valve in use. Robust and versatile, they are used in on/off applications for a variety of fluid services. Plug valves can cut through thick fluids and slurries and are popular in the oil industry becuase they can handle erosive media such as mud and cement. This is due to their large sealing surface, so that if damage should occur then any leakage is kept to a minimum.
The plug valve shares many similarities with the ball valve, but the main difference is the closing element which, as the name suggests a plug!
(most plug valves made today are used to replace existing plug valves; only a limited number are specified for new applications)
The diaphragm valve
Resembling a pinch valve in that they contain a flexible sleeve, this valve is often seen in slurry or process applications, and is not generally suited to high temperature applications. A known disadvantage is that the diaphragm can be easily worn.
The pinch valve
Think of this valve as a bit like pinching a garden hose to stop the flow of media passing through it. The pinch valve consists of a flexible sleeve which when the valve is open effectively forms a continuation of the adjacent piping system. Because the closing mechanism is outside the valve itself, the pinch valve is very suitable for handling abrasive slurries or dry powers - they can be seen in applications for fertilisers, cement, granulate handling, ceramics, wastewater, pulp & paper etc.. Whilst I compared the pinch valve to a rubber garden hose, the rubber linings in these valves are need to protect the body from highly corrosive applications, so the linings are highly engineered products. Common sleeve materials include Nitrile, Neoprene and natural rubber to name just a few.
Need to know more? The Valve World Essentials course is ideal for (relative) valve newcomers who need a fast-track, authoritative understanding of valves and their usage. For absolute beginners, the “Headstart” section of our website will ensure you arrive at the course ready to hit the ground running.