The wide selection of valves offered by Swagelok may seem overwhelming at first: ball valves, needle valves, check valves, safety valves …
How do you make the choice that best fits your application? Let’s take a look at some of your options.
- The most common valve for the job is a ball valve. A metallic ball sits in the path of flow and the ball has a large hole drilled horizontally through the center. Turning the valve so the hole lines up with the path of flow lets the fluid flow. Giving the ball a quarter-turn stops the flow. Ball valves get the job done quickly, and the position of the handle makes it easy to see whether the valve is open or closed.
- Check valves make sure that the fluid goes only in one direction. Typically, the upstream pressure compresses a spring inside the valve to push open a poppet to allow flow. If the downstream pressure rises, it forces the poppet back into its seat and cuts off any reverse flow. Some check valves are adjustable, so you can set them to a more specific cracking pressure.
- Diaphragm valves are packless and good for fast shutoff and precise actuation speeds. Each valve has a thin diaphragm of metal or plastic that flexes up and down to create a seal over the inlet. They are typically used in high-purity applications such as biopharmaceutical and semiconductor manufacturing. Diaphragm valves are usually small, with internal pathways of two inches of less, but they provide the highest cycle life among all valve types.
- Bellows valves have a welded seal dividing the lower and upper halves. The stem is encased in a metal bellows, which seals the inlet by moving up and down without rotating. Bellows valves are packless, making them a good choice when it's critical to have a good seal to the atmosphere and you have limited access for maintenance. Bellows valves are often specified for the containment area in nuclear power plants.
- Gate valves are used most often for process control, particularly for lines larger than two inches. A sealing mechanism -- the "gate" -- sits in the flow path. Rotating or raising the handle raises the gate and lets the fluid flow. The stem going from the handle into the valve might have packing such as gaskets or O-rings to prevent leaks, or it may have "packless" metal-to-metal seals.
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