Which Valve Is Right For Your Fluid System? Let Us Show You
by Jeff Hopkins, on 4/20/17, 8:30 AM
Don't let yourself get overwhelmed by the wide variety of styles and sizes
Swagelok's selection of valves can seem overwhelming at first. There are ball valves, diaphragm and bellows valves, as well as check valves, excess flow valves, fine metering, multi-port, needle, plug, relief, rising plug, and safety valves.
Each type comes in many sizes, configurations, materials of construction, and actuation modes.
So let's break it down into some more manageable chunks of information.
First, there are the kinds of valves used for the most basic on-off functions. That list includes ball valves, diaphragm valves and bellows valves. The right choice depends on how large your line is, how quickly you need to start or stop flow, how "clean" your system is, whether you need a good seal to the atmosphere, among other factors.
Sometimes you want to do more than just turn the flow on or off. Let's say you want to set the handle for a particular rate of flow and have it stay there. The most common choices are needle valves, fine metering valves and plug valves. For the most precise control, choose a type of needle valve called a fine-metering valve. Be aware that some models are not designed to shut off.
Then there are quarter-turn plug valves, commonly used for low-pressure throttling applications. A closely related design is the rising plug valve. They aren't as effective at providing fine gradations of flow, but they are roddable, which makes them a good choice if the valve gets clogged.
If you need to control the direction of fluid flow, you'll probably want a check valve. When downstream pressure rises, the valve closes on its own. Some check valves are adjustable to a specific pressure. Flow can also be directionally controlled with some ball valves and diaphragm valves that have multiple ports for outflow.
While valves don't regulate pressure, some types do prevent the buildup of system pressure beyond a certain point. One type is the relief valve, with a vent that opens to the atmosphere when the system pressure exceeds a point that you've selected. A variation on the design is the safety relief valve, designed to open quickly and release large amounts of fluid.
Rupture discs also vent to the atmosphere. They are used mainly on sample cylinders to guard against over-pressurization, such as when temperature rises. Once the disc bursts, it must be replaced.
Then there are excess flow valves to stop uncontrolled release of fluids if the line ruptures downstream.
Know your system
Beyond that, there are a few common-sense pieces of information to have ready when you make your selection. First, know the chemical composition of the system media and the full range of pressure and temperatures that the valve will have to deal with. Make sure your valve is made of the right materials for your application.
Whenever you add a valve or other component to your system, it usually produces a drop in pressure. You need to be aware of the cumulative pressure drop.
Lastly, the true cost of a valve is the purchase price plus the cost of owning, maintaining and replacing that valve over time. Note that some valves are much easier to service than others. Some can be serviced in place; others must be removed from the process line. Different valves have different maintenance schedules, an obvious point that is often overlooked. Are you willing to service that valve once every 20 days when it is 100 feet in the air?
Now that you know the basics, you are ready to move on to some details. Get off to a good start by downloading three essential resources:
- Matching Valve Type to Function: A Tutorial in Valve Selection. This four-page PDF covers on-off valves, directional flow valves, flow-control valves, overpressure protection valves, excess flow valves, and a selection guideline summary.
- Valve Sizing Technical Bulletin. An 11-page PDF that shows how flow can be estimated well enough to select a valve size—easily, and without complicated calculations.
- Valve Selection Worksheet. Our two-page PDF covers valve function, system conditions, flow, and many other factors such as actuation method, lock-out / tag-out, position indicator, materials of construction, and more.