The right valve for controlling flow direction? Check.
by Neil Ide, on 10/27/16 9:00 AM
Also known as non-return valves, they’re more complex than you might think.
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Where and when chemical mixing occurs is often critical, and it rarely happens without some sort of check valve on the lines. As chemistry professor Frank Westheimer once said, "a month in the laboratory can often save an hour in the library.”
Preventing upstream mixing
Whether the finished products are petrochemicals, biopharmaceuticals or household cleaners, they’re made by mixing chemicals. Where and when the mixing occurs is often critical, especially if the mixing results in a chemical reaction. And it rarely happens without some sort of check valve on the lines to prevent the chemicals from mixing upstream.
Check valves (also called non-return valves) come in a variety of types and are used in all sorts of applications beyond chemical processing. Their function is simple: Stop fluid from flowing in the reverse direction. How they do that, however, can get complex.
Check valves 101: the basics
Swagelok makes two basic types of check valves: poppet check valves and lift check valves. Poppet check valves have a spring-loaded valve poppet that is compressed against an elastomeric seal. Fluid flows through when there’s enough upstream pressure to overcome the spring pressure, shifting the poppet off the closed seat.
The amount of required pressure to open the check valve is called the cracking pressure. This type of action is very similar to the function of a proportional relief valve.
Lift check valves depend on gravity. In a lift check valve, the poppet is positioned so that forward flow through the valve lifts the poppet off of the valve seat. When higher pressure is felt on the outlet of the check valve, gravity pulls the poppet shut with the assistance of the fluid.
Since there is no spring on the poppet, it cannot fully seal and no elastomer material is required for the seat. A lift check valve can only restrict flow, not stop it completely.
Crack and reseal
In poppet check valves, cracking and reseal pressures determine when the valve opens and closes. Remember, cracking pressure (PCr) is the minimum pressure differential needed in the forward direction to open the check valve.
Reseal pressure (PRs) is the minimum pressure differential in the reverse direction that is required to reseal the check valve. Until PRs is reached, the check valve will not shut off under reverse flow conditions.
Failure to meet PRs is one of the most common errors when a poppet check valve is used. (The other common error is installing them backward.)
Because resealing is aided by a spring, the lower the cracking pressure the higher the reseal pressure required. If you are using a 1/3 PSI cracking pressure check valve, the reseal pressure can be as high as 20 PSI backpressure before the valve shuts off.
Failing to meet the reseal pressure means the check valve may bleed fluid backward, which can be downright dangerous in some systems.
The power of two
So what do you do if you can’t tolerate any backflow through the check valve before it shuts off? The answer is surprisingly simple: put two check valves in series. The combination is called a backflow prevention device.
You might not realize how often you see backflow prevention devices. Next time you go past a building, look for a pipe coming out of the ground with a couple of valves and then returning right back to the soil.
On public water systems, backflow prevention devices protect our drinking water from lawn sprinkler systems that add fertilizer to the water. Backflow prevention devices also protect us from fire sprinkler systems that use anti-freeze.
Code-compliant backflow devices have a little bit more to them than two check valves in a row; however, adding a second check valve in line with the first changes the functional dynamic of the valves and together you get a reseal pressure that is effectively 0 PSID.
At your disposal
If you are currently engaged in a gas or liquid system design and want some expert advice, get in touch. There is a good chance that we’ve already encountered a problem you may be struggling with, maybe even in a different industry.
At Swagelok, we pride ourselves as a high performance resource for fluid systems technology, and serving multiple industries gives us a lot of exposure to the troubles of others. As chemistry professor Frank Westheimer once said, “a month in the laboratory can often save an hour in the library.” Contact us the next time you start heading to the lab to solve a problem.
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About Neil Ide
Neil has always been fascinated by the technical side of things. When he grew up, Ide served on Navy submarines as a nuclear plant operator. At Bloom Energy, a startup that makes power generation systems based on fuel cells, he worked with the R&D science teams to develop and build test equipment. At Swagelok Northern California, he built up the assembly solutions program to handle more complex assemblies and created the popular Customer Briefing Center in our Fremont office.