Why flow coefficient in valve selection matters

Happy New Year and welcome to my first official learning blog post! Couple my current interest in water treatment with my inherent love for writing, and it should come as no surprise that I’m ringing in the New Year with a blog of my own. I’ve benefitted so much from the blogs of others. Here is my small part and piece to add to it all from the perspective of a relatively new learner.

In the world of pumps, pipes, and valves that I’m in these days, there is a question of how to size a valve for a process system allowing for optimal performance. Undersizing valves can mean restriction upstream and back pressure buildup which can lead to flashing or cavitation. Oversizing via a too large flow/valve coefficient can lead to the opposite problem: a drastic pressure drop and velocity speed up. Here again, there is the real chance of encountering flashing or cavitation. Trim parts inside a valve can start eroding, causing a “bathtub stopper effect” wherein the closure element of a valve gets sucked into its seat. 

What are flashing and cavitation? These are pressure problems at opposite ends of each other, both with real potential for damaging effects to anything in their paths. When we’re thinking about the concept of pressure, it has to due with a strong or weak flow of water.

  • With high pressure, i.e. low/weak water flow, we want to boost water flow up to meet demand.
  • It is the opposite with strong/high water flow. We need to do the opposite and decrease it.
  • Too fast of a velocity or speed boost of the water flow results in a large pressure drop. A large pressure drop can lead to flashing, whereby water “flashes” into a partially vaporized state with the potential for equipment damage.
  • Slow the speed, i.e. velocity, down too far and we get a large pressure increase. This is where cavitation comes in. Now we have too much pressure, so we’ve got water static pressure dropping below vapor pressure instead. The vapor evaporation collapses on itself and that’s cavitation. Think of caved in or deformed pipes, valves, or equipment and you’ve got the idea here.

This, in part, all has to do with the valve “restriction”. What is restriction? Drumroll, please…

You say you’ve gotta valve in the path of a pipe, do you? Drumroll square roots, because they are all over this and so many other of these doggone mechanical engineering problems. There is an equation called the “Square-Root Law”, whereby flow = restriction * square root of pressure drop. I need to figure out how to get these symbols in my posts properly. Until then, there are calculators and charts that punch these out in no time.

Back to where we started here: valve coefficients. There is a sizing formula for this, known as Cv. Cv = flow * square root of (specific gravity/pressure drop).  Don’t get hung up or caught up in this. And please don’t let anyone try to say that you *just can’t* possibly understand or grasp this due to it being formulaic. Everyone uses calculators. Get your hands on one and you’re as good to go as anyone else making this proclamation. They’re not doing anything else we wouldn’t.

We need to get this right to size valves properly to optimize performance and avoid expensive wear and tear. Flow characteristics come into this, where there are inherent flow and installed flow characteristics. WTH, you might ask. I know I did. Inherent flow does not take into account the effects of piping, while installed flow does. There is more to this, but I am putting a close to this post for now. Thanks for reading. Please feel free to add or comment. Where is the rest of the topic here? I’m not done yet! If there is interest, please stay tuned for more.

All my best wishes for health, happiness, and exciting new learning and development in the New Year!

In gratitude,

Jennifer Zadka

Good company in a journey makes the way seem shorter. — Izaak Walton

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