How To Select A Valve For Cost Savings
Tim Fallon, Product Manager at the Henry Pratt Company, explains how valve selection affects efficiency of flow, thereby influencing total electric costs for pumping water.
The following is an excerpt from a Q&A with Water Online Radio. Click on the Radio Player above to hear the full interview.
Water Online Radio: Talk about the typical kind of application where a plugged valve is more appropriate than some other kind of isolation valve.
Tim: Plug valves come into play when we're dealing with a media that has a lot of solids. It might be stringy material where another type of valve (like a butterfly valve) would not be appropriate because the solids could get caught up in the disk and foul up the valve. The plug valve is full port, so solids can easily pass through the valve without any type of fouling.
Water Online Radio: Walk us through the engineering or design attributes a design engineershould declare when writing specification language for a plug valve.
Tim: A plug valve is a standard valve in the water industry, but sometimes we see a little bit of confusion about ball or plug valves. There are also other types of plug valves, like a lubricated plug valve which is used in oil and gas applications.
The plug valve itself will last a long time if it's built right, so we look at attributes like a nickel seat, where corrosion won't play into the life of the valve. A rubber face with a Buna elastomer is best for most applications involving sewer applications.
Ductile iron or cast iron for the valve body is very good. We try to specify ductile iron because of the yield property of the ductile versus cast iron. We would also want to specify ductile on the plug. Cast iron will fail over time if it's over-torqued.
We get into the offices of the consulting engineers and help them specify the product just so everyone can land on what we ultimately want to do. I know there are some folks out there that like to write proprietary specs, such as one physical attribute that they have solely on their design that nobody else has. At Henry Pratt, we open it up to make the spec worthy of use for everybody out there who has a plug valve.
Water Online Radio: Can you give us a definition and explanation of the pluses, minuses, and differences in consideration surrounding a full port, 100% port, round, and rectangular?
Tim: The plug valve has a bunch of different port shapes (round port, rectangular port, or square port). The port shape doesn't really play into the efficiency of the valve.
What you should focus on, in my opinion, is the efficiency of the valve, or the CV, the efficiency of the flow. In our industry, we have a measurement of how efficient a valve is, and that's the CV value of that valve. The higher the CV -- which is gallons per minute with one psi of pressure drop across a valve -- the higher that value is, and the more efficient the valve can be. This means that the pumping cost is decreased because the flow isn't impeded.
To get into a debate over port shape doesn't make any sense. The engineer should focus on the efficiency or CV of the valve. Let the port shape go, and focus on the true physical attribute that can be measured and understood once that valve gets in line.
The number one accounts payable aspect at water treatment plants is the electric cost to pump water. You have these impedances in the line, such as the isolation valve or checked valves. These all create head loss in the line and physical impedances, which require more pumping. Instead, look at the CV value, and base your choice of valve on that.
Water Online Radio: Walk us through some different types of shaft seals and how they ensure a leak-tight seal.
Tim: The valve has got a shaft that penetrates through the top of the valve body, and it has to be sealed off. Some approaches are standard O-rings, or what you call chevron packing, which is a multi-layer type of self-sealing seal. You have a braided packing, which needs to have some kind of follower on it to create a seal between the shaft and the top of the valve body.
What we have found most effective is primarily the chevron packing. It's self-energizing, so when the water pressure comes up in the valve, the chevron will compress against the shaft seal and valve body, creating a leak-tight seal at that point. There's no need to tighten them down, and they'll last a long, long time.
Water Online Radio: You say a long time. How often do they need to be replaced?
Tim: Depending on the severity, temperature plays a big role in that. If you have a valve in the ground, buried service application, the shaft seals will stay sound for 20 to 25 years without any need to replace them. Now, if you've got a valve that you're operating on a daily basis, you might need to look at shaft seal replacement every five or ten years...