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  • Choosing The Right Valve Part 3: Flow Rate


This post is all about choosing the right material for the valve you are needing. This post is part of a series. Click here to read Part 2 So, we’ve settled on the kind of valve and the material it is going to be made out of. Now for some of the other considerations you might want to consider. QUESTION 3: WHAT ELSE IS IMPORTANT WHEN CHOOSING A VALVE? I have saved the discussion of valve seals and seats for this article. This is not going to be as exhaustive as the previous article, but these materials can be an important factor in your decision. They can have just as much of an effect on the valve’s properties. Although some valves come with metal seats/seals, the vast majority of valve seats and seals are going to be made of plastics. We discussed a few of those in the previous article, but the list of possible seal materials is as endless as the chemical combinations that can be dreamed up by really smart people in white coats. Since we are talking about synthetic materials, there is always a new material in the works and you should do research into the specific properties (and any legal/health regulations) of any valve seal material before using it. However, to give you some idea of the variety out there, we’ll talk about a few. DELRIN® is capable of withstanding pressures up to 6,000 psi and nuclear radiation up to 106 rads. But it should not be used with oxygen service or steam. The temperature range for the valve is -70° to 180°F. PEEK® (polyether ether ketone) on the other hand can work up to 480°F and can withstand pressures up to 4,500 psi – although not at the same time. PEEK has excellent chemical and abrasion resistance. PTFE (polytetrafluoroethylene), commonly known as “Teflon”, is the most common seal material. It has a temperature range of -50° to -320°F and works well up to 1,000 psi. A close cousin to PTFE is Reinforced PTFE (RPTFE), which uses a glass filler to improve the polymer’s life cycle and temperature rating. QUESTION 4: WHAT SIZE AND THREAD STYLE DO YOU NEED? When you buy a pair of pants at the store, you can use a tape measure to check your waist size and inseam, then make your choice. Measuring a valve connection isn’t quite as simple. As covered in much greater depth in this article.  If you don’t know the connection size, you need to measure the outside diameter (for male connections) or the inside diameter (female connections), then check it against a chart that list the correct size. That goes for both NPT (National Pipe Thread - Tapered; America’s standard thread type) and BSPT (British Standard Pipe - Tapered; the international standard). There are other thread types, including NPSC, NPTR, NPSM, NPSL and BSPP, which rarely show their heads in United States plumbing uses. NPT is the most common thread type for industrial plumbing use in the United States and the threading we sell in the United States. The threads are tapered to interlock better, helping prevent leaks with the assistance of pipe tape or pipe dope. NPTF (National Pipe Thread - Tapered for Fuel; sometimes called DrySeal) is very similar to NPT threading with the additional benefit of a better seal in fuel operations without the use of sealant. So, if NPT is pretty much the “end all, be all” in the U.S., why ask this question at all? Well, because if you don’t, you are opening yourself up to buying the wrong connection type, which will cause leaks and cost money. If you are adding this valve to an existing system, you have to be sure you are getting the right thread type, just in case. If you are building a new system, you want to make sure you have all the right parts before you begin. QUESTION 5: FULL PORT, STANDARD PORT OR REDUCED PORT? This is another question that you might not have a lot of input into, but bears consideration in certain circumstances. Like thread size, the flow restrictions of many valves will be pre-determined by their function. That is especially true of control valves, which limit flow by design. And although you may have the full, standard and reduced port options on other valve types, the most common place you will run into this is in ball valves. Since there aren’t really any twists and turns in most ball valve designs, ball valves have the potential for unobstructed flow. That is only true with full port valves, however. That’s what full port – sometimes called “full bore” - means. The opening in a full port ball valve’s center is the same, or very nearly the same, as the pipe connection size. A good rule of thumb is that full port valves are a minimum of 90% full size. You can compare that with standard port size – also known as “standard bore” - which reduces the center ball size by about one NPT pipe size. An informal survey of the industry shows that standard port valves are usually 75%-90% of full size. Some manufacturers offer valves with an even smaller bore size, usually listed as “reduced port”. These valves can be less than 75% of full size and are used mostly as backups or secondary valves. They can serve well in those roles, but in instances like the ones outlined below, you really need to go with a full port valve. Full port valves are usually more expensive than standard and reduced port valves, but they offer better flow rates and less potential for prohibitive chemical buildup along internal edges. Full port valves also minimize the chance for cavitation. Cavitation happens when tiny bubbles, or “cavities,” form in the process material and then implode from the pressure inside the valve. That implosion causes a shockwave which can be noisy and cause major damage to the valve over time. The bubbles are produced as a result of going from a higher speed flow area (such as the narrower area of a standard port valve) into an area of slower flow (when the valve opens back into the regular pipe size). Because full port valves do not have that internal change in flow rate, are less likely to cause cavitation. However, in most applications where full flow is not a major concern, standard port valves work just as well. It depends on the material being transported and the flow rate. Dense, pure liquids with high surface tension and transported at higher speeds are more likely to produce cavitation. They would benefit from full flow, whereas less viscous liquids moving at a slower rate would be fine in a standard port or reduced port valve. In that case, you could realize some cost savings and, depending on the design, save some space as well. The DuraChoice standard port valves sold on DirectMaterial.com as significantly smaller than the full port variety, making them easier to fit into some projects. You can learn more about the difference in full and standard port valves in this article. Conclusion Well, I hope these past few articles have given you a little bit to think about when it comes to valve choice. You will probably have a lot more project-specific questions as you move forward, but these questions should get you started down the right path. Be sure to search the DirectMaterial.com knowledge base if you have any other questions or write us at service@directmaterial.com if we can be of help. Read Part 1 of this series Read Part 2 of this series