A Guide on the Types of Needle Valves and How They Apply to You
Needle valves are specified and categorized for several reasons. Some of the most common include shut off, isolation, flow control, and dampening of pressure spikes. Users oftentimes only look at the pressure, temperature and size of a valve when making their selection. There are other features within the valve that can affect whether it will meet your specific needs. Let’s take a look at what is available in needle valves to make certain you are utilizing the best valve for your application.
Different needle valves are supplied with different stem tips. The variety of stem tips allow for optimal output, depending on the user’s needs. Figure 1 depicts different stem tips. The stem tips illustrated here are, in order, 1) regulating, 2) vee, 3) soft or blunt, and 4) ball. Each of these stem tips need to be selected and implemented based on where you plan to apply the valve and how you plan to use it.
Many of us think of using a needle valve to accurately control the flow. Control applications require us to specify a regulating stem tip (Stem 1 as shown in Figure 1). Regulating stems tips are sometimes known as equal percentage stem tips. Each turn of the stem increases flow an equal amount. Figure 2 (courtesy of Swagelok) shows flow through a needle valve with its relationship to turns opening the valve (“0” would be the valve fully closed and “8” or “9” would be the valve fully open.) The regulating stem tip (blue lines) show continuously varied flow (increases in Cv) with eight or nine full turns (a turn is 360 rotation of the valve handle), allowing you to precisely control the flow coming out of the valve. When selecting a stem tip, using a regulating stem tip provides the best chance for controlling the flow.
Needle valves have been around for many years. Needle valves predate ball valves by several years and have long been used for shut off. When used for shut off, needle valves can help minimize or eliminate the surge pressure (also known as fluid hammer, water hammer, surge or hydraulic shock) in fluid piping systems. The Joukowsky equation was developed to predict peak surge pressure and was predicated on the fast closing of valves. The vee, blunt, and ball tips are all used for shut off, with some (very) limited flow control capability. These stem tips offer fairly fast shut off with minimum regulation. The vee stem is the most common of stem tips (Stem 2 in Figure 1). The ball stem tip (Stem Tip 4 in Figure 1) is a non-rotating stem tip. Once the ball engages the valve seat, the ball stops turning, even as you turn the valve stem to close it more tightly. This feature can help reduce the chance of damaging the valve seat by over-tightening. A rotating stem tip will grind into the valve seat and can scar it, making bubble-tight shut off difficult or impossible to achieve. The blunt stem tip (Stem 3 in Figure 1) is often designed with a soft goods tip (Kel-F, PCTFE or other soft good) which will help achieve bubble-tight shut off on gases. The chart below shows that with the vee and soft seat stem you are no longer regulating flow after 1 1⁄2-2 turns. The ball stem tip curve is very similar to this.
Once you have selected the type of control tip you need, be certain to check the temperature and chemical compatibility of the valve packing. High or low temperature service can be accommodated with the selection of the proper valve packing. PTFE, Grafoil and other materials are commonly available. A quick search of the internet can help you determine the compatibility of the valve packing with your service conditions, as well as the chemical compatibility. The location of the packing, either above or below the stem threads, can influence the valves suitability for your application. Valves with packing above the stem threads normally work best with lubricating processes. The process will lubricate the stem threads, making the valve easier to operate. Packing below the stem threads isolates the lubrication on the stem threads from the process and helps make the valve easier to operate in non-lubricating processes, such as gas lines.
A variety of handles are normally available. Handles will typically be plastic or metal, and either round or bar-shaped. Plastic handles are often available in a variety of colors, which can make selecting the correct valve easier for the operator. They also transmit less heat on hot applications, preventing burns. However, metal handles offer advantages as well. They usually can withstand the rough application of a wrench or pliers, which may break a plastic handle.
There are a wide variety of needle valves on the market. Needle valves offer features that can affect whether your needs will be met or not. Different needle valves are specified and categorized based upon their features and how they should be applied. Take time to be certain the valve you select will work for your intended application. Applying the correct needle valve offers a much higher chance of production and satisfaction.