Control Valve Technology. How Do I Choose?
on Tue, 05/11/2021
For example, the question should be posed, “how accurate does the control need to be?” Initially, this question sounds a bit silly. Of course, if we’re controlling a flow, it should be accurate, right? Well, it may need to be, but on the other hand, it really may not need to be very accurate. The reason this is so important is the accuracy level has a direct correlation to the price tag.
As control packages become more precise and speed of response increases, the cost typically rises as well. There are certainly operations in the field where a more expensive control package is in place than is required based on the level of control needed. Alternatively, there are control packages in operation that don’t make the grade based on what's required. How does one know if what they're trying to control dictates a more expensive, more precise approach, or if a less expensive, less accurate option will do a sufficient job? There are several factors that should guide this decision, allowing plant operators to select the best option.
The Available Options
Globe valves have more severe service options than other valve types, so there are times when nothing but a globe valve will survive and function properly. They also offer replaceable trim sizes for each line size, allowing an improperly sized valve to be corrected, or letting users start with a reduced-trim valve and change to a larger trim as capacity increases.
However, the inherent design of a globe valve can create drawbacks in some applications. They don't tolerate suspended solids well and can jam and quit working. They also tend to be heavy and physically large, which can be a problem in a system requiring tight packaging. Finally, they also tend to be the most expensive type of valve for a given size.
More recently, quarter-turn or rotary valves have been used in many control applications. These can include a butterfly valve, ball valve, segment valve or an eccentric plug valve. While there were some failures when used in processes where they were applied inappropriately, several decades of progress in materials and design enhancements have greatly increased the range of processes in which quarter-turn valves can be safely used.
Butterfly and segment valves (Figure 3) are physically small and light relative to their CV, making pipe design easier than accommodating and supporting a heavier valve. Not coincidentally, they're also less expensive because they require less metal and machining time than globe valves.
What are the areas we need to consider when determining if a valve type is a good fit for a process? Temperature and pressure are two of the most important.
Temperature, Pressure and SteamWhen discussing temperature limitations, one must focus on the extremes. If temperature requirements are moderate/mild, many soft-seated rotary valves will do the job. However, once temperatures reach or exceed 400 ºF, soft-seated valves start to have trouble handling the stress, and a metal-seated valve may be required. A metal-seated valve is typically higher in price. If temperatures approach 800 ºF, it may make the most sense to recommend a globe control valve.
There are competing factors that can affect the decision-making process regarding pressure. Rotary valves are available with ANSI pressure class ratings of 2500 and higher, just as for globe valves. But rotary valves typically generate more noise and are more prone to cavitation if control differential pressures are high. Proper sizing and noise calculations are required to verify a rotary valve will work in a high-pressure system. It's typically easier to find a globe valve for high-pressure and especially high-pressure-drop applications.
However, rotary valves also can have an advantage in high-pressure systems. The required actuator size for a globe valve increases quickly with increasing pressure, even with more expensive balanced designs. Many rotary designs have less increase in required actuator size as pressure increases, saving money and reducing assembly size.
Steam can be worrisome as it tends to do a great job of eroding materials. The characteristics of basic rotary valves are well-suited for air and water, but can have problems with steam. You may need to use a special severe-service rotary valve for steam applications with a 50-psi pressure drop or more. High-performance butterfly valves can be remarkably effective for low and medium differential pressure applications and be extremely cost effective. High differential pressure applications, such as steam turbine power plants, usually will require a specially designed and very expensive globe valve.
Control RequirementsOnce the owner understands the temperature, pressure and material conditions the process needs to operate under, the next question is “how precise does the control need to be?”
There are some applications when using a rotary valve for control will work perfectly. A good example is a simple water temperature control system for creating a warm output. If the process has a lot of capacity and temperatures don’t change very quickly, a ball valve will work well. This can also be done with a three-way globe control valve, and it will do a more accurate job of controlling the outlet. However, this approach will be far more expensive than using a basic ball valve. The question is, how accurately do you need to control the temperature? Does it really matter if the water is exactly 100 °F, or will 98-102 °F work? If it really doesn’t matter for a specific application, a three-way globe control valve may be unnecessary and an unwarranted expense.
Part of this discussion also lies in rangeability, which is the difference between lowest flow and highest flow. What is that difference? One can control a globe control valve down to 5% of its flow and up to 90% without too much concern. There is a rangeability of 10-20:1 for a valve with linear trim, and as much as 50-100:1 with equal percent trim.
With a butterfly valve, due to its design, one should not operate it below 10% and above 70% open, where the gain curve flattens and the valve is effectively all the way open.
Fortunately, there are rotary valves that do offer wide rangeability. A segmented ball valve, commonly called a V-Ball, has a deeply characterized v-shaped control element, and can provide the same 50-100:1 rangeability as an equal percent globe valve. A segmented ball valve is more expensive than a butterfly and most other ball valves of the same materials and pressure class, but it will typically be half the price of a globe valve with the same flow capacity.
Price is a significant factor at this stage of the decision-making process. In general service, a ball valve will increase in price faster than a butterfly valve as the size increases. At small sizes, typically under 4-in., there may be little difference in the price of a ball valve or a high-performance butterfly control valve. However, once you reach 6-8-in., the ball valve will be more expensive than a butterfly valve, and for large sizes (greater than 12-in.), you'll definitely want to use a butterfly valve if it will handle the process conditions.
A globe control valve, on the other hand, may be three to four times the price of the butterfly valve, with similar increases in the total cost of ownership (TCO) over the life of the valve.
The decision should always be determined by the application requirements. If the process conditions and rangeability requirements allow the use of a rotary valve, it will likely be much less expensive than a globe valve. If the conditions can only be met by a globe control valve, then that is what should be used.
On the other hand, globe control valves are often specified when they simply don’t need to be. If a rotary valve can handle the temperature, pressure, nature of the product flowing through the line and meet control requirements, then it should be considered. It will cost the operation far less in TCO over the life of the valve.
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