Figure 3: Butterfly control valve designs accommodate the highest flow rates for their size, so are a cost-conscious option if performance requirements are met. (Neles L6 Series Butterfly Valve; photo courtesy of Neles)
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Similar to temperature, increasing differential pressure requirements reach a point when it becomes difficult to find a rotary valve that will work. Depending on the
process fluid and conditions, this can be at 100-30-psi differential pressure. If there are higher pressure drops across the valve, however, it's likely that a properly designed globe control valve is the only option that will work.
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.
Once 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.
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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.
Behind the byline
Peter Jessee, P.E., is a process control application engineer and Rick Ferdon is a technical support representative for Valin Corp., a leading technical solutions provider for the technology, energy, life sciences, natural resources and transportation industries. Valin offers personalized order management, onsite field support, comprehensive training and applied expert engineering services, utilizing automation, fluid management, precision measurement, process heating and filtration products.
Eccentric half ball valves are suitable for waste water treatment, paper plant, chemical treatment, oil & gas pipeline, as well as power plants, hydraulic slag removal.
With 90 degree rotation by eccentric shaft, the valve can achieve open and close movement and plays a role in cutting off the media. Eccentric half ball valve can avoid the friction between the seat and spherical cap and because of the adoption of eccentric crank, there is an eccentricity between the center line of the spherical cap and valve runner. When the valve is open, the crank turns at a slight angle. At this moment, the spherical cap will leave the seat and having no more connection; on the contrary, in the close process, only in the close moment can the spherical cap contact with the seat.
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