Valve wars

Rising stem vs. rotary.

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By Jon F. Monsen

PlantServices.com

In recent years, the growing interest in rotary-action control valves resulted in at least a perceived battle between globe valves and the various styles of rotary valves. In a few industries and applications there is a clear winner. For example, globe valves are really not suitable for handling pulp stock in pulp and paper mills. As a general rule though, it cannot be said that one style of control vale is the best. If there were one "best" control valve type, it would be the only one that the valve manufacturers would make, and that has not happened.

The globe valve is the most mature of the control valve types and offers a wide choice of options. High pressure and temperature ratings are available from most manufacturers as is a broad range of trim materials. A variety of cavitation and noise reduction options are available, ranging from moderately severe duty to extremely severe service. The capacity or Cv rating of a globe valve, as well as the inherent flow characteristic, can be changed simply by changing the trim. Most modern globe valves are of the top entry design. This means that the internals can be replaced while the valve is in-line (although many plant standards do not permit the repair of valves in-line). The globe valve is an extremely versatile design. It is also a very expensive valve, especially for larger line sizes.

The ball valve, both full ball and segment ball, has approximately twice the flow capacity of the globe valves and has the added advantages of long-life rotary stem seals and of being less expensive, size-for-size, than globe valves. Ball valves, with the exception of one-inch segment valves, do not have the option of changing trim size. It is impossible to change the inherent flow characteristic of ball valves. Although there are some options for noise and cavitation reduction, the choices are not as extensive as with the globe valves. Because of their relatively unobstructed straight-through flow path, ball control valves are well suited to media such as slurries and pulp stock.

High-performance butterfly valves gained widespread acceptance as control valves. They offer high flow capacities comparable to that of ball valves. Of the common control valve styles, these have the lowest price and weigh the least. They are also the least versatile, being more prone to cavitation problems than other control valve styles. They have no options for reduced trim and practically no options for noise or cavitation reduction.

Rotary eccentric plug valves combine several of the features of globe valves and rotary control valves into a single design. They offer much of the ruggedness and the higher pressure recovery factor of globe valves along with the compactness, lower cost and longer stem seal life of rotary valves. Some years ago, one manufacturer began referring to its rotary eccentric plug valves as rotary globe valves and successfully displaced globe valves in a variety of applications. Although less versatile than globe valves, rotary eccentric plug valves accommodate interchangeable reduced trim and provide some cavitation and noise control options. Unlike the other rotary valve styles that have approximately twice the flow capacity of globe valves, rotary eccentric plug valves have flow capacities that are comparable to those of globe valves.

Selecting a valve style

In the past, selecting a control valve style was relatively simple. The philosophy was simple: "Whatever valve style we traditionally use in this plant is what we will use this time, too."
Those who specify control valves today are expected to minimize expenses while simultaneously improving product quality and process efficiency. Doing so requires a discussion of some of the factors that need to be considered when selecting the best control valve style for a particular application.

First cost

The first cost or purchase price of a control valve is, of course, only part of the story. The selection process must consider the total life cycle costs, including installation, maintenance and eventual replacement cost. Nevertheless, the first question that is always asked in a discussion of control valve types is how they compare on the basis of price.

Figure 1 compares the approximate first costs of typical high-performance butterfly, segment ball, eccentric rotary plug and globe valves. Note that because two-inch high-performance butterfly valves are not very common, no price is shown for that size. The rotary valves are clearly less expensive than the globe valves, with the difference becoming greater as the size increases. If a 10-inch globe valve has the right flow capacity for your application, you should at least check to see if one of the rotary valves would also be suitable for the application.

Keep in mind that, size-for-size, a segment ball valve or high-performance butterfly valve has approximately twice the flow capacity of the globe valves. Most likely, the comparison would be between a 10-inch globe valve costing nearly $19,000 (1998 prices) and an eight-inch high-performance butterfly valve or segment ball valve, either of which would cost under $5,000.

Figure 2: Control valve weight comparison.

Weight

Figure 2 demonstrates the correlation between the size and the weight of a valve. A significant portion of the cost of a control valve is in the materials. The 10-inch globe valve weighing close to 2,000 pounds would certainly be more costly to install than the alternative high-performance butterfly valve that weighs closer to 300 pounds. Because control valves are among the hardest working components in a piping system, it is almost assured they will need to go to the instrument shop or valve shop for maintenance. Given the choice, most maintenance people would prefer to remove and transport a 300-pound valve than a 2,000-pound valve.