October 1, 2009

Proper Valve Selection Reduces Downtime, Increases Process Efficiency

Tom Beaulieu

Proper valve sizing and selection saves facilities money by improving plant uptime and decreasing maintenance costs. One way to improve the selection of valves for a particular application is to take advantage of valve manufacturers’ wealth of information and experience. Valve manufacturers can work with designers to gather all the necessary information to specify the best valve and size for their applications, thus preventing common, costly mistakes that can reduce process efficiency and increase process downtime.

Many customer quotation requests provide only the line size, pressure class rating, and valve type. A typical request might read: size 4, Class 900 globe valve. Though this may be enough information to produce a valve quote, it rarely is enough information to size the best valve from both a performance and cost perspective. A reputable and experienced valve manufacturer will want to know not only the size and pressure-class rating, but also the following:

• Design pressure

• Design temperature

• Upstream operating pressure

• Upstream operating temperature

• Flow rate

• Allowable pressure drop

• Pipe size, schedule, and material

• Media flowing through the valve

• End connection type

• Alternate operating conditions

For example, a butt-welded, stainless steel system used to transfer caustic fluids will require a significantly different valve construction than a flanged valve in a carbon steel pipe system throttling steam or water. A generic request for a size 4, Class 900 globe valve does not communicate this critical information to the valve manufacturer (Figure 1).

1.    Globe varieties. Globe valves are distinguished by an element that closes parallel to the direction of flow. Available styles are T-pattern, angle, and Y-pattern (shown). Courtesy: Flowserve

Valve Selection: Isolation Valves

Gate and globe valves are used for initiating flow in a static line or for isolating flow in a dynamic line based on input from a handwheel or actuator. They are typically placed in a full open or full closed position, although most globe valves can be positioned partially open for rough throttling applications.

The throttling ability of a globe valve can be of significant interest for an end user, although it comes at a relatively higher price. A globe valve, size-for-size, will have a higher pressure drop than a gate valve. The straight-through design of a gate valve offers negligible pressure drop but is a poor choice for throttling applications (Figure 2).

2.    Gate varieties. Gate valves are distinguished by an element that closes perpendicular to the direction of flow. Styles include wedge, flex wedge, double-disc, and split wedge (shown). Courtesy: Flowserve

It is not uncommon for a new design engineer to specify the requirement for a gate valve in a throttling application due to the lower pressure drop at the full open position. However, the closure element of a gate valve is prone to damaging vibration in partial stroke positions and is therefore not appropriate for throttling applications.

Sizing a gate or globe valve to match the line size of a system is usually a safe practice, but it may result in extra cost to the user. If the valve is allowed to develop a higher pressure drop than that provided by a line-sized valve, using a smaller valve could produce several benefits: The cost of the valve will be lower due to the smaller body size and, if the valve is to be operated with an electric, pneumatic, or hydraulic actuator, the size of the actuator will be smaller and thus less expensive.

Another consideration in choosing and sizing isolation valves is stroke time. On a size-for-size comparison, a globe valve will reach the full open position faster than a gate valve. A globe valve stroke length is approximately one-third of its nominal size, whereas a gate valve stroke length is approximately 100% of its nominal size. Standard actuation speeds for rotary, electric-actuated gate, and globe valves are 12 inches per minute for gate valves and 4 inches per minute for globe valves. Although this suggests an equal overall stroke time for both valve types, this is rarely true in application. Many gate valves need to stroke more than 100% of their nominal size, while many globe valves stroke under one-third of their nominal size to reach the full open position. Applications exist where gate and globe valves have very short stroke times — on the order of 3 to 10 seconds — but these valves rely on special, linear actuation instead of multi-turn actuation.

Globe valves used in throttling applications may need hardened seats, plugs, and stem and a body material that is more resistant to damaging erosion and cavitation. In these cases, a valve body material that is dissimilar to the pipe material is most likely appropriate; common valve body materials selected for erosion resistance in cavitation service include chromoly and stainless steel, which both possess a higher content of damage-resistant chromium.

If the piping system is constructed from carbon steel but the valve needs to be made with a different material, a full-service valve manufacturer will be able to weld carbon steel ends on the valve inlet and outlet (commonly called pipe pups or safe-ends). The post-weld heat treatment required after welding dissimilar metals is much easier to perform on the valve while it is still in the factory. When the valve is received by the end customer, the weld required to install the valve will be between similar materials, which eliminates the need to perform complicated post-weld heat treatment in the field.

In an effort to save money, design engineers commonly attempt to use a globe valve for an application that requires a control valve. Although control valves may use a globe valve design, they are in fact very different than globe valves designed for isolation services. Control valves are specifically designed to provide finite and stable flow control at mid-stoke positions, whereas a globe valve is only suited for rough adjustments to flow. Specifying a control valve for applications requiring stable flow control is worth the extra acquisition cost and can result in reduced maintenance and increased process efficiency.