CONTROL VALVES
Figure 4: Certain valve applications involve erosion, corrosion, flashing, or cavitation, all of which can destroy valve internals. In these cases, it is wise to engage expert help to properly select the correct valve body style, materials of construction, and necessary internal components.
Users must be wary of stacking worse case scenarios during the design process. While it might be technically possible for a perfect storm of process conditions to occur simultaneously, it usually makes little economic sense to pay two or three times more for a valve to handle a set of design conditions that will probably never occur in actual operation. Users should avoid overdesign but also avoid under design. Most plants have different modes of operation that subject the valve to very different conditions. For instance, during shutdown or start up, the valve pressure drop may be significantly higher, the flows much reduced, and the fluid composition may be significantly different. All these conditions should be evaluated to size the valve. A complicating factor for many valve sizing applications is choosing the inlet, outlet, and flow values because they are often interrelated. For example, increased flow may reduce inlet pressure and increase outlet pressure due to line pressure drops, so all three values may change with flow. However, if the inlet and outlet pipes are short or oversized, these pressure drops may become insignificant, and the inlet and outlet pressures could remain the same regardless of flow. Therefore, knowledge of the surrounding piping and how the valve is installed is critical for choosing the correct process data. Flow can be a difficult parameter to select, and it is important that a range of flows be entered. The normal flow is usually easy to determine, but it is the high and low flows that ultimately determine the size of the control valve. The low flow value should be the minimum flow that the valve must accurately control, while the high flow is the maximum flow the valve must pass. This range of flows will usually determine both the valve size and
style since different valve styles have broad variations in their rangeability.
Further complications
Valve sizing for severe service is very detailed topic, so it cannot be adequately addressed here. However, it is worth mentioning several conditions that will likely require a much more involved valve sizing calculation. In this scenario, expert valve sizing help from a knowledgeable vendor is encouraged as incorrect valve sizing, body style, and materials of construction can result in catastrophic consequences( Figure 4). When encountering these red flag conditions, users should seek additional help:
• High pressure drops across the valve, which in liquid service often result in flashing and cavitation. This impacts valve selection to avoid damaging valve internals and downstream piping. High pressure drops in gas / vapour service create choking within the valve and usually generate excessive noise and vibration, which also damage the valve. Valve experts can suggest the right combination of valve style, trim, and materials of construction to address these conditions.
• Slurries or two and / or multi-phase flow applications require specific combinations of valve internals and materials of construction to avoid valve damage and plugging. Note that International Standards for control valve sizing models only considers single state flowing media.
• Specialty applications like compressor antisurge or emergency vent valves. Valves for these services are usually custom engineered for a particular application and must be carefully selected to provide the necessary response time and required precise flow control.
Engage experts and take part in the process
It is certainly wise to engage knowledgeable automation vendor experts when sizing control valves. However, the user is strongly encouraged to take an active part in the control valve sizing endeavour, as the effort requires a complete knowledge of the normal and abnormal process conditions, as well as a broad understanding of the control valves themselves. Both the automation provider and the user have only a part of the knowledge they need to correctly size and select the right control valve, so it often takes a collaborative combination of both parties to make the correct design decisions.
Figures all courtesy of Emerson.
About the author
Mark Nord is the control valve solution architect for Emerson’ s Flow Controls Products in Marshalltown, Iowa. He is responsible for solving the most challenging control valve applications using his 35 years of experience in plant operation, control valve and steam conditioning applications. Nord holds a BSME degree from the University of North Dakota.
38 Valve World November 2026 www. valve-world. net