Choosing the Right Size Control Valve

To ensure optimal system performance, safety, and reliability, it’s critical to ensure the right size valve is used. Proper valve sizing allows for reduced costs, less downtime, and fewer labor needs — keeping operations running smoothly and efficiently.

Below are some key points to keep in mind when sizing control valves.

What Parameters Are Needed to Properly Size Control Valves?

A control valve should be sized to allow for operation between 60% and 80% open on max flow and approximately 30% open at minimum flows. This is considered a general rule and will be times where some applications will require higher operating levels. For optimal operation and life expectancy, the valve should be within the operating range of 30% to 80% open. Anything outside this range may cause premature wear on the internal components. When the 30%- 80% rule needs to be greater, it is best to have your control valve expert review your process conditions and valve calculations case by case.

The Basics of Control Valve Sizing

There are several factors to consider when selecting and sizing a control valve for a given application, such as:

Types of Control Valves

Which valve type is best for an application depends on things including fluid type (i.e., gas, steam, liquid) and viscosity, operating pressures and temperatures, requirements for precise flow control or tight shut off, and other application specifics. Some of the most common types of control valves include:

• Ball: This is a rotary type of valve in which a ball with a hole through it rotates inside the valve. The amount of fluid that flows through the ball is determined by how much of the hole is exposed. Ball valves allow for tight shut off and can be used with high-pressure steam.
• Butterfly. This rotary valve type controls fluid flow with a rotating disc. It is often used for lower-pressure applications.
• Linear: This type of valve contains a sliding stem that moves a plug into or out of a sealed port. Globe valves are a common type, used widely for gas and liquid applications.
• 2-port: With a range of internal mechanisms to control flow, these valves can fully open or close, and restrict the flow of fluid to varying degrees.
• 3-port: In continuous flow applications, a three-port valve allows for mixing or diverting fluids with different configurations of inlets and outlets to adjust temperatures or pressures. These valves are often controlled with either pneumatic or electric style actuation.

Flow Coefficient (Cv)

Flow coefficient (CV) is an expression of the capacity of the valve and trim (i.e., stem, plug, seating, or sealing area) and how much fluid can flow through it in gallons per minute. This is important because it helps determine the size valve needed to maintain the desired flow rate without excessive vibration, noise, cavitation, flashing, or other factors that impede the process fluid.

If the CV is too small, meaning the valve or trim is undersized for the application’s flow needs, adequate process fluid will not be supplied and pressure may be too low. An oversized valve can lead to oscillation, higher noise levels, cavitation, and valve damage caused from vibration.

Inherent and Installed Flow Characteristics

Inherent flow characteristic looks at how a valve’s flow rate changes as the opening position (or valve lift) changes when pressure is constant. The installed flow characteristic looks at the inherent flow in the context of how pipeline pressure and fluid temperature affect the valve’s operation once it is installed in a pipeline system.

There are three types of flow characteristic behavior and they are largely related to the shape of the plug:

• Fast Opening: These approach binary on or off in operation with little intermediate adjustment.
• Linear: These have a directly proportional relationship between how the amount of lift and the flow rate vary under a constant pressure differential.
• Equal Percentage: For every increment of lift, the flow rate increases by a certain percentage.

Selecting a control valve with a linear flow characteristic is ideal in many applications because it can accommodate a range of flow rates, allows for good flow regulation in operation, and results in a stable and predictable flow in the pipeline.

Ensuring Accuracy

Choosing the right size control valve is vital to achieving the highest degree of process control for gas, liquid, or multi-phase fluid. To ensure accuracy, determine the following factors for proper control valve sizing:

Noise

Noise can be caused by drops in pipeline pressure, changes in the direction or rate of flow that cause turbulence, cavitation, or valve design factors. Vibrations from noise can cause damage to the valve and other pipeline components or downstream equipment. It can also cause distractions for personnel or disrupt the general work environment, which can lead to safety and productivity concerns. Selecting an appropriately sized valve can greatly reduce noise and associated problems.

Cavitation

Cavitation is the formation of bubbles in a liquid due to pressure drops and increases in liquid velocity. These conditions create areas of low pressure and cause vapor bubbles to form and collapse. Cavitation can cause noise and vibrations in the pipeline system, which can lead to equipment damage, or valve damage. Note that cavitation is generally not relevant for applications with steam or gas.

Actuator Selection

Many control valves use an actuator to operate the stem and plug inside the valve. An actuator can be manual (such as a handle or lever) or driven by a pneumatic or electric power source. Pneumatic valves generally use pistons or a diaphragm that moves with compressed air pressure. Piston actuators may be used for small spaces or with high pressures or when high-speed movement is needed. Diaphragm actuators are controlled with compressed air that forces the plug down into or up out of the seal. It’s important to pre-determine what position the plug should revert to in case of air failure – either open or closed.

An electric valve uses a motor to power the actuator that moves the plug relative to the seal. Here it is important to prevent damage to the valve by using a torque switch or position limit switch. This allows the plug to be seated firmly without applying excessive force. It’s also important to consider the valve’s torque requirement relative to its opening angle, pressure, and the actuator’s torque output so that movement is smooth.

A positioner can be used along with an actuator in some cases. These devices can be used to increase actuator speed, positional accuracy, the pressure against which the valve can be closed, or to retain plug position when differential pressures vary.

What Are the Consequences of Incorrect Valve Sizing?

Incorrect valve sizing can result in unnecessary, rapid wear on the internal components, and can also cause valve chattering and, in some cases, catastrophic damage. At the very least, improper initial sizing can force users to start from scratch in order to correct the problem. This can result in loss of production time, an increase in labor, additional costs for shipping and associated expenses, and, if the original product can't be returned, cost increases for the end products themselves.

At Baelz North America, we have assisted our customers with sizing their valves properly, allowing our customers to optimize performance and longevity. For this reason, we prefer that our clients send us all relevant information prior to ordering valves so we can assist with sizing from the start. We can provide our recommendations within 24 hours of the request.

What's Involved in the Baelz Valve Sizing Process?

To properly size your valve, Baelz will need the following information:

• Operating temperature — The operating temperature at the valve location.
• Operating flow rates — minimum, normal and maximum with units.
• Operating pressure — The maximum psi/bar at the valve and the differential pressure as well.
• Type of fluid and specific gravity (if known) — The type of fluid used can determine special requirements that the valve may need.

Our valves can support a wide range of special process needs, including chemicals and hazardous materials, and can easily handle heavier flows with the use of special cage plugs and stainless-steel components.

Source Properly Sized Control Valves from Baelz NA

As the official North American supplier of industry-leading valves and valve components from Baelz Automatic, Baelz North America is proud to offer a wide range of valve solutions to meet specific application needs. Our expert team is well-versed in valve sizing and can work with you to ensure that you select the ideal size for your unique job.

To learn more about control valves or for help in sizing your control valve, contact us or request a quote today. For tips on how to avoid valve chatter due to improper orientation, download our free guide.