Control Valves

A control loop is made of a controller, sensing component, and a control valve. A sensing component is responsible for transmitting signals to a DCS (distributed control system) or a single controller.

The controller examines the setpoint and signal, and will make necessary adjustments through transmitting a signal to the valve. The adjustment is confirmed by the sensing component. Thus, completing the control loop.

An effective control valve must achieve the following:

• A wider rangeability, meaning it must be able to function over different flows;

• Display minimum hysteresis or dead time;

• React to signals within its operational range accurately;

• React with the needed stroking speed;

• Respond to additional adjustments properly

For a tighter control, a positioner could be added. This device improves the performance of the valve by enhancing the signal of the controller. Therefore, a more accurate response is achieved. Also, this helps in controlling the effects of any friction between the valve and stem, achieving a better sealing.

The quality of a control valve can be measured in terms of time constant, dead-time lag, and its gain. Among these three factors, the gain is the most significant.

Selecting materials must include the trim, hard body, seal and packing, and soft gasket materials. As a basic requirement, the trim and body must be suitable for the materials of the connecting pipeline.

Apart from cost considerations and client specifications, the properties of the fluid flow should also be considered. Also, take note whether or not the application is handling corrosive and erosive media. Hard-facing the internal parts of the valve with chromium-cobalt alloys or nickel may slow down erosion.

Moreover, high-temperature (>800°F) and low-temperature (below freezing point) applications must also be considered. For instance, at a high temperature, a control valve is exposed to increased leakage and stress as a result of internal parts expanding.

Fluids that rapidly go through the control valve could cool down to below zero temperatures. This condition is particularly true when controlling high-pressure hydrocarbon fluids. A flash computation should be done to inspect the outlet temperature in the lower pressure.

For other low-temperature applications, such as atmospheric moisture and cryogenic fluids, the moving parts of the control valve are subject to freezing, making them inoperable.

Therefore, a control valve used in such conditions need insulation. The body and packing must be made to endure high pressure. In applications having greater than 1,000 psi, graphite is used to assist soft packing to avoid extrusion in small openings.

Determining the system and flow properties will help select the right control valve. The specified maximum flow rate must have the proper design margin, which is usually 10%. In specifying the max flow rate, these characteristics must be considered: the system’s size, pipe schedule number, geometry, and construction materials.

Flow coefficient is the most essential factor in terms of measuring the size of a control valve. The calculation of flow coefficient relies on whether the process flow is compressible, incompressible, or mixed phase. There are formulas to determine these characteristics that allow proper measurement of the valve size.

The following are some factors that will help you decide which flow characteristic is suitable for your application:

Equal Percentage

• When the majority of the pressure drop of a control system is through the control valve;

• When the majority of the pressure drop of a control system is not through the control valve;

• When the pressure drop across the control valve is anticipated to stay constant;

• For pressure- and temperature-control linear loops; and
For flow-control or liquid-level services.

Fast Opening

For repeated on/off operation, such as semi-continuous services, or where a quick huge flow is needed, such deluge systems.

Bonnet

A bonnet is a part of a control valve that encloses the valve packing and actuator. A bonnet is usually designed to suit various ranges of temperature. Therefore, special considerations must be done when choosing the right bonnet for your control valve.

An extended bonnet is used for high temperatures, such as 450°F, and below freezing temperatures. This type of bonnet makes sure that the valve packing is away from crucial temperatures.

In cryogenic service, an extended bonnet isolates the stem-valve packing from the sub-zero flow. This prevents the packing from getting damaged.

Trim

A trim is made of the internal components of a control valve that come in contact with the process flow. The components that are not part of the trim are the bonnet, packing, and bottom gaskets and flanges.

A trim maintains a stable connection between the valve-plug lift and flow capacity. It also guarantees a positive shut-off of the control valve.

The seat is mainly responsible for achieving a tight shut-off. However, a tight shut-off and proper lift depend on other components of the control valve. These include actuator design, valve-stem packing, and shape of the valve body.

The preferred level of tight shut-off relies on the nature of the service. Measuring shut-off is based on the percentage of leaks in the flow when the control valve is in the closed position. There are different industry standards that specify the requirements for shut-off for a range of applications.

A typical standard that determines leakage classes is ANSI/FCI 70-2-1998 (3) or Control Valve Seat Leakage. These classes are between Class II (weak shut-off) to almost zero leakage (Class VI). Depending on the requirements of the application, the operator sets the TSO or tight shut-off requirement in one of the classes (in most cases, Class IV, V, or VI are chosen).

The selection of the valve trim is mainly based on the following: operation conditions of the fluid, basic flow characteristics for a specific trim (by the manufacturer), and the effects of various operating conditions at the basic flow characteristics. These factors indicate a trim’s flow characteristics that are used as reference when choosing the right trim.

A reduced trim helps in achieving accurate control of low flows and provides enough room for high flows in the near future. The design of this trim lessens the flow through the opening of the valve. However, there is increased precision in controlling flow due to the reduced distance between the plug and lift.

There is no rule that indicates a reduced-capacity trim must be used lower than a specific turndown rate. But when accurate control is needed at 20% to 25% valve capacity, a reduced trim may be an excellent solution.

The use of cages is common in trims as it provides several advantages:

• A cage may be designed to guarantee an equal distribution of fluid forces on the stem and plug. This design results to a balanced, cage-guided trim.
• A cage guides the plug to make sure that it is positioned properly and makes proper contact with the valve seat.
• A cage may be used to change the original flow characteristics of the control valve.

The stem and plug in a sliding-stem valve encounter pressures that may affect the control of the actuator. This could result to high dead-bands and improper stem movement. The fluid surrounding the stem can lift it up, either sideways or downward, and may even provide torsional pressure.

There are several trim designs that could balance and counteract these pressures. A balanced trim utilizes mechanical adjustments to the plug or a cage-guided trim to distribute and balance the pressure.

Noise

A control valve creates noise as a result of aerodynamic effects, mechanical vibrations, or cavitation. Unsteady flow, pressure oscillations, and high velocities generate vibrations that are typically below 100 dB (decibel). A decibel refers to the strength of sound at max level from a portable radio’s earphones.

Noise as a result of cavitation varies on its degree. An increase in pressure drop through a control valve increases noise. A control valve creates a rattling noise in full cavitation.

Once a type of valve and flow characteristics are determined, an initial size can be specified by calculating the stroke of the valve for every design flow.

A valve stroke refers to the ratio of the actual flow capacity to the computed flow capacity for a certain valve. Select a valve type that functions between 10% to 80% of the valve stroke through the anticipated operation range.

Guidelines When Choosing the Size and Type of Control Valve:

• Pressure drop distributed in a pumped circuit to a control valve must be equivalent to ⅓ of the dynamic losses in a process system at 15 psi or at a rated flow, whichever is higher.
• Pressure drop distributed in a control valve from a centrifugal compressor’s discharge or suction line must be 50% of the system’s dynamic losses or 5% of the actual suction pressure, whichever is higher.
• Pressure drop in a control valve in steam lines to process vessels, reboilers, and turbines must be 5 psi or 10% of the steam system’s actual pressure, whichever is higher.
• In an operation wherein static pressure transfers fluid from one pressure container to another, pressure drop distributed to a control valve must be 50% of the dynamic losses of the process system or 10% of the lower-terminal container pressure, whichever is higher.
• The gain on the valve must not be lower than 0.5.
• Do not use the above 90% and below 10% of the valve stroke. A valve is simpler to control when it is between 10% and 80%.

A control valve may need more maintenance when it is poorly designed as it cannot function properly under certain service conditions. Problems that could occur include wear of the seat, packing, actuator diaphragm, and valve body. The chances of these problems can be decreased by choosing the appropriate design and materials of the control valve.

For instance, a control valve that handles mixed liquids and solids should be cleaned frequently to get rid of dirt. In this situation, a globe valve might not be ideal as dirt could go through the stem seal. This could restrict control and potentially damage the valve. Instead, a rotary valve may be a more suitable choice.

Friction made between the valve stem and packing may cause wearing. A sliding stem usually causes more packing wear compared to a rotary stem. This is due to a sliding stem collecting deposits and may carry them through the valve packing.

For services that may cause erosion, a metal seat is preferred. If a soft seat is necessary, then it must be positioned in a way that it is protected from the flow path. If the seat and plug don’t have great contact, then it might be a good solution to lap the seat.

Lapping is only applicable to a metal seat. It is a process wherein the seat and plug are ground together manually so their surface finish match each other. This process can achieve a leak-free connection. Furthermore, selecting a control valve with distinguished features could help decrease maintenance.

Smart positioners and valves transmit unique parameters, such as stem travel and actuator pressure, to software. This software uses these parameters to compute performance indicators, like torque and stem-packing friction.

A reduced-port control valve is typically used than a full-port. This is because a reduced port generates a pressure drop that can achieve the proper flow capacity. Moreover, a reduced port is more affordable since it can fit a smaller valve body. Prevent from asking rare sizes of valves, such as 2.5, 3.5, and 22. These are difficult to find and may cost more compared to standard sizes.

A control valve can have different end connections. The most commonly used is the RF or raised face. An RTJ or ring-type joint is common in several high-pressure classes. A control valve may be welded to achieve a tight connection. Welding a valve into place removes the weight and expenses of using flanges. However, it may become an issue when the valve needs to be removed for maintenance or repair.