Throttling Ball Control Valves

What Is a Throttling Ball Valve?

A standard on-off ball valve is designed for low resistance when fully open and tightness when fully closed. When such a valve is used in intermediate positions, flow rises very quickly in the first degrees of opening, the seats are exposed to high-velocity flow and the control loop can become unstable. A throttling ball valve is a control valve designed with these issues in mind.

Ball valves used for throttling have a contoured V-notch on the ball. As the ball turns, the flow area grows gradually from a small opening, so flow changes in a proportional and predictable way with opening angle. Splined connections between shaft and ball and between shaft and actuator help transfer even small changes of the control signal to the ball without loss.

These valves can be used for both continuous throttling and automated on-off service, with many rotary actuators and manual operators. This allows the same valve platform to serve both control and isolation points in a plant and reduces the variety of spare parts.

How Is Throttling Achieved with a Ball Valve?

Throttling is a chain in which the control signal is converted into valve position and valve position into flow. Every link in the chain affects control quality.

1.      Measurement and control decision: A flow, pressure, level or temperature transmitter measures the process variable; the controller produces a valve position signal according to the setpoint.

2.      Positioning: A digital valve controller or positioner converts the signal into actuator air pressure and verifies ball position through feedback.

3.      Transfer of motion without play: The actuator turns the ball through the splined shaft. Splined connections and a clamped actuator lever reduce lost motion so that small signal changes reach the ball.

4.      Characterized flow: The V-notch creates a flow area that grows gradually with opening; the modified equal percentage characteristic gives fine control especially at small openings.

Why the characteristic matters

The valve characteristic defines the relationship between ball opening and flow coefficient. Equal-percentage type characteristics help linearize the installed characteristic when the system has pressure losses, and make it easier for the loop to respond similarly at different loads. The ratio of the maximum flow coefficient to the minimum usable flow coefficient is called rangeability and shows over how wide a flow range the valve can control.

Cavitation and noise during throttling

During throttling the pressure drop concentrates in the narrow passage between ball and seal. In liquids this can lead to cavitation, in gases to aerodynamic noise. A rotary attenuator reduces these effects in both liquid and gas services; an anti-cavitation trim can be used in liquids with severe cavitation and a noise-attenuating trim in gas and steam. With an attenuator installed, the V-notch no longer acts as a high-velocity erosion point.

Features That Define Throttling Performance

·         Contoured V-notch: Precise contouring gives a modified equal percentage characteristic.

·         Splined shaft connection: A splined drive shaft and clamped actuator lever keep lost motion to a minimum.

·         Pressure-balanced seal design: Precision-machined parts and pressure-balanced seals allow smooth, accurate ball movement.

·         Small-notch option: Small-notch ball on 1 in valves for very low flows; lower and higher capacity notch variants.

·         Field-reversible action: When a rotary diaphragm or piston actuator is used, the action can be switched in the field between push-down-to-close and push-down-to-open.

·         Severe service trims: Rotary attenuator, anti-cavitation and noise trims for cavitation and noise created during throttling.

·         Wiping seal: Because the seal wipes the ball surface continuously, no deposits form during throttling.

Technical Specifications

The values below belong to V-notch ball control valves used for throttling service.

Flow coefficients, flow coefficient ratio and noise levels are determined by size and trim from separate catalog data.

Key Advantages

·         Stable control loop: A characterized ball and low lost motion reduce oscillation and process variability.

·         Throttling and on-off in one: The same valve can be used for continuous control and automated isolation.

·         Accuracy at low flow: Control is possible at small flows with the small-notch ball.

·         Rotary-valve capacity: The straight-through flow path provides high capacity for the line size.

·         Suitable for demanding services: Use in high pressure drop throttling with cavitation and noise trims.

Applications

Throttling ball valves are used as the final control element of flow, pressure, level and temperature loops in process industries.

Process liquids

Flow and level control of clean and dirty liquids; with anti-cavitation trim on lines at risk of cavitation.

Steam and gas

Pressure and flow control on steam and gas lines; with noise trim at noise-critical points.

Slurry

Non-clogging throttling of fibrous slurries.

Low-flow dosing

Accurate control of very low flows with the small-notch ball.

Selection Criteria for Throttling Service

In throttling service the valve should be selected as part of the control loop; valves chosen by line size alone are usually oversized and operate at small openings.

Control requirement

·         Minimum, normal and maximum flow

·         Target ball opening at normal flow

·         Required rangeability

·         Speed and accuracy expected from the loop

Process conditions

·         Pressure drop and cavitation / noise risk

·         Solids and fiber content

·         Seal suitable for process temperature

·         Possibility of reverse flow

Actuation and positioning

·         Actuator type and torque capacity

·         Fail position

·         Digital valve controller and diagnostics needs

·         Need for a manual operator

Standards

Main standards applicable to throttling ball control valves:

Applicable standards must be confirmed for the valve series.

The TLY Enerji Control Loop Approach

Many throttling performance problems stem not from the valve itself but from oversizing, the wrong characteristic or a mismatch between actuator and controller. TLY Enerji therefore handles throttling ball valve selection across the whole control loop.

During sizing with process data, the opening at normal flow, rangeability and cavitation/noise risk are assessed together. Actuator, digital valve controller and, where needed, a severe service trim are selected on the basis of this analysis. If existing loops show oscillation or instability, valve size and characteristic are reviewed and improvement proposals are prepared. Support needed during commissioning to check position feedback and loop response together is also included in the project plan.

·         Loop-oriented sizing

·         Characteristic and trim selection

·         Actuator and digital valve controller selection

·         Performance review of existing loops

·         Coordination of commissioning support

Frequently Asked Questions

Can a standard ball valve be used for throttling?

When standard on-off ball valves are used in intermediate positions, flow rises very quickly in the first degrees of opening and the seats are exposed to high-velocity flow, which can wear them quickly. For continuous throttling, control-oriented ball valves with a characterized ball and low lost motion should therefore be preferred.

What is lost motion and why does it matter?

Lost motion is the play and flexibility that lets the actuator move without the ball changing position. As lost motion increases, small changes of the control signal do not reach the valve and the loop may oscillate. Splined shaft connections and clamped actuator levers reduce this play and improve control accuracy.

Which characteristic do throttling ball valves use?

V-notch characterized balls use a modified equal percentage characteristic. With this characteristic, flow increases slowly at small openings and faster at larger openings. Considered together with system pressure losses, it helps make the installed characteristic more balanced.

Can very low flows be controlled with a ball valve?

On 1 in valves, a small-notch ball option allows accurate control of very low flows. Chrome-plated stainless steel, solid cobalt alloy and ceramic balls are available; the ceramic version is limited to 93 °C. Lower- and higher-capacity notch variants can also be evaluated per project.

What should be done if cavitation occurs during throttling?

First assess cavitation risk from the pressure drop and liquid properties. If there is a risk, a rotary attenuator or, for severe cavitation, an anti-cavitation trim can be used; this trim can reach a Kc of 1.0 at extreme conditions. For it to work effectively, the valve has to be mounted so that flow enters in the forward direction.

Can the same valve be used for throttling and on-off service?

Yes. V-notch ball control valves can be used for high-performance throttling and on-off service with various rotary actuators and manual operators. For on-off duty, the required shutoff class should be evaluated separately together with the seal choice.