Figure 1. Globe flow control valve schematic cross-section. Source: Assured Automation

Characterized ball valves provide enhanced flow control along with several additional advantages compared to globe valves.

Globe valves have long been the standard for many commercial and industrial flow control applications due to their acceptable equal percentage flow characteristic. Ball valves have several desirable advantages over globe valves (Fig. 1 schematic courtesy Assured Automation); for example, ball valves offer tighter zero leakage close-offs, higher rangeability ratios, high recovery, low-pressure drop, higher flow capacities and an improved turndown in less-costly, lighter and smaller bodies that are resistant to higher pressures. Ball valves also typically provide higher flow capacities (Cv) compared to globe valves of the same size.

The sliding or wiping action of the ball against the seat provides a self-cleaning feature, allowing ball valves to handle slurries and high-fiber media. In the open position, a full port (round bore or cylindrical hole) ball with a bore the same diameter as the process pipe or tubing provides unimpeded, low-turbulence flow. This superior flow pattern allows the use of low-cost, undersized ball valves in some process piping applications. The smooth bore of full port ball valves are “piggable,” which enables the passage of pigs or pigging systems for inspection and cleaning. Globe valves have regulating trim (plug or cage), which impede flow and cause turbulence in process fluids depending on the trim shape or level of characterization.

Figure 2. Characterized ball valves use wedge-shaped holes or V-ports to modify flow characteristic curves for flow control applications. Source: Assured AutomationFull port ball valves are excellent and superior to globe valves for on-off shut-off service and leak performance. Unlike globe valves, they also prevent sudden changes in inlet flow upon opening. They can achieve higher close-off ratings, such as bubble-tight shut-off valve leakage class ratings (such as ANSI/FCI 70-2 Standard for a Class IV shut off, or the more stringent TightShut-off (TSO) class of API 508/ISO 5208API). Bubble-tight leakage means that no bubbles are be observed in a prescribed time span during leak testing.

Ball valves are typically less costly and more compact compared to globe valves, but conventional ball valves with a full port ball have inherently poor flow characteristic with an extremely high flow coefficient. In throttling or flow control applications, the sharp edges on the bore in full port balls cause noise, cavitation and erosion of the trim components. Globe valves are also less prone to cavitation at higher flows, but cavitation can still be a problem at lower flow rates.

Ball valves with characterized trim (balls or seat inserts) can alleviate these problems and modify valve characteristics or flow characteristic curves (Fig. 2 schematic courtesy Assured Automation). In a characterized ball valve, the ball or seats with V-shaped holes, (v-ports or v-balls), slots or slotted holes, and custom-shaped holes are used in place of round through bored trim. Characterized ball valves also offer more flow capacity (C_v) options, which configure a valve more closely to the required specifications of a flow control application. Characterized ball valves are also called modulating ball valves, throttling ball valves, control ball valves, characterized control ball valves, slotted ball valves, linear ball valves, equal percent ball valves and V-port ball valves. In order to understand how a ball valve can be characterized, an understanding of flow characteristics curves is needed first.

Valve Flow Characteristic Curves

Figure 3. Commom types of flow characteristic curves. Source: Engineered SoftwareValve flow characteristics describes the relationship between the valve flow capacity or flow coefficient with changes in the valve stroke or stem opening of the valve. Valves are typically grouped into several major types of flow characteristic curves, such as quick opening, linear, equal percentage, parabolic and hyperbolic. Flow control applications typically require valves with linear, equal percentage or parabolic flow characteristics.

Quick opening flow characteristic valves have large, initial increases in flow when opened only a small percent. The characteristic curve is steep or has a large gain. These valves are fine for on-off service where a maximum flow is needed promptly. Quick opening valves would be fine for applications requiring periodical refilling of a process vessel, dip coating tank or heating bath. The gain of quick opening valves is too high for flow control. In valves with linear flow characteristic curves, the flow capacity changes linearly with percent opening (lift) or stem movement. Linear characteristic valves are useful in applications when flow control occurs at intermediate-to-high flow rates or percent valve openings, as well as in flow control applications requiring a valve with constant gain. Quick opening characteristic, and in some cases linear characteristic, valves are not useful for flow control at low flow rates or low percent opening, because small opening changes result in large changes in flow. Linear control valves are suitable for liquid level control applications in continuous process operations.

Flow capacitFigure 4. Valve characteristic curves for various valve types. Source: Valiny increases exponentially with valve trim level or opening. Equal percentage valves open progressively more area with valve travel to produce the same percentage change in flow per fixed increment of valve opening or angular position on the characteristic curve. A percent change in valve opening produces a constant percent change in flow rate. For example, if a 5% opening increases flow 20%, then another 5% opening would increase flow another 20%. Equal percentage flow characteristic valves provide more precise flow control or good throttling at low flow capacities or small-percent openings. In some applications, a valve needs to be oversized. Equal percentage valves are tolerant of oversizing. Equal percentage characteristic valves have more constant gain with load changes, which enables a more stable control loop. Modified equal percentage or parabolic characteristic curves lie between linear and equal percentage curves. They provide fine throttling at low flow capacities and then transition to linear behavior at high flow capacities.

Figure 5. Shift of inherent to installed characteristic curves for various static heads or pressure drops. Source: Engineered SoftwareManufacturers provide the inherent flow characteristic curves of their valve products, which are determined under constant pressure conditions across the valve, so installed flow characteristics can be different. A valve’s installed flow characteristic curve is different than the inherent curve due to the pressure drop across the valve from the process piping. In a system with extensive piping and process equipment-induced pressure drops, the flow characteristic curve will be shifted up, or additional valve opening will be required to compensate for the reduced flow from the pressure drop. The pressure difference across a valve is also a function of flow, which changes with valve position. An inherent equal percentage characteristic valve may have an installed characteristic curve closer to a modified linear curve shape. An inherent linear curve might be shifted to a curve approaching the undesirable quick opening type, which is not useful for throttling.

Characterizing Valve Trim Components

Figure 6. Standard flow performance curves for valves having characteristized V-port balls with various angles or linear characterization. Source: Plast-O-Matic ValvesThe ball valves can be flow optimized or characterized by using modified hole geometry such as a V-port or wedge-shaped hole, slotted, U-port, parabolic shaped or a combination of slots and small bores. Alternatively, ball valves can be characterized by changing round bored seats or sealing elements with V-shaped, U-shaped or parabolically shaped holes. V-variant port balls have a hole shape combining a slot with a wedge. A segmented ball with a V or parabolic contour will also alter flow characteristics. Segmented balls consist of a portion of a sphere. Segmented balls have less mass and surface area in sliding contact with seats or seal, so they are easier to rotate in position, which makes them useful in large industrial ball valves.

V-type or V-port ball valves are the most common type of characterized ball valve. The angle of the V-shape or V-notch in characterized ball valves alters the flow characteristic. V-port ball valves with 90° and 60° angle balls provide an equal percentage inherent flow characteristic, which is only slightly different compared to full port ball valves. Intermediate angle V-Port valves, such as a 30° or 45° balls, produce a modified equal percentage inherent flow characteristic. Slotted balls or V-shaped holes with small angles result in a ball valve with a linear characteristic curve. Figure 6 shows how the flow characteristic varies with characterized ball valves with different wedge angles.

Custom Characterization in Valve Design

Figure 7. Interchangeable characterized seats can modify flow characteristic curves of valves in the field for specific control needs. Source: FlowserveIn some cases, ball valves are designed with customizable characterizability, where it can be adjusted in the field by exchanging balls and inserts to modify the flow characteristics as needed. The swappable or exchangeable balls or inserts are sometimes called flow optimizers, flow optimizing trim, characterizing trim, flow optimizing ports or characterizing ports. A valve with characterizability or a characterizable valve can have a ball or seat swapped to vary the flow characteristic in the field for specific process applications.

Custom-designed ports are also available to meet special control requirements. Control characteristics and flow capacities can be easily changed by exchanging the ball. Balls can be designed with multiple passages or a series of smaller through-holes, slots or plates to reduce flow turbulence and flow characteristics. The number and position of passages or small bores can be selected to provide the proper modification to the flow characteristic curve. Balls are also designed with tortuous paths to allow gas expansion and control pressure drop to reduce fluid velocity, noise, cavitation and flashing gas ball valve applications. Custom characterized balls with combinations of V-ports, small round bores and slots are shown in figure 7.

Many factors are involved in the custom characterized trim geometry to deliver the required flow characteristics while avoiding turbulent flow and cavitation. The parameters or factors analyzed during trim design include:

• Type and properties of the fluids (for example, Reynolds number, density and viscosity)
• Fluid or media state (temperature, pressure and fluid velocity)
• Loss coefficient for valve and trim shape
• Size and shape of flow paths or resistance coefficient
• Flow characteristics curve versus valve stem stroke (C_v or percent flow versus percent opening)
• Control area and open area

Characterized ball valves can provide linear or equal percentage flow characteristics equivalent to or better than globe valves, along with excellent control stability. The in-line nature of characterized ball valves allows fluid media to flow linearly through piping systems with minimal flow disturbance, which enables increased media control and faster response times to controller commands.

Figure 8. Custom-designed characterized port shapes can be cut into valve balls to produce unique flow characteristics for specialized ball valve flow control applications. Source: Past-O-Matic ValveCharacterized ball valves are designed with a combination of control area and open area to provide exceptional flow control and high rangeability. The control area of the characterized ball valve can produce high pressure drop and low flow conditions. The open area delivers the low pressure drop, high flow coverage and uninterrupted flow capacity required in many applications. Valves with characterized balls or ball segments have high rangeability. Rangeability is a factor calculated by dividing the maximum flow capability of the valve by the minimum controllable flow. The rangeability factor is measured with a constant pressure differential across the valve. Overall valve design and manufacturing tolerances also impact rangeability. Characterized ball valves can have rangeabilities from 200:1 to 800:1.

Conclusion

Characterized ball valves may cost more than full port or standard port ball valves due the complex drilling or forming of wedge or V-shaped holes. Custom characterized trim geometry might add even more cost. However, the performance benefits of characterized ball valves far outweigh the added cost. In addition, trim characterization can be applied to other valve types to improve flow characteristic curves. For instance, plug and globe valve flow characteristic curves can also be modified by using characterized trim such as characterized cages or contoured plugs.

Find Characterized Ball Valves

Select the characterized option in the features section on the ball valves SpecSearch filter.

Characterized Ball Valve Products and Suppliers

Assured Automation Ball Valves
Plast-O-Matic Valves, Inc. Ball Valves
Belimo Americas Ball Valves
Bray Commercial Characterized Ball Valves
CRANE ChemPharma Flow Solutions Ball Valves
Flowserve Corporation Ball Valves
TrimTeck Characterized Ball Valves

Further Reading

Modulating Control Valves
Part I: An Insider’s Guide to Control Valves and Process Variability
V-Port Ball Valves vs. Globe Valves for Control Applications
Globe Valves vs Ball Valves
Valve Trim Design Using Control Valve Performer
Linear or equal percentage valves: When should I use which?
Specifying Control Valve Data
Controlling Flow Ball Valves
Equal Percentage Control Valves and Applications