Control Valve Characteristics

Illustrative examples of control valve characteristics and their intrinsic properties can be found in Figures 6.5.1 and 6.5.2.

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Quick-Opening Characteristic

Quick-opening valves exhibit a significant change in flowrate with minimal valve lift from the closed position. For instance, a 50% valve lift may lead to an orifice pass area and flowrate as high as 90% of its full potential. Often referred to as having an ‘on/off’ characteristic, these valves lack a standardized curve shape. Thus, two quick-opening valves may differ, one showing 80% flow for a 50% lift and another 90% for a 60% lift.

Typically, these valves are electrically or pneumatically operated for ‘on/off’ control. Self-acting control valves, however, demonstrate a nearly similar plug shape but allow for high rangeability and precise flowrate changes, hence not classified strictly as quick-opening.

Linear Characteristic

Valves with a linear characteristic maintain a direct proportionality between valve lift and flowrate at constant differential pressure by ensuring a linear relation between valve lift and orifice pass area. For instance, a 40% valve lift translates to a 40% orifice size allowing 40% of the maximum flow.

Equal Percentage Characteristic

Each increment in valve lift for these valves increases flowrate by a certain percentage over the previous flow, forming a logarithmic relation rather than a linear one. This control is governed by mathematical expressions such as Equation 6.5.1.

Example 6.5.1:

Assuming a maximum flowrate of 10 m³/h and a turndown of 50:1, at constant pressure:

• At 50% lift, flowrate reaches 1.414 m³/h, a 48% increase from 0.956 m³/h at 40% lift.
• At 60% lift, flowrate hits 2.091 m³/h, again a 48% increase from 1.414 m³/h at 50% lift.

This ensures a 48% rise for every 10% valve lift increment within a 50 rangeability. Higher rangeability, such as 100, further increases this percentage to 58% for identical conditions. Table 6.5.1 can clarify this flowrate variation across the valve lift range for equal percentage valves.

While other inherent valve characteristics exist, like parabolic or hyperbolic, fast opening, linear, and equal percentage remain the most frequently produced types.

Aligning Valve Characteristics with Installation Requirements

Applications inherently exhibit different installation characteristics correlating fluid flow with heat demand, often influenced by varying valve pressure differentials:

• In water systems, reduced flow boosts upstream valve pressure.
• In steam systems, pressure drop across the control valve adjusts to the heating load requirement.

Chosen valve characteristics must ensure a direct relation between valve opening and flow over its operational travel. Linear characteristics suit water systems, while steam systems leverage equal percentage valves for optimal performance.

1. Three-Port Valve in Water Circulating Heating Systems

Maintaining balance in such systems involves using a linear characteristic valve for constant or diverted water flow to keep pressure loss over the valve stable.

Conclusion: A linear characteristic valve often aligns best with inherent characteristics, offering minimal gain to the control loop.

2. Two-Port Valve in Boiler Water Level Control Systems

Water systems using two-port valves, such as described in Figure 6.5.6, vary flow rates impacting differential pressure due to:

• Pump characteristics and frictional resistance changes with flowrate.
• Boiler pressure variations based on load and control system type.

Example 6.5.2:

Simplified boiler rated at 10 tonnes steam per hour; analyzing Table 6.5.2 reveals differential pressure dynamics affecting valve sizing. Unsuitable sizing might result in undersized valves missing required valve capacity (Kvr) specific to installation demands described by installation curves.

3. Steam Temperature Control with Two-Port Valves

In heat exchangers using steam for primary heating, a two-port valve modulates steam flow based on heat transfer rates. Understanding this dynamic through data such as heat loads, heat transfer coefficients, steam pressure, and secondary water flowrates helps align valve characteristics with installation specifics.

Example 6.5.3:

Illustrating steam-to-water heat exchange analysis through determining heat load, heat transfer areas, and using tables (6.5.7 – 6.5.10) helps properly size and select valves ensuring optimal heat exchanger performance.

Exploring in-depth performance reviews for linear versus equal percentage valves across typical scenarios helps identify suitable control solutions for varied applications.

Expanded Information on Industrial Linear Control Valve Products

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