CONTROL VALVE FUNCTIONS AND TECHNOLOGY EXPLANATION

The rated flow rate passing through a valve under specified conditions.

A constant related to valve geometry, used to quantify flow capacity. It is defined as the flow rate, in US gallons per minute (US gal/min), of 60°F (approx. 15.6°C) clean water that passes through a fully open valve with a pressure drop of 1 psi across the valve ports. A higher Cv value indicates greater flow capacity and lower pressure loss.

The flow coefficient (Cv value) of a valve at its rated travel.

The ratio of the flow coefficient (Cv value) at a specified travel to the flow coefficient (Cv value) at rated travel.

The ratio of the maximum controllable flow to the minimum controllable flow, provided the valve meets specified flow characteristic deviation requirements. It is often expressed as the ratio of the maximum flow coefficient (Cvmax) to the minimum flow coefficient (Cvmin), i.e., R = Cvmax / Cvmin. Typical rangeability for standard control valves is 30:1 or 50:1.

The minute flow that passes when the flow control element (e.g., valve plug) is not fully seated against the valve seat, occurring below the minimum controllable flow.

The fluid quantity passing through the valve sealing interface when the valve is in the fully closed position under specified differential pressure and temperature conditions. This is an important indicator of a valve's shut-off tightness.

The point where fluid velocity is at its maximum, and static pressure and flow area are at their minimum as fluid passes through a valve. It is typically located slightly downstream of the valve seat.

A valve construction (e.g., ball valve, butterfly valve) featuring streamlined internal contours and low fluid turbulence, allowing downstream pressure to recover to a high percentage of the inlet pressure beyond the vena contracta.

A valve construction (e.g., conventional straight-through single-seat valve) that, due to its flow path geometry, causes significant fluid turbulence, resulting in weaker pressure recovery downstream of the vena contracta. The recovered pressure represents a lower percentage of the inlet pressure.

The functional relationship between the relative flow (Q/Qmax) through a valve and the relative travel (l/L) as the percentage rated travel varies from 0% to 100%. This term must be clearly distinguished as either inherent flow characteristic or installed flow characteristic.

The relationship between flow and travel measured under test conditions where the differential pressure across the valve remains constant.

The relationship between flow and travel under actual piping system conditions, where the differential pressure across the valve varies with flow due to factors such as line resistance.

An inherent flow characteristic where the flow change per unit change in travel is proportional to the current flow. In other words, the logarithm of flow is linear with travel. Suitable for applications requiring wide-range regulation and significant system pressure drop variations.

An inherent flow characteristic where the flow change per unit change in travel is constant. That is, flow is linear with travel. Suitable for applications where most of the system pressure drop occurs across the valve.

An inherent flow characteristic where flow increases rapidly during the initial valve opening (small travel), after which the flow increase tapers off. Commonly used for on/off or two-position control.

A composite flow characteristic that provides an approximately equal percentage characteristic at low travel positions of the flow control element and an approximately linear characteristic at high travel positions.

The driving device of a control valve that generates thrust or torque based on a control signal to move the valve stem or shaft.

An actuator that can provide driving force in either direction (open or close) using external power (e.g., pneumatic or hydraulic pressure).

The adjustment range of the spring inside an actuator, used to balance the process-induced unbalanced force under actual operating conditions, ensuring the valve can be stably positioned at the required travel.

The difference between the high and low pressures applied to the diaphragm of a diaphragm actuator required to produce rated valve plug travel.

The diaphragm pressure range measured with the valve body at atmospheric pressure (i.e., no process pressure influence).

The pressure range required on the diaphragm to produce rated valve plug travel while the valve is subjected to actual process pressures. Due to dynamic unbalanced forces generated by the process fluid, this range typically differs from the inherent diaphragm pressure range.

In a diaphragm actuator, the portion of the diaphragm area that effectively generates output force. This area varies with diaphragm travel, with molded diaphragms exhibiting less variation than flat diaphragms.

The net force acting on the valve plug (or valve poppet) at any specified valve opening due to the action of process fluid pressure.

The net force generated on the valve stem at any position due to fluid pressure.

The change in spring force per unit change in spring compression (or extension). In diaphragm actuators, this is typically expressed in pounds-force per inch (lbf/in).

A safe state: when the driving air supply to a pneumatic actuator is interrupted, the spring force within the actuator drives the valve flow control element to the closed position.

A safe state: when the driving air supply to a pneumatic actuator is interrupted, the spring force within the actuator drives the valve flow control element to the open position.

A critical characteristic of a valve and its actuator: upon interruption of the driving energy source (e.g., air supply, electrical power), the valve flow control element automatically moves to a preset safe position. This safe position may be fully closed, fully open, or remain in the last position (Fail-in-Last-Position), depending on process safety requirements. Achieving a complex “hold-last-position” function typically requires auxiliary control systems (e.g., air tanks, solenoid valves).

Synonymous with “Fail-Close.”

Synonymous with “Fail-Open.”

A straight-through valve construction where the flow control element is located between the actuator and the valve seat. When the actuator stem moves downward (extending), it pushes the flow control element against the seat, thereby closing the valve. This construction is also referred to as direct acting.

A straight-through valve construction where the valve seat is located between the actuator and the flow control element. When the actuator stem moves downward (extending), it pulls the flow control element away from the seat, thereby opening the valve. This construction is also referred to as reverse acting.

The distance the valve flow control element moves from the fully closed position to the manufacturer’s recommended fully open position.