Key Questions
The following key questions are answered in this module:
What is the valve coefficient?
The valve coefficient is a measure of how much fluid can flow through a valve in a given time period.
What is the valve stroke?
The valve stroke is the distance the stem must move to go from fully closed to fully open.
What is the flow characteristic?
The flow characteristic describes the relationship between the stem's movement through its stroke and the corresponding changes to the valve's flow rate.
What are valve pressure classes?
Every valve can handle a certain amount of internal pressure. This is the valve's pressure rating. Valves can be classified by how much pressure they can handle. The most commonly used standards for valve pressure classes are set by the American National Standards Institute (ANSI). There are six standard pressure classes and additional special classifications and intermediate classifications.
What are valve shutoff classes?
Valves are also rated by the amount of leakage they permit when sealed. This leakage is measured as a percentage of the valve's rated flow capacity. ANSI has created six shutoff classes, with Class I being the weakest seal and Class VI the strongest.
What are actuator-controlled valves?
Valves that are not controlled by a manual operator, such as a lever or handwheel. Instead, actuator-controlled valves are controlled by an automatic actuator. Automatic actuators are frequently a part of a control loop that also includes a sensor and a controller.
What are control loops?
In a feedback control loop, the sensor monitors downstream conditions and sends that information to the controller. This then controls the valve. There are feedforward and feedback control loops.
Whtat is rangeability?
Rangeability is a measure of how effectively an automatic control valve can control a flow after receiving a signal from the controller. It is defined as the ratio of the maximum controllable flow divided by the minimum controllable flow through a valve.