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Control Hardware: Electric/Pneumatic Actuation
Control valve capacities and characteristics are investigated, along with theory and practical advice on how to size them for water and steam systems. Actuators, positioners, and controllers are introduced plus their overall effect on the control loop.
This tutorial briefly describes the basic components of different types of linear and rotary action control valves available for use in steam and water systems.
Valves need to be measured on their capacity to pass fluid. To enable fair comparison, valves are sized on a capacity index or flow coefficient. This tutorial explains the different types of flow coefficient in use, how they are established, how they compare, and typical values for different sized valves.
This tutorial briefly describes how to use flow coefficients to size valves for water systems, the difference between using two-port and three-port valves and the effect of these valves on pressure drop, flow and water system characteristics. Also explained is the importance of valve authority, and the cause and effects of cavitation and flashing under certain conditions.
Sizing a control valve for a steam application can be a complex matter. This Module attempts to throw light on the subject by using first principles to explain the relationship between flow and pressure drop. It uses a simple nozzle to explain the phenomenon of critical pressure, and how this can be predicted for steam flow through a control valve. It continues by discussing other properties such as noise, erosion, and how steam is dried or superheated as it passes through a valve, and gives various examples of such calculations. It also briefly compares shell & tube and plate heat exchangers, and shows how to use simple Kv charts to size steam valves.
Various types of flow characteristics are available. This tutorial discusses the three main types used in water and steam flow applications: fast opening, linear, and equal percentage flow; how they compare, and how (and why) they should be matched to the application in which they are used.
Control valves need actuators to operate. This tutorial briefly discusses the differences between electric and pneumatic actuators, the relationship between direct acting and reverse acting terminology, and how this affects a valve's controlling influence. The importance of positioners is discussed with regard to what they do and why they are required for many applications.
Controllers and sensors are important parts of the control system; without information from the sensor, the controller cannot make a decision and instruct the valve to move. This tutorial briefly discusses the different types of controllers and sensors available and how they operate. A brief explanation of digital and analogue control signals is also given.