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Pressure Independent Control

Pressure Independent controls in valves an additional input to the control of the VAV terminal unit. The pressure independent control options also monitor and respond to the velocity of the air flow, generally at the inlet of the unit.

The terminal unit damper is positioned from a signal sent from the zone thermostat through a velocity reset controller to the damper actuator. The velocity reset controller then responds to changes in the inlet pressure conditions to maintain the required airflow. Pressure independent controls are frequently used for single duct variable volume control. For a given thermostat setting the controller can position the damper further open if the air flow at the inlet is insufficient to meet the requirement or it can position the damper further closed if the inlet air flow is greater than the requirement.

Example-
If the people in room A were to disperse and leave the room, the cooling load would decrease and the thermo-stat would respond by sending a signal to the VAV terminal unit damper actuator to close down the damper and reduce the airflow. The result would be an increase in the system static pressure which would increase the amount of airflow to the terminal unit serving room B. The terminal unit controller would immediately sense the increased air flow through the inlet sensor and begin to reposition the damper to maintain the required airflow.

Pneumatic and Electronic pressure independent controllers do have their limitations. Selection of air flows must be given careful consideration. There set controllers respond to an input signal from the differential inlet sensor in the range of 0.0”WC to 1.35” WC which reflects the CFM range of a given terminal unit size. Both pneumatic and electronic controllers can not accurately control the air flow when the differential pres-sure signal falls much below 0.03” WC. For this reason, Carnes publishes minimum air flow setting limitations and suggested air flow ranges for each unit size.

Another design consideration when using pressure independent controls is the state of the central system. When the central system is shut down or not supplying adequate air to meet the design requirements, the primary flow control damper can drive open looking to satisfy a minimum air flow condition as long as the controls remain active. This feature can be beneficial by providing open dampers at the start of the morning warm-up cycle. However, it may not be desirable in some fan terminal unit applications. Re circulated air could short circuit and flow backup stream.

Pneumatically, the terminal unit damper can be configured to fail in the open or closed position on a loss of main control air pressure irregardless of the thermostat action required. Electrically or electronically the damper must be powered and driven to either of those conditions.

Flow Control Valve

Operation
Control valves are normally fitted with actuators and positioners. Pneumatically-actuated globe valves and Diaphragm Valves are widely used for control purposes in many industries, although quarter-turn types such as (modified) ball, gate and butterfly valves are also used.

Control valves can also work with hydraulic actuators (also known as hydraulic pilots). These types of valves are also known as Automatic Control Valves. The hydraulic actuators will respond to changes of pressure or flow and will open/close the valve. Automatic Control Valves do not require an external power source, meaning that the fluid pressure is enough to open and close the valve.

Automatic control valves include: pressure reducing valves, flow control valves, back-pressure sustaining valves, altitude valves, and relief valves. An altitude valve controls the level of a tank. The altitude valve will remain open while the tank is not full and it will close when the tanks reaches its maximum level. The opening and closing of the valve requires no external power source (electric, pneumatic, or man power), it is done automatically, hence its name.

Applications
Process plants consist of hundreds, or even thousands, of control loops all networked together to produce a product to be offered for sale. Each of these control loops is designed to keep some important process variable such as pressure, flow, level, temperature, etc. within a required operating range to ensure the quality of the end product. Each of these loops receives and internally creates disturbances that detrimentally affect the process variable, and interaction from other loops in the network provides disturbances that influence the process variable.

To reduce the effect of these load disturbances, sensors and transmitters collect information about the process variable and its relationship to some desired set point. A controller will then process this information and decides what must be done to get the process variable back to where it should be after a load disturbance occurs. When all the measuring, comparing, and calculating are done, some type of final control element must implement the strategy selected by the controller. The most common final control element in the process control industries is the control valve. The control valve manipulates a flowing fluid, such as gas, steam, water, or chemical compounds, to compensate for the load disturbance and keep the regulated process variable as close as possible to the desired set point.

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