Steam Pressure Reducing Valve-This valve must have a turndown somewhat greater than that of the system; it must respond to plus and minus control signals even at maximum and minimum flow rates. These valves are selected for a useful control range of 20 to 80% of maximum flow. They are normally an equalpercentage type or have equal-percentage characteristics. This type of valve has the best inherent flow characteristic and range needed for proportional control.
Water Control Valve - This valve must have sufficient rangeability to meet the application. Two valves in parallel may be needed to get this range-one large and one small. Where a large water pressure differential is encountered, be careful of possible cavitation. Consider using a pressure control valve upstream of the main flow valve.
Atomizing Steam Shutoff Valve - This valve is used only with Type 6970-72 desuperheaters in the on-off, not throttling mode, as in above valves. The valve is sized on a constant flow, dependent on unit size and a nominal pressure drop.
Temperature Controller - Must have an adjustable proportional control band wide enough to match response characteristics of the entire desuperheater system. Automatic reset prevents drift in control point. Rate action is seldom needed, but if it is provided, complete cutoff should be possible.
Pressure Controller - Must prevent large pressure variations which might interfere with temperature control; therefore, it needs an adjustable throttling range and automatic reset.
Control Valve Actuators - Pneumatically operated control valve actuators are the most popular type in use, but electric, hydraulic and manual actuators are also used. The spring-and-diaphragm pneumatic actuator is most commonly specified due to its dependability and simplicity of design. Pneumatically operated piston actuators provide an integral positioner capability and high stem force output for demanding service conditions.
Temperature Switches - They are used in connection with alarm systems for high and low temperatures. The temperature sensor uses the expansion principle in which the fluid or element in the sensing bulb reacts to the line temperature.
Pressure Switches - These switches, as above, are generally used in connection with alarm systems for high and low pressures. The alarm can be either audio or visual. Pressure applied to the sensor actuates a mechanism and its movement is then used to control the operation of an electrical snap acting switch, or other actuating medium.
WARNING: A control system cannot successfully hold a temperature that is no higher above saturation than the controller's degree of sensitivity and deadband. |