11th May 2007
The controls are easily adapted for remote timed on/off modulating and the ‘safeguard’ option protects processes by automatically shutting off heat at the source in the event of temperature overrun or system failure.
“Self-acting control valves require no external power source or instrument signal. It relies on the vari- able process conditions to respond in some predetermined way,” explains Spirax Sarco marketing manager Bobbie Brooke-Taylor.
Self-acting controls give stable, modulating control and are suit- able for steady loads, such as storage tanks, oil outflow heaters and hot water storage.
This type of control responds well to gradual changes in the pro- cess condition, holding the variation in the process very close to the set point and avoiding large swings that can lead to system hunting.
However, for applications with rapidly fluctuating temperatures, self-acting controls may not react quickly enough to provide close control. It is also unsuitable where a temperature ramp-up is needed, as when starting a process. Self-acting controls cannot generally be interfaced with external systems.
These valves are generally used when the process has one set point condition. The self-acting valve is tuned to that set point and, as the process varies, responds in a way to return conditions to the set point. Pressure and temperature are the two most popular process conditions to whicha self-acting valve is applied.
The Spirax Sarco self-acting control valves can be used in a wide variety of environments from the hospitality industry to the food pro- cessing industry. They also enjoy a long maintenance-free life with repeatable performance, as they oper- ate through liquid expansion with friction-free bellows.
Where speed of response is not essential and electricity is the preferred source, electric actuation offers reliable and accurate control.
Electric controls are suitable for applications requiring slower res- ponse times, such as drying, preheating, feed tanks and space heating.
Spirax Sarco electric controls are offered with a sensor, controller, actuator, positioner and valve in one complete system. A plug-in positioner card provides accurate valve movement irrespective of varying forces.
In the majority of applications, temperature or pressure control needs pneumatic actuation to maintain accuracy under rapidly changing load conditions.
Pneumatically actuated controls are ideal for dusty and hazardous conditions and offer powerful actuation for high-pressure applications.
Edited by: Laura Tyrer