CONTROL VALVE APPLICATIONS
TEMPERATURE CONTROL of a SEMI-INSTANTANEOUS HEATER using a Temperature Control Valve
PURPOSE:
For accurate control of the temperature of a product being heated when the benefits of a temperature control valve are required.
OPERATION:
A temperature control valve offers response times and characteristics suitable for semi-instantaneous heating applications. The temperature sensor (RTD or Thermocouple) senses the temperature of the liquid being heated and allows modulation of the valve for appropriate supply of steam through the complete control loop. Pneumatic or modulating electric actuators may be used.
INSTALLATION GUIDELINES:
(see Figure15)
• Care should be given to the installation of the temperature sensor to ensure full immersion in the liquid. The sensor should be placed as close as possible to the heater tank to ensure accurate temperaturecontrol of the process fluid. • When using a pneumatically-actuated control valve, a valve positioner will provide enhanced control, stability, and shut-off capability in extreme flow applications. The positioner is mounted to the valve (integral on certain electric actuators) and receives a signal from an external control source, compares the control signal to the actual position of the valve plug, and then sends a corrected signal to the valve's actuator, thereby positioning the valve plug for optimum flow modulation. • Eccentric reducers, if required, are used on valve inlets to prevent accumulation of condensate which could become entrained with high-velocity steam, possibly resulting in dangerous waterhammer. • While a separator is appropriate for protection of the Regulator, it is not always required, as a properly sized drip leg with steam trap may be sufficient. It is recommended for systems where steam is known to be “wet” and the entrained moisture could affect valve performance and/or result in component damage. • Consider low-cracking pressure (1/4 PSI opening pressure) check valves after steam traps when discharging intocondensate return lines. Check valves eliminate the possibility of condensate backing up through the steam trap into the system. • A vacuum breaker and auxiliary air vent (not shown) should be properly installed to promote proper drainage and optimum heat transfer. The vacuum breaker allows system equalization with atmospheric air to allow gravity condensate drainage when vacuum is formed from condensing steam. The air vent improves heat-up times and overall heat transfer by expelling accumulated air on start-up. • A safety relief valve (SRV, not shown) is appropriate where applicable codes dictate their requirement, or anywhere protection of downstream piping and equipment from over-pressurization is desired. Consult the factory for appropriate SRV sizing guidelines. • The use of semi-instantaneous water heaters in conjunction with a storage tank or mixing station is typically not needed when using fast-acting control valves with high rangeability. Because the storage tank or mixing station will generally result in less demand variation, the control valve may cycle unnecessarily resulting in erratic control and or damage. Care should be taken to properly size the valve and select the appropriate actuator if a storage tank or mixing station is used. Consult the factory for sizing and selection guidelines.
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