Column
Control valve Q & A
About the author
Dr . Hans D . Baumann is an internationally renowned consultant with extensive experience in the valve industry . Throughout his career , he held managerial positions in Germany and France , and his innovative spirit led to the creation of 10 novel valve types , including the well-known Camflex valve . Dr . Baumann has authored 8 books , including the acclaimed " Valve Primer ," and has been granted 115 US patents . He also founded his own valve company , which he later sold to Emerson , and served as Vice President at Masoneilan and Fisher Controls Companies .
Q : Process controllers play a crucial role in control systems , but their functions and settings can be complex . Could you explain the basics of process controllers in a straightforward manner , without relying on complex equations and graphs ?
A : Let ' s break down the functions of process controllers and their role in control systems in a simple way : Controllers are the final part of a process control loop . They compare a signal from a process transmitter , which indicates the actual process conditions , with a desired value . Any difference between the two ( an offset ) is then modulated , and a final output signal is sent to the control valve ( or other final control elements ) to eliminate such an offset . Now , let ' s examine the controller settings that modify the offset to create a fitting output signal . ( Don ' t try to look this up in a handbook on automatic control ; you may be confused by the many equations and graphs .) Here ' s my simple explanation : 1 . Proportional band : This is how your room thermostat operates . All that ' s needed is the setting ( the temperature you want ) and feedback ( the room temperature ). You ' ll notice that there ' s always a small difference between the room temperature and the desired one ; that is the proportional band . It ' s typically set by the thermostat manufacturer but is adjustable in process controllers .
2 . Rate action ( derivative ): This controller action is performed when the process ( transmitter signal ) is above the controller ' s set point . Here , the valve is asked to slowly decrease flow .
3 . Reset action ( integral ): This correction is made when the process signal falls below the controller ' s set point . Here , the valve is asked to slowly increase flow . All Rate and Reset actions are made in steps per second . Their rate of change may be non-linear , depending on the time constant of the system . A typical diaphragm-actuated control valve has a time constant of 4.7 seconds . This limits the rate of travel to 14 % per second , which is too slow for a fast ( pressure control ) system . As a rule of thumb , a factor of 3 should separate the Time Constant of a system from that of the control valve for optimum loop stability . This means that with a diaphragm-actuated control valve ( without positioner ), a controlled system should have a time constant above 14 seconds , meeting most temperature control systems . Note : Proportional band is equivalent to ' gain '.
S = COMMAND SIGNAL in PERCENT of TRAVEL per SECOND
10
8
6
4
2
0
GRAPHICAL DISPLAY of RATE ACTION from TEXT
O 10 20 30 40 50 60 70 80 = PERCENT of DIVIATION FROM SET POINT
Example : To better understand the above , consider this example : In a heating system comprising :
• A control valve with 1.5 inch travel , controlling the flow of heating medium to a heat exchanger
• A temperature transmitter measuring the temperature downstream of the heat exchanger
• A process controller
When everything is in balance :
• The controller set point is 12 mA
• The transmitter signal reads 12 mA
• The valve is at 50 % travel , corresponding to a controller signal of 12 mA
Now , assume there ' s a sudden decrease in temperature demand , requiring less flow of heating medium . As a result :
• The transmitter signal drops to 10 mA
• The controller senses this as a 17 % error below the set point
• This calls for a rate action response from the controller , requiring a change in the signal to the control valve to reduce travel
The speed of valve closure is determined by the following equation : S = ( percent error / 10 ) 2 × 0.15 Where S is the speed of travel in inches per second . In this case : S = ( 17 / 10 ) 2 × 0.15 = 0.4 inch per second towards valve closure Assuming the valve has an installed linear flow characteristic , a 17 % reduction in flow ( and travel ) could be accomplished within 0.6 seconds ( assuming no dead times in the system ).
Conclusion
Understanding process controllers is key to optimising control systems . While the underlying mathematics can be complex , the basic principles are straightforward . Proportional , Rate , and Reset actions work in concert to maintain desired process conditions . By grasping these concepts , engineers can better fine-tune their systems , balancing these actions to match specific system characteristics and process dynamics . This knowledge forms the foundation for effective control across various industrial applications , from simple heating systems to complex chemical processes .
12 Valve World November 2024 www . valve-world . net