Level Control of Simple Vessels

The analysis employed here is further simplified in that the eflfects of variable valve-pressure drop are omitted, as are transmitter dynamics. 

For this case Ka, Gj, s) becomes simply Then equation (16.5) becomes  

If the input span of the valve positioner is the same as the transmitter output span (as, for example, 3-15 psig), and if the valve has an installed linear ffow characteristic with a wide-open capacity approximately equal to four times flowsheet flow, Q s, then  

This last equation is very useful for finding the desired holdup, A iHr, provided a value of is specified. Usually we choose Ka, = 2 and bias the 

Signal flow diagram for simple level control system 

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