Pressure Compensation of Distillation Tray Temperature

Many process measurements are sensitive to pressure changes. By incorporating the pressure measurement into the PV we can ensure that the controller takes the correct action. We have already covered one example of this as Equation (4.45) — a means of reducing the effect of swell in steam drums by conditioning the level measurement to reduce 

If a distillation tray temperature controller keeps the temperature constant as the pressure changes, the composition wiU move away from target. We can resolve this by using the pressure to condition the temperature measurement. The subject of pressure compensated temperatures is covered in full in Chapter 12. 

If the flow measurement is in actual volumetric units, i.e. reported at actual (rather than standard) pressure and temperature, then the formula becomes 

However these formulae should be applied with care. In Chapter 10 we show how their application to gaseous fuels can worsen problems with combustion control. 

So the flow of hydrogen, in standard volumetric units, is given by 

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