Level measurement damping chamber

If level-measurement nozzles are not protected by an internal damping chamber, they should have an orientation no more than 90° from the vapor-retum nozzle and should not be located under the last downcomer. 

For base level measurements, internal damping chambers should be used if sealing only is required. For severe plugging applications such chambers are, of course, inappropriate. 

The natural period of any control loop containing a resonant element cannot exceed that of the resonant element. The phase shift of a resonant element is exactly -90 at its natural period, no matter how heavily damped it may be. Since the integration of flow into average level represents an inherent phase shift of —90°, the process, from flow to measured level, will exhibit 180 at the natural period of the vessel. To damp the measuring chamber by throttling its connecting valves will not change this period, but will onfy reduce the amplitude of the resonance. 

As discussed later, if it is desired to have a liquid purge and damping, the purge rate must be fixed and the liquid level elevation in the chamber known. One could deliberately use the upper connecting line as a condenser, send a fixed amount of condensate via a metering orifice or capillary to the measurement chamber, and overflow the remainder back to the column. 

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