(237f) Performance of Vibronic Level Switches in Corrosive Services

Vibronic level switches are capable of detecting the presence of liquid and solid products. More generally, the principle of operation of a vibronic level switch provides differentiation of a dense phase from a lighter phase, typically a gas or vapor. Low energy vibration at a fixed frequency is imparted to the metallic level switch probe when it is contacted by the lighter phase, and the vibrational frequency is reduced when a contacting dense phase adds to the mass of the system.

In corrosive services, the metallic probe may lose mass over time due to loss of corrosion by-products and therefore may realize a higher vibration frequency during time in service. The microprocessor for the level switch can include diagnostic alarm settings to indicate an abnormally high vibrational frequency due to corrosion damage.

In highly corrosive services, a corrosion-resistant liner can be applied to the metallic probe. The corrosion-resistant liner, by itself, adds to the mass of the system. Diffusion of corrosive chemicals through the liner material during time in service will result in a slow but finite corrosion rate to the metallic probe. Rather than losing mass over time due the corrosion mechanis, a lined level switch will gain mass over time and will therefore realize a reduced vibration frequency over time. A sufficiently low vibration frequency will be interpreted by the microprocessor as a "tripped" condition; in other words, a condition of being wetted by a dense phase. Lined level switches can be monitored periodically to determine if slow corrosion is impacting the vibration frequency of the instrument.

Processing facilities which handle highly corrosive chemicals must understand the potential for false trips due to corrosion to lined level switches. A false trip due to corrosion to a lined level switch can give the appearance of a unreliable instrument, which can cause operating personnel to doubt the validality of level switch instrument signals.