Magnetic Level Indicator- Instrument bad Actor

Why will a Magnetic Level indicator give False readings randomly?

Quench system in the Coker units main purpose is to take Overhead Vapors from the off-line Coke drum. It is used during cooling off, de-pressurizing, and quenching stage of getting a drum (700 degrees, down to 200 degrees) before we can take the head off the drum, and De-coke the drum.

The overhead vapor line is tied to the Quench Tower, and goes in close to the middle/bottom of tower. The oil that is in the Quench tower, that accumulates, is sent from tower to the Quench circulation pump. This oil travels from the pump, to a cooler, then to the manual circulation valve, and back in at the Top of the Tower.

The oil falls down the tower cooling off the vapors coming from the Coke Drum. Off that same line from the pump, it splits and goes to a Level control Valve. Lets say the valve is in automatic and the set value is 80% on the valve.

When the level on tower shows 80% this valve will automatically open. If the tower is reading wrong, lets say it says its 100%, sometimes it lies and its really bone dry. The valve opens up and cavitates the pump

More info would be useful. However if this is a local control loop using pneumatic controllers and a displacer level it could be sticking. I work in an ethylene plant and it is not uncommon for us.

If you are using a smart transmitter and the control is from the DCS or what have you it is likely you are getting below the measurable range of the meter and it is going to a fail high state. In this case you need to know what your configured parameters are and measure tap to tap. Once you have done that math it all out and that should be able to tell you if you are going below the zero reference.