Pressure vessel level control

A petrochemical plant had a very difficult application for level measurement in a very hot high pressure vessel. The vessel has a vapor layer, a large amount of turbulence and interior obstructions. The exteremly large volume of product moving through this vessel creates a constant vibration of everything on its platform. Because of the corrosiveness of the product and the dirty nature of the process, there are also oxides and other solids which tend to settle in the bottom of the vessel. The customer had tried a number of level measurement devices over a period of decades, but none were able to provide a long-lasting solution in this service. Differential Pressure transmitters seemed to perform the best of everything they tried, but these would plug up in a period of weeks or months.

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We recommended a magnetic level gauge for this application. The level gauge offers features that overcome the problems presented by this vessel.

The magnetic level gauge consists of a vertical float chamber with appropriate process connections to fit the vessel. Inside the chamber is a “float” which is designed for the test pressure and temperature of the vessel, and the specific gravity of the product. As the level in the chamber rises and falls, the float rises and falls with it. The float has a number of magnets mounted inside. External to the chamber, there is a glass indicator tube with a magnetic indicator that runs the length of the chamber. The indicator rises and falls with the float due to their magnetic attraction.

The float chamber is simply pipe fabricated to specifications and welded to code. This makes the level gauge a simple and safe installation, even on high pressure applications. The float must be engineered for proper weight, stability (length) and sink (about 80% of the float sinks into the process fluid with 20% above the fluid level). The float must have the ability to withstand the pressure and temperature of the process. 

As you can see from this diagram, the indicator is outside the process. There are no special gaskets or thick glass plates to leak or break. This design makes the level gauge very safe.

In addition to a local indicator, magnetically-actuated switches and/or a transmitter may be strapped on to the float chamber. The switches and transmitter are also outside the process, so that maintenance and calibration may be performed without disturbing the process or posing a hazard to personnel.

The level gauge for this application was built to pressure vessel specifications. The level gauge did not have to be fitted around the interior obstructions of the vessel since it is mounted on the exterior. It was made of Alloy 20 to withstand the corrosive fluid. The temperature was not a problem, but the gauge was fitted with an insulating blanket to protect personnel. The vapor and the turbulence in the vessel have no effect the operation of the level gauge. The level gauge was fitted with a 2-wire 4-20mA transmitter for feedback to the plant DCS. The only thing at issue was the problem of solids build-up causing clogging.

The customer installed automated valves on the process connections, vent and drain of the level gauge (not shown in the diagram). At the push of a button, the process block valves close, isolating the level gauge. The vent and drain valves open. The level gauge is flushed out through the drain, eliminating any accumulation of solids.

Redundant level gauges were installed on this critical vessel, along with redundant dP transmitters. The dP transmitters were both inoperable within 3 months. The level gauges have been performing well for 3 years. One level gauge transmitter circuit board needed repair after 6 months in service. The failure was attributed to vibration.

Advantages:

•Safe. 
•Reliable Operation. 
•Minimal Maintenance. Also, the transmitter can be removed from the gauge for calibration without disturbing the process. 
•Widely Applied. The level gauge can be fabricated for many kinds of level and interface applications. 


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