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By Bill Sholette and Ricardo Chavez
|Web server capability lets engineers and operators monitor tanks and receive alarms at a mobile device.|
Recent environmental-news events in West Virginia have reinforced the worry that our water supply is in jeopardy of contamination from storage- and processing-tank leaks and overfills at chemical, petroleum, water/wastewater and other facilities.
In Charleston, W.Va., a tank containing 4-methylcyclohexane methanol leaked, causing contamination of the Elk River. The Elk River provides over 300,000 people with drinking water. These people were without tap water for at least five days. The total effect of the spill may not be known for years.
The company that owned the storage facility where the tank leaked — Freedom Industries — faces at least 31 lawsuits, along with state and federal investigations. As a result, Freedom Industries filed for bankruptcy.
Instrumentation exists to monitor vessel contents and alarm in the event of a leak or overfill event. These well-established and reliable solutions have been used for years. Figure 2 shows typical instrumentation for monitoring tank levels to prevent overfills and leaks.
Each instrument has a specific function for keeping the contents inside the tank:
1 High-level overfill prevention switch is installed to indicate when the liquid in the tank reaches a dangerously high condition. It’s often called a high-high level switch, as it is mounted above the high-level switch used to indicate the tank’s normal stop-fill point.
If the high-level switch fails, the high-high level switch is there to prevent the tank from overfilling. High-high level switches typically include a way to function test the switch to ensure its integrity. Because high-high level switches are mounted above the normal maximum fill point, they can be in service for years without ever “seeing” liquid in the tank. Because of this, the ability to test the switch on a periodic basis to verify its function is critical.
In far too many tanks, a high-high switch is the only protection against spills. This is unacceptable. Additional spill and overfill detection methods are needed.
2 Radar level gauge continuously monitors level in the tank. To detect leaks, spillage or overfills, it is extremely important that the level gauge be very accurate. A high degree of accuracy is needed to allow indication of level decreases when the tank liquid is not actively being transferred or pumped out. If the level starts to decrease during inactivity, a leak in the tank is indicated and an alarm should sound. Given the application, accuracy needs to be measured in fractions of millimeters. Radar level gauges can provide accuracy of 0.5 millimeters, sufficient for detecting even minor leaks.
The radar level gauge also acts as a backup to the high-high level switch.
3 Temperature sensor measures tank liquid temperature because the volume of most chemicals expands or contracts with temperature change. Without compensation, these volume changes look like level changes when, in fact, the actual tank contents have not changed. Again, temperature measurement accuracy is very important, in this case to provide proper compensation. Temperature sensors with multiple measurement points and accuracy of 0.1 degree C are required for this application.
4 External level switch mounted inside the retention dike indicates if a liquid is accumulating. The level switch needs to be able to detect any liquid present. Even rainwater accumulating after a storm should be detected as it must be removed to maintain the appropriate volume of the dike. Essentially, any accumulation of liquid within the dike, water or a chemical, requires a response. As such, switches that reliably indicate liquid presence, such as tuning forks, are best suited for this application.
5 Tank-side monitor performs corrected-volume calculations using the temperature probe output and the radar transmitter signal to determine the height of the material in the tank. It also provides intrinsically safe loop-power to the level gauge and the temperature transmitter, reads data from all connected devices and displays the values of each instrument.
6 PC-based monitoring software (Figure 3) is a typical PC-based HMI software package that displays tank output in a tank farm, as well as volume calculations. Such software packages are available from several suppliers. These software packages typically have Web-server capability built-in, so an operator or engineer can quickly and easily access tank information from any PC or handheld device via a browser.
The software also monitors tank level and can react to any level change. The software should have a feature that allows an inactive tank to be “Locked Down.” If the level in a locked down tank drops, it would indicate a leak, and the software would produce an alarm.
As with all alarms and events, this information can be pushed out to users via an Ethernet line so that information registers on the appropriate device such as a PC, a tablet or a smartphone (Figure 4).
7 receiver tester because it is critical to test high-high and external level switches to ensure they are functioning properly, as described above. Although many level switches have continuous self-checking to monitor their health, the receiver tester allows an operator to test the switch manually via a pushbutton.
The tester can detect a short circuit, an interruption in the signal line to the measuring sensor, vibrator corrosion in the sensor, or a defect in the input circuit. As a backup to the main automation system, the tester also includes relays to provide an output to an alarm or a control function such as a diversion valve to prevent overfilling.
In the wake of the recent West Virginia incident and other unfortunate occurences, it’s clear that additional scrutiny will be coming to chemical storage facilities, along with reviews of existing regulations. But with a few precautions and a relatively minor investment, this event could have been identified early on, and action could have been taken to mitigate the leak and its destructive aftereffects
Bill Sholette is level products business manager and Ricardo Chavez is solutions business manager at Endress+Hauser, a global leader in measurement instrumentation, services and solutions for industrial process engineering.