Leak-Preventing Instrumentation Keeps Chemicals in the Tank
Leaking or overfilled tanks can cause environmental problems, contaminate drinking water and cost a company millions of dollars. Proper instrumentation, monitoring and control can help prevent these problems.
Recent events in West Virginia have shown that the potable water supply is in jeopardy of contamination from leaks or overfills of storage and processing tanks at chemical, petroleum, wastewater and similar facilities.
In Charleston, W.V., a tank containing 4-methylcyclohexane methanol leaked, causing contamination of the Elk River. The Elk River provides more than 300,000 people with drinking water, and local residents were without tap water for at least five days. The total effect of the spill may not be known for months or even 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.
In the wake of this incident, it is 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, the risk of this event could have been identified early on, and action could have been taken to mitigate the leak and its destructive aftereffects.
Instrumentation exists that will monitor the contents of a vessel and provide an alarm in the event of a leak or overfill event. These products have been used for years and are well established and reliable. Figure 1 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.
The seven specific functions provided by the instruments include the following.
1. High Level Overfill Prevention Switch
This switch is installed to indicate when the liquid in the tank reaches a dangerously high condition. It often is called a high-high level switch because it is mounted above the high level switch used to indicate the normal stop fill point of the tank. 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 and additional spill and overfill detection methods are needed.
2. Radar Level Gauge
A radar level gauge continuously monitors level in the tank, and because of its role, it must be be very accurate to detect leaks, spillage or overfills. If the level decreases when the liquid in the tank is not actively being transferred or pumped out, this indicates a leak in the tank. The monitoring system should provide an alarm.
Because of these operating parameters, 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
It is important to measure the temperature of the liquid in the tank because the volume of most chemicals expands or contracts with changes in temperature. Without compensation, these changes in volume will look like changes in level when, in fact, the actual contents of the tank have not changed. Again, in this case, accuracy of the temperature measurement is important to provide proper compensation. Temperature sensors with multiple measurement points and accuracy of 0.1°C are required.
4. External Level Switch
The level switch is mounted inside the retention dike to indicate if a liquid is accumulating. The level switch needs to be able to detect any liquid that is present. Even rainwater accumulating after a storm should be detected because it is necessary to remove this water to maintain the appropriate volume of the dike. Essentially, any accumulation of liquid within the dike, be it water or a chemical, requires a response. As such, switches such as tuning forks that will reliably indicate the presence of any liquid are best suited for this application.
5. Tank-Side Monitor
The tank-side monitor performs corrected-volume calculations using the output from the temperature probe and the signal from the radar transmitter 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
A typical PC-based HMI software package displays the output of each tank in a tank farm as well as volume calculations (figure 2). Such software packages are available from several suppliers. These software packages typically have web server capability built-in, so an operator or engineer can 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 link so that information registers on the appropriate device such as a PC, a tablet or a smartphone.
7. Receiver Tester
As described, it is critical to test high-high and external level switches to ensure that they are functioning properly. 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.
With an effective instrumentation and monitoring system, spills and leaks are detected at multiple levels — with tank monitors, HMI software and receiver testers providing redundancy.
The events in Charleston, W.V., make it clear that chemical storage tanks need to be secure and that spills and leaks must be prevented where possible. This should extend to all industries that store chemicals that could contaminate water supplies. These industries include chemical, oil and gas, water and wastewater, foundries and any other industry where toxic chemicals are stored.
In the petroleum industry, the American Petroleum Institute (API) has recommended practices for preventing and responding to spills in its document, API2350 recommended practices for above ground storage tanks.In the chemical industry, most facilities that manufacture chemicals are required to have a spill prevention control and countermeasure (SPCC) plan in place.
Unfortunately, because the facility in West Virginia only stored chemicals and did not manufacture them, it was not required to have the site inspections and permits that a manufacturing facility would require. Lack of these requirements and the failure of the owners to provide proper level monitoring led to the incident, which was serious enough to bankrupt the company.
Proper instrumentation for monitoring storage vessels can prevent spills and identify leaks. Where these incidents cannot be prevented, it is critical that they are quickly identified, and that a response plan is in place to mitigate damage and environmental impact. Outfitting tanks with overfill prevention switches and tank monitoring instruments will provide the security that is needed to prevent catastrophes.