Preventing Storage-Tank Overfills with a Wireless Network

Emerson's Chris WomackHi, it’s Chris Womack again with another guest post about terminals.

So, how do you quickly modify an acid terminal to dramatically reduce overfills—and their attendant clean-up costs, product losses, and environmental compliance headaches—to save a bit more than $100,000 after the first year?

Emerson’s Carlos Matos and Brian Cleary spell it out in a presentation from the fall’s Emerson Global Users Exchange—and it’s a clever, simple wireless solution that uses three main Emerson parts in conjunction with a PLC—in this case a Red Lion Data Station Plus.

Using wireless devices and a PLC at an acid terminal filling rackThose three parts are the Rosemount 2160 Vibrating Fork Liquid Level Switch, which wirelessly detects when a tank-truck’s or railcar’s fill-level reaches a predetermined point; the Model 1420 Gateway, which connects the PLC to the wireless network, allowing it to issue commands based on level measurements; and the Model 702 Discrete Output Switch, which conveys PLC commands to a loading arm’s valve. (See the artist’s conception at the right—nope, it’s not my drawing.)

At the customer site where Emerson’s team helped to install this solution, this combination served 14 loading arms, and each arm had a maximum rate of 45 gallons per minute. To save battery power, while still ensuring the wireless control loop could stop filling within about 25 gallons of each tank’s capacity, the team relied on a measurement-update rate that’s at least three times as fast as the process’s time constant—which comes to one update every four seconds.

SmartWirelessGatewayAccording to Carlos Matos and Brian Cleary, the $100,000 that the terminal’s owner saved in the first year of wireless operation stemmed from a dramatic decrease in clean-up costs and environmental fines. That is, it doesn’t take into account how much the terminal operator didn’t spend on wires for connecting and powering devices—or for the labor it takes to lay wires. After all, it took just half a day to update each of those loading arms.

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