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Hot tanks protected

06 November 2008

Emerson Process Management’s Smart Wireless network is protecting hot asphalt tanks at the Hunt Refining Company, Tuscaloosa, Alabama, USA. Several hot tanks at the refinery are fitted with wireless temperature transmitters as a cost-effective means of keeping control room operators informed of conditions inside the tanks.

Emerson Smart Wireless network is protecting hot asphalt tanks at Hunt Refinery Company
Emerson Smart Wireless network is protecting hot asphalt tanks at Hunt Refinery Company

This enables them to maintain temperature uniformity in the tanks to prevent formation of “cold spots” where trapped moisture can lead to extensive damage with repairs costing up to $200,000 per tank.

At Hunt Refining, operators need to keep asphalt at process temperatures above 100º C in about 10 tanks. This avoids formation of cold spots. When very hot asphalt, at 148º C to 204º C, is added to a tank, the hot fluid may “melt through” the currently stored asphalt and reach cold pockets where any moisture present can flash off violently. There is a real potential for tank roof failure if operators do not recognise these cold spots and take corrective action.

A decision was made at Hunt to install temperature sensors in these tanks and connect the measurement data into the refinery’s distributed control system so that cold spots could be avoided, and easily identified if they do occur. After analysis showed that traditional installation of wired monitors would be very costly, a trial of Emerson’s Smart Wireless technology was initiated.

Three Rosemount wireless temperature transmitters were positioned 120 degrees apart around the outer circumference of the tank and 30 to 60 cm above the bottom. The Smart Wireless Gateway for these transmitters was located about 120 metres away near the I/O building with a repeater between the tank and gateway to ensure reception of all signals transmitted on the self-organising wireless network. Two-wire Modbus communication carries the temperature data from the Smart Wireless Gateway to the refinery’s distributed control system. Emerson’s AMS Suite predictive maintenance software is connected along with the Plant Information (PI) network, so the wireless field devices are added to all wired devices managed by this software. Wired or wireless, the data appears the same.

“These transmitters just work; they don’t fail,” said Hunt Refining Process Control Engineer Dennis Stone. “The wireless network proved its value. The installation was simple, and the transmitters came up immediately and talked with the gateway as soon as power was applied. The gateway was easily connected to the distributed control system via a 2-wire Modbus communication. Now, any computer with access to our Plant Information network can obtain this information.”

Each wireless device on a self-organising network can act as a router for other nearby devices, passing messages along until they reach their destination. If there is an obstruction, transmissions are simply re-routed along the mesh network until a clear path to the Smart Wireless Gateway is found.

Although the transmitters at Hunt Refining are spaced evenly around the tank and not within sight of each other, their transmissions easily “bend” around the tank and then on to the repeater and gateway. As conditions change or new obstacles are encountered in a plant, such as temporary scaffolding, new equipment, or a parked construction trailer, these wireless networks simply reorganise and find a way. All of this happens automatically, without any involvement by the user, providing redundant communication paths and better reliability than direct, line-of-sight communications between individual devices and their gateway.

This self-organising technology optimises data reliability while minimising power consumption. It also reduces the effort and infrastructure necessary to set up a successful wireless network.

“This wireless technology proved to be so reliable and robust,” Stone said, “it has already been expanded with temperature monitors on more tanks networked to a second Smart Wireless Gateway. That gateway also receives transmissions from a single wireless temperature transmitter monitoring the water being returned to the Black Warrior River to be certain of compliance with environmental regulations.”

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