Efficient Energy


June 7, 2012

TXI’s Mill Creek, Okla., plant targets energy savings through variable frequency drives on its motors.

It’s no secret that commodity prices in the aggregates markets are sensitive to ever fluctuating economic conditions. While producers may have little control of external market variations, many are taking steps to lower operating costs to get a leg up on the competition.

By installing variable speed drives on the pumps to move water between its wash plant and fresh water supply, TXI was able to realize immediate energy savings.

Reducing transportation, personnel, and other overhead costs are often targets for cuts. But one company has identified another way to reduce operating costs and, at the same time, improve plant reliability.

In 2008, TXI Operations, L.P. (TXI), a leading supplier of aggregate, cement, and consumer product building materials, began a company-wide initiative to enhance efficiency and reduce overall operating costs. As part of the initiative, TXI took a hard look at its Mill Creek, Okla., crushed stone operation. Opened in 2002, the facility supplies crushed stone to the Dallas/Fort Worth and Beaumont markets, primarily by rail. The Mill Creek operation boasts a very large stone reserve — with annual production capacity of 5 million tons and a 100-year lifecycle.

Targeting improvements

Increasing energy prices and energy supply concerns have placed pressure on TXI and other companies to reduce energy consumption. On a national scale, the industrial sector consumes one-third of the energy in the United States, making it a prime target for energy consumption reduction. And, since industrial electric motors represent the single largest consumer of energy in the industry sector today (65 percent), TXI identified them for reductions.

Gary Allen, TXI crushed stone general operations manager, says that, since the Mill Creek plant opened, the pumps controlling the water flow to wash the aggregate were operating at fixed speeds. Output flow was controlled manually via valves on the outlet lines, while the motor continued to run at full speed and amperage.

When considering the total cost of ownership for an industrial electric motor system, 95 to 99 percent of the cost is expended on motor energy requirements. As with TXI, many industrial motor systems had inefficient mechanical control methods. This means the motor operates at 100-percent speed, while the load does not necessarily require 100-percent output — needlessly wasting energy. By optimizing the speed of motors to correspond to the load requirements, energy savings can be immediately achieved. One method to control motor speed is with energy-efficient variable frequency drives.

“Crushed stone plants are, historically, high movers of water,” Allen says. “With the original arrangement, we had to turn the whole system on to run even small amounts of water, with no way to decrease the cost of pumping. It left a big carbon footprint, consumed a lot of energy, and put a strain on the mechanical system.”

The cost-saving strategy to replace the “historic monster,” Allen says, was to rip out the electro-mechanical, fixed-speed, flow control system and install Sinamics G150 variable frequency drives (VFD) from Siemens Industry, Inc.

The drive is enclosed, air-cooled, and designed for applications that do not require regeneration back into the power supply system. It includes an AC/AC power module with IGBT power semiconductors and a cooling concept that provides low-loss operation for high energy efficiency. All units include a controller with a PROFIBUS port, an advanced operator panel, and a terminal module for digital and analog inputs and outputs. The G150 is available in several versions covering a broad range of voltages and power ratings, with variety of standard options that allow it to meet industry and site-specific needs.

TXI’s variable-frequency drive system, up and running since January 2011, consists of 10 G150 VFDs. Five of the drives provide up to 15,000 controlled gallons-per-minute flow of fresh water to wash the aggregate. Four drives control the flow back to the fresh water supply. One drive is available on an as-needed basis.

“We went to the G150 VFDs because we wanted to lower our carbon footprint and energy costs,” Allen says. “We knew there would be a distinct savings we could achieve by going to a variable-frequency water supply versus mechanical.”

Mill Creek Plant Manager Ken Caldwell says an important part of the VFD system is the reliability of the drive software package that simplifies commissioning and configuration.

“The software helped us to utilize the horsepower as needed on our freshwater and return water pumps,” he adds. “Before, we used an across-the-line starter that used a lot of amps and horsepower we didn’t need. Now, we can control and maintain the flow rate of the pumps.”

Reaping benefits

There have been no maintenance issues since the drives were installed. “There is less strain mechanically on the pump because we are not turning it at maximum RPM,” Caldwell says.

“We have five pumps that now only come on as needed,” Allen adds. “As the demand for water increases, we are able to keep up with it. The system is more reliable since we are not running all pumps all the time, and that greatly reduces the stress on the motor controller. If you have only 20-percent demand, you only need one pump out of five. Obviously, the reliability of the other four has to be pretty high.”

Caldwell and Allen say the drive installation has been a success, and the energy savings have been significant.

“As every successful company must do in these times, we have a plan to not only reduce our carbon footprint, but to become more competitive,” Caldwell adds. “We’ve been able to achieve both.”

Article courtesy of Siemens Industry, Inc.

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