Mill automation upgrade

by Tim Little
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For more than a century, Carr’s Flour Mills Ltd., has produced a wide variety of products at its wheat milling facility in Maldon, England.

In addition to white and whole wheat flour, Carr’s Flour Mills supplies U.K. supermarkets and bakeries with specialty blends such as chapatti flour and malted wheat flour as well as cake mixes and pizza flour. Located on this same site for more than 120 years, the company now ships 150 tonnes of flour each day — more than 54,000 tonnes annually. That’s enough flour to make more than 195 million pizzas.

With such a high production volume, Carr’s depends on precise inventory and process control throughout its milling and packaging operations. Grain from farms around the U.K., as well as from farms in France and even Canada, arrives at Carr’s in 31-tonne-capacity trucks. On busy days, the facility will see up to 15 trucks unloading their grain into the 68- and 75-tonne-capacity storage silos.

From these storage silos, grain moves throughout the facility to be cleaned and conditioned, broken up by roller mills, and then sieved through a series of sifters to remove impurities and determine the flour’s consistency.

After the flour is blended, pipes move the finished product to 20 storage silos before the flour is packaged into 11- to 55-pound (5- to 25-kilogram) bags and shipped by truck to customers throughout the U.K.


The demand for Carr’s products is high, and the facility runs 24 hours a day. However, because the mill is situated in a mainly residential neighborhood, trucks only enter the facility during the day. This calls for precise inventory tracking, since technicians don’t want to run out of raw grain during the night shift.

Previously, Carr’s used manual level measurement to monitor inventory. A worker would climb the eight flights of stairs and use a tape measure to manually dip the silos and convert the resulting measurement into a volume of grain. This type of manual checking takes a lot of time. And that adds up over days, months and years.

With an eye toward increasing the facility’s automation, Carr’s electrical engineer, Harvey Sykes, wanted to find a better method of inventory management. Partnering with Siemens via their technical partner, Process Instrumentation Sales Ltd., Sykes installed SITRANS LR560 radar transmitters on top of all 26 silos for solids level measurement.


Fourteen of these silos pose a challenge for any level measurement device. That’s because the bottom of each 70-year-old concrete silo has a “cheese wedge” shape, which narrows down into a sharp point. However, with the transmitter’s narrow four-degree beam angle, SITRANS LR560 has no trouble measuring into this sharp cone area.

Located away from the silo’s fill point, the transmitter was easy to install and set up, using the device’s Quick Start Wizard. The transmitters relay their level readings through a 4 to 20 mA connection to a central location on the ground, where operators can easily monitor grain inventory without climbing a single flight of stairs.

“It’s the simplicity of the device, combined with the many advanced features SITRANS LR560 offers, that make it attractive,” says Sykes.


Carr’s uses the complete wheat kernel. After machinery separates the different parts of the wheat, the leftover wheat midds (also known as millfeed) don’t go to waste. Rich in protein, fiber, and phosphorus, the wheat midds become animal feed for local livestock.

The company stores this byproduct from the production process in their “midds bins,” three connected silos, which are also monitored with SITRANS LR560 radar transmitters. Despite the silos’ dusty environments created by the flowing grain particles, the 78 GHz high-frequency transmitter has no problem “seeing” the material and reporting reliable levels to operators.


In addition to the success Carr’s has experienced with the radar transmitters, the company has installed an assortment of other process instrumentation from Siemens to simplify and automate operations.

A SITRANS AS100 acoustic sensor notifies operators in case of a no-flow situation in the pipe carrying raw grain to the screw conveyors loading the storage silos. Operators know immediately if a blockage has occurred and can fix the problem with minimal interruption to the production process.

SITRANS LPS200 level paddle switches monitor high and low levels on a small stainless steel service hopper containing gluten. When this fine powder reaches a low level, gluten is added to the hopper from the main gluten storage silo, which then stops filling the hopper once material reaches the high level. From here, the gluten is dosed into the main finished-product flour stream, based on individual recipes’ needs.

The SITRANS LPS200 switches have been in service for more than four years without any problems. With this process instrumentation from Siemens, Carr’s Flour is able to produce thousands of tonnes of flour every year.

Added to that is the absence of instrument maintenance, improved health and safety by removing operators from locations that contain moving grain, and alerts that tell operators if and when a stoppage in production occurs.

Tim Little is product manager level at Siemens in Peterborough, Canada,