Dean Foods installed its first AllenBradley programmable logic controllers (plcs) in 1981 at its cottage cheese plant in Rockford, Ill. The units worked out so well that the company subsequently decided to standardize on AllenBradley plcs throughout all of its plants.

"We were very satisfied with the AllenBradley units right from the beginning," says Bob Coutlee, corporate manager of process design and automation. Individual units rarely break down and the systems hold up very well. In addition, Allen-bradley keeps upgrading its line. We installed a new Model 560 processor at the Cream o'Wber plant in Salt Lake City that is about the size of a hardcover book and controls the whole plant."

Standardization pays off

Standardizing on one line of plcs has tremendous advantages. We do all of our own programming, and after using these systems for 10 years, we have experienced just about everything that could happen with them, so we don't get many surprises. We've worked the bugs out of the programs, so our engineers can go into any Dean plant in the country and know exactly what to expect."

At least one of each type of programmable controller used in all the Dean plants is also installed at its corporate engineering center in Rockford, 111. This enables the staff to duplicate any operation going on in the field.

"If a plant is having problems, we can tie right into their system with a modem and troubleshoot the operation. We can adjust set points and make other changes in the controls right over the phone. If necessary, we can even change the programming from Rockford," says Coutlee.

Cream o'Wber was using plcs made by other manufacturers when it was acquired by Dean. However, Dean replaced almost all of those units with Allen-Bradley models as part of a plantwide control plan.

With so many existing plcs at the plant, we seriously considered retaining what was there rather than replacing them. However, after evaluating the alternatives, we decided it was more cost effective to install the Allen-Bradleys," s Coutlee. Among the considerations were the cost of adapting Dean programs to the alternate units and of training Dean engineers and operators from other plants.

The model 560 processor at the Salt Lake plant controls pressures and flow rates on the separators and heat exchangers, adjusts the speed of the pumps in the process operations, and controls product flow to the fillers. It also is tied into monitoring panels (AllenBradley Ready Panels) located in the milk receiving bay, the tank room and the process area, on every filler, and in other strategic locations. These panels allow operators to determine levels in tanks and select which tanks are fed into or drawn from.

The system also incorporates programming to ensure that correct decisions are made. For example, the filler operator can enter the code of the product he is running and the tank he is drawing product from, and the system will cheek to make sure that he has selected the proper tank.

As another example, the operator in the milk receiving bay can select which pump to empty the truck, which tanks the milk will be pumped into and which CIP system should be used to clean the truck. In each case, the system will check to ensure that the selection is compatible with the existing conditions in the plant. For more information on Allen-bradley programmable logic controllers, circle number 411 on the reader reply card.