Just the Right Amount of Starch

Jeff Mueller, PE, is the manager of engineering in the Electronic Materials Division of the National Starch and Chemical Company in Salisbury, N.C. His company uses Profibus PA and Profibus DP to connect field devices to a Siemens PCS7 system.

National Starch digitally monitors pressure, flow, level, conductivity, pH and weight instruments via fieldbus. “Most instrument vendors now provide Profibus as a communication option or sell a card to retrofit their instruments for Profibus. This helps keep vendor options open so we can stay with plant standards,” observes Mueller.

Mueller gives some good design and installation tips:

1. Make sure your instrument vendors have the GSD files to talk Profibus;
2. Get the maximum mA draws from each vendor so you can layout optimum Profibus connection sequences;
3. Choose automatic valves with solenoid actuators that can use less than 7.5 mV to allow use of intrinsically safe output cards so no separate barriers are needed;
4. Work closely with your electrical/instrumentation installation contractors to make sure they understand the reduced installation time/cost of Profibus;
5. Bid/price each DP/PA instrument group (five to seven analog instruments) as one cable pull and five to seven terminations;
6. After installation, have the systems integrator or plant control engineer come back and unscrew the downstream leg of each T-connector and assign an address to each successive instrument in the group;
7. Each instrument should be configured online instead of in the field.

Benefits realized by National Starch include lower installation costs and remote instrument access. “Profibus costs less to install because we have only one cable run to connect five to seven instruments, the maximum allowed to stay under 85 mA total and to remain intrinsically safe,” reports Mueller.

“Each instrument needs someone to enable/unlock software communication, set dip switches, or otherwise prepare the instrument to receive its new Profibus address. But after that, programming can be accomplished from the comfort of the air-conditioned control room. Anyone who has sat on top of 10,000 gallons of rocket fuel at 95 ºF and 95% humidity, programming a tank level instrument can appreciate this,” concludes Mueller.