The digital differential pressure architecture eliminated the impulse line and pneumatic repeater between the two sensors; both had caused many problems. The system was installed in the same manner as the earlier instrumentation, so no changes were needed to the vessel connections.
Once the new level measurement system was up and running, plant engineers and operators noticed a drastic improvement in response time, and they were able to accurately and quickly track changes in the vessel level that previously went unnoticed. Additionally, the measurement accuracy improved, with error decreasing from 3 to 5 percent to less than 0.5 percent of span.
By consistently measuring the level in the vessel, the facility is now able to produce more enzymes in the fermenters. "This vessel went from being our worst performing to our best," an instrument technician at the facility said.
Impulse line elimination
The system installed at the biotech facility eliminates the impulse line and addresses other issues by linking the two pressure sensors with a non-proprietary electrical cable. Either sensor can be designated to calculate differential pressure (dP) and level, and this data along with readings from each sensor can be sent back to the control system via a single 4 to 20 mA signal with HART digital information imposed.
No programming is needed at the control system to infer level because these calculations are made at either sensor, with the user selecting the "master" sensor at installation and configuration. No errors are introduced from changes in static pressure, and the system is easily zeroed and calibrated.
A dP level measurement system with a digital link between the pressure sensors can be less expensive to install than traditional two-transmitter solutions because no impulse line is needed, and because only one transmitter has to be wired back to the control system. When compared to dP measurement with an impulse line, a digital system yields more accurate and repeatable level measurement with quicker response time, along with reduced maintenance.
Nathan Stokes is a senior product manager for global Rosemount dP Level products with Emerson Process Management in Shakopee, Minnesota. Stokes holds a degree in mechanical engineering from the University of North Dakota and an MBA from the University of Minnesota.