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Bypass and analyzer flow indication is accomplished with 1 psig differential pressure (dP) sensors measuring pressure drop across restricted orifice plates embedded between the sensors and the substrate blocks on which they’re mounted. The sensor from Honeywell Sensotec has a micro-machined silicon structure that changes resistance (ohms) when force is applied. A wheatstone bridge, arranged to measure diaphragm movement from pressure changes up and downstream of the restricted orifice, provides 1% accuracy over a wide temperature range. The sensor’s transmitter sends a 4-20 mA output to the DCS system to make flow conditions visible.
Representative sample delivery in PPM moisture analyzer applications is susceptible to temperature influences throughout the system because moisture cools stainless-steel surfaces, and levels equilibrate throughout the system. As a result, electric-traced sample transport lines and an enclosed, heated SHS maintaining a constant 50°C sample temperature were specified. With the first NeSSI-based system, Parker’s “pegboard” backplane-design feature supported Intertec Instrumentation Ltd.’s (www.intertec-inst.com) Class 1, Division 1 “smart” conductive heater block clamped between the pegboard’s backside and the top of the enclosure’s rail mounting. This allows the system’s inflexible conduit to be installed independently of its flexible tube and power runs. The enclosure’s instrument air purge also is preheated via the pegboard.
The plant’s NeSSI system was commissioned in June 2005 and provided analyzer and bypass flow-rate information to the control room’s DSC terminal. This allowed users to remotely actuate the stream-switching valves to validate the analyzer’s calibration.
The bypass flow signal indicates inlet pressure fluctuations, and it initializes when nitrogen reference gas flows through the cell for 9 min 25 sec. This is followed by a 5-sec flow interruption, when the switching valves—actuated from the control room—shut off the reference gas and open the sample flow. The sample flow signal is a slightly different value, revealing the specific gravity difference between the sample and the nitrogen and their influences on the dP sensor. This signature repeats as 9 min 25 sec switching intervals continue during the one-hour snapshot.
Training operators on the new tools was straightforward because they were involved early project. The validation function sends an “OK to use analyzer data” signal through the DCS, which eliminated costly lab-sample validations. Technicians’ time spent checking properly functioning systems also has been reduced. Now, a flag is sent to the operator and technician requesting correction when a validation doesn’t pass. Understanding signal limits and signatures based on normal events provides benchmarks of normal operating conditions, while abnormal conditions present signatures that are easily detected as different. A library of these condition signatures was cataloged, and these indicate corrective actions.
Consequently, confidence in the analyzer and SHS has increased, and the analyzer’s data now plays a much bigger role in Sarnia’s process. Since startup, the plant’s sampling system has posted a 100% uptime record, and the analyzer’s data is regarded as absolute and conclusive.
Process instrumentation hasn’t been installed to run closed loop yet, but the new system recently proved its worth when an unexpected moisture event was detected. Following protocol, a validation cycle was initiated from the DCS, including flow confirmation to the analyzer, which reported the same uncharacteristic moisture condition that prompted correction of the process. The plant’s staffers agree that if this event had occurred before its new SHS tools were installed, they would have faced a costly plant shutdown. The tool that overcame the shutdown was the analyzer’s sample flow indicator over the DCS. Sarnia’s process engineers had requested this dP flow-inference solution for many years.
The project’s design, implementation and commissioning were a collaborative effort between Lanxess, Parker and their local distributor, Viking Instrumentation. Lessons learned during the project included:
Jamie Canton is an analyzer specialist at Lanxess Inc. in Sarnia, Ontario. He can be reached at (519) 337-8251 or by email at firstname.lastname@example.org.