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By Jamie Canton
Accurate moisture measurement and control is necessary to maintain profitability in many chemical and petrochemical processes. While on-line moisture analyzers provide moisture level and trending visibility through a distributed control system (DCS), many process engineers and operators remain skeptical of these data because sample handling systems (SHS) historically have problems that compromise their analyzers’ accuracy.
This article shows why and how Lanxess deployed a modular sample system to monitor moisture in a sensitive butyl rubber process at its plant in Sarnia, Ontario. The system uses compact SP76 modular flow-control components in a heated enclosure with remote sample validation and bypass and analyzer flow indication over the DCS. These tools let operators assign more weight to the analyzer’s results.
Legacy moisture analysis systems at Sarnia used conventional sample systems with 3/8-in. and ½-in., carbon-steel pipe that increased sample delivery lag times to the analyzer. These high-volume SHSs require high-flow and fast loops to deliver representative process parts-per-million (PPM) moisture conditions. These high flows led to high steam-trace energy requirements to prevent moisture absorption to the sample-transport pipe wall. Steam is a cost-effective heat trace method at Sarnia, but steam trap or other system failures commonly caused SHS and analyzer downtime.
While operators could see the moisture analyzer’s data via the DCS, they were basically blind to the SHS’s health and operation. As the analyzer reported moisture nearing out-of-specification levels, the operators typically disbelieved the analyzer, reported the drift to the analyzer maintenance department and sent a technician to physically inspect the analyzer and SHS. A manual validation would be done by cycling stream-switching valves to a reference gas and reading local flow indicators to determine if sample and reference gas flows had been introduced to the analyzer. Callouts for technicians were common, and these SHS inspections could be as long as six hours. These lags were especially costly when moisture levels exceeded specification, causing poor product quality and downtime.
In 2000, the Sarnia plant participated in Lanxess’ deployment of an analyzer and SHS downtime measurement system to benchmark its overall performance. A DCS subroutine was programmed internally, providing visibility to an analyzer’s online or offline status. However, only the technicians were given status-change authority, and all status changes required inputs documenting the downtime’s cause. The program let the plant focus on eliminating out-of-service conditions for its nine process gas chromatographs (GCs). The success of this GC initiative in late 2003 inspired Sarnia to establish future-state-analyzer and SHS requirements to support a butyl rubber plant prone to downtime due to excessive moisture.
Sarnia’s SHS layout features two separators and valves that direct sample and reference gasses.
The analyzer project team adopted a customer-service perspective and asked: “How can we instill confidence in the process engineers and operators (the customers) to trust and act upon the analyzer’s data (the deliverable product)?” They answered: “Include these customers on the project team.” This resulted several key deliverables, including:
Consequently, the New Sample System/Sensor Initiative’s (NeSSI) technology was identified as an alternative for SHS fabrication early in the project cycle. Its features included:
The team researched the capabilities of several NeSSI-based manufacturers and chose Parker Hannifin’s IntraFlow system, as well as its R-Max air-actuated DBB system to deliver remote validations. Also, these stream-switching functions gave Sarnia’s engineers the confidence to conduct previously prohibited, multi-stream analysis on one analyzer. They also developed a remote flow-indication device, which promised to provide analyzer flow information. Parker undertook Sarnia’s butyl rubber PPM moisture analyzer SHS project in early 2005.
From a sample conditioning standpoint, the plant’s rubber moisture analyzer sampling system is relatively simple (Figure 1). It features two membrane separators in series to protect the moisture analyzer from costly flooding due to heat-trace failures. The separators are configured in-line without traditional bypass legs. Nitrogen is dried in an on-board desiccant dryer and used as a standard reference gas for calibration and validation. A normally closed (NC) R-Max switching valve directs sample flow to the analyzer, while an adjacent, normally open (NO) R-Max directs the standard reference to it. The NO and NC valves are controlled by solenoid-operated pilot valves on-board the SHS, and their orientation provides a power or instrument-air failsafe condition for reference gas flow.