Utilizing Advanced Measurement Diagnostics to Improve Powerplant/ Powerhouse Availability

Advanced diagnostics in todayís smart field devices will significantly reduce unscheduled outages and improve overall plant availability, say Scott Mawhinney & Mark Menezes

Traditional Approaches when Modernizing Field Devices
When modernizing field devices, engineers have been driven by the desire to improve the measurement performance and/or device reliability and traditionally used a combination of three approaches:

Use more Robust Transmitters
Using robust transmitters that fail less frequently will directly improve the measurement performance and device reliability.
For example, a plant with 400 transmitters each with an MTBF of 100 years can expect about 4 transmitter failures each year.
Upgrading to higher quality transmitters with an MTBF of 400 years will reduce this to 1 failure each year.

Improve Measurement Accuracy and Repeatability
Accurate and repeatable measurements minimise undetected failures and false trips. Consider the example of a control application such as fuel-air cross limiting where the user needs to maintain the ratio between fuel and air flowrates. Most users operate with an excess air “safety buffer” since the environmental and safety consequences of excess fuel greatly outweigh the purely economic consequences of excess air. Although many factors contribute to make a system non-ideal, it should be apparent that a system with a 5% flow measurement uncertainty will require a minimum 5% safety buffer. Improving the flow measurement from 5% to 1% improves consistency and provides the user with the opportunity, at no increased environmental or safety risk, to improve process efficiency and reduce excess air and therefore reduce fuel cost.

Employ “Best Practices” in Installation and Maintenance
Many measurement failures can be attributed to poor installation or maintenance practices. For example, using very long impulse lines in a differential pressure flowmeter installation will increase the likelihood of impulse lines plugging and freezing. Using shorter impulse lines or converting to direct mount will improve both measurement performance and reliability and should actually cost less.
Most users have continuously improved their measurements using the approaches outlined above. Unfortunately, many users are finding that while upgrading an older field device to state-of-the-art can deliver significant improvements; further upgrades will show little or no returns.
In the example above, while the improvement from 5% to 1% flow measurement uncertainty may have significant benefits, it should be clear that future improvements, even to perfection, will return no more then one-third of the benefits of the previous upgrade. Similar comments apply to improvements in installation “best practices” and transmitter robustness.

The Next Round of Improvement – Increase Overall Plant Availability
Diagnostics present in today’s smart transmitters allow the user to remotely diagnose field devices and increase plant availability. These diagnostics are of limited usefulness unless the user can economically and conveniently access diagnostics alerts and information. Open Digital Protocols and Asset Management Software enable a user to access this information in an economic and convenient way. For an example, how useful is a “Sensor failure detection” if the user must climb a ladder to read that message on the transmitters local display? This diagnostic information is much more useful if it could be easily accessed at a PC online in the maintenance shop or at the local console of the control system.

Fig. 1: Global Pressure Transmitter Shipments (by Protocol) – (Thousands). Download

Fig. 2: Asset Management System. Download

Open digital protocols such as HART or FOUNDATION Fieldbus which allow a field device to transmit additional information on the same pair of wires that carry the process variable and allow access to diagnostic alerts and information online without any additional wiring.
While proprietary digital protocols have existed for years, these continue to be abandoned by users and suppliers in favour of open protocols Fig. 1).
Once the diagnostics have been generated in the field device, and..

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