Instrumentation and Control Journal

Best Measurement Practices for Better Power-Plant/Powerhouse Safety, Availability & Efficiency

Using published specifications and expected conditions, it is possible to calculate prior to installation, the impact of ëreal-worldí effects on installed measurement repeatability, explains Mark Menezes.

Accurate, repeatable and reliable measurements are vital to safe and efficient power-plant operation. Modern measurement technologies typically boast of excellent accuracy and repeatability under ‘reference’ or‘laboratory’ conditions.
Unfortunately, measurements are rarely made under laboratory conditions – as a result, performance is always worse in the ‘real world’.
Using DPflowmeter examples, this article presents the key reasons for this deviation between laboratory and real Safety, Availability & Efficiency Best Measurement Practices for Better Power-Plant/Powerhouse Using published specifications and expected conditions, it is possible to calculate prior to installation, the impact of ëreal-worldí effects on installed measurement repeatability, explains Mark Menezes It also presents tools that allow the user to quantify expected deviations prior to installation– in real applications, with real products.
Finally, the tools allow the user to calculate the impact of different maintenance and installation practices on measurement accuracy and repeatability, and ultimately to determine the ‘best practice’ for any given application.

How Can Better Measurement Improve Power Plant Safety, Availability & Efficiency?

In many control or interlocking applications, the user needs to balance competing safety and efficiency motivations.
For example, for fuel-air cross-limiting control, the user needs to maintain the ratio between fuel and air flowrates.
Excessive fuel can cause ‘smoking’– an environmental hazard - and unburned fuel in the stack can cause a safety hazard.
Excessive air, while safe and non-polluting, is expensive – instead of being used to make steam, combustion energy is used to heat air.
With an ‘ideal’ measurement and control system, the user would maintain fuel and air at the exact stoichiometric ratio.
In practice, there is no such‘ideal’ system.
Since the environmental and safety consequences of excess fuel greatly outweigh the purely economic consequences of excess air, most users operate with an excess air ‘safety buffer’.
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.
A reduction in flow measurement uncertainty does not directly reduce operating costs. Instead, it improves consistency, and provides the user with the opportunity– at no increased environmental or safety risk – to reduce excess air and hence fuel costs.
Therefore, one dollar of reduced gas flow uncertainty yields one dollar of opportunity to reduce fuel usage.

Why is Accuracy and Repeatability Worse in the Real-World than in the Laboraty?

Even with a well-installed and wellmaintained transmitter, real-world accuracy can be significantly worse than laboratory accuracy, for any measurement technology.
The reason for this is that real-world transmitters are not installed and operated under ‘laboratory conditions’.
Using the example of a differentialpressure flowmeter, ‘real-world’ effects may include:

Ambient Temperature Variation
In the vast majority of ‘real-world’flow measurements, the transmitter operates at a very different ambient temperature than the temperature at which it was caliberated.
In some outdoor applications, ambient temperatures can vary more than 50°F from calibration temperature.
These variations can have a significant effect, which is easily simulated on the bench – blow warm air over a transmitter, and watch its output change.

High Static Line Pressures
A high line pressure could, significantly affect the differential pressure transmitter used to infer flow.
To simulate this effect on the bench, the user should apply a small differential pressure across a transmitter.
Then, add several hundred pounds of additional static pressure to both sides of the transmitter.
In theory, the measured differential pressure should not change.
In reality, it does.

Drift/Stability
The output of any analog component will vary over time.
As with the ambient temperature effect described earlier, this can affect all flow technologies.
Better, smart transmitters are more stable than older, analog transmitters or transducers.
Within regulatory or contractual restrictions, a more stable transmitter will allow the user to

Accurate, repeatable and reliable measurements are vital to safe and efficient power-plant operation.
obtain equivalent accuracy and repeatability when calibrated less frequently.
An inferior device will need to be calibrated more frequently to maintain acceptable performance Accuracy.

How Can the User Quantify the Impact of ‘Real-World’ Sources of Error?

Reputable suppliers publish specifications, which allow the user to calculate and predict the impact of these and other “real-world” effects on installed flow accuracy and repeatability.
For the purposes of this paper, a spreadsheet was designed (Appendix 1) which uses published specifications to calculate flow error caused by a DP transmitter in an orifice meter installation.