In power generation, improving efficiency often comes down to finding opportunities to get more from the energy already being produced. One area that can sometimes be overlooked is flow measurement. Differential pressure (DP) flow measurement is used throughout power plants to measure steam, water, air and gas. While these measurements are critical to plant operation, some traditional DP technologies create a restriction in the flow that results in permanent pressure loss. That lost pressure represents energy that was generated by the plant but is no longer available to the process. For applications with large pipe sizes and high flow rates, reducing permanent pressure loss can make flow measurement an important part of the overall efficiency equation.

Understanding Permanent Pressure Loss

DP flow technologies measure flow by creating and measuring a differential pressure. Traditional primary elements, such as orifice plates and flow nozzles, create a restriction within the pipe to generate that differential pressure. However, not all of that pressure is recovered downstream. The difference between the pressure before the restriction and the pressure recovered afterward is known as permanent pressure loss. In a steam application, for example, pressure lost across the flow meter means less energy is available downstream. Maintaining process conditions can require additional energy, which over the life of a measurement point can contribute to substantial operating costs.

This makes permanent pressure loss an important consideration when selecting a flow technology, particularly for large diameter applications such as:

• Steam
• Circulating and cooling water
• Combustion air
• Flue gas
• Gas turbine inlet air

Measuring Flow with Less Pressure Loss

Rosemount™ Annubar™ Flow Meters use averaging Pitot tube technology to measure differential pressure at multiple points across the pipe. Multiple sensing ports measure the flow profile across the pipe diameter and provide an averaged differential pressure measurement.

For large pipe applications, this approach can provide several advantages:

• Reduce permanent pressure loss by minimizing obstruction in the flow path
• Simplify installation with an insertion style design that requires a single pipe penetration
• Support reliable flow measurement by averaging differential pressure across multiple sensing points

These characteristics can be especially valuable in large pipes, where installation requirements and permanent pressure loss become increasingly important considerations.

Turning Reduced Pressure Loss into Energy Savings

The potential impact can be significant in high energy steam applications. During an upgrade at a combined-cycle power plant, existing main steam flow nozzles were replaced with Rosemount 585 Annubar Primary Elements for Severe Service. The original flow nozzles created approximately 8.2 psi of permanent pressure loss, compared with approximately 0.73 psi with the Annubar technology. After one year of operation, the customer calculated approximately $43,000 in annual savings from increased generation and improved efficiency. This example demonstrates how selecting a flow measurement technology suited to the application can deliver benefits beyond the measurement point, helping improve efficiency while reducing energy loss. Read more about this customer success story here.

Better Flow Information Can Support Better Control

Accurate flow information can also help plants identify opportunities beyond the measurement point itself. At a coal-fired power plant, operators were estimating combustion airflow using induced draft fan performance curves. The resulting airflow estimates had errors of approximately ±25% to 30%, making it difficult to effectively control the air-to-fuel ratio. The application was particularly challenging because the inlet air traveled through a large 4-foot by 5-foot duct with multiple elbows and limited straight run. By using Rosemount 485 Annubar Primary Elements along with flow profile characterization, the plant improved airflow measurement accuracy to approximately ±5%. Better airflow information allowed engineers to more effectively optimize the air-to-fuel ratio, and the plant ultimately verified a 10% reduction in fuel consumption. The improvement demonstrates an important point: the value of flow measurement isn’t limited to the measurement itself. Better information can give operators greater confidence when making decisions that affect overall process performance.

Consider the Full Cost of Flow Measurement

Accuracy and initial equipment cost are important factors when selecting a flow technology, but they aren’t the only considerations. Permanent pressure loss, installation requirements and ongoing maintenance can all contribute to the lifecycle cost of a measurement point. In large diameter power applications, these factors can become especially significant. Averaging Pitot tube technology provides a way to maintain reliable DP flow measurement while minimizing unnecessary pressure loss. For power producers working to improve efficiency, that means more of the energy generated by the plant can remain where it belongs: in the process.

Read the full article here. Learn more about Rosemount Annubar Flow Meters and other Emerson differential pressure flow measurement solutions at: Emerson.com/DPFlow