White Paper - Engineered Solutions for Extreme Applications: Mitigating Abrasive Wear in a Lignite-Fired Utility Plant

A White Paper by Delta Ducon

1. Executive Summary: The Imperative for Durable Material Handling

The operational integrity of Circulating Fluid Bed (CFB) boiler systems, particularly those that utilize highly abrasive fuels like lignite, hinges on the reliability of their bulk material handling components. Standard rotary valves, often specified for these systems, are frequently inadequate for the extreme conditions they face, leading to premature wear, frequent maintenance, and costly downtime. This white paper analyzes the specific challenges of handling lignite in a utility plant environment and presents a comprehensive, engineered solution. By examining the successful, long-term implementation of the PERMA/flo™ Extreme Rotary Valve at a 305 Megawatt CFB plant in Texas, we demonstrate how an investment in a robust, purpose-built valve can not only withstand everyday abrasion but also survive catastrophic events, ensuring consistent performance and a significant return on investment.

2. The Challenge: Abrasive Fuels and Operational Instability

The lignite-fired utility plant in Texas faced a classic industrial challenge: a demanding application that was rapidly consuming standard equipment. Lignite, a low-ranking brown coal, is a complex fuel to handle. It is not only abrasive and sticky but also flammable, requiring a rotary valve that can effectively isolate high boiler pressure while also feeding the gravimetric system. The plant's initial rotary valves, supplied by a competitor, failed to meet these criteria, creating a host of operational problems.

A detailed review of the application revealed several critical failure points inherent in the initial, lower-cost solution:

• Inadequate Material Durability: The existing valves were simply not made of a material capable of withstanding the abrasive nature of lignite. This led to rapid erosion of critical components, including the rotor tips, housing interior, and shaft. This premature wear compromised the valve's sealing capabilities, allowing boiler pressure to escape and disrupting the feed process.

• High-Maintenance Design: With no provisions for external adjustment, the plant was forced to engage in a continuous and expensive cycle of replacement. The valves were a "throwaway" component, with a service life of six months or less, resulting in unsustainable maintenance costs and an increased burden on plant personnel.

• Risk of Catastrophic Failure: The abrasive nature of the lignite, combined with the high-pressure environment of the boiler, posed a constant threat of process upset. A failed valve could lead to a loss of boiler pressure isolation and a hazardous feedback loop of material and air.

  
  

The plant's initial decision, based on a lower capital expenditure, proved to be a costly mistake in the long run. The need for a more robust and reliable solution became a top priority.

3. The Engineered Solution: A Paradigm Shift in Valve Technology

Delta Ducon's recommendation was the 24″ PERMA/flo™ Type “XL” Extreme Rotary Valve, a product designed specifically for the most demanding applications. This valve is a testament to over a century of engineering experience and a philosophy that prioritizes durability, reliability, and ease of maintenance. The valve's key features directly address the shortcomings of the competitor's model:

• P-33 Chrome-Iron Alloy: The core of the valve's durability lies in its material of construction. Both the rotor and the seal shoe are cast from P-33, a special chrome-iron alloy with a hardness of 500-600 Brinell. Unlike surface-hardened metals, this alloy's superior abrasion resistance is consistent throughout the casting, ensuring that it maintains its integrity even as it wears. This eliminates the need for frequent replacement and provides unparalleled longevity.

• External Adjustability for Extended Life: The "XL" and "XT" series valves feature a unique design that allows for external field adjustment between the rotor and the seal shoe. This critical functionality enables technicians to easily restore the vital rotor-to-seal-shoe gap as wear occurs, maintaining a tight seal and extending the valve's operational life without the need for a costly and time-consuming removal from the process line.

• Optimized Design: The valve is engineered to maintain tight clearances without allowing the rotor and shoe to come into contact. This minimizes frictional wear while ensuring the necessary pressure seal to isolate the boiler from the feed system.

  
  

4. The Result: A Test by Fire and a Decade of Performance

To validate the proposed solution, the utility plant purchased one Delta Ducon Extreme Rotary Valve for a two-year performance evaluation in 2007. The plant committed to replacing additional units only if the valve's performance surpassed that of the existing models.

Six months into the evaluation, the valve was put to the ultimate test. An unexpected process upset caused a fire to spread through the fuel feed chute, exposing the valve to estimated temperatures of upwards of 1800° F (982° C).

In an incredible demonstration of its rugged durability, the PERMA/flo™ Extreme Rotary Valve continued to operate during the fire. Once the fire was extinguished, the valve was inspected and, with no major damage detected, was immediately put back into service. This event proved the valve’s exceptional ability to withstand conditions far beyond its typical operational parameters.

Well over a year after the fire, the valve had required no major maintenance and no adjustment to its clearances. The plant considered the two-year evaluation a resounding success and, based on its exceptional performance, opted to purchase three additional units.

Today, more than a decade later, the 24″ PERMA/flo™ Extreme Rotary Valves continue to operate with minimal maintenance requirements. Their success has led to a lasting partnership between Delta Ducon and the utility plant, with subsequent installations including Pneumatic Sand Transport and Injection Systems and Hydrated Lime Unload and Injection Systems, all featuring the reliable Extreme Rotary Valve. This success story stands as a powerful testament to the value of investing in quality and engineering to solve the toughest material handling challenges.