Structural Improvement of Tee Valves in Powder Material Conveying Systems

Overview of the Study

The paper by Jiang Zhijun from Hubei Xiangfan Power Generation Co., Ltd., published in Boiler Technology (2000, Vol. 31, No. 7, pp. 21–22), proposes structural improvements to tee valves used in the powder material conveying systems of coal-fired power stations. While this topic is situated in the field of power plant auxiliary systems rather than traditional pipe fitting manufacturing, it addresses a practical engineering problem that involves the selection, design, and operation of tee-type flow control devices under demanding conditions of abrasive powder transport.

Operating Environment and Challenges

Powder material conveying systems in coal-fired power stations handle pulverized coal, fly ash, or limestone powder at flow rates of 5–50 t/h through pipes with diameters of 80–300 mm. The operating temperature ranges from ambient to 250°C depending on the process stage, and the material velocity is typically 15–25 m/s to prevent settling and blockage. These conditions create severe erosive wear on all internal surfaces of the tee valve, particularly at the branch root and the outer surface of the bend where the powder stream impinges directly.

The conventional tee valve design, which uses a simple butterfly or gate damper at the tee junction, suffers from rapid wear of the damper blade, uneven wear on the tee inner surface, and difficulty in achieving precise flow control due to the asymmetric flow pattern. The wear rate can exceed 1 mm/month for unprotected carbon steel components, necessitating frequent shutdowns for replacement and creating significant unplanned maintenance costs.

Proposed Structural Improvements

Engineering Analysis of the Wear Mechanism

The wear in powder conveying tee valves is primarily erosive, governed by the kinetic energy of the impacting particles, the angle of impact, and the hardness of the surface material. At the branch root, the powder stream impinges at an angle of 30–60 degrees, which is the most erosive regime for most engineering materials. The erosion rate follows the Ashby relationship, where the wear rate is proportional to the particle kinetic energy raised to a power between 1.5 and 2.5.

The proposed hardfacing overlay with Stellite 6 (a cobalt-chromium alloy with hardness of HV 400–500) is particularly effective because it combines high hardness with good ductility, allowing it to absorb the impact energy of the powder particles without spalling. The overlay should be applied by submerged arc welding or plasma transfer arc welding with a single pass thickness of 1.5–2 mm, followed by a second pass to achieve the total overlay thickness.

Maintenance and Quality Control Considerations

The improved tee valve design should be incorporated into the preventive maintenance schedule of the power station. The inspection interval for the hardfaced tee should be set at 6 months for systems handling pulverized coal and 12 months for systems handling fly ash, based on the established wear rate data. During inspection, the remaining overlay thickness should be measured at multiple points around the branch root using an ultrasonic thickness gauge, and the overlay should be renewed when the remaining thickness falls below 2 mm.

The flow control performance of the improved damper should be verified during commissioning by measuring the flow rate at various damper positions and comparing with the design flow characteristic curve. A deviation exceeding 10% from the design curve indicates that the damper profile or the tee internal geometry requires adjustment.

Study Insights and Practical Implications

While this paper is relatively brief and focused on a specific application, it illustrates an important principle in engineering: the design of flow control components must account for the actual operating conditions, not just the nominal flow requirements. The conventional tee valve design, which is adequate for clean fluid service, fails prematurely in abrasive powder service because the wear mechanism is not addressed in the original design.

The proposed improvements represent a practical engineering approach to extending component life without requiring a complete redesign of the conveying system. The hardfacing overlay and wear-resistant lining are well-established technologies that can be applied by qualified welders and fitters, making them accessible to power stations without specialized manufacturing capability.

The study also highlights the importance of field experience in driving technical improvements. The author, working directly in a power station, identified the wear problem through operational experience and proposed solutions based on practical constraints. This bottom-up approach to engineering improvement is valuable and should be encouraged in industrial settings where academic research may not address the specific challenges of a given application.

For engineers involved in the design of powder handling systems, this study serves as a reminder that tee valves and similar flow control components should be designed with erosion resistance as a primary requirement, not as an afterthought. The selection of wear-resistant materials, the design of internal geometries to minimize particle impingement, and the provision of maintenance access should all be considered during the initial design phase.