Improving the Manufacturing Process for High-Temperature, Wear-Resistant Gate Valves

Abstract: Catalytic reactions typically occur under high-temperature, high-pressure, and high-velocity environments. Valves used in fluid catalytic cracking (FCC) units are prone to erosion from the process medium. Several factors significantly shorten the service life of gate valves, including the degradation of metal mechanical properties at high temperatures, rapid wear caused by catalyst particle impingement, valve jamming due to catalyst accumulation in the valve cavity, and sealing surface failure resulting from erosion.Three purge valves are installed to inject nitrogen or steam, mitigating incomplete valve closure caused by catalyst buildup.To combat erosion damage to both sealing surfaces and flow-path surfaces, the study optimized and compared various material selections and manufacturing processes. It proposes that, under typical FCC unit operating conditions, the optimal treatment is thermal spraying of Al₂O₃ onto the flow-path surfaces and plasma hard-facing of the sealing surfaces with Stellite 20 (STL.20).

A catalyst is a substance that speeds up or slows down a chemical reaction without being consumed in the process. Used in over 90% of industrial processes—ranging from chemical manufacturing and petrochemicals to biochemistry and environmental protection—catalysts play a pivotal role in advancing both the chemical industry and society.

An FCC unit is composed of three systems: the reaction-regeneration system, the fractionation system, and the absorption-stabilization system. Within the reaction-regeneration system, feedstock oil comes into contact with the catalyst in a riser or reactor, where the reaction takes place. The resulting products are then sent to the fractionation system. Coke generated during the reaction deposits on the catalyst, which subsequently enters the regenerator. There, air is introduced to burn off the coke and restore the catalyst's activity. Heat released during coke combustion is transferred via the regenerated catalyst to the reactor or riser, thereby supplying the energy required for the reaction. Given that most catalytic reactions occur under conditions of high temperature, high pressure, and high velocity, valves in FCC units are particularly vulnerable to erosive wear caused by the process medium and catalyst particles. These harsh operating conditions significantly shorten the service life of gate valves, directly impacting the operational performance of the reaction-regeneration system and indirectly affecting the economic efficiency of refining and chemical enterprises.

Given this context, this study adopts a combined approach of experimental research and process trials to examine material formulations, forming processes, and surface strengthening techniques for critical wear-resistant components—namely, the valve core and seat—of high-temperature, wear-resistant gate valves. This study aims to overcome the technical challenges inherent in existing valves, which include a tendency to wear, rapid deterioration of sealing performance, limited service life, and elevated maintenance costs when operating under complex conditions. This research provides a practical process solution for the efficient, low-cost production of these valves. Furthermore, it improves operational stability and durability in high-temperature, high-wear environments—including metallurgy, power generation, and chemical processing—reduces equipment failure rates and associated maintenance costs in industrial production, and promotes the upgrading of manufacturing processes as well as technological progress within the industry.

Table 1 presents the basic operating parameters of the process medium along with the corresponding equipment requirements. These conditions necessitate that valves employed in fluid catalytic cracking (FCC) units demonstrate resistance to high temperatures, wear, and erosion, as well as reliable sealing capability.

Tables 2 and 3 summarize the valve design, manufacturing, and basic specifications.

Installed on an elevated platform adjacent to the regenerator, the valve is subject to prolonged radiant heat from the unit, in addition to ambient outdoor conditions including solar exposure, precipitation, and diurnal thermal cycling. In winter, low temperatures can make metal components brittle, while summer heat accelerates aging of the sealing materials.

Figure 1 Partial layout of the valve pipeline

This is a typical heavy-oil FCC process built around a riser–regenerator configuration, with integrated reaction–regeneration, fractionation, absorption–stabilization, and energy recovery systems. A comprehensive description based on the above flow diagram is presented below:

(1) Core Reaction-Regeneration System (Riser Reactor + Regenerator): This constitutes the core of the FCC process, enabling the cyclic conversion of feedstock via cracking reactions and concurrent catalyst regeneration.

(2) Catalyst Regeneration (Regenerator): Spent catalyst is directed into the regenerator (designated as item 5 in the figure), with compressed main air introduced at the bottom to fluidize the catalyst and oxidize coke accumulated on its surface.

(3) Fractionation system (fractionation column): High-temperature oil and gas exiting the settler are routed to the fractionation column (labeled 8 and 9), wherein fractionation is achieved through a combination of trays and reflux streams.

(4) The absorption and stabilization system—interfacing with downstream units as indicated in the diagram—processes the rich gas and raw gasoline withdrawn from the top of the fractionation column, directing them sequentially to the absorption, stripping, and stabilization towers.These streams are further fractionated into three products: dry gas (composed primarily of H₂ and C1–C2 hydrocarbons, routed to fuel gas systems or hydrogen production units); liquefied gas (consisting predominantly of C3–C4 hydrocarbons, utilized as chemical feedstock or domestic fuel); and stabilized gasoline — a specification-compliant product suitable for direct blending into the gasoline pool or further refining.

(5) Energy recovery system: This system recovers energy from the regeneration flue gas. High-temperature flue gas (approximately 650°C) generated in the regenerator is fed into a waste heat boiler, where the resulting steam is used for power generation or to drive blowers.The cooled flue gas is directed to the stack for atmospheric discharge via an induced draft fan; excess flue gas, when present, is routed to the flare system for incineration (identified as "Flare" in the diagram).

(6) Steam system: Steam generated in the unit is superheated and utilized either to drive steam turbines (for applications such as blowers or power generators) or as process steam for downstream operations. Deoxygenated water, serving as boiler feedwater, is preheated and circulated back to the waste heat boiler to complete the loop.

(7) The unit is outfitted with auxiliary systems, including those for catalyst transport, pressure control, and emergency response, among others.

(1) Catalyst deposits tend to accumulate in the lower portion of the valve cavity, potentially resulting in valve jamming. Under severe conditions, such accumulation impedes complete closure of the gate valve; forced actuation with high torque subjects the gate bottom to impact forces, which may induce fatigue fracture or abrasion of the sealing faces.

(2) Erosion and wear of sealing surfaces. The catalyst discharge line from the third-stage cyclone separator (located within the reaction/regeneration section of the catalytic cracking unit) is provided with a cylindrical storage vessel featuring a perforated bottom plate. The storage vessel holds a specified inventory of catalyst (in solid particulate form, at approximately 100% concentration) and is provided with bottom gas injection ports for supplying 1.5 MPa steam as the carrier fluid. Given that the catalyst medium comprises solid particulate matter, these particles are conveyed by the steam flow from the vessel into the gate valve passage. The sealing surfaces of the valve are thereby subjected to prolonged and aggressive erosion, which progressively impairs sealing integrity.

(3) Erosion-induced wear of internal flow-path surfaces. The medium is subject to operating temperatures of up to 700°C. Under these severe thermal conditions, progressive grain coarsening takes place; sustained stress concurrently induces grain deformation and microstructural evolution, resulting in a marked reduction in erosion resistance. Continuous erosion by the flowing medium gradually abrades the internal flow-path surfaces, giving rise to erosion pits which, under severe conditions, may progress to perforation. Examples of gate fracture morphologies and sealing surface degradation are presented in Figure 2.

Figure 2 Gate bottom fracture types and sealing surface damage (schematic)

To address the foregoing issues, this study proposes systematic improvements in structural design, sealing materials, and manufacturing processes—three critical areas.

To address incomplete valve closure resulting from catalyst buildup, a purge valve system is proposed. Purge valves are to be installed on the left and right sides of the bonnet as well as at the bottom of the valve body. A minimum of three purge ports are arranged at 45° intervals around the valve body to support the purge system. Nitrogen or steam is delivered via these ports to comprehensively purge the valve cavity, with particular attention to dead zones susceptible to catalyst buildup, thereby averting incomplete gate seating and guaranteeing effective shutoff.

Six specimens were prepared with hard-faced cobalt-based alloy coatings (STL.6, STL.12, and STL.20). After grinding and polishing, hardness tests were conducted, and the results are presented in Table 5. All measured hardness values fell within the ranges specified in Table 4. Upon manufacture and delivery to a petrochemical facility for operational validation, field feedback revealed that valves coated with STL.20 cobalt-based alloy achieved a substantially extended service life, thereby demonstrating the alloy's enhanced resistance to erosion and galling.

(1) Welding Process

Cobalt-based alloys are typically deposited using manual arc, tungsten inert gas (TIG), or plasma hardfacing processes. Previously, TIG and manual arc hardfacing were the primary choices, yet both approaches had their limitations. TIG hardfacing, for example, was prone to defects including non-conforming geometry, insufficient penetration and fusion, burn-through, cracking, porosity, slag entrapment, bead overheating and oxidation, and arc blow.Manual arc hardfacing was associated with several deficiencies, such as porosity, excessive dilution, slag entrapment, cracking, undercutting, incomplete penetration, distortion, and substandard bead morphology.

Sealing materials are required to exhibit superior erosion resistance. Inadequate hardness of the sealing surface renders it susceptible to erosive wear from particulate media in service, ultimately compromising sealing integrity.In severe instances, this may result in loss of seal integrity and internal leakage, eventually necessitating valve shutdown for component replacement or refurbishment. Traditionally, sealing surfaces were hardfaced with STL.6 or STL.12 cobalt-based alloys via TIG welding. However, the resulting hardfaced layer frequently exhibited inadequate or non-uniform hardness, which compromised performance. To achieve improved erosion and galling resistance on the sealing surfaces, the material selection has been transitioned to STL.20 cobalt-based alloy. The chemical composition, mechanical properties, and hardness of the three cobalt-based alloys are presented in Table 4. As can be observed, higher Cr and W contents in the alloy correspond to increased hardness of the surfaced sealing layer.

Tungsten inert gas (TIG) surfacing and manual arc surfacing have several common problems, including uneven hardness, cracking, and high dilution rates in the deposited layer.To address the problem of inconsistent surfacing quality, plasma surfacing is now being used for sealing surfaces. It offers the following advantages:

• The deposited layer forms a high-strength metallurgical bond with the underlying substrate.
• High surfacing speed and low dilution rate.
• The deposited layer has a dense microstructure and produces a clean, aesthetically pleasing bead.
• High plasma arc temperature, concentrated energy, and good stability, resulting in minimal residual stress and deformation in the workpiece.
• Good controllability. By adjusting parameters such as power, gas type and flow rate, and nozzle dimensions—thereby controlling the plasma arc atmosphere and temperature—efficient automated production can be achieved, enhancing overall productivity.

Process parameters that affect plasma surfacing include welding current, working gas composition, shielding gas, travel speed, standoff distance between the nozzle and the workpiece, and torch oscillation frequency.To achieve high-quality surfacing deposits, all operations must be carried out in strict accordance with the plasma surfacing process specifications given in Table 6. Owing to the superior crack resistance inherent to plasma surfacing, preheating is not required, and the component may be air-cooled directly after welding without inducing cracks, thereby substantially reducing the overall welding cycle. Moreover, the superior quality of the plasma-deposited layer effectively prevents sealing failures that would otherwise arise from non-uniform surface hardness.

To better demonstrate the appearance and welding quality of the surfacing layer, Table 7 compares the surface quality produced by different surfacing processes.

(2) Flow-path Surface Spraying

Since flow-path surfaces are frequently subjected to severe erosion by the medium, which leads to wall thinning and perforation, an Al2O3 coating layer is applied to the flow-path areas of the valve body, bonnet, and gate.The total coating thickness is 0.3–0.5 mm, with hardness controlled at HRC 65–60 for HV0.1 measurements and HRC 59–55 for HV0.2–HV0.3 measurements. The coating bond strength must be ≥40 MPa, ensuring that the flow-passage surfaces possess sufficient strength to withstand erosion by particulate media.Table 8 presents the Al2O3 spraying results for the various components.

Figure 3 Surfacing Quality

Gate surface

Valve body flow-passage surface

Valve body central cavity surface

Table 9 presents a comparison of the performance of three valve groups pre- and post-modification. The best performance is obtained with Al2O3   (corundum) thermal spray on the flow channel and STL.20 plasma overlay weld on the sealing surface. If on-site process conditions impose objective limitations, the combination of Al2O3 thermal spraying on the flow channel surface and STL.12 overlay welding on the sealing surface may be selected to ensure that the valve's post-maintenance performance meets the project's technical requirements.

Fluid Catalytic Cracking (FCC) is a core process technology in the petrochemical industry. Often likened to the "microchip" of the chemical sector, it is characterized by high technical sophistication and significant product value-added.  As such, it is a critical determinant of the technical standards and economic performance of petrochemical production facilities, where valve quality directly impacts the economic returns of refining and chemical enterprises.This paper analyzes the damage patterns of gate valves used in FCC units to identify operational issues and propose targeted improvements. To address valve jamming, three purge valves were installed—positioned at 45°angles on both sides of the bonnet and at the center of the valve body bottom—to allow nitrogen or steam purging, which mitigates incomplete valve closure caused by catalyst deposition.Furthermore, to address erosion damage on the valve sealing surfaces and flow-path surfaces, the study also optimized the materials and manufacturing processes;By comparing the performance of three sets of flow-path coating materials, sealing surface materials, and manufacturing techniques, it was determined that for FCC operating conditions, the preferred configuration for purge-equipped gate valves involves an Al2O3 spray coating on flow-path surfaces and a plasma-surfaced Stellite 20 (STL.20) cobalt-based alloy on sealing surfaces.