Natural gas, which may or may not contain sulfur, is extracted from the ground. This sulfur-containing gas has different degrees of corrosion on commonly used metal materials. At the same time, the high ejection pressure has a strong scouring effect on equipment structural parts. Therefore, the material is required to have good comprehensive performance and excellent anti-sulfur corrosion performance. For example, as the large-area valve plate and valve seat of the high-pressure anti-sulfur gas wellhead sealing components, they are very critical structural parts, requiring: flexible opening and closing, safe and reliable, and resistant to acidic corrosive media H2S-CO2-H2O and (20%-25%) HCI medium corrosion; at the same time, as the wellhead opening and closing parts, they must be hard, HRC＞45, resistant to strong wear, resistant to high-speed airflow scouring, resistant to high specific pressure friction, low friction coefficient, small sealing ratio; tensile strength 40MPa-70 MPa. In the late 1980s, research by the Guangzhou Research Institute of Nonferrous Metals showed that the surface hardness, smoothness and dimensional accuracy of sealing assemblies made from TC4 alloy after nitriding treatment met the design requirements and were superior to the products of Cameron Corporation of the United States. The prepared sealing assemblies had reliable sealing, small opening and closing torque, and could operate safely and reliably under high pressure.
Sealing component preparation process
TC4 flat valve seal preparation process: forging blank → stress relief annealing → machining and sealing surface grinding → shaping treatment → sealing surface grinding → ion treatment. Ion treatment is a very important process in the preparation of high-performance sealing surfaces. Its treatment parameters are: treatment temperature, 880℃-900℃ for valve plates, 840℃-860℃ for valve seats; treatment time 6h-8h; maximum heating rate <15℃/min; maximum cooling rate <18℃/min; reaction atmosphere N2+H2 mixed gas, with trace oxygen added, N2 content 66%-90%; working pressure 1.33MPa-1.73MPa; gas flow 100L/h-150L/h; current density 3mA/cm2-7mA/cm2. In order to ensure the dimensional accuracy of the sealing components, in addition to the reasonable selection of treatment process parameters, the glow discharge stability should also be improved, and the furnace should be reasonably loaded. Ion treatment can significantly improve the wear resistance and corrosion resistance of TC4 alloy, while maintaining the toughness of the matrix and the carburized layer.
Sealing performance
Hu Youzhen and others from Guangzhou Nonferrous Metals Research Institute tested some properties of four titanium alloy flat valve plates (Table 1). All performance indicators met or exceeded the design requirements. The surface hardness and dimensional accuracy were higher than those of the same specification American FMC-OCT valve plates, creating stable and reliable conditions for use. The process effect has good repeatability, and no waste products were left after processing.
On-site test
Guangzhou Nonferrous Metals Research Institute conducted strength and sealing water pressure test, torque test, ground static natural gas test, and wellhead dynamic natural gas production test on the assembled titanium alloy TC4 flat valve body, and all achieved or exceeded the set value and expected effect.
