Key points for installing and the working principle of large-caliber gate valves

　　Large-diameter gate valves are suitable for small-caliber pipelines operating at Class 150 to 2500, with service temperatures ranging from -29°C to 425°C (for carbon steel) or -29°C to 500°C (for stainless steel). These valves are designed to either shut off or open the flow of media within the pipeline. By selecting different materials, they can handle a wide variety of media, including water, steam, oil products, nitric acid, acetic acid, oxidizing agents, urea, and more. They are particularly commonly used in petroleum pipelines.

　　3. Before installation, valves must undergo a visual inspection. The valve nameplate should comply with the provisions of the current national standard "General Valve Marking" GB 12220. For valves with a working pressure exceeding 1.0 MPa or those serving as shut-off devices on main pipelines, both strength and tightness tests must be conducted prior to installation. Only valves that pass these tests are permitted for use. During the strength test, the test pressure should be 1.5 times the nominal pressure, held for at least 5 minutes; the valve body and packing must show no leakage to be considered qualified. For the tightness test, the test pressure is set at 1.1 times the nominal pressure, and the duration must meet the requirements specified in GB 50243. The test is deemed qualified if there is no leakage observed across the sealing surfaces of the gate.

　　Installation tips:

　　1. The installation location, height, and inlet/outlet direction must comply with design requirements, and the connections should be secure and tight.

　　2. All manual valves installed on insulated pipelines must have handles positioned facing upward, never downward.

　　3. Before installation, valves must undergo a visual inspection. The valve nameplate should comply with the provisions of the current national standard "General Valve Marking" GB 12220. For valves with a working pressure exceeding 1.0 MPa or those serving as shut-off devices on main pipelines, both strength and tightness tests must be conducted prior to installation. Only valves that pass these tests are permitted for use. During the strength test, the test pressure should be 1.5 times the nominal pressure, held for at least 5 minutes; the valve body and packing must show no leakage to be considered合格 (qualified). For the tightness test, the test pressure is set at 1.1 times the nominal pressure, and the duration must meet the requirements specified in GB 50243. The test is deemed合格 if there is no leakage observed across the sealing surfaces of the gate.

　　How it works:

　　Based on the sealing surface configuration, gate valves can be divided into wedge-type and parallel-type. The wedge-type gate valve, in turn, is further categorized into single-gate, double-gate, and flexible-gate designs; while the parallel-type gate valve comes in single-gate and double-gate variants. Additionally, gate valves can also be classified according to the thread position of the valve stem—specifically, as either rising stem or non-rising stem valves.

　　1. When the large-diameter gate valve is closed

　　The sealing surface can rely solely on the medium pressure to achieve a seal—specifically, the medium pressure forces the gate’s sealing face against the valve seat on the opposite side, ensuring a tight seal. This is known as self-sealing. Most gate valves, however, use forced sealing: when the valve is closed, an external force is applied to press the gate firmly against the valve seat, guaranteeing a secure seal. In gate valves where the gate moves linearly along with the stem, the design is referred to as a rising-stem gate valve (also called an exposed-stem gate valve). Typically, the rising stem features a trapezoidal thread, which converts rotational motion into linear movement via a nut at the top of the valve and guided slots in the valve body—effectively transforming the operating torque into actuation thrust.

　　2. When the large-diameter gate valve is opened

　　When the gate's lifting height equals 1:1 of the valve's bore diameter, the fluid passage is fully open—but this exact position cannot be monitored during operation. In practice, the top point of the valve stem is used as the indicator, marking the point where the valve can no longer turn further, which defines the fully open position. To account for potential locking caused by temperature changes, it’s common to open the valve slightly past this top position—typically by reversing it back by 1/2 to 1 full turn—and then lock it in place. Thus, the valve’s true fully open position is determined based on the actual position of the gate (i.e., its travel).