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1, Introduction Gate valve usually has single gate, double gate, wedge, parallel type and other structural design, wedge double gate (Z42, Z62 series) and wedge elastic gate (Z40, Z60 series) the most widely used , The former closed automatically when the gate on both sides of the valve seat automatically compensate for the wedge angle of the processing position error; the latter relies on elastic gate in the central slot, by the axial thrust of the valve stem to compensate for the wedge angle processing position error, both Have achieved better sealing effect. Precisely due to its excellent sealing on both sides, an abnormal increase in pressure in the middle chamber can occur in some cases. That is, when the high-temperature and high-pressure fluid (liquid or gas) is blocked in the valve cavity, if the upstream side ) Fluid temperature, the cavity (see Figure 1) fluid will be heat transfer simultaneously increased, due to the cavity can not expand the volume, blocking the cavity fluid from cold to hot, the liquid may vaporize rapidly, resulting in pressure Rapidly rising, increasing pressure is often a geometric series. The consequences of over-pressure valve work is very serious. When abnormal pressure is raised in the middle of the valve, the working stress of the pressure-receiving parts and the opening and closing parts (such as the stress of the valve stem and the gate frame) will increase sharply, and the driving force of the driving mechanism will be overwhelmed or even unable to start, When the stem pull off the rake rack rupture, the motor burned, these phenomena are common in many high-pressure large-diameter gate valve, many users often complain that this is the gate "killed", in fact, "bite dead" the real reason is often Cavity abnormal boost this "invisible killer." Figure 1 chamber abnormal pressure increase Description Typical cases such as Z962Y series wedge gate valve used in thermal power plant water supply system and its bypass, such valves are generally closed after the cold for the hydraulic test, when the unit starts When the system temperature rises to 250 ~ 300 ℃, due to the sharp rise in temperature, the closed middle of the cold water temperature will be synchronized with a sharp rise in vaporization, the fluid volume increases, the pressure increases, this time to open the valve or drive torque Large enough, or the stem assembly strength is high enough, or often appear stem rupture, racks fracture, gate T-slot flowering, fracture, so that the pump can not start, leading to serious downtime downtime. 2. Harm of Abnormal Boost The formation of abnormal booster occurs alarmingly and similarly in many gate valve applications because its two major elements are similar in many industrial systems. That is, after system power is switched on, Hot; Shutter closed in cold, hot open. Therefore, if the system does not take measures to abnormal damage to the system is almost impossible to avoid the damage, which has three hazards: (1) the destruction of the valve itself Valve housing, valve cover and stem the strength of the general parts of the valve Nominal pressure design strength, abnormal pressure, the cracking pressure will increase exponentially, resulting in the use of related parts double the stress, when the actual stress exceeds the allowable stress, the high-stress parts will produce fracture damage, resulting in the valve Can not be opened, the valve will be damaged or scrapped. (2) the destruction of the system safety Obviously, the shell, bonnet and other pressure overpressure is very dangerous, once the overpressure of the weak parts may be the first perforated, causing media leakage; its packing and self-sealing ring Parts of the high-pressure fluid will often be washed out, causing a large number of media leakage. When the medium is a hot gas, toxic gases, harmful gases will be more serious, and even cause equipment and personnel injuries. (3) huge loss of production control process The normal opening and closing of the valve is the key to the control of all kinds of industrial processes. Once this control can not be realized, the system will be shut down and shut down for maintenance. This will result in huge direct or indirect losses. 3, protective measures From the design, installation, commissioning and other aspects, to fundamentally eliminate the abnormal pressure in the cavity is entirely possible, generalized three sets of programs for the general user reference: (1) the valve to open the pressure relief hole to solve Cavity anomalous pressure is the fundamental balance of pressure in the cavity, the pressure relief hole is the most economical and effective solution, shown in Figure 2, respectively, on the upstream side of the gate and the inlet side of the valve seat opened a relief hole, when the cavity pressure When it rises, the cavity pressure will be automatically released to the upstream side, and the cavity pressure will always be equal to the pressure on the upstream side so as to avoid abnormal step-up. (a) Gate Pressure Relief (b) Block Relief