What are the hazards of abnormal gate valve pressure rise?
Introduction to the structure of gate valves：
Gate valves usually have a single gate, double gate, wedge type, parallel type, and other structural designs, of which wedge type double gate and wedge type resilient gate is most widely used.
The former closes when the gate automatically anastomoses both sides of the seat, automatically compensating for the processing position error of the wedge angle; the latter relies on the resilient slot in the middle of the gate, relying on the axial thrust of the stem to compensate for the processing position error of the wedge angle, both of which achieve a better sealing effect.
It is because of its excellent sealing on both sides, on some occasions will produce abnormal pressure rise in the cavity phenomenon, 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 rises, the cavity (see Figure 1) fluid will be heat transfer synchronous rise, due to the cavity Hugh product can not be expanded, blocking the cavity fluid from cold to hot state, the liquid may quickly vaporize, resulting in a sharp increase in pressure The increased pressure is often geometric.
The consequences of the valve over-pressure work are very serious. Valve cavity abnormal pressure, the pressure-bearing parts and the work of the opening and closing parts stress (such as the use of the stem and gate frame stress) will increase sharply, the driving force of the driving mechanism will be overwhelmed, and even unable to start, serious stem pull off, gate frame fracture, motor burnout, these phenomena are common in many high-pressure large-diameter gate valve, many users often complain that this is the gate “bite Die”, in fact, “bite” the real reason is often the cavity of the abnormal pressure rise of the “invisible killer”.
In a typical case, a wedge double gate valve is used in a thermal power plant water supply system and its bypass, this type of valve is generally first in the cold state for hydraulic test after closing, when the unit starts when the system temperature rises to 250 ~ 300 ℃, due to a sharp rise in temperature, closed in the middle of the cold water temperature will be synchronized with a sharp rise in vaporization, so that the fluid volume increases, the pressure rises, at this time to open the valve either the driving torque is large enough, or the stem assembly strength is high enough. Either the stem assembly strength is high enough, otherwise, there are often valve stem fractures, gate frame fractures, gate T-slot bloom, and fractures, so that the feed pump can not start, resulting in serious furnace shutdown accidents.
What are the hazards of abnormal pressure rise?
(1) the damage to the valve itself
Valve shell, bonnet, and stem parts of the strength of the valve are generally designed to the nominal pressure of the valve strength, abnormal boosting, the opening pressure will increase exponentially, resulting in the use of the relevant parts of the stress multiplied, when the actual stress of the material exceeds the allowable stress, the high-stress parts will produce fracture damage, resulting in the valve can not open, the valve will be damaged or scrap.
(2) Damage to system safety
Obviously, the shell, valve cover, and other pressure-bearing parts overpressure are very dangerous, once the overpressure of its weak parts may first occur perforation, causing media leakage; its packing and self-sealing ring parts are often washed out by high-pressure fluid, causing a large number of media leakage. When the medium is high-temperature gas, toxic gas, and harmful gas will be more serious, and even cause equipment and personnel injury.
(3) on the production control process to cause huge losses
The normal opening and closing of the valve is the key to the control of various industrial processes, once this control can not be achieved, the system is paralyzed to shut down for maintenance, which will cause huge direct or indirect losses.
What are the protective measures to prevent hazards?
(1) the valve’s internal pressure relief hole
The fundamental solution to the abnormal pressure rise in the cavity is to balance the pressure in the cavity, the opening of the pressure relief hole is the most cost-effective solution, Figure 2 shows the gate on the upstream side and the inlet side of the valve seat opened pressure relief hole when the cavity pressure rises, the cavity pressure will automatically release to the upstream side, always keep the pressure in the cavity and the upstream side pressure equal to avoid the occurrence of abnormal pressure rise.
(2) valve installation outside the bypass pressure relief
The valve can be installed with an external bypass and external pressure relief valve to reduce the pressure in the cavity, as shown in Figure 3. The external bypass with a shut-off valve is used to connect the cavity with the upstream side, and the shut-off valve can be closed when the main gate is closed (depending on the temperature change in the cavity, it must be opened when it is too high). When the main gate valve is opened, the bypass shut-off valve should be opened first to reduce the pressure in the cavity before starting the main gate valve.
(3) Installation of a special pressure relief valve outside the valve
The pressure range of the pressure relief valve outside the gate valve cavity, set the pressure relief valve discharge pressure is the rated working pressure of the main valve, in the cavity overpressure, automatic discharge to the set pressure, so as to maintain the safe operation of the main gate valve.
The structure design of the pressure relief valve (for reference). A shut-off valve is installed in front of the pressure relief valve to facilitate adjustment and maintenance of the pressure relief valve. The pressure setting of the pressure relief valve can usually be considered at 1.33 PN (PN is the nominal stress of the system valve).
In addition, the valve commissioning, especially the electric valve commissioning, should pay attention to the closing stroke and torque control, as far as possible to close the torque is smaller, to prevent wedging of the gate; high temperature valves should consider the high temperature state stem thermal expansion caused by wedging of the gate, it is recommended that high temperature and high pressure large diameter gate valve commissioning, the gate in place after the appropriate back valve stem to avoid the occurrence of real bite accident.
Abnormal over pressure is the invisible killer of double-seated class gate valve, a serious threat to the safe operation of equipment and systems, must be paid attention to; gate valve, especially high temperature, high pressure, large diameter gate valve design and installation must consider the occurrence of abnormal over pressure, and take the necessary preventive measures.
