Fisher Fisher - Hazards and Protection of Abnormal Pressure Boost in the Cavity of Gate Valves

Gate valves usually have single rams, double rams, wedges, parallel and other structural designs, among which wedge double rams (Z42, Z62 series) and wedge elastic rams (Z40, Z60 series) are widely used, the former when the ram is closed, the ram automatically fits the valve seats on both sides, and automatically compensates for the processing position error of the wedge angle; The latter relies on the elastic groove in the middle of the ram and the axial thrust of the valve stem to compensate for the machining position error of the wedge angle, both of which achieve a better sealing effect.

It is precisely because of its excellent sealing on both sides that the pressure of the cavity will rise abnormally in some occasions, that is, when the high temperature and high pressure fluid (liquid or gas) is blocked in the middle cavity of the valve, if the temperature of the fluid on the upstream side rises, the medium cavity fluid will be synchronously increased by heat transfer.

The consequences of overpressure operation of the valve are very serious. When the valve cavity is abnormally pressurized, the working stress of the pressure bearing parts and opening and closing parts (such as the use stress of the valve stem and the ram frame) will increase sharply, and the driving force of the driving mechanism will be overwhelmed, or even unable to start, and the valve stem will be broken when the valve stem is broken, the ram frame is broken, and the motor burns out, these phenomena are not uncommon in many high-pressure large-diameter gate valves, and many users often complain that this is the gate "bite", in fact, the real reason for the "bite" is often the abnormal boost of the cavity.

Typical case such as Z962Y series wedge double gate valve used in thermal power plant water supply system and its bypass, this kind of valve is generally closed after the hydraulic test in the cold state, when the system temperature rises to 250-300 °C when the unit is started, due to the sharp increase in temperature, the cold water temperature in the middle of the closure will increase sharply synchronously and vaporize, so that the fluid volume increases, the pressure increases, at this time to open the valve either the driving torque is large enough, or the stem assembly strength is high enough, otherwise the valve stem breakage, ram frame breakage, The T-slot of the ram blooms and breaks, preventing the feedwater pump from starting, resulting in a serious furnace shutdown.

The hazards of abnormal pressure increases

The formation of abnormal pressure boost will occur surprisingly similar in many gate valve applications, because the two major elements of its occurrence are similar in many industrial systems, that is, the system medium changes from cold to hot after start-up; The ram closes in the cold state and opens in the hot state. Therefore, if no measures are taken against the system, the damage to the system caused by abnormal voltage boost is almost unavoidable, and it has three hazards:

(1) Damage to the valve itself

The strength of the valve shell, valve cover and valve stem parts is generally designed with the nominal pressure of the valve, and when the pressure is abnormally boosted, the opening pressure will be doubled, resulting in the use stress of the relevant parts being doubled, and when the actual stress of the material exceeds the allowable stress, the high-stress part will be fractured and damaged, resulting in the valve being unable to open, and the valve machine will be damaged or scrapped.

(2) Disruption of system security

Obviously, it is very dangerous when the pressure-bearing parts such as the shell and the valve cover are overpressurized, and once the overpressure is overpressurized, the weak parts may be perforated first, causing the leakage of the medium; The packing and self-sealing parts are often washed out by high-pressure fluids, causing a large amount of leakage of the medium. When the medium is high-temperature gas, toxic gas, and harmful gas, it will be more serious, and even cause damage to equipment and personnel.

(3) Huge losses to the production control process

The normal opening and closing of valves is the key to the control of various industrial processes, and once this control cannot be achieved, the system is paralyzed and must be shut down for maintenance, which will cause huge direct or indirect losses.

Protective measures

Starting from the aspects of design, installation, and commissioning, it is entirely possible to fundamentally eliminate the abnormal pressure boost in the middle cavity, and summarize 3 sets of commonly used schemes for the reference of the majority of users:

(1) A pressure relief hole is set inside the valve

The solution to the abnormal pressure of the cavity is to balance the pressure of the cavity, the opening of the pressure relief hole is the most economical and effective solution, in the upstream side of the ram and the inlet side of the valve seat outside the circle of the opening of the pressure relief hole, when the cavity pressure rises, the pressure of the cavity will automatically release to the upstream side, always keep the pressure of the cavity and the pressure of the upstream side equal, so as to avoid the occurrence of abnormal pressure rise.

(2) The valve is installed with an external bypass to relieve pressure

For the valves that have left the factory, the external bypass and the external pressure relief valve can be used to reduce the pressure in the middle chamber during installation

The bypass with a shut-off valve is used to connect the middle cavity and the upstream side, and the shut-off valve can be closed when the main gate is closed (depending on the temperature change of the middle cavity, it must be opened when it is too high). When the main gate valve is opened, the bypass stop valve should be opened first, and the pressure in the chamber should be reduced before starting the main gate valve.

(3) A special pressure relief valve is installed outside the valve

A pressure relief valve is set up outside the chamber of the gate valve to control the pressure range, and the discharge pressure of the pressure relief valve is set to the rated working pressure of the main valve, and when the cavity is overpressurized, it is automatically discharged to the set pressure, so as to maintain the safe operation of the main gate valve.

A shut-off valve is installed in front of the pressure relief valve, which is convenient for the adjustment and maintenance of the pressure relief valve. The pressure setting of the pressure relief valve can usually be considered as 1.33PN (PN is the nominal stress of the system valve).

In addition, when debugging the valve, especially when the electric valve is debugging, attention should be paid to the control of the closing stroke and torque, and the closing torque should be adjusted as small as possible to prevent the wedge of the ram; It is recommended that when the high-temperature and high-pressure large-diameter gate valve is debugged, the gate should be properly retracted after the gate is in place to avoid the occurrence of real bite accidents.

Conclusion

Abnormal overpressure is the invisible killer of double-seat gate valves, which seriously threatens the safe operation of equipment and systems, and must be paid attention to; The occurrence of abnormal overpressure must be considered when designing and installing gate valves, especially high-temperature, high-pressure, and large-diameter gate valves, and necessary precautions must be taken.
Fisher Fisher - Hazards and Protection of Abnormal Pressure Boost in the Cavity of Gate Valves

Fisher Fisher - Hazards and Protection of Abnormal Pressure Boost in the Cavity of Gate Valves

Fisher Fisher - Hazards and Protection of Abnormal Pressure Boost in the Cavity of Gate Valves

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