Among the usual structural designs of single-gate, double-gate, wedge, parallel and so on, the wedge double-gate and the wedge elastic gate are the most widely used ones. The former’s gate automatically coincides with the two sides of the seat, while the latter relies on the elastic groove in the middle of the gate wedge to seal. The former has a automatic compensation, while the latter depends on the axial thrust of the stem to compensate the machining position error of the wedge angle. As a result of the excellent sealing performance of both of them, in some cases, the pressure in the middle cavity will rise abnormally. Since the high-temperature and high-pressure fluid (liquid or gas) is blocked in the middle cavity of the valve, the fluid inside, heated up by the upstream, may vaporize rapidly, resulting in a sharp increase of pressure. It is often a geometric progression.
The consequence of valve over-pressure is of great magnitude. The working stress of the bearing parts and the switch gears (such as the working stress of the valve stem and the gate frame) will be pressurized by the abnormal cavity, thereby burdening the driving force of the driving mechanism. Even the abnormal pressure may make it worse by a failed start, a cracked valve stem, a broken gate frame and a burnt motor. These cases are common in many high-pressure large-diameter gate valves. Many users blame the gate valve “stuck” . In fact, the “stuck” is rooted in the abnormal surge of pressure in the cavity, which is the “invisible killer” to the gate.
Typically suppose that the wedge double gate valve is used in a water supply system of thermal power plant and its bypass. The cold water left by the pressure test is heated under 250~300℃. Flash vaporizing upsizes the volume and surges the pressure of the fluid. Open the valve with either large enough driving torque or high enough strength of the stem assembly, otherwise, the valve stem fractures, the gate frame breaks or the gate T-slot blows up. Then the water pump fails to start, leading to a serious shutdown accident.
First, hazards of the abnormal pressure.
The formation of abnormal pressure in diversified applications of gate valves is surprisingly alike, for its two major factors in many industrial systems are similar. They are the heated media and the heat start following the cold stop. Therefore, it is almost inevitable, if no measures are taken, the damage to the system caused by the abnormal pressure. Here are 3 hazards.
1. The destruction to the gate valve
In general, the strength of the valve shell, the bonnet and the stem is in accordance with the nominal pressure of the valve. Abnormal pressure raises the opening pressure, which times the working stress of the related parts. Once the actual stress overwhelm the allowable stress, parts with high stress will fracture, leading to a failed opening, a damaged the valve gate or even a scrapped overall machine.
2. The destruction to the safety system
The pressure bearing pieces like the shell, the bonnet and so on once over-pressured is obviously dangerous for the medium easily breaks its vulnerable zones then leaks out. The high pressure fluid rushes out from its packing and the self-sealing ring, causing a massive leakage. The case will be worse by bringing about damages and casualties if the medium is any gas of the high temperature, toxic or harmful ones.
3. The destruction to the operation system
A normal gate valve is the key to all kinds of industrial operation control. Once the operation control fails, the whole system must be shut down for maintenance, which will result in huge direct or indirect losses.
Second, protective measures.
It is possible to totally eliminate the abnormal pressure in the middle cavity by dealing with the gate valve’s design, installation and debugging. Here are three solutions commonly adopted for reference:
1. A pressure relief hole inside
The pressure relief hole, set inside the valve to balance the pressure in the middle cavity, is the most economical and effective solution. As shown in picture 2, the pressure relief hole is set outside of the upstream wedge and the inlet valve seat respectively. The increased pressure in the middle cavity will be released to the upstream automatically, always keeping a balanced pressure in both sides, thereby killing the abnormal pressure.
2. A bypass pressure relief outside
For the already-made gate valve, install it with an external bypass as an outlet relief, as shown in picture 3.
A bypass block valve is used to connect the middle cavity to the upstream. Only when the main gate is off can it be switched off (switch it on when the cavity is overheated). The bypass block valve should be switched on first to reduce the pressure in the cavity before opening the main gate valve.
3. A special relief valve attached
The special relief valve is attached outside the cavity as shown in picture 4. Set the discharge pressure of the relief valve the same as the rated working one of the main gate valve. Once the cavity is over-pressured, it is automatically discharged to the set pressure, thereby maintaining a safe operation of the main gate valve.
To facilitate the adjustment and maintenance of the relief valve, the block valve should be installed in front of it. The rated pressure of the relief valve is usually set at 1.33PN (PN is the nominal stress of the system valve).
In addition, as to the debugging, especially in the case of the electric valve, attention should be paid to the switch-off stroke and torque control. Minimize the switch-off torque to prevent wedge from being stuck. To take the thermal expansion of the stem into account, pull back the stem after the wedge is in place during the debugging of the high temperature gate valve with large diameter to avoid the stuck accident.
Attached importance to the abnormal overpressure, it is a hidden killer of the double-seat gate valve, which threatens the operation of the equipment and the safety of the system. Therefore, the design and installation of the gate valve, especially the high temperature, high pressure and large diameter ones, shall be able to prevent the abnormal overpressure.
