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Safety valve selection operation
Aug 29, 2018

The function of the safety valve:

The safety valve is a safety protection device of the closed system. Its opening and closing member is normally closed under the action of external force. When the medium pressure of the closed system rises, it will automatically open when the safety valve exceeds the specified value. External discharge medium to reduce system pressure to prevent damage to the system caused by high pressure of medium in the system. Safety valves are automatic valves. They are mainly used in boilers, pressure vessels, air conditioners and solar pipes. The control pressure does not exceed the specified value and plays an important role in the protection of machines and equipment.

Classification of safety valves:

There are two main types of safety valve structures: spring type and lever type. The spring type refers to the force of the spring of the valve disc and the valve seat. The lever type is the force of the lever and the weight. With the need for large capacity, there is a pulse-type safety valve, also known as a pilot-operated safety valve, consisting of a main safety valve and an auxiliary valve. When the medium pressure in the pipeline exceeds the specified pressure value, the auxiliary valve is opened first, the medium enters the main safety valve along the conduit, and the main safety valve is opened to lower the pressure of the increased medium.

The discharge of the safety valve is determined by the diameter of the valve seat and the opening height of the valve flap. It can also be divided into two types: the micro-opening height is the inner diameter of the valve seat (1/15) ~ (1/20), full-opening It is (1/3) to (1/4).

In addition, there are closed and unclosed types depending on the requirements of use. The enclosed or discharged medium is not leaked and is discharged all along the prescribed outlet and is generally used for toxic and corrosive media. Non-closed is generally used for non-toxic or non-corrosive media

I. According to its overall structure and loading mechanism, it can be divided into three types: heavy hammer lever type, spring type and pulse type.

1. Heavy hammer lever type safety valve

The heavy-duty lever-type safety valve uses a weight and a lever to balance the force acting on the valve flap. According to the principle of the lever, it is possible to obtain a larger force by the increase of the weight of the weight using a smaller weight hammer, and to adjust the opening pressure of the safety valve by moving the position of the weight (or changing the mass of the weight).

The heavy-duty lever type safety valve has a simple structure, is easy to adjust and is relatively accurate, and the applied load is not greatly increased due to the rise of the valve flap. It is suitable for occasions with high temperature and has been used in the past, especially in the past. It is used in boilers and high temperature pressure vessels. However, the heavy-duty lever type safety valve is relatively bulky, the loading mechanism is easy to vibrate, and often leaks due to vibration; its return pressure is low, and it is difficult to close and keep tight after opening.

2. Spring micro-open safety valve

The spring micro-opening safety valve uses the force of the compression spring to balance the force acting on the valve flap. The amount of compression of the spiral coil spring can be adjusted by turning the adjusting nut on the upper side. With this configuration, the opening (setting) pressure of the safety valve can be corrected as needed. The spring micro-open safety valve is light and compact, has high sensitivity, is not limited in installation position, and can be used in mobile pressure vessels because of its low sensitivity to vibration. The disadvantage of this type of safety valve is that the applied load changes with the opening of the valve, that is, as the valve flap increases, the amount of compression of the spring increases, and the force acting on the valve flap also increases. This is detrimental to the rapid opening of the safety valve. In addition, the spring on the valve will reduce the spring force due to the long-term influence of high temperature. When used on containers with higher temperatures, it is often necessary to consider the thermal insulation or heat dissipation of the springs, complicating the structure.

3. Pulse safety valve

The pulse type safety valve is composed of a main valve and an auxiliary valve. The pulse of the auxiliary valve drives the main valve to operate, and its structure is complicated. It is usually only suitable for boilers and pressure vessels with a large amount of safety relief.

Among the above three types of safety valves, spring type safety valves are commonly used.

II. According to different media discharge methods, safety valves can be divided into three types: fully enclosed, semi-closed and open.

1. Fully closed safety valve

When the fully closed safety valve is exhausted, the gas is completely discharged through the exhaust pipe, and the medium cannot be leaked outside, and is mainly used for the medium. a container for gas.

2. Semi-closed safety valve

A part of the gas discharged from the semi-closed safety valve passes through the exhaust pipe, and a part of it leaks from the gap between the valve cover and the valve stem, and is mostly used for a container in which the medium is a gas that does not pollute the environment.

3. Open safety valve

The bonnet of the open safety valve is open, allowing the spring chamber to communicate with the atmosphere, which helps to reduce the temperature of the spring, and is mainly suitable for containers in which the medium is steam and high temperature gas which does not pollute the atmosphere.

III. According to the ratio of the large height of the valve opening and the diameter of the safety valve runner, the safety valve can be divided into two types: the spring micro-opening high-pressure safety valve and the spring full-open safety valve.

1. Spring micro-opening high pressure safety valve

The opening height of the micro-opening safety valve is less than 1/4 of the diameter of the flow passage, and is usually 1/40 to 1/20 of the diameter of the flow passage. The operation process of the micro-opening safety valve is proportionally active, mainly used for liquid applications, and sometimes for gas applications with small emissions.

2. Spring full-open safety valve

The opening height of the full-open safety valve is greater than or equal to 1/4 of the diameter of the flow passage. The discharge area of the full-open safety valve is the small cross-sectional area of the seat throat. The action process belongs to the two-stage action type, and must be fully opened by means of a lifting mechanism. The full-open safety valve is mainly used for gas medium applications.

3. Middle opening safety valve

The opening height is between the micro-open and the full-open. That is, it can be made into two stages, or it can be made into a proportional action.

IV. According to the principle of action, it can be divided into direct acting safety valve and non-direct acting safety valve.

1. Direct acting safety valve

The direct acting safety valve is opened under the direct action of the working medium, that is, the mechanical load applied to the valve flap by the loading mechanism is overcome by the action of the working medium pressure, so that the valve is opened. The safety valve has the advantages of simple structure, quick action and good reliability. However, because of the structural loading, the load size is limited and cannot be used in high pressure and large diameter applications.

2. Non-direct acting safety valve

These safety valves can be divided into pilot-operated safety valves and safety valves with power assists.

The pilot operated safety valve is driven or controlled by the medium that is discharged from the pilot valve. The pilot valve itself is a direct-acting safety valve, and sometimes other types of valves are used. Pilot-operated safety valves are suitable for high pressure, large diameter applications. The main valve of the pilot-operated safety valve can also be designed to be sealed by means of a working medium, or it can apply a much greater mechanical load to the valve flap than a direct-acting safety valve, thus providing a good sealing performance. At the same time, its movements are rarely affected by back pressure. The disadvantage of this type of safety valve is that its reliability is related to the main valve and the pilot valve. The action is not as fast and reliable as the direct-acting safety valve, and the structure is complicated.

The safety valve with power assist is forced by the power assist device to force the safety valve to open below the normal opening pressure. This safety valve is suitable for applications where the opening pressure is very close to the working pressure, or where the safety valve needs to be opened periodically to check or blow off adhering, frozen media. At the same time, a means of forcibly opening the safety valve in an emergency is also provided.

V. Whether the pressure can be adjusted according to the classification, can be divided into fixed non-adjustable safety valve and adjustable safety valve

1. Fixed non-adjustable safety valve pressure value has been set at the factory, can not be changed when used, commonly used in central air conditioning air conditioning, boiler wall-hung boiler, solar energy and other systems, such as S10 series safety valve.

2. Adjustable safety valve take-off pressure can be arbitrarily set within a certain range according to different needs of users. It is often used in situations where the system protection pressure needs to be changed frequently, such as S10 series safety valve, but the price is generally higher.

VI. According to the working temperature, it is divided into normal temperature safety valve and high temperature safety valve.

1. Normal temperature safety valve is generally a 110 °C temperature-resistant safety valve installed only on HVAC, air conditioning or water systems, such as S10 series safety valve.

2. High temperature safety valve refers to the safety valve of 180 °C, which is specially used in solar energy system and mold temperature system, such as S10 series safety valve.

The actual operation method of the safety valve:

i.  Adjustment of the opening pressure of the safety valve

① Before the safety valve is delivered from the factory, the opening pressure should be adjusted one by one to the user's required setting value. If the user proposes the spring working pressure level, it should be adjusted according to the lower limit of the pressure level.

② The user must re-adjust the safety valve at the installation site before installing the safety valve on the protected equipment or before installation to ensure that the set pressure value of the safety valve meets the requirements.

③ Within the range of the spring working pressure level indicated on the nameplate, the opening pressure can be adjusted by changing the spring compression amount by rotating the adjusting screw.

④ Before rotating the adjusting screw, the valve inlet pressure should be reduced to less than 90% of the opening pressure to prevent the valve flap from being rotated when the adjusting screw is rotated, so that the sealing surface is damaged.

⑤ In order to ensure that the opening pressure value is accurate, the medium conditions during adjustment, such as the type of medium and temperature, should be as close as possible to the actual operating conditions. The type of medium changes, especially when the medium is in a different accumulation state (for example, from a liquid phase to a gas phase), and the opening pressure often changes. When the operating temperature rises, the opening pressure generally decreases. Therefore, when it is adjusted at normal temperature and used for high temperature, the set pressure value at normal temperature should be slightly higher than the required opening pressure value. The degree to which it is high is related to the valve structure and material selection, and should be based on the manufacturer's instructions.

⑥ Conventional safety valve is used to fix the additional back pressure. When the opening pressure is adjusted after inspection (the back pressure is atmospheric pressure), the setting value should be the required opening pressure value minus the additional back pressure value.

ii: safety valve discharge pressure and return pressure adjustment

① Adjusting the valve discharge pressure and the return pressure must perform the action test of the valve to the full opening height. Therefore, it is only possible after the large-capacity test device or after the safety valve is installed on the protected device. The adjustment method varies depending on the valve structure.

② For the structure with the recoil disc and the seat adjusting ring, the valve seat adjusting ring is used for adjustment. Unscrew the adjusting ring fixing screw and extend a tool such as a thin iron rod from the exposed screw hole to push the gear teeth on the adjusting ring to make the adjusting ring rotate left and right. When the adjustment ring is rotated counterclockwise to the left, its position is raised, and the discharge pressure and the return pressure are reduced. Conversely, when the adjustment ring is rotated clockwise to the right, its position is lowered, and the discharge pressure and the return pressure are increased. For each adjustment, adjust: the amplitude of the rotation of the circle should not be too large (generally rotate a few teeth). After each adjustment, the fixing screws should be screwed so that the end is located in the groove between the two teeth of the adjusting ring, which can prevent the adjusting ring from rotating and does not generate radial pressure on the adjusting ring. For safety reasons, the safety valve inlet pressure should be appropriately reduced (generally less than 90% of the opening pressure) before the adjustment ring is turned to prevent the valve from suddenly opening during the adjustment, resulting in an accident.

③ For the structure with the upper and lower adjustment rings (the guide sleeve and the valve seat have one adjustment ring), the adjustment is more complicated. The seat adjustment ring is used to change the size of the passage between the valve flap and the adjustment ring, thereby changing the amount of pressure accumulated in the chamber between the valve flap and the adjustment ring when the valve is initially opened. When the valve seat adjustment ring is raised, the degree of pressure buildup is increased, so that the phase of the valve proportional opening is reduced and the sudden rapid opening is achieved relatively quickly. Therefore, raising the seat adjustment ring can reduce the discharge pressure. It should be noted that the seat adjustment ring must not be raised too close to the valve flap. In this way, the leakage at the sealing surface may cause the valve to suddenly open prematurely, but since the medium pressure is not enough to keep the valve flap in the open position, the valve flap is closed again, and the valve jumps. Valve seat adjustment: "The ring is mainly used to reduce the valve ratio, the opening phase and adjust the discharge pressure, and also affect the return pressure.

The upper adjustment ring is used to change the angle at which the flowing medium is deflected after the lower side of the valve flap is reflected, thereby changing the magnitude of the fluid force, thereby adjusting the return pressure. When the upper adjustment ring is raised, the folding angle is reduced, and the fluid force is reduced, thereby increasing the return pressure. Conversely, when the upper adjustment ring is lowered, the return pressure is lowered. Of course, while the upper adjustment ring changes the pressure of the seat back, it also affects the discharge pressure, that is, raising the upper adjustment ring causes the discharge pressure to increase, and lowering the upper adjustment ring to reduce the discharge pressure, but the degree of influence is not as good as The seat pressure is as obvious.







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