Why There Is Double Valves?
1. Set the intention of the double valve
According to the interpretation of Article 7.2.8 of the Petrochemical Regulation GB 50160, "The low point of a continuously operated flammable gas pipeline is provided with two drainage valves, the first valve (near the pipeline side) is a normally open valve, and the second valve is a regularly operated valve." When the second valve leaks, close the first valve and replace the second valve."
2. Set the usage of the double valve
According to the provisions of the "Petrochemical Regulation" GB 50160, after setting two valves, not both valves are closed during use, but only one valve is kept closed and the other valve is normally open, as a backup measure after the first valve leaks. At present, in actual production, we can require two valves to be opened and closed, and we can also require both valves to be closed. This is to take into account that first, the manufacturing accuracy of the valve itself is insufficient, and there is an inherent risk of leakage if the sealing surface is not tight; Second, the frequent use of the valve will also lead to internal leakage due to wear of the sealing surface; Third, the material in the equipment and pipeline is not strictly closed due to crystallization (scale), and there is leakage phenomenon; Fourth, the operator's misoperation may cause the valve to not close in place and bring leakage. For the discharge outlet directly to the atmospheric environment, it is still required that both valves be closed.
3. Double valves should be set
Considering that the fundamental purpose of setting double valves is to prevent leakage and avoid accidents caused by material leakage, the following discharge ports and sampling ports involving flammable and explosive, toxic and harmful, and corrosive media should be strengthened by setting double valves:
(1) The low point of the equipment and pipeline where liquid may flow out to the external environment.
This is because the liquid medium is easy to accumulate in front of the low discharge valve by static pressure, and the possibility of leakage is greater.
Although these discharge ports can be preferred in the form of single valve and blind plate, for discharge valves that need to be operated frequently, in order to avoid frequent removal of the blind plate, it can still be in the form of double valves.
(2) Manually operated liquid and gas sampling ports.
This is due to the frequent operation of these valves, which may be loosely closed.
4. It is not appropriate to set double valves
Double valves should not be installed at the pipe opening for gas drainage. This is because the release pipeline itself is used for emergency release when the system overpressure, the setting of double valves instead reduces the timeliness of emergency treatment, and may even cause accidents because the release is not timely.
5. The following parts may not require double valves
(1) The exhaust pipe port installed on the top of the liquid conveying pipe or equipment.
This is to take into account that the system exhaust exists in the opening and stopping or production of abnormal adjustment conditions and other scenarios, these valves operate at a low frequency, and the liquid conveying equipment pipeline accumulated less gas, low pressure, gas escape leakage is small, so there is no need to force the installation of double valves.
(2) Pipeline discharge outlets involving public works (water, steam, gas).
Although these pipes also have the possibility of media leakage, from the perspective of risk, it is necessary to analyze the possible consequences of water, steam and gas leakage. Especially when enterprises in the implementation of the process of increasing the double valve operation, production and shutdown, hot operation and other riskier operations, the increase of the double valve is not worth the loss.
(3) For the sampling points involving flammable and explosive, toxic and harmful materials, priority should be given to using closed sampling to permanently solve the leakage problem, rather than simply requiring the setting of double valves. Double valves may not be required for closed sampling ports.
6. Double valve selection requirements
For the parts that need to set double valves, the valve type should be scientifically selected. In general, choose flange valve instead of welding valve, should choose ball valve, gate valve and try not to choose stop valve. This is mainly to take into account the later replacement, easy maintenance and easy dredging after the occurrence of pipeline blockage. For the need to control emissions, the front hand valve can consider the ball valve, the rear hand valve is recommended to use gate valve or needle valve. Once it is found that the back hand valve is not tightly closed, the front hand valve can be quickly closed, and the ball valve is not easy to be blocked by solid impurities.
7. The choice of on-site liquid level gauge shower valve should be based on the material
The drain valve installed at the low point of the field level gauge should be determined according to the characteristics of the material being measured. Ball valves, gate valves or Cock valves should be installed at low points where crystallization is easy to block the pipeline. For measuring the low point of the liquid level gauge that is not easy to crystallize and clog the pipeline material, the form of valve + wire plug can be taken.
