I. Definition
Check valves, also known as back flow valves, backstop valves, check valves, are valves used to prevent the back flow of media in pipelines or equipment, allowing fluid to flow in one direction only, relying on the pressure of the media in the pipeline and automatically open and close, belonging to a kind of automatic valve. In the heating and water supply system, check valves are often installed in the back end of the trap, the pump outlet, the back end of the pressure reducing valve, as well as other locations that do not allow the reverse flow of fluids, the installation of a suitable check valve, the steam heating equipment trapping system has a very good safeguard for the pipeline water hammer protection is of great help!
(1) Prevent the medium back flow; prevent water hammer;
(2) To prevent the pump and its driving motor reversal;
(3) To prevent the leakage of the medium in the container, such as some vacuum class equipment, to prevent the destruction of vacuum. It should be noted that, when the check valve is closed, although it can effectively prevent the media back flow, but can not be used in place of the shut-off valve, that is to say, the check valve can not guarantee zero leakage, can not completely cut off the medium.
(1) Lift: can be installed in the horizontal or vertical pipeline, applicable to DN100 and the following calibre.
(2) Rotary opening type: can not be installed in the flow down the vertical pipeline; applicable to large calibre.
(3) Clamping type: can be installed in the horizontal or vertical pipeline, can be used for a variety of calabrese.
Check valves have a very wide range of applications, we will focus here on discussing the most common applications on the steam system, especially to focus on the need to explore the installation of check valves after the trap.
1.To protect critical valves from back flow, such as pressure reducing valves, flow meters and control valves after the installation of check valves to avoid pressure reducing valve failure and many other problems;
2.The pump outlet must be installed check valves to prevent the pump from stopping due to water back flow leading to motor reversal, which in turn causes damage to the pump body, the impeller is damaged by steam corrosion and other issues.
3.Steam trap must be installed after the check valve?
The answer is not necessarily!
(1) If there is a possibility of condensate back flow, a check valve must be installed; conversely, if condensate is discharged independently to the atmosphere and the drain is horizontal or downward, a check valve is not required.
(2) When the condensate from multiple traps is recovered to the same common recovery line, if no check valve is installed, the condensate from the high-pressure equipment may flow backward to the low-pressure equipment, which may result in water hammer damage to the traps and heating coils, or cause water to accumulate in the equipment, making it difficult to raise the temperature, so check valves must be installed after the traps.
(3) When condensate from multiple traps is recovered to the same common recovery line, if no check valve is installed, the condensate from the equipment that is being used may flow backward to other equipment that is not shut down, and the high-temperature condensate will dioxide and corrode the trap and heating coil, so it is necessary to add a check valve after the trap.
(4) If the condensate piping behind the trap climbs, once the equipment is shut down, or if the pneumatic valve in front of the equipment cuts off the steam, a vacuum will occur in the equipment, resulting in the back pressure behind the trap exceeding the pressure at the front end so that back flow occurs. In this case, check valves must also be installed behind the trap to effectively prevent water hammer.
(5) If there is a control valve or shut-off valve in front of the heating equipment, and the condensate piping after the trap climbs, and there is also a long horizontal pipe in front of the climbing pipe, then the condensate flowing backward from the climbing pipe may collide with the newly discharged condensate, which can lead to water hammer; in this case, it is necessary to install a check valve in the beginning of the lifting pipe, which is of great help in preventing the occurrence of water hammer.
(6) In the condensate transport piping, low-temperature condensate pulse flow produced by the water hammer can also be used to help solve the check valve. For example, additional check valves are required when multiple condensate branch lines are brought together in the condensate recovery main.
- Does the installation of a check valve after the steam trap necessarily prevent water hammer?
The answer is not necessarily!
There are many factors that cause water hammer. In most cases, one of the causes of water hammer is the back flow caused by the lifting pipe in the recovery line, and the installation of check valves in these places can be of great help in preventing water hammer caused by the back flow, but not in the following cases, i.e., the installation of check valves in these cases will not work.
(1) When high-pressure condensate is discharged to a lower pressure area through a trap, a certain amount of flash steam is generated. If this flash steam flows into a recovery line filled with low-temperature condensate, the flash steam transfers its latent heat to the condensate, which condenses rapidly and generates water hammer. Under these conditions, the installation of a check valve will not be effective, and the low-pressure condensate will not flow back anyway, so it can be left out.
(2) If the low-pressure condensate wants to be collected into the high-pressure condensate recovery pipe, it is not possible to do so, and it is useless to install a non-return valve.
Because a check valve prevents back flow but does not reduce back pressure, it does not allow condensate to be discharged from the low-pressure area to the high-pressure line. Even if a check valve is installed after the trap, condensate cannot be discharged if the primary pressure is less than the secondary pressure. Therefore, condensate needs to be recovered separately from high and low pressure!
- Installation of check valves is not recommended in the following cases
If a check valve is installed after a trap with a very small differential pressure, the check valve itself will become a point of resistance, which is equivalent to a slight increase in the back pressure, which will further reduce the differential pressure before and after the trap and lower the trap’s discharge capacity. Therefore, careful consideration is needed for the question of whether or not a check valve needs to be installed behind a trap with a very small differential pressure. For this type of equipment, it is recommended that no check valves be installed, and that condensate be discharged to the atmosphere alone, so as to smooth out the trap and ensure that the material warms up.
However, please note that in actual production, users often do not strictly follow the principle of separate recovery of condensate at different pressures, and pool all the condensate in the plant in the same recovery pipe or pipes without making any pressure distinction. Therefore, in order to avoid miscalculation, it is recommended that check valves be installed behind each trap! Prevent high pressure condensate from flowing back towards the low pressure!
V. Conclusion
Overall, check valves play a very important role in the trapping system, effectively preventing back flow of media and protecting the safe operation of piping systems and equipment, and are an integral part of the piping system.
