Technical Articles
Notes for Installation of Decompression Valves
Posted: 01/07/2015 14:53:23  Hits: 504
Decompression valves are newly invented and applied to practice in recent years. They achieve extensive attentions in construction and fire-control water supply fields.
Decompression valves are installed on the outlet pipes of fire pumps. They are closed in normal times and automatically open when pressures of fire-control water supply pipeline systems go beyond preset values in case water supply systems breakdown. Over-pressure of water supply pipeline systems are mainly caused by the following reasons. First, the Q-H curves of water pumps selected according to pre-designed flow of fire-control system are steep type. Therefore, these valves would burden over-pressure when facing small flow (during self-inspection or initial stages of fire). Second. If water pumps happened to stop (because of black out or other reasons) and no measures were taken to outlet pipes, then pump-stopping water hammers would lead to over-pressure.
When installing decompression valves, issues like positions, selection of calibers, values of pressures and relevant accessories should be taken into consideration.
First. Decompression valves can be installed near fire-control water pumps or water tanks. When setting decompression valves on outlet pipes of fire-control water pumps, one can choose positions before or behind check valves. Decompression valves before check valves can eliminate over-pressure caused by small flows of water pumps. While decompression valves set behind check valves can not only eliminate over-pressure caused by small flows of water pumps, but also those caused by pump-stopping water hammers. Even if no measures, such as quick closed check valves, slow closed check valves, buffer tanks and others, are taken to outlet pipes of fire-control water pumps, everything will be fine all the same. It should be noticed that, positions before or behind check valves here are based on flow direction.
Second. There are two kinds of calibers designed for decompression valves. One has the same or smaller diameters, compared with fire-control water supply pipes. Another one is DN100 or DN150.
In view of pressure relief, the bigger calibers are, the greater effects of pressure relief are. Nevertheless, from the perspective of economy, prices of decompression valves increased along with the size of calibers. Thus, costs of construction will increase naturally. Therefore, decompression valves with fixed calibers are the most preferred at present. Even though the actual pressures will be slightly higher than those of decompression valves with big calibers when relieving pressures, comprehensive results of decompression valves with fixed calibers are still feasible. As a consequence, it can be stipulated that in fire-control water supply system, pipes with 150mm diameter can adopt DN 100 decompression valves while those with diameters bigger than 150mm should adopt DN150 decompression valves. 
Third. The preset pressures of decompression valves should be higher than those of fire-control primary pumps and regulating pumps. With regulating pumps, two pressure valves exist when opening and closing them. The pump-stopping water hammers are likely to open decompression valves mistakenly. Hence, for fire-control water supply systems that suffer from stable and high pressures, there should be reliable and perfect technical measures that can eliminate mistakenly operation of pump-stopping water hammers. What’s more, technicians should repeatedly check residual pressures of water supply systems after running of hammers. Only when the residual pressures are lower than preset pressures of decompression valves, the latter won’t be opened mistakenly.
Fourth. Decompression valves should be installed on the branch pipes that connected with primary pipes of fire-control water supply systems. For the sake of maintenance and normal on-off of control valves, signal valves or signal butterfly valves should be set before decompression valves. Besides, be-pass pipes should be installed to make sure normal flow during maintenance periods of decompression valves.