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What is the difference between air chamber and surge tank in pressurized pipelines?

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Air chambers and surge tanks are normally installed in watermain to ease the stress on the system when valves or pumps suddenly start up and shut down. A surge tank is a chamber containing fluid which is in direct contact with the atmosphere. For positive surge, the tank can store excess water, thus preventing the water pipes from expansion and the water from compression. In case of downsurge, the surge tank can supply fluid to prevent the formation of vapour column separation. However, if the relief of surge pressure is significant, the height of surge tank has to be large and sometimes it is not cost-effective to build such a large tank. On the contrary, an air chamber can be adopted in this case because air chamber is an enclosed chamber with pressurized gases inside. The pressure head of the gas inside the air chamber can combat the hydraulic transient. The volume of liquid inside the air chamber should be adequate to avoid the pressure in the pipelines falling to vapour pressure. The air volume should be sufficient to produce cushioning effect to positive surge pressures. In essence, air chambers can usually be designed to be more compact than surge tanks. Air chamber has the demerits that regular maintenance has to be carried out to check the volume of air and proper design of pressure level of gas has to be conducted.

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This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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