Introduction to Water Hammer
Water hammer is a commonly observed phenomenon taking place during a fluid flow. Presence of water hammer can be easily detected by the noise it makes. Noise is not the final effect of water hammer but just an indication of it. Water hammer has multiple adverse effects on steam systems. Water hammer can damage equipments like flow meters which are installed on the steam network. Instances of rupture and disruption of piping on account of water hammer are also quite common. In a few cases, water hammer has resulted into catastrophic hazards. Water hammer is not only a system issue but it is also a safety issue. Water hammer can be defined as follows-
Water hammer is a pressure surge or wave caused when a fluid (usually a liquid but sometimes also a gas) in motion is forced to stop or change direction suddenly (momentum change).
As soon as steam leaves the boiler, it starts losing heat. As a result, steam stats condensing inside the pipe work. The rate of condensate formation is high particularly during the start ups when the system is cold. As a result of the condensation, the droplets of water are formed. These droplets of condensate get built up along the length of steam pipework forming a solid slug. When this slug encounters any obstacle such as a bend, it will be brought to a halt abruptly. All the kinetic energy of the condensate slug will get converted into pressure energy which has to be absorbed by the pipe work. This gives rise to the phenomenon of water hammer.
Formation of water hammer
Formation of water hammer can be understood very well from the diagrams below.
Formation of water hammer
After condensate is formed, the flow inside the pipe has two components, steam and the condensate. The flow velocity of steam is much higher than that of the condensate. During such dual phase flow, the heavy condensate which flows at the bottom of the pipe is pulled by high speed steam. This results in formation of water slug which is much denser than steam travelling with the velocity of steam. When this slug is stopped by any abruption like a bend or equipment, the kinetic energy of the slug will be suddenly converted into pressure energy which will create a shock wave in the entire pipework. The pipework will keep on vibrating until this energy is dissipated in the structure.
The impact of water hammer
One might wonder why water hammers are thought to be a serious problem. The destructive nature of water hammer can be realized through the following illustration:
Recommended velocity of saturated steam in pipe network = 20-35 m/s
Recommended velocity of water in pipe network= 2-3m/s
In case of water hammers, condensate is dragged by steam and hence, the water slug travels with velocity equal to that of steam which is around ten times more than the ideal water velocity. As a result, the total pressure impact exerted by water hammer is very high.
Best practices to avoid water hammer
Though water hammer cannot be completely eliminated from steam systems, it can certainly be avoided. There are certain best practices, which when followed, ensure least chances of occurrence of water hammer. Some of these practices are-
- Steam lines should always be installed with a gradual slope (gradient) in direction of flow.
- Installing steam traps at regular intervals and also at the low points. This ensures removal of condensate from the steam system as soon as it is formed.
- Sagging of pipes should be avoided by providing proper support. Sagging pipes can form pool of condensate in the pipework, increasing the chances of water hammer.
- Operators should be trained to open isolation valve slowly during the start-up modes.
- Drain pockets should be properly sized to ensure that condensate just not jumps over it. Instead, the drain pockets should be sized enough so that all the condensate reaches the trap.
- Reducers- Eccentric reducers should be used against concentric reducers