WATER HAMMERING

A 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).

Water hammer in a steam system is witnessed by a repetitive metallic noise accompanied by vibration of steam mains. Noise is not the final effect of a water hammer but just an indication of it.

When the water hammer occurs an instantaneous and abrupt pressure change (6-10 bar) occurs inside the piping.

As soon as steam leaves the boiler, it starts losing heat. As a result, steam starts condensing inside the pipework. The rate of condensate formation is high, particularly during the start-ups when the system is cold. As a result of the condensation, 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 an obstacle such as a bend, it will be brought to a halt abruptly. The vibration and noise are a result of the pressure energy absorbed by the pipe. The pressure energy is nothing but the kinetic energy of the condensate or slug. This gives rise to the phenomenon of water hammers.

The formation of the water hammer can be understood very well from the following diagrams. After condensate is formed, the flow inside the pipe has two components, steam, and 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 the formation of a water slug which is much denser than steam traveling 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 vibrating until this energy is dissipated in the structure.

Impact of water hammer

Water hammers are thought to be a serious problem. The destructive nature of the water hammer can be realized through the following illustration:

Recommended velocity of saturated steam in pipe network = 20-35 m/s (65.61-114.82 ft/s)

Recommended velocity of water in pipe network= 2-3m/s (6.5-9.8 ft/s)

In the case of water hammers, condensate is dragged by steam, and hence, the water slug travels with a 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 the water hammer is very high.

Water hammer has multiple adverse effects on steam systems. Water hammers can damage equipment like flowmeters that are installed on the steam network. Instances of rupture and disruption of piping on account of water hammers are also quite common. In a few cases, water hammer has resulted in catastrophic hazards. Water hammer is not only a system issue but it is also a safety issue.

Best practices to avoid water hammer

Though water hammers cannot be completely eliminated from steam systems, it can certainly be avoided. There are certainly best practices, which when followed, ensure the least chances of occurrence of water hammer. Some of these practices are:

1) Steam lines should always be installed with a gradual slope (gradient) in direction of flow.

Water Hammer occur

 

 

Water hammer does not occur

2) Installing steam traps at regular intervals and also at the low points. This ensures the removal of condensate from the steam system as soon as it is formed.

3) Sagging of pipes should be avoided by providing proper support. Sagging pipes can form pools of condensate in the pipework, increasing the chances of water hammer.

Sagging pipes

4) Operators should be trained to open the isolation valves slowly during the start-up modes.

5) 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.

 

 

Drain pocket Arrangement

6) Reducers- Eccentric reducers should be used against concentric reducers

7) Moisture separators- Process steam should be as dry as possible for efficient heat transfer. Wet steam is the cause of many problems like reduced batch times, corrosion, water hammer, etc. Wet steam can also damage expansion line equipment like flowmeters, control valves, etc.
Moisture separator removes residual moisture suspended in the steam flow, it ensures that the highest dryness fraction is achieved at the outlet.
Water drops have more mass and hence more inertia than steam. The moisture separator works on this phenomenon of inertia difference between steam and water.