Water Hammer in Process Plant
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). A water hammer commonly occurs when a valve closes suddenly at an end of a piping system, and a pressure wave propagates in the pipe. It is also called hydraulic shock.
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 equipment’s 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. It is possible to reduce the effects of the water hammer pulses with accumulators, expansion tanks, surge tanks, blow off valves, and other features.
Water Hammer – Steam System
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. 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 and this creates ripples. If the level of condensate is high enough, the ripples may fill the entire pipe cross section. Under pressure, and with nowhere else to go, the ripples are transformed into a mass of water (often referred as slug) which is much denser than steam and travelling with the velocity of steam. When this slug is stopped by any obstacle 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.
Water Hammer – Condensation Induced
Condensation-induced water hammer may be caused either by steam entering a piping system that contains water (cooled condensate) or by the injection of water into a piping system containing steam.
Below Figure depicts the four-step sequence that causes condensation-induced water hammer.
(a) As condensing steam loses its heat to the pipe wall and cooler condensate, it changes phase from a vapor to a liquid.
(b) Working together, the condensation and flow of steam produce waves that build until they fill the pipe cross section, trapping steam between their peaks. The trapped steam then condenses rapidly.
(c) Resulting liquid, due to condensation of steam, occupies up to 1,000 times less space than it did as steam. The pressure in the void falls to a much lower level than that of the steam surrounding it. The void then collapses as the water, under steam pressure, rushes in.
(d) The collision of the water and condensate produces a local, pressurized pulse of water that rebounds down the pipe.
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= 2-3 m/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, 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 ahead of any risers. 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.
- Standard start up procedures are required for cold start of the plant. Operators should be trained to open isolation valve slowly during the start-up modes.
- Do not allow steam into any line filled with subcooled condensate or into any cold steam line suspected of containing condensate.
- 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.
- Eccentric reducers should be used against concentric reducers.
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