Waterhammer: How much do you know?
As soon as steam leaves the boiler, condensate takes place in the pipe due to heat loss. This situation is even more intense during the initial supply of steam and when the steam is cold.
Figure 1 demonstrates how tiny droplets of condensate can build up with increasing length of the pipe and eventually, it becomes a mass of liquid (solid slug) carried along at high speed.
When the slug encounters any obstacle such as a change in the pipe direction, it will stop immediately. The kinetic energy of the high speed condensate is suddenly converted into pressure energy which has to be absorbed by the pipework. If the speed is very high or there is a lot of weight, the amount of energy output may be strong enough to crack pipes and rupture the fittings. Even if there is not much speed and weight, the noise created in the system by the impact can be a severe nuisance.
The incidence of waterhammer will be encouraged if pockets of condensate are allowed to build up in low points of the steam system. Common sources of trouble are sags in the pipework and incorrect use of concentric reducers as shown in Figure 2. If the eccentric reducer is installed correctly as shown in Figure 3, collection of condensate will not be allowed. Even a strainer fitted as in Figure 4 is a potential cause of waterhammer. It is better to fit strainers on their sides in steam lines and will prevent the formation of condensate which can be picked up by fast moving steam.
To reduce the possibility of waterhammer, steam lines should be arranged with a gradual fall in the direction of flow and drain points installed at regular intervals and at all low points. Check valves should be fitted after any traps which would otherwise allow condensate to run back into the steam line or equipment during shut down.
Waterhammer can occur in submerged steam heating coils commonly found in tanks and vats. Although the coils do not have the long uninterrupted lengths of steam mains or space heating pipes, incoming steam still can be condensed very quickly. This results in a large weight of water being carried forward by steam which has a fairly high speed due to the heavy condensate rate. If condensate is lifted to the trap, it is important that the coil falls continuously along its length and is fitted with a loop seal and small bore riser.
Difficulties arise when there is insufficient pressure available at the trap inlet if condensate has to be lifted after the steam trap. The equipment will then waterlog and waterhammer may happen when the steam pressure builds up again. This is especially the case with equipment fitted with automatic temperature controls. The best arrangement in such cases is to drain the condensate by gravity to a vented receiver and use a pump to lift to a higher level.
It is recommended to fit steam traps which have a strong resistance such as the thermodynamic or bucket types wherever there is a risk of waterhammer.
Content provided by Spirax Sarco, originally published in Steam News Magazine.