Water Hammer: The Mechanism
Water hammer generated in steam and condensate recovery systems is usually classified into two main categories:
- caused by high-speed condensate slaming into piping, etc.
- caused by the sudden condensation of steam, which produces walls of condensate that crash into each other
Water Hammer caused by high-speed condensate
Radiant heat loss causes condensate to form inside steam transport piping. Steam flowing at high speeds within this piping draws this condensate forward and causes ripples. From this turbulence, slugs of condensate gradually begin to form and are carried along with the steam. This is similar to the high waves formed by very strong wind.
In this case, water hammer occurs when these slugs of condensate strike a curve or valve as they travel through the piping.
Water Hammer caused by the sudden condensation of steam
When steam loses its heat, it turns into condensate, whose specific volume is more than 1000 times smaller than that of steam. So when steam comes into contact with colder condensate and condenses; its volume is instantly reduced to next to nothing.
During the condensation process, the space occupied by the steam momentarily becomes a vacuum and the condensate inside the piping surges toward this vacuum. This is the second form of water hammer, which occurs when these surging walls of condensate crash into each other.
In short, it is dangerous for piping to contain a mixture of cold condensate and steam. This is the norm, however, in condensate recovery piping and similar systems, which makes this form of water hammer difficult to resolve.
Note that this type of water hammer isn't limited to condensate recovery systems, but also occurs in steam distribution lines and steam-using equipment.
Powerful impacts can occur in both of the above-mentioned types of water hammer; however, these impacts occur with much greater frequency in the second case.
How does condensate temperature affect water hammer?
Previously, it was believed that the lower the temperature of the condensate, the greater the resulting water hammer. However, experiments carried out at TLV revealed a surprising fact. It was discovered that the most severe impacts from water hammer occur when the condensate is at a temperature only slightly lower than that of the steam.
More specifically, at a steam temperature of 100 ºC, it was found that condensate between 70 ºC and 80 ºC caused water hammer on a larger scale than condensate between 50 ºC and 60 ºC.
In fact, the impact caused by water hammer can be mathematically calculated, and the results of such calculations show a strong relationship between the intensity of the water hammer and the volume of the condensing steam (= called ‘pockets of steam’).
Taking a closer look at the graph, three zones of condensate temperatures can be identified:
- On the left side of the graph, steam comes into contact with cold condensate and immediately condenses. In this case, condensation happens on the scale of tiny steam bubbles and large 'pockets of steam' cannot form, hence only small waterhammer occurs.
- The middle section is of greater concern. Due to the relatively small temperature difference of 20-30°C between the condensate and steam, the steam does not condense all at once, but gradually. As the condensation process slowly occurs, it will reach a point where suddenly all the steam condenses. The delay created between the time the steam comes into contact with condensate and the time it suddenly condenses is what allows the formation of bigger pockets of steam, and hence bigger water hammer.
- On the right side of the graph, steam comes into contact with condensate of the same temperature. In this case, it does not instantly condense and water hammer does not occur. This can be confirmed from the fact that water hammer does not appear right at the outlet of a steam trap where saturated condensate coexists with flash steam of the same temperature.
We know that condensate between 70ºC and 80 ºC causes an increase in size of the 'pockets of steam' and with this the most severe water hammer. So what triggers the process? Find out in Water Hammer: Cause and Location.
