In a domestic environment, water hammer is probably most commonly associated with faucets being turned on or off. In industrial environments, it can occur in a variety of situations, particularly those involving steam systems, which is why it is also known as steam hammer. In principle water/steam hammer could affect a domestic steam shower. But, in practice, these have minimal risks associated with them. In industrial environments, however, steam hammer has the potential to be lethal. It is more than capable of causing injury and can easily lead to expensive damage to piping and housing, particularly at junctions where it could quite feasibly destroy actual valves as well as their flanges and gaskets.
What causes water hammer?
Water hammer is essentially a product of condensation, which is what happens when steam is cooled and turns back into water (albeit often hot water). It generally comes about in one of two main ways.
The tidal effect
Piping without effective insulation can lose a lot of heat. As the temperature lowers inside the pipe, condensation begins to form. This is carried forward by the flow of steam, rather like raindrops in a wind. As time passes the amount of condensation increases until it reaches a volume where it creates a significant impact whenever it encounters an obstacle in its path such as a valve (or a curve in the piping). It’s rather like high winds creating waves which crash against tidal barriers.
The vacuum effect
Like many substances, water expands when heated, hence steam has a much greater volume than water (even hot water). In practical terms this means that steam occupies a much greater amount of space than the equivalent quantity of liquid water. This means that as steam condenses, it creates a vacuum and, as we were all taught at school, nature abhors a vacuum and tries to fill it as soon as possible. If there is cold condensate inside the piping it will be sucked into the vacuum and as streams of concentrate bump into each other, they end up knocking into the piping and creating the hammering effect.
Preventing water hammer
In an ideal world one of the steps taken to avoid water hammer would be to avoid having hot steam and cold condensate in the same pipe. In the real world however, this are a variety of practical reasons why this is currently standard practice in condensate recovery piping (and other systems of a similar nature). In these situations, preventing water hammer is largely a matter of understanding the practical implications of its causes and designing systems which minimize its triggers (or improving existing systems to eliminate or at least reduce places which are known to trigger water hammer).
Water hammer in steam lines is generally due to the tidal effect and the key point to remember is that water hammer only occurs when a line is pitched upwards. If it is flat or pitched downwards, there is no hammer. In a perfect world, piping systems would be designed without upward pitches. In the real world, drip trap stations should be installed every 300 to 500 feet as well as at the end of the main and just before any risers and all regulator valves. Additionally, Y strainers should be mounted in such a way that the screen and dirt pocket are horizontal as this prevents condensation from collecting in them.
Condensate return lines
Water hammer in condensate return lines is usually due to the vacuum effect. There are a variety of solutions available depending on the exact nature of the system. Possibly the best solution is to use correctly-sized gravity return lines. These allow for what could be termed natural separation of condensate and steam since the heavier condensate sinks to the bottom of the pipe while the lighter steam rises to the top. It is also advisable to look at heat exchanges and ensure that they operate in a way which minimizes the likelihood of water hammer and also to look at the use of vacuum breakers.