Treat steam with respect!
Published: 10 September, 2015
Steam is a very versatile and efficient carrier of energy and it is used in a wide range of industrial processes. It is intrinsically safe and frequently used in food and pharmaceutical production; however, it must be treated with respect. The apparatus used to convey and control steam must be properly designed and maintained to avoid energy losses as well as damage to boilers and other plant by water hammer, which can be particularly destructive when left unchecked. Ian Webster, hygienic processing field segment manager at Bürkert UK and Ireland looks at this condition and how it should be avoided.
Steam has many positive attributes; it is readily available, inexpensive, clean and easy to control. Providing the system used to transport it around the facility is properly designed, steam can deliver an efficient and reliable source of energy to a vast number of applications. However, the ability of steam to store so much energy also makes it very dangerous and in some cases destructive. The most obvious sign of a potential problem within a steam system is water hammer, a condition which should not be tolerated.
The effects of water hammer can cause physical damage to equipment as well as injury to plant personnel. In fact, nearly two thirds of premature process component failures can be attributed to the effects of water hammer, so effective control of this condition should be implemented to avoid unnecessary maintenance costs in the future. In order to control it, it is first necessary to understand the causes and therefore the remedies.
What is water hammer in steam systems?
Heat loss from the steam transport piping causes condensate to form and the velocity of the steam flowing over the condensate causes ripples in the water. Turbulence builds up until the water forms a solid mass, or slug, filling the pipe. This slug of condensate can travel at the same speed as the steam and will strike the first elbow or valve in its path with a force comparable to a hammer blow. In fact, the force can be great enough to break the back of an elbow joint.
In addition, water hammer can be caused by the sudden condensation of steam which reduces its specific volume by more than 1000 times, so when steam comes into contact with colder condensate and condenses, its volume is instantly reduced to next to nothing. This sudden reduction in volume causes the condensate inside the pipework to surge towards this point. When the converging walls of condensate crash into each other the resulting noise is called water hammer.
This should not be confused with hydraulic water hammer, which occurs in pumped feed water and pumped condensate lines when a valve suddenly closes and abruptly stops the flow. The result is vibration in the pipework, which, if it occurs on a repetitive basis, can cause severe damage to the pipework.
The key to resolving these issues is to design and maintain a good condensate management regime and eliminate water hammer. As a general rule of thumb, pipe sizes are typically calculated using 25m/sec as the reference speed. This forms a crucial part of the design, to reduce the impact of water hammer, pitting and wear.
Professional design tips
Even with the most efficient lagging, condensate will form in steam pipework, due to radiated losses to the surrounding air. It is recommended to fit a drain pocket at regular intervals of 30 – 50m and at the base of a lift. Fitting a drain pocket and trap set before an on/off valve or control valve will remove the risk of trapped condensate being released at high velocities, which can result in valve damage and water hammer. Y-type strainers are needed to protect expensive and process-essential equipment from damage and faults due to debris in the steam. However, installed incorrectly they are a potential source of condensate pooling and therefore, water hammer. When installed in the steam line, Y-type strainers should be installed NOT in the 'belly-down' position, but with the "belly" of the strainer in the horizontal plane – an issue Bürkert has seen on several occasions while fault finding on systems it has been called in to assess.
It may be necessary to reduce the pipe diameter as part of the steam system design. On steam systems, it is essential that eccentric reducers are used, rather than concentric parts, with the flat side at the bottom. Concentric reducers have a funnel-like profile and are sometimes installed by less informed Engineers or when costs are being cut; however this will lead to pooling of condensate and can be a prime source of water hammer.
As a general rule, when installing a drain pocket, it should be of the same pipe diameter as the steam main – forming "equal T's", up to a size of DN100. If a smaller bore drain pipe is fitted then the velocity of the condensate will allow it to skip over the drain pipe connection. In addition, the reduced volume of the drain pipe may cause it to overflow, thereby becoming another source of water hammer.
Qualified and experienced personnel
For any company which operates a steam system, it is essential that it employs personnel with the proper training and if necessary uses qualified contractors to design and install any modifications to an existing system. The consequences of operating a poorly maintained steam process can be extremely costly and in the same way making a poorly designed addition to a satisfactory system can also lead to similar problems.
For further information please visit: www.burkert.co.uk