Dealing with thermal movement in riser pipes
Supporting pipe risers subject to thermal expansion and contraction in multistorey buildings can present major challenges for design engineers. The traditional methods for allowing and supporting movement are expansion loops or expansion joints. Here, Adam Fox, director at building services and plant engineering specialist Mason UK, argues that these two methods have major drawbacks for vertical pipe runs and that supporting the pipes on springs is a far more effective solution to the problem presented by thermal movement.
All pipes expand when they get hotter and shrink when they get colder. The rates of expansion vary depending on the material. For example, the rate of expansion in stainless steel is approximately 10mm/10m/100 degrees Fahrenheit. In other words, if you have a stainless steel pipe that is ten metres long and it was 70 degrees Fahrenheit, raise the temperature by 100 degrees and the pipe would grow approximately 10mm longer. For plastic pipes, the movement is tremendous in comparison – an additional 86mm in the scenario above.
On the face of it, these might seem like small amounts, but the problems created by this movement should not be underestimated. Even one tenth of a millimetre in change can lift a pipe off from a support, meaning that support is no longer doing its job. If a pipe is clamped and therefore cannot move, you introduce the risk of buckling and pipe failure, potentially leading to flooding and enormous costs.
Given you cannot prevent expansion of the pipe, how do you deal with this problem? Firstly, it is important to adopt a holistic approach. By that I mean you need a concerted design effort that looks at the entire pipe system or pipe run. The two traditional approaches to this problem are a system of expansion loops, or a system of expansion joints. I’ll explain briefly how they work, before telling you why they provide a suboptimal solution at best.
Expansion loops and expansion joints
Figure A shows a system of expansion loops, supported by anchors either side of the loop. During expansion, the loop bends and stresses the pipe, but it accepts the movement. Figure B depicts a system of expansion joints. Here you locate bellows between each anchor point that act as an expansion compensator, absorbing the forces generated by thermal expansion.
The system of expansion loops has multiple drawbacks. Firstly, once the pipe goes into or out of the riser chase and into the building, it is occupying valuable, rentable space. Secondly, each loop has four elbows and two additional vertical runs that are adding resistance to flow. This means you require a higher horsepower pump and greater energy consumption to deliver the same volume of water.
The expansion joints may allow the engineer to keep the riser straight, but potential failure becomes a major issue. If an expansion joint fails, it means not only the loss of heating or cooling, but a high possibility of extensive water or steam damage. The expansion joints must also remain accessible for periodic inspections; something that is not always possible. Finally, both these systems require multiple anchor points which present the engineer with a difficult task, as the load at each point is indeterminate.
The alternative is springs
As a supplier of expansion joints, you might find it strange that we would question their efficacy. However, while expansion joints are ideal for horizontal pipe runs in many applications, for the scenario described in this article there is a superior alternative. Supporting the pipe run on springs is both more affordable and more effective. I will try and illustrate the basic concept here, using the scenario of a vertical pipe run in a multistorey building. However, the same concept can also be applied in horizontal pipe runs where appropriate.
The spring support system typically uses a single anchor — located as close as possible to the middle of the riser — to direct the pipe to expand away or contract toward the anchor point. Alternatively, the system can be designed as totally free-standing and without any anchor point, but control is far more difficult to achieve in these circumstances.
The number of spring mounts may vary, depending on the desired load distribution. Importantly, you can calculate the load at each support point. The engineer will know the load at the installation phase, when empty, when full, and when operating at different temperature extremes. This allows you to calculate which type of spring mount to select, and precompress it to the correct level during the installation phase.
The spring mounts are precompressed to a carefully calculated ”initial deflection” rating, to resist the anticipated load when water enters the system. It is during this stage of the installation, where the pipe is empty, that the anchor has to resist the most force, the uplift force presented by the springs. When the pipe is full and the system is operating, the load on the anchor point is neutral, as the spring forces pushing up, and the combined weight of the water and pipe pushing down, negate each other.
If the pipe were to expand, the spring mounts above the anchor lose deflection while those below would gain deflection. Again, the system remains in balance with the load at the anchor zero. The system also performs perfectly from an acoustic point of view, as the springs effectively absorb any vibration. As the anchor is neutral during operation, this means it does not operate as a significant vibration transmission path.
If you read any of the literature online about dealing with expansion in pipe systems, you would be led to believe that your only options were either expansion loops or expansion joints. Both these systems have major drawbacks and for scenarios like the one described in this article, they will soon become obsolete. There is an alternative system, which involves supporting the pipe on a system of springs. This system was developed by Mason Industries and is common in many high-rise buildings in the US, Abu Dhabi and Dubai. Hopefully, engineers in the UK will soon see the benefits too.