This blog provides a general discussion on the considerations for selecting pipe insulation thickness for steam pipelines.
Steam is regularly used in industry for the purpose of generating process heat. In general, steam should be generated and distributed at the highest possible pressure and utilized at the lowest possible pressure. There are two main factors which determine pipe sizing in a steam system. The first factor is the initial pressure at the boiler plant and the allowable pressure drop of the complete distribution system. It is said that the pressure drop across the total system shouldn’t exceed 20% of the total maximum pressure at the boiler. This includes all pipe losses, valve, bend, tee losses etc. The second factor is the velocity of the steam. The flow through a given steam line should be within certain design velocity limits meaning distribution pipe sizing requires careful consideration to ensure the system is within these limits. High velocity leads to pipe erosion whereas low velocities tend to increase the amount of condensate and hence system energy losses.
Undersized pipes result in high line velocities which leads to insufficient pressure at the demand point due to higher pressure drops and steam starvation as well as pipe erosion as already noted. Oversized pipes, on the other hand, result in lower line velocities, higher capital installation costs and of course higher heat losses due to the larger surface area of the pipe. The higher heat loss, of course, causes an increase in the amount of condensate which increases in the size of steam traps and condensate return lines. The diameter of the pipe will also have a knock-on effect on the selected diameter of insulation required for the pipeline.
On the topic of pipe insulation, it is recommended that an appropriate insulation thickness should be determined for a given system that would save money. The thicker the insulation on the pipe, the lower the heat loss which saves energy. However, in economic terms, thicker insulation is relatively more costly than a thinner layer and has a higher payback period. Maintenance costs also rise with greater insulation thickness. So, in an ideal scenario, the insulation thickness should be optimised so that the investment on insulation is recovered through the resultant energy savings achieved.
Therefore, there should be an optimization study carried out when deciding on the pipe diameter and insulation thickness.
Insulation of any system requires capital expenditure. As such, from an economic standpoint, the most important factor in any insulation system is to analyse the thermal insulation with respect to cost. The effectiveness of insulation follows the law of decreasing returns. Hence, there is a definite economic limit to the amount of insulation which is justified. An increased thickness is uneconomical and cannot be recovered through small energy savings. This limiting value is often described as an economic thickness of insulation and is illustrated in Figure 1.
A common approach often used is the static economic method for determining minimum cost functions. Annual costs of heat losses are added to annual costs of maintenance and investment in heat insulation. The economic insulation thickness is determined by searching for the minimum cost and an assumption that interest rates remain constant during the service life of the insulation. A dynamic method can also be considered where the rise in annual costs of heat loss due to inflation is considered in spite of what can be a relatively constant heat loss through the insulation. The dynamic method is based on supposed costs which may appear during the service life of the insulation. This method considers inflation effects on producing heat, maintenance etc.
As part of the overall design of the piping system, in addition to the practical operational design requirements, the system requires economic considerations, i.e. the pipe diameters and insulation thickness should be optimised based on the points discussed.
Pipe sizing and the selection of optimum levels of insulation are very important aspects of the design of steam distribution piping systems in order to save energy. Incorrect pipe sizes and insulation can result in heavy losses in terms of steam generated as well as additional fuel and associated costs.
Steam pipes need to be properly insulated with optimum thickness to obtain the maximum performance and energy saving.
It is said that when considering new construction projects, retrofit of existing piping systems and equipment, specifiers and or owner-operators should discard any existing or old tables of insulation thickness and create new tables using current energy cost data. Using the original older insulation thickness tables can result in unnecessary waste of revenue for heating energy.
- Minimizing Energy Loss by Optimizing Pipe Diameter and Insulation Thickness in Steam Distribution Pipelines, SLEMA Journal.