How Efficient Is Your Steam Distribution System?

Frederic A. Hooper, Jr. and Ronald D. Gillette

There's an old management saying, "If you don't measure it, you don't get it." Nobody measures steam distribution system efficiency because there is no generally accepted method. So your system's efficiency is probably low - not the 70-90% of the boiler, but instead the 30-60% of a neglected system. This paper proposes how to measure it.

Steam distribution system efficiency percentage

The purpose of a steam distribution system is to deliver energy from the boiler to where it is needed. A perfect steam distribution system would deliver it without any delivery losses. This would require perfect piping insulation, perfect steam traps and perfect condensate recovery systems, with no maintenance needed.

Real steam distribution systems are built of components that decay, and are maintained by human beings. Real systems have losses from less-than-perfect insulation, leaking steam traps, flash steam and dumped condensate. Maintenance is expensive and may be delayed by lack of funds or personnel. The delivery loss of a real steam distribution system is much higher than the zero delivery loss of a perfect system.

Having described an "ideal" and an "actual" value, we could find the system efficiency by dividing the energy needed by the energy supplied. Notice this is "ideal ÷ actual", the reverse of most percentage calculations. This reversal is necessary because we are measuring what we provide, not what we get.

We also have difficulty finding the "ideal" value because its calculation is complex. A computer model of the steam distribution system will make it possible to calculate system efficiency from the easier-to-obtain numbers in the model. This computer model will need to calculate some values so they can be compared with actual values for calibration. It should also use currency, as this is the only common basis for comparison across a complex system.

Modeling the steam distribution system

Condensate flowing through steam traps represents the energy leaving the distribution system. The computer model can calculate the amount of energy that is leaving by combining the condensate flow and steam pressure at the traps. Classifying the steam traps into "working" and "drip" traps separates those using energy from those transporting energy.

Energy needed: Condensate from the working traps represents the energy the steam system was built to provide. Estimating their condensate flow is the first step towards building the necessary model. By combining the condensate flow and steam pressure at the working traps, the model can calculate the energy provided by a perfect system. For systems that deliver to customers, the energy in delivered steam must be added to the total of the working traps' energy.

Energy transport: Drip traps' condensate flow results from less-than-perfect piping insulation. Combining the condensate flow and steam pressure at the traps in the same manner as above provides the energy transportation cost of the system. The model can also calculate the energy loss from less-than-perfect piping insulation. This second measure of energy loss provides calibration of this portion of the model.

Lost condensate: When condensate is lost from the system it must be replaced. If it were not lost, it would be returned to the boiler still heated. If it is lost, its components - water, heat and chemicals - must all be replaced.

Flash steam: Steam is lost from the distribution system through flash steam and leaking steam traps. Flash steam occurs when the condensate from a steam trap is above the boiling point. Without a flash recovery system in place, the flash steam will be lost.

Steam traps: Leaking steam traps are a major source of energy and condensate losses in most distribution systems. As these losses are hidden, many companies don't maintain their steam traps. Those that attempt to maintain them typically do so with annual test-and-replace, so the average leaking trap has been leaking for six months before it is found. More sophisticated maintenance methods are rarely employed at present. The model should include various steam trap maintenance methods with their labor and parts costs as well as lost steam.

Condensate recovery system: Steam from leaking traps quickly overwhelms any flash recovery systems and leaves the distribution system. However, it does not do so constantly. Non-condensable gases are mixed with the steam through vents and become mixed with the condensing condensate. If the condensate is later heated to boiling by leaking traps, the gases are released and cause the condensate piping to deteriorate much more rapidly than it should. Also, if live steam reaches condensate pumps they need more frequent and extensive maintenance.

A steam system modeling program: Steam$$™

Steam$$ is a program that models a steam distribution system, calculates a value for system efficiency and allows calibrating the model. Using limited information, Steam$$ calculates the steam distribution system's actual cost from the losses discussed above. It also calculates the system's ideal cost, divides the two numbers, and gives the percent result. This accurately represents the steam distribution system. In addition to efficiency measures, it provides figures for calculating the benefit (and some costs) of improvements to the steam system.

Conclusion

The calculation of steam distribution system efficiency is more complex than calculating boiler efficiency but is straightforward with a computer model. An additional benefit of this calculation is that it helps evaluate possible system improvements. Finally, comparing steam distribution system efficiency across companies and industries would provide an important competitive benchmark.

Trapo home page, More information, Download files, FAQs, Prices, Links