Benchmarking steam system efficiency

The second task is to benchmark the flow rates in steam letdown valves. A larger letdown flow than the benchmark value could indicate the need for a driver switch between the motor and steam turbine to minimize letdown flow. However, the driver switch must obey the trade-off between fuel and power cost. The detailed discussions and examples for the above two tasks will be provided in Chapter 19. 

Opened bypass valves, opened steam-bleed valves, steam leaking out of steam traps, visible waste condensate, and system water hammers are the signs that steam consuming equipment are not optimal. An opened bypass valve usually indicates equipment limitation, so operators open bypass valves to compensate. Bleeders are often visible at turbines and in steam jacket tracing applications. This could be a sign that operators may not have confidence in the installed CDL, so they bleed valuable steam to achieve the required operation. 

The fourth task is to benchmark heat recovery across the process areas that are not directly related to steam usage however, process heat recovery could lead to reduced steam usage and increased steam generation using process heat effectively. In most cases, increased process heat recovery results in the largest energy savings. The benchmark for process heat recovery can be determined by pinch analysis, which is discussed in Chapters 9, 10 and 11. 

No matter how much the steam system could be optimized, one relevant and important question is What is the maximal efficiency that the steam system can achieve The answer will be provided by the concept of cogeneration efficiency that is discussed next. 

A steam system configuration implies the number of steam headers and their pressure and temperatures, the number of steam turbine drivers and generators, as well as their operating ranges and efficiency, the connections of turbines with steam 

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For an existing steam system, if the best performance of energy efficiency can be predetermined, this could set a benchmarking basis for the purpose of monitoring and comparison. The performance gap can be readily identified by applying the methods presented in this chapter. 

Benchmarking the fuel cost for steam generation is an important step in managing the steam system. For a given amount of steam production, the total fuel cost depends on boiler efficiency, steam pressure, fuel type and cost, and feedwater temperature. The benchmark can serve as a tracking tool for monitoring the boiler performance. The benchmark calculations are provided (DOE, 2012), which are briefly illustrated next. 

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