Best Practices - Steam
Steam is prevalent in many manufacturing facilities. According to the U.S. DOE Office of Industrial Technologies Best Practices, “Over 45% of all the fuel burned by U.S. manufacturers is consumed to raise steam.” Steam is used for process heating, pressure control, mechanical drive and space heating, among other applications. Within a plant, steam is utilized as a third utility like electricity and gas. Unlike traditional utilities, the cost of steam is not typically measured or tracked. A typical industrial facility can realize steam savings of 20% by improving its steam system.
The presentation and workshop materials developed as part of the Illinois IOF program are intended to motivate steam system operators to scrutinize their plants for energy saving opportunities and then provide the equations and tools necessary to quantify the energy and cost savings potential.
Steam System Characteristics
The whole system must be considered for improvements to best pursue reducing operating costs. Upstream inefficiencies will affect process heating and cost of producing steam, while downstream inefficiencies (leaks, bad traps, poor load control) can also affect process heating and have severe effects on the boiler and cost of producing steam. Example opportunities for savings are found in:
Losses in each of these system components can add up to significant inefficiencies.
As shown in Figure 1a, an inefficiency system can be only 40% effective
in delivering the energy from the primary fuel (natural gas in most cases)
to the process users. Minimizing losses in the boiler (combustion and
cycling losses) and distribution systems (traps and leaks) can increase
this total system efficiency to 70% as shown in Figure 1b.
Figure 1a. Energy Loses in an Inefficient Steam System
Figure 1b. Energy Loses in an Efficient Steam System
Steps to Improving System Efficiency
Steam system operators generally improve system efficiencies by addressing issues such as boiler combustion efficiency, steam leaks and faulty steam traps. However, these activities are generally performed without a true sense of the potential savings and direct impacts. In order to motivate these and more capital-intensive projects, operators should first determine the cost of producing steam through a benchmarking exercise.
First, a review of natural gas utility bills for the previous 12 to 24 months should be able to separate seasonal heating loads from the baseline process boiler usage. This may be difficult as some process loads, including outdoor tank heating and pipe tracing, will require additional steam during winter periods. Then, operators should determine the actual cost of steam in terms of $/thousand lbs of steam. This cost includes not only boiler fuel, but water costs, chemical treatment costs and condensate pump energy. For a typical system, total costs can be approximated using the equation:
Total Steam Cost ($/MMBtu) = Fuel Cost ($/MMBtu) x 130%
Knowing the operating steam pressure (psig), this number can be converted into $/thousand lbs by multiplying by the enthalpy of vaporization found from steam tables in engineering reference books or online.
A more thorough calculation of the cost of steam requires
an extensive analysis of a steam system including condensate return rates,
blowdown practices, etc. The U.S. DOE OIT Best Practices Program publishes
the authoritative resource for this exploration called the Steam
System Assessment Tool.
Illinois IOF Workshop Materials
US DOE Resources