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:
• Steam Generation through cogeneration applications, boiler
controls, and water treatment;
• Steam Distribution through checking steam leaks, installing
insulation and proper steam trap maintenance;
• Steam End Use through heat exchanger maintenance;
• Steam Recovery through condensate return.
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.
Benchmarking
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.
