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Ask an Energy Expert: Optimizing Industrial Steam Systems

From the Fall 2008 issue of Energy Matters

Ask an Energy Expert

Riyaz Papar, our featured DOE Energy Expert, regularly conducts Save Energy Now energy assessments to improve efficiency of steam systems at industrial plants. In this issue of Energy Matters, Riyaz addresses some common questions that arise during assessments.

How do I identify steam energy saving opportunities in my plant?

This is a very simple question but the answer can be difficult to comprehend and implement, if not done in a systematic manner.

First and foremost, one must take a systems rather than a component-based approach to evaluate any steam system. DOE's Industrial Technologies Program (ITP) divides a steam system into four areas: generation, distribution, end-use, and recovery. Every industrial system will include one or more of the above four areas. All the four areas work together as a system, and modifying operations in any one area will have an impact both downstream and upstream of that area. Hence, it is very important to use a systems approach for identifying energy-saving opportunities in steam systems.

When I visit a plant, I use DOE's Steam System Tool Suite to evaluate and identifying steam system improvements. The Steam System Scoping Tool (SSST) quickly gives me an overall understanding of the level of best practices at the plant. This identifies potential areas of energy-saving opportunities on a qualitative basis. Next, I use the Steam System Assessment Tool (SSAT) to model the steam system at the plant. This is the detailed model which allows me to perform a "what-if" system-level analysis for different projects that I select in the SSAT. The SSAT provides me with quantified energy-saving opportunities for projects. This first level due-diligence can then lead to further project development on select energy-saving opportunities. Lastly, I also use the 3EPlus® insulation tool to get more specific on energy-saving insulation-related opportunities in the steam system. Learn more about how these steam software tools work (PDF 1.3 MB). Download Adobe Reader.

Currently, DOE is working together with industry experts, the American Society of Mechanical Engineers, and the American National Standards Institute to develop energy assessment standards. Watch the ITP Web Site for news on their progress, reviews, and release.

How do I calculate my cost of steam production ($/klb)?

This question comes up very often during energy assessments. Calculating the cost of steam production can be a very challenging task in a plant. Nevertheless, it is the first thing an energy engineer should do.

There are several factors that directly impact steam production cost, including:

  • Energy (fuel) cost
  • Water cost (includes chemical treatment cost)
  • Electric utility cost for fans, controls, etc.
  • Equipment maintenance cost
  • Emissions control / mitigation cost
  • Labor cost
  • Equipment capital cost
  • Insurance cost

In almost all circumstances, we should be concerned with the marginal or variable cost of steam rather than the fixed cost. The marginal cost truly represents the actual energy cost savings opportunities for projects that may be undertaken at a plant. For our discussion here, we will use the fuel and water (including chemical treatment) costs to calculate the steam production cost. This logic can be extended to include the other cost factors as listed above.

Let us start with the data required to calculate our cost of steam production:

  • Time period: This is the time period over which the average cost of steam production is required. It can be an hour, a day, a month or a year. (8,760 hours)
  • Average steam production: Typically from a steam flowmeter profile over the time period.(80,000 lb/hr)
  • Average fuel consumed: Typically from a fuel flowmeter profile over the time period. (100 Mcf/hr)
  • Fuel cost: Average cost of fuel over the time period. ($10 per Mcf)
  • Average water usage: Typically from a water flowmeter profile over the time period. (50 gpm)
  • Water cost: Average cost of water (including chemical treatment) over the time period. ($2.50 per kgal)

This figure consists of three formulas. The first formula reads like this: Steam cost equals Total cost of fuel and water divided by the Total steam produced. The second formula reads like this: Steam cost equals (in parentheses) 100 times 8,760 times 10, plus (in parentheses) 50 times 60 times 8,760 times 2.50 divided by 1,000. This total represents the dollar figure. This dollar figure is divided by (in parentheses) 80,000 divided by 1,000 times 8,760 (thousand pounds). The third formula reads like this: Steam cost equals 12.59 per thousand pounds of steam.

This is the most direct method to calculate the steam cost. In the event certain parameters are not available, other methods (for example, boiler efficiency) can be used to calculate the steam production costs. We will discuss those in a future column on steam system efficiency.

Is power generated from a steam back pressure turbine free, since I am using the exhaust steam in my process anyway?

This is a very interesting question and I will explain the premise using the figures below.

This figure is divided into two parts by a vertical line between two drawings. The one on the left consists of two triangles meeting tip to tip. Above the triangle figure are the words "Steam inlet" next to an arrow pointing down, with P1 and T1 next to the arrow. Underneath the triangle figure is an arrow pointing down, with the words "Steam outlet" and the letters P2 and T2. In a box underneath this triangle shape are the following statements: P1 is greater than P2; T1 is greater than T2; and H1 equals H2. The drawing on the right is of a trapezoid with the parallel sides running vertically. Above the trapezoid are the words "Steam inlet" with an arrow pointing down next to the letters P1 and T1. To the right of the trapezoid are the words "Steam horsepower turbine speed" next to a rod that appears to be running through the length of the trapezoid. An arrow indicates that the rod turns counter-clockwise. Underneath the trapezoid are the words "Steam outlet" with an arrow pointing down next to the letters P2 and T2. In a box underneath this drawing are the following statements: P1 is greater than P2; T1 is greater than T2; H1 is greater than H2.

In a typical steam system, a Pressure Reducing Valve (PRV) reduces the steam pressure from P1 to P2. This pressure reduction happens in such a manner that the total energy content (enthalpy – Btu/lb) does not change (H1 = H2) and no shaft work is done.

On the other hand, when steam goes through a steam turbine, it expands and the steam pressure reduces from P1 to P2. The steam turbine produces shaft horsepower and as a result, the steam exit energy content (enthalpy – Btu/lb) is lower when compared to the PRV case.

Steam is used for heating purposes in the plant. The process heat duty (Btu/hr) is fixed by the plant demand. Since the steam supplied to the process has a lower enthalpy, an additional amount of steam (lb/hr) is required to ensure the same available heat duty. This additional amount of steam has an associated cost. Hence, power generated from a backpressure steam turbine is not free.

Nevertheless, using a backpressure steam turbine can improve the overall plant and global energy efficiency and more importantly, it can reduce total operating costs. Hence, it can be a very good energy-saving opportunity. In a future column on steam systems, we shall investigate the factors that impact steam turbine cost effectiveness.

Additional Resources

For more information on how to improve your plant's steam system efficiency, please see the following ITP resources.

Questions/comments about this column? Contact Riyaz Papar at rpapar@hudsontech.com.

Photo of Riyaz Papar.

Mr. Papar is Director of Energy Assets and Optimization at Hudson Technologies, and also serves as an energy consultant, steam end-user training instructor, and Qualified Specialist for the U.S. Department of Energy. He is a Registered Professional Engineer with more than 15 years of experience in industrial energy infrastructure and energy asset management of process and utility systems in refineries, chemical plants, and manufacturing facilities. He specializes in performance monitoring and optimization of energy systems. Mr. Papar received his Bachelors degree in Mechanical Engineering from the Indian Institute of Technology, and his Masters degree from the University of Maryland.

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