Steam System Survey Guide: Improving Industrial Steam System Efficiency, Study notes of Designs and Groups

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ORNL/TM-2001/

Steam System Survey Guide

Greg Harrell, Ph.D., P.E.

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iii

CONTENTS

Page LIST OF FIGURES ................................................................................................................... v LIST OF TABLES..................................................................................................................... vii NOMENCLATURE .................................................................................................................. ix

iv

REFERENCES .......................................................................................................................... R-

Appendix A. STEAM PROPERTIES........................................................................................ A- Appendix B. STACK LOSS TABLES ...................................................................................... B-

vi

vii

LIST OF TABLES

• ABSTRACT............................................................................................................................... 1- ACKNOWLEDGMENTS xi
• 1. INTRODUCTION............................................................................................................... 1-
• 2. PROFILING THE STEAM SYSTEM 2-
  • 2.1 OVERVIEW AND GENERAL PRINCIPLES 2-
  • 2.2 UTILITY COSTS....................................................................................................... 2-
  • 2.3 BENCHMARKS 2-
  • 2.4 CALL TO ACTION—STEAM SYSTEM PROFILING 2-
• 3. IDENTIFYING STEAM PROPERTIES FOR THE SYSTEM 3-
  • 3.1 OVERVIEW AND GENERAL PRINCIPLES 3-
  • 3.2 CALL TO ACTION—IDENTIFYING STEAM SYSTEM PROPERTIES.............. 3-
• 4. OPPORTUNITIES FOR BOILER EFFICIENCY IMPROVEMENT................................ 4-
  • 4.1 OVERVIEW AND GENERAL PRINCIPLES 4-
  • 4.2 BOILER EFFICIENCY 4-
    • 4.2.1 Example Boiler.............................................................................................. 4-
    • 4.2.2 Economics of Boiler Efficiency Improvement.............................................. 4-
    • 4.2.3 Stack Losses 4-
    • 4.2.4 Blowdown Loss............................................................................................. 4-
    • 4.2.5 Shell Loss...................................................................................................... 4-
    • 4.2.6 Unburned Fuel Loss 4-
    • 4.2.7 Boiler Efficiency Summary........................................................................... 4-
  • 4.3 BOILER LOADING 4-
  • 4.4 BOILER FUEL FLOW ESTIMATE 4-
  • 4.5 CALL TO ACTION—BOILER EFFICIENCY......................................................... 4-
• 5. EFFECTIVENESS OF RESOURCE UTILIZATION 5-
  • 5.1 OVERVIEW AND GENERAL PRINCIPLES 5-
  • 5.2 FUEL SELECTION 5-
    • 5.2.1 Fuel Switch Example 5-
  • 5.3 SYSTEM BALANCING............................................................................................ 5-
    • 5.3.1 Vent Steam 5-
    • 5.3.2 Combined Heat and Power............................................................................ 5-
  • 5.4 PROCESS INTEGRATION....................................................................................... 5-
  • 5.5 STEAM SYSTEM PRESSURE................................................................................. 5-
  • 5.6 CALL TO ACTION—EFFECTIVENESS OF RESOURCE UTILIZATION 5-
• 6. STEAM DISTRIBUTION SYSTEM LOSSES 6-
  • 6.1 OVERVIEW AND GENERAL PRINCIPLES 6-
  • 6.2 STEAM LEAKS 6-
    • 6.2.1 Pipe Failures.................................................................................................. 6-
    • 6.2.2 Steam Traps................................................................................................... 6-
  • 6.3 INSULATION............................................................................................................ 6-
  • 6.4 CONDENSATE RECOVERY................................................................................... 6-
  • 6.5 FLASH STEAM RECOVERY 6-
  • 6.6 CALL TO ACTION—DISTRIBUTION SYSTEM LOSSES 6-
    • 1 Typical fuel properties 2- Table Page
    • 2 Steam properties 3-
    • 3 Flue gas oxygen content control parameters............................................................ 4-
    • 4 Stack loss of No. 2 fuel oil....................................................................................... 4-
    • 5 Boiler water chemical limits 4-
    • 6 Steam leak rates 6-
    • 7 Pipe surface heat transfer 6-
• A.1 Saturated vapor properties—pressure A-
• A.2 Saturated vapor properties—temperature A-
• A.3 Saturated liquid properties—pressure...................................................................... A-
• A.4 Saturated liquid properties—temperature A-
• A.5 Superheated steam properties A-
• A.6 Subcooled liquid properties A-
  • B.1 Natural gas stack loss............................................................................................... B-
  • B.2 No. 2 Fuel oil stack loss........................................................................................... B-
  • B.3 No. 6 Fuel oil stack loss........................................................................................... B-
  • B.4 Typical bituminous coal stack loss B-

ix

NOMENCLATURE

The following is a list of symbols used throughout this text.

A flow area E^ & energy flow rate HHV fuel higher heating value h enthalpy K^ & operating cost m^ & mass flow rate P pressure Q^ & heat transfer rate T operating period V^ & volume flow rate W^ & power X thermodynamic quality (mass basis) η efficiency κ energy unit cost ρ density σ savings Λ loss rate λ loss φ factor or ratio

x

xii

STEAM SYSTEM SURVEY GUIDE

Greg Harrell, Ph.D., P.E.

ABSTRACT

This Steam System Survey Guide provides technical information for steam system opera- tional personnel and plant energy managers on some of the major opportunities available to improve the energy efficiency and productivity of industrial steam systems. The guide covers five main areas of investigation: (1) profiling a steam system, (2) identifying steam properties for the steam system, (3) improving boiler operations, (4) improving resource utilization in the steam system, and (5) investigating energy losses in the steam distribution system. The guide discusses major areas where steam systems can be improved and outlines calculations that can be per- formed to quantify steam system improvement opportunities.

1. INTRODUCTION

This Steam System Survey Guide is intended for steam system operational personnel and plant energy managers. Often operations personnel and energy managers are unaware of the opportunities available for energy and productivity savings in their steam systems, or they are unsure of the calculation procedures required to determine the savings opportunities. The purpose of this guide is to assist operations personnel and energy managers in identifying significant opportunities to improve their steam systems. The Steam System Survey Guide does not attempt to guide steam system users in the imple- mentation phase of improvement projects. In some cases, improvements may be simple to make, but others will require the assistance of qualified steam system experts. However, if the guidance of this document is followed, many possible opportunities for improving the steam system should be identified. These guidelines are organized to assist steam users to take the following steps in identifying opportunities to improve their steam systems:

• First, the analysis basis must be determined; guidelines are provided for profiling individual steam systems. Methods are presented to estimate the fuel costs and operating characteristics of the facility and to identify improvements in energy efficiency that translate to operational cost savings.
• Second, the steam properties of the facility are identified to allow calculations to be performed in latter sections of the analysis.
• Third, the boiler operation is investigated. This analysis centers on evaluating the fuel-to- steam conversion efficiency of the boiler.
• The fourth analysis area is concerned with resource utilization throughout the facility. The main concerns in this area are to use the most appropriate fuel, to maintain the proper steam balance throughout the system, and to integrate process energy.

2. PROFILING THE STEAM SYSTEM

2.1 OVERVIEW AND GENERAL PRINCIPLES

In general, operational changes are based on economic factors. Thus, the economics of the steam system should be determined. The main factors in this evaluation are associated with the fuel supplied to the boilers. The total cost of fuel supplied to the boilers will provide an order of magnitude of the economic potential associated with a proposed operational change. The unit cost of fuel is also important in the evaluation of system performance and operational changes. From the standpoint of managing the steam system, more measurements will allow more informed management. The energy management principle “You cannot manage what you do not measure” holds true.

2.2 UTILITY COSTS

The total cost of fuel supplied to the facility should be determined. Typically, this value is known from fuel invoices. This is a very important value from an operations and analysis stand- point. As boiler and steam system efficiency improves, the amount of fuel purchased decreases for a given steam production. This becomes the justification for any economic investment. Cau- tion should be exercised to include only fuel supplied to the boilers. Many facilities have only one fuel metering device, and fuel may be used in process equipment or in heating and air condition- ing equipment. If the amount of fuel supplied to the boilers is not metered, the fuel consumption can be estimated. The methods used in the estimation process will be discussed in Sect. 4.4, “Boiler Fuel Flow Estimate.” The time that the boilers operate also needs to be determined to allow savings and cost evaluations to be based on the appropriate operating hours. Many facilities operate 24 h/d and 365 d/year. The operating hours, T , for this facility would be calculated as follows.

T = 24 h/d (365 d/year) = 8760 h/year. (1)

A determination of fuel cost is essential for the efficient management of a steam system. Gaseous fuels are typically sold in units of 1000 standard cubic feet (i.e., 10 3 std ft 3 , 10 3 ft 3 , 1000 scf, and Mcf). Gaseous fuel pricing is also provided based on 100 standard cubic feet (Ccf). Fuel oils are typically sold in terms of gallons, while coal is sold primarily based on tons. A steam system survey investigates the use of energy throughout the steam system. Therefore, it is beneficial to determine the fuel cost on an energy basis. To accomplish this, some properties of the fuel must be known. The main property required is the fuel energy content that is termed the “fuel heating value.” In the United States, the higher heating value (HHV) is commonly used; in Europe and many other parts of the world, the lower heating value is used. The difference in the values is in the fuel analysis and, in particular, the state of the water involved in the combustion process. This guide will use the fuel HHV for all calculations. Fuel heating value and the fuel sales price are used to determine the fuel unit cost. Three examples of this calculation are provided below. The first example is for natural gas with a purchase price of $7.00/10^3 ft^3. The example natural gas has an HHV of 987,124 Btu/10 3 ft 3 (23,000 Btu/lbm). This results in a fuel cost, κnatural gas, of $7.09/10^6 Btu.

κnatural gas = $7.00/10 3 ft^3

10 3 ft 987,124 Btu

1,000,000 Btu 10 Btu

3 6

F

HG^

I

KJ^

= $7.09/10 6 Btu. (2)

The next example determines the fuel cost, κNo. 2 , for No. 2 fuel oil with a purchase price of $1.00/gal. The example No. 2 fuel oil has an HHV of 139,874 Btu/gal.

κNo. 2 = $1.00/gal

1 gal 139,874 Btu

1,000,000 Btu 10 Btu^6

F

HG^

I

KJ^

= $7.15/10 6 Btu. (3)

The final example is for coal that is purchased with a price of $50.00/ton and an HHV of 13,500 Btu/lbm.

κcoal = $50.00/ton

1 ton 2,000 lb

lb 13,500 Btu

1,000,000 Btu 10 Btu

m m^6

F

HG^

I

KJ^

F

H

G

I

K

J

= $1.85/10 6 Btu. (4)

The energy-based cost of coal, κcoal , is typically much lower than the common liquid and gaseous fuels. These examples and several other common fuels are summarized in the Table 1.

Table 1. Typical fuel properties

Fuel

Typical sales unit

Example price ($/sales unit)

Energy content (Btu/sales unit)

Energy content (Btu/lb (^) m)

Unit price ($/10^6 Btu)

Fuel “density” (lb (^) m/sales unit)

Natural gas 10 3 standard ft 3 7.00 987,124 23,000 7.09 42. No. 1 fuel oil Gallon 1.18 134,510 19,810 8.77 6. No. 2 fuel oil Gallon 1.00 139,874 19,400 7.15 7. No. 4 fuel oil Gallon 0.76 146,731 18,860 5.18 7. No. 5 fuel oil Gallon 0.60 146,891 18,760 4.08 7. No. 6 fuel oil Gallon 0.51 145,485 18,300 3.51 7. Coal Ton 50.00 27,000,000 13,500 1.85 2,000.

The information contained in Table 1 is not representative of fuel prices and properties univer- sally. The table demonstrates typical data for the purpose of providing examples. Each fuel has a wide range of properties and costs. These properties are provided as examples, and each facility should use data for that particular site. Most fuels maintain consistent properties and are supplied based on certain specifications. In general, coal can have the widest range in properties because most other fuels are produced with certain tolerances. Coal storage and handling typically provides the opportunity for the fuel to contact water. Generally, coal is specified on a “dry” basis. However, the coal is not supplied to the burner on a dry basis. Therefore, to determine the actual performance of the boiler, periodic “as-fired” coal samples should be analyzed to determine the qualities of the coal supplied to the boiler. Periodic “as-received” coal samples should also be analyzed to determine what is actually being purchased. In these analyses, care must be given to the surface or extrinsic water. The surface water is part of the commodity purchased and supplied to the boiler. It does affect the boiler performance; therefore, surface water should not be lost in the analysis. Coal analysis typi- cally grinds the coal prior to analysis. This grinding evaporates the surface water, which can provide test results with erroneously low moisture values. Care should also be exercised in understanding the fuel pricing structure. When determining the economic benefit of an efficiency improvement, only the incremental cost of fuel is affected. For example, the first 500 10^3 ft^3 of natural gas purchased each month may carry a price of $7.50/10^3 ft^3 ; the remaining natural gas purchased may carry a lower price of $6.75/10 3 ft 3. If the

calculation demonstrates the method used to calculate the relative steam cost for the facility, the benchmark. The fuel cost is $1.00/gal, and conversion factors must be introduced into the calculations to maintain appropriate units.

κ

κ

steam

steam

m m

m

fuel consumed steam produced

fuel price

gal d lb d

gal

lb lb

lb

a f

a f

3 58 10^3

The unit cost of steam (κsteam) provided by Eq. (7) is not the steam “sales” price or the cost of steam distributed to the facility because some portion of the steam produced from the boiler must be used internally in the production of steam. This steam is used in deaeration, feedwater heating, possibly sootblowing, and other internal activities. The steam cost benchmark is a concrete marker that represents the boiler’s performance. The information in the example is a part of a reference example used throughout this document. Additional information concerning this “example system” is provided throughout the text with the majority of the information given in Sect. 4.2.1, “Example Boiler.” As indicated previously, a common reference is total fuel expense or the fuel portion of the boiler operating cost. Boiler operating cost ( K &boiler ) is calculated below.

K^ &boiler = fuel consumed (fuel price) = V &fuel κ (^) fuel.

(8) K^ &boiler = 27,780 gal/d ($1.00/gal) 365 d/year = $10,140,000/year.

2.4 CALL TO ACTION—STEAM SYSTEM PROFILING

1. Determine the total cost of fuel supplied to the boilers ($/year, $/month, and $/season).
2. Calculate the unit cost of fuel based on energy ($/10^6 Btu).
3. Compare the unit cost of fuel to other available fuel supplies.
4. Determine the unit cost of electricity supplied to the facility ($/10 6 Btu).
5. Compare the unit cost of fuel to the cost of electricity supplied to the facility.
6. Determine the typical steam production for the facility (lbm/h and lb (^) m/d).
7. Determine the production cost of steam for the facility ($/10^3 lbm).
8. Determine the amount of steam required to produce a product (lb (^) m steam/lbm product ).

3. IDENTIFYING STEAM PROPERTIES FOR THE SYSTEM

3.1 OVERVIEW AND GENERAL PRINCIPLES

A steam system analysis investigates the energy transfer of the fuel to the steam and the steam to the process. To complete the analysis, steam properties must be known. Steam properties are provided in tabular, graphical, and computerized form. 1–3^ Typically the values used to determine properties are the steam temperature and pressure if the steam is superheated. When steam is dry and saturated, pressure or temperature can be utilized to determine the steam properties. If dealing with saturated condensate, pressure or temperature are also the common properties used to provide fluid information. Finally, temperature and pressure are used to determine the properties of water below the saturation temperature (subcooled). Many other methods can be used to determine steam and water properties, but temperature and pressure are the most common measurements. Table 2 provides some typical steam properties for a boiler operating at 600 psig and producing superheated steam (750°F). Atmospheric pressure is 14.7 psia for the example.

Table 2. Steam properties

Location Temperature(°F) Pressure(psia)

Specific volume (ft 3 /lb (^) m)

Internal energy (Btu/lb (^) m)

Enthalpy (Btu/lb (^) m)

Entropy (Btu/lb (^) m°R)

Quality (%)

Boiler outlet 750 614.7 1.10357 1,253.42 1,378.95 1. Medium pressure 621 214.7 2.90619 1,216.42 1,331.88 1. Intermediate pressure 407 54.7 9.26341 1,143.99 1,237.76 1. Low pressure 429 23.8 22.08426 1,155.16 1,252.43 1. Boiler blowdown 489 614.7 0.02018 472.35 474.64 0.67502 0. Makeup water 60 14.7 0.01600 28.02 28.07 0. Condensate return 180 14.7 0.01650 147.87 147.91 0. Deaerator outlet 237 23.8 0.01690 205.61 205.68 0.34938 0. Feedpump exit 241 915.2 0.01690 206.80 209.66 0.

Makeup water is the water introduced into the system because steam or condensate is lost from the system; this water is relatively cold. Condensate return is water returned to the boilers from steam users. Usually this water is relatively warm, unless returned from turbine condensers. Most calculations completed on steam systems are investigating the energy associated with an activity. Typically, the thermodynamic property used to determine energy flow is enthalpy. Enthalpy is expressed in terms of specific energy content for a given mass of material; the common English units are British thermal units per pound mass (Btu/lbm). Enthalpy is expressed in equations as the variable h ; this is the convention used throughout this text. Additional data are provided in Table 2, which is used in some calculations.

3.2 CALL TO ACTION—IDENTIFYING STEAM SYSTEM PROPERTIES

1. Determine the properties of the steam generated in the boilers (temperature, pressure, saturated, superheated, enthalpy, and the remaining thermodynamic properties).
2. Determine the properties of boiler feedwater (temperature, pressure, enthalpy).
3. Determine the properties of boiler blowdown (pressure, enthalpy).
4. Determine the properties of condensate return (temperature).
5. Determine the properties of makeup water (temperature).