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 STEAM DELIVERY SYSTEM UGRADE AND REPAIR


    The delivery of steam throughout a manufacturing plant is essential to many processes in industry. If the steam distribution system has leaks or is not properly insulated the boiler generates more steam than necessary.  The system maintenance is essential to preventing a significant amount of energy from being lost to the plant surroundings.  In steam distribution a defective steam trap can typically waste 50 MMBtu/yr, at a cost of between $100 and $1000, depending on the source of the fuel.  This module contains the following recommendations to illustrate the energy savings potential.

General Rules of Thumb:

  • The average cost of electricity is $0.05/kWhr ($15/MMBtu)
  • The average cost of natural gas $0.35/CCF
  • The average cost of #2 fuel oil is $4/MMBtu
  • There are 2000 hours per year per shift (based on the assumption that one shift is 8 hours per day, 5 days per week, 50 weeks per year)
  • A typical boiler or furnace has a combustion efficiency of 80%
  • 90% of the heat loss from a hot, uninsulated surface can be economically eliminated by installing insulation.
  • Cost of high pressure (125 psig) steam leaks are on the order of $150 to $500/leak/shift/year
  • Cost of low pressure (15 psig) steam leaks are on the order of $30 to $110/leak/shift/year
  • Cost of heat lost through hot, uninsulated pipes: (associated per 100 feet of uninsulated pipe)
                25 psig:  $375/100ft/shift/year
                50 psig:  $430/100ft/shift/year
                75 psig:  $480/100ft/shift/year
                100 psig:  $515/100ft/shift/year
  • Switching from electric heat to natural gas or #2 fuel oil can reduce heating costs by 78%
    Notes:
                    Before choosing the following targeted recommendations READ THE FOLLOWING:     Pay back estimates for the following recommendations will use the equation below.  They will vary depending on the, application, type of installation, and purchase quantity of material and labor associated with each recommendation.  It will be up to the person doing the analyses to use the URL references below each equation to help estimate an implementation cost.

    The data correlating to the variables below each equation will be prompted for in order to execute a calculation.  Frequently the fuel cost (FC) associated with the specific recommendation will be prompted for in order to calculate the annual cost savings (ACS). Unless otherwise specific to a particular recommendation the ACS will be calculated as follows:
 

    Table 1: Heat transmission coefficients for bare steel pipes*.

                                   Btu/(hr)(linear ft) (Fº difference between pipe and surrounding air)

        No allowance for fittings.  This table applies only to straight runs of pipe.  When numerous fittings exist, a suitable safety factor must be included.  This added heat gain at the fittings may be as much as 10%.  Generally this table can be used with out adding this safety factor.

        Other insulation:  If other types of insulation are used, multiply the above values by the factors shown in table 3 (below this one).

--------------
---------------
-----------Hot
Water---------
---------------
Steam---------
---------------
------Steam
Temp. inside pipe
120 ºF
150 ºF
180 ºF
210 ºF
227 ºF 
(5 psig)
  300 ºF         (50 psig)
338 ºF 
(100 psig)
Nominal 
Pipe
---------------
---------------
----------------
Temperature
Difference------
---------------
-------------
Size (in)
50 Fº
80 Fº
110 Fº
140 Fº
157 Fº
230 Fº
268 Fº
1/2
0.46
0.50
0.50
0.55
0.58
0.61
0.76
3/4
0.56
0.61
0.61
0.67
0.72
0.75
0.93
1 
0.68
0.74
0.74
0.82
0.88
0.92
1.15
1 1/4
0.85
0.92
0.92
1.01
1.09
1.14
1.43
1 1/2
0.96
1.04
1.04
1.15
1.23
1.29
1.63
2
1.18
1.28
1.28
1.41
1.51
1.58
1.99
2 1/2
1.40
1.53
1.53
1.68
1.80
1.88
2.36
3
1.68
1.83
1.83
2.01
2.15
2.26
2.84
3 1/2
1.90
2.06
2.06
2.22
2.43
2.55
3.22
4
2.12
2.30
2.30
2.53
2.72
2.85
3.59
5
2.58
2.80
2.80
3.08
3.30
3.47
4.39
6
3.04
3.29
3.29
3.63
3.89
4.07
5.16
8
3.88
4.22
4.22
4.64
4.96
5.21
6.61
10
4.76
5.18
5.18
5.68
6.09
6.41
8.12
12
5.59
6.07
6.67
6.67
7.15
7.50
9.53

    Table 2: Heat transmission coefficients for insulated pipes*.

Btu/(hr) (linear ft) (Fº-diff between pipe and ambient space)**

 
Iron Pipe
--------------------
85 % Magnesia Insulation***
--------------------
 
 
Size (in)
1 in thick
1 1/2 thick
2 in thick
 
 
1/2
0.16
0.14
0.12
 
 
3/4
0.18
0.15
0.13
 
 
1
0.20
0.17
0.15
 
 
1 1/4
0.24
0.20
0.17
 
 
1 1/2
0.26
0.21
0.18
 
 
2
0.30
0.24
0.21
 
 
2 1/2
0.35
0.27
0.24
 
 
3
0.40
0.32
0.27
 
 
3 1/2
0.45
0.35
0.30
 
 
4
0.49
0.38
0.32
 
 
5
0.59
0.45
0.38
 
 
6
0.68
0.52
0.43
 
 
8
0.85
0.65
0.53
 
 
10
1.04
0.78
0.64
 
 
12
1.22
0.90
0.73
 

    Table 3: Insulating material conversion factors*

 

 
MATERIAL
PIPE COVERING FACTORS
 
 
Corrugated Asbestos (air cell)
 --
 
 
4 ply per inch
1.36
 
 
6 ply per inch
1.23
 
 
8 ply per inch
1.19
 
 
Laminated Asbestos 
(sponge felt)
0.98
 
 
Mineral Wool
1.00
 
 
Diatomaceous Silica
1.36
 
 
Brown Asbestos Fiber 
(Wool Felt)
0.88
 
    * Reference:  Carrier System Design Manual, part 1 - load estimating, Ninth Printing, Carrier Air conditioning Co., Syracuse,     New York, 1972, pp. 107 - 108.

Table 4: Heat loss from steam leaks'''
 
--------------------
--------------------
--------------Steam
Pressure----------
--------------------
-------------------------
Hole 
20 psig
50 psig
100 psig
200 psig
400 psig
Size (in)
------------------------
Annual Heat Loss
(MMBtu/yr)
--------------------
----------------------
0.05
20
25
100
150
375
0.10
100
200
500
800
1500
0.25
250
1000
2025
4000
> 4000
0.50
1600
3250
4000
>4000
> 4000

        ''' Reference:  Data from Figure 2-1.  Heat Loss From Steam Leaks.  Source:  U.S. Department of commerce, Energy Conservation Program Guide for Industry and Commerce,  NBS handbook 115 (Washington, D.C.:  Government Printing Office, 1974), p. 3-24.
    1. Use Correct Size Steam Traps
    2. Reduce Steam Delivery System Losses
    3. Reduce Heat Losses by Insulation of Exposed Surfaces
    4. Increase Amount of Condense Returned
    5. Use Steam Condensate for Preheating
    6. Flash Condensate to Produce Low-Pressure Steam
    7. Use Minimum Necessary Operating Steam Pressure
    8. Reduce Demand for Steam
    9. Reduce Steam Loss

1.  Use Correct Size Steam Traps
               Steam traps are used to remove air, carbon dioxide, and condensed steam and to prevent steam from flowing freely into the outside air from steam distribution systems.  If these are not removed, they can cause corrosion of the pipes, valves and coils of the steam distribution system.   Correctly sized steam traps can prevent high (temperature and pressure) steam from being released into the condensate return lines thereby keeping the energy of the steam within the system.  Excess steam that can be released translates into an energy and monetary loss.  When a steam trap fails it is usually stuck shut or open, and the design consideration of most maintenance interests is proper drainage. .A correctly sized steam trap will allow only condensate to be returned to the boiler or condensate tank and keep maintenance to a minimum.  The following equation can be used to calculate the correct size for a steam trap.

2. Reduce Steam Delivery System Losses
         The repair or replacement of defective steam traps, the creation of a steam trap maintenance program, the repair and/or elimination of steam leaks in steam lines and valves, high pressure reducing stations, and process equipment, and shutting off traps on superheated steam lines when not in use will reduce the energy the boiler will consume.   These actions reduce the fuel required to generate the steam.  The following equation illustrates the potential savings.

3. Reduce Heat Losses by Insulation of Exposed Surfaces
         Exposed surfaces radiating heat can contribute to energy losses that are not always obvious.  Inspection of surfaces of a temperature above 150 ºF should be conducted to ensure that they are adequately insulated.   The installation, upgrade or repair of insulation on condensate storage tanks, condensate and steam lines, and oven surfaces can significantly reduce delivery system heat losses thereby reducing boiler fuel costs.  The following equation illustrates the potential savings.

4. Increase Amount of Condensate Returned
       Condensate from process or heating use that is delivered to the sewer or wasted in any way can contribute to an increase in fuel consumption by the boiler.   When this occurs the boiler uses more municipal or ground water as makeup.  The installation of steam traps or other equipment to remove condensate from steam lines, the increase in steam pressure, the repair of leaks and the installation of a condensate tank can significantly increase the delivery system's efficiency.  This efficiency  increase is a direct result of the condensate being returned at a higher temperature than the municipal or ground water supply.  The following equation illustrates the potential savings.

    Power Engineering Books
    ASHRAE
    Thomas Register .

    CR = condensate returned, lb/hr (measured by plant personnel)
    Cp = specific heat of water, 1 BTU/lb-Fº
    Tc = temperature of condensate, ºF
    Tm = temperature of makeup water, ºF
    HY = operating hours per year of boiler providing steam
    h = efficiency of boiler providing steam
    ACF= boiler fuel cost ($/MMBtu)
    IMPC= foresighted implementation cost ($)

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5. Use steam condensate for preheating
       Steam condensate that is a by product of a process or heating system can be used to preheat a variety of fluid systems.  Typically, it is used to preheat makeup water that is required to feed a boiler or a process steam generator.  Condensate returned to the boiler room to preheat makeup water can create energy savings achieved through the decrease in requirements for raw water and chemicals needed in boiler feed water treatment; and effluent charges for discharge of condensate.  If applicable; use condensate for non-potable hot water supply or for heating domestic hot water.  The following equation illustrates the potential savings that can result.

    Power Engineering Books
    ASHRAE
    Thomas Register

    Q = volumetric flow rate of condensate, ft3 /hr
    d = density of condensate, lb/ft3 (62.4 for liquid water)
    Cp = specific heat of condensate, BTU/lb-Fº (1.0 for liquid water)
    T1 = current feed water temperature, ºF
    T2 = anticipated feed water temperature, ºF
    HY = operating hours per year
    HL = fraction of energy loss during heat transfer
    h = efficiency of boiler providing steam

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6. Flash condensate to produce low-pressure steam
            In applications where low pressure steam can be used as opposed to high pressure steam there can be an energy savings derived.  Typically when a condensate line runs past a process that demands steam, out of convenience, it is connected without thought given to the fact that that process may actually require less pressure.  The following equation illustrates the potential savings that can be achieved.
     
    Power Engineering Books
    ASHRAE
    Thomas Register

    FS = fraction of flash steam, lb/lb steam
    SH = sensible heat in the condensate at the higher pressure before discharge, BTU/lb steam (see Thermo-Tables )
    SL = sensible heat in the condensate at the lower pressure to which discharge takes place, BTU/lb steam (see Thermo tables)
    H = latent heat in the steam at the lower pressure to which the condensate has been discharged, BTU/lb (see thermo tables)
        The percentage of mass converted to flash steam in a flash tank can also be obtained from tables, given the steam pressure (before discharge) and the flash tank pressure. (ref.- Industrial Energy Management and Utilization, Witte, Schmidt and Brown, pg. 282)

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7.  Use minimum necessary operating steam pressure
        Reducing the pressure a boiler will have to operate will reduce the firing temperature required to generate the steam.  Many processes and heating systems that are oversized are using steam that is too high in pressure.  The following equation shows the savings that can result from reducing steam generating pressure.

    Power Engineering Books
    ASHRAE
    Thomas Register

    Q = volumetric flow rate of steam, ft3 /hr
    d = density of steam, lb/ft3 (inverse of specific volume see Thermo-Tables)
    DT = difference between anticipated and current temperature, Fº
    HY = operating hours per year
    h = boiler efficiency

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8.  Reduce demand for steam
         Steam that is purchased or generated for process use may be redundant if there are other primary heat sources available in a plant.
        Substituting hot process water or other fluids for steam can reduce the fuel consumed by the boiler.  Also, the use of heat exchange fluids instead of steam in pipeline tracing systems can reduce the demand of the generated resource.  The following equation illustrates the potential savings that can be achieved.

9.  Reduce steam loss
         Steam leakage is typical among large processes or heating systems in a manufacturing plant  Neglect of this problem can result in an energy and monetary loss.
        Removal or closure of unneeded steam lines permanently or when not in use; reduction of  excess bleeding of steam; elimination of steam tracing during mild weather (install a self-contained temperature actuated valve to shut off steam to the tracing automatically when it is not needed) all can significantly reduce the fuel consumed by the boiler.  The following equation shows the savings that can be obtained.