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    Steam Tracing Appendix

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    Amin Bagheri
    The document provides calculations for heat tracing of process pipes to prevent freezing or component separation. It describes two methods: bare steam tracing using a half-inch steam pipe attached to the process pipe, and cemented steam tracing where heat conductive cement increases contact between the pipes. Formulas are derived to calculate the heat transfer areas and rates between the steam tracer, insulation, and process pipe to determine steam requirements.

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    Steam Tracing Appendix

    Uploaded by

    Amin Bagheri
    384 views13 pages
    The document provides calculations for heat tracing of process pipes to prevent freezing or component separation. It describes two methods: bare steam tracing using a half-inch steam pipe attached to the process pipe, and cemented steam tracing where heat conductive cement increases contact between the pipes. Formulas are derived to calculate the heat transfer areas and rates between the steam tracer, insulation, and process pipe to determine steam requirements.

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    The document provides calculations for heat tracing of process pipes to prevent freezing or component separation. It describes two methods: bare steam tracing using a half-inch steam pipe attached to the process pipe, and cemented steam tracing where heat conductive cement increases contact between the pipes. Formulas are derived to calculate the heat transfer areas and rates between the steam tracer, insulation, and process pipe to determine steam requirements.

    Copyright:

    Attribution Non-Commercial (BY-NC)
    Download as PDF, TXT or read online from Scribd
    Download as pdf or txt
    384 views13 pages

    Steam Tracing Appendix

    Uploaded by

    Amin Bagheri
    The document provides calculations for heat tracing of process pipes to prevent freezing or component separation. It describes two methods: bare steam tracing using a half-inch steam pipe attached to the process pipe, and cemented steam tracing where heat conductive cement increases contact between the pipes. Formulas are derived to calculate the heat transfer areas and rates between the steam tracer, insulation, and process pipe to determine steam requirements.

    Copyright:

    Attribution Non-Commercial (BY-NC)
    Download as PDF, TXT or read online from Scribd
    Download as pdf or txt
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    The passage describes steam tracing, which uses steam pipes to prevent heat loss from process pipes. There are two main types - bare steam tracing and cemented steam tracing.

    Bare steam tracing uses a half-inch or three-quarter inch steam pipe attached directly to the process pipe with straps. Cemented steam tracing uses heat conductive cement placed around the steam tracer to increase the contact area for heat transfer.

    Bare steam tracing simply uses a steam pipe attached to the outside of the process pipe. Both pipes are then insulated together to minimize heat loss.

    Explanation of the calculations behind the sheet (revised edition) By Andr de Lange

    Heat tracing is used to prevent heat loss from process fluids being transported in process fluid pipes, when there is risk of damage to piping, or interference with operation such as fouling or blockage, caused by the congealing, increase in viscosity, or separation of components in the fluid below certain temperatures, or when there is risk of formation of corrosive substances or water due to condensation in corrosive services. This prevention of heat loss is accomplished by employing electrical tracing or steam tracing, and insulating both the process fluid pipe and the tracer together, using appropriate insulation lagging, in an attempt to minimise heat loss from the pipe and tracer to their surroundings. Steam tracing is described by attaching a smaller pipe containing saturated steam, also known as the tracer, parallel to the process fluid pipe. The two pipes are then also insulated together with the specified insulation and jacketed if necessary. Steam tracing is more labor intensive to install than electrical heat tracing, but there are very few risks associated with it. The temperature of the tracer also cannot exceed the maximum saturation temperature of the steam, as it operates at specific steam pressures. Steam tracing may be done in one of two ways. Bare steam tracing is the most popular choice as it is fairly easily installed and maintained and it is ideally suited to lower temperature requirements. It is simply composed of a half inch or three quarters of an inch pipe containing saturated steam attached to the process fluid pipe by straps and both pipes are then insulated together. The other available option is to make use of cemented steam tracing, during which heat conductive cement is placed around the steam tracer running parallel to the process fluid pipe, (shown in figure 1b), in an attempt to increase the contact area available for heat transfer, between the tracer and the process fluid pipe. Because the area around the process fluid pipe and tracer cannot be accurately described simply by assuming perfect cylindrical geometry, provision had to be made for a realistic impression of the true geometry. (See figures 1a and 1b)

    For bare tracing, the following formulas were derived: The calculations for the cemented tracer are not shown, but are derived from the same principles

    Figure 2: Schematic diagram to calculate Lai

    = cos 1

    r1 r2 r1 + r2

    tan =

    Lai (r1 r2 )
    with 2Lai being the area of annulus space exposed to insulation per metre of pipe length, and r1 and r2 being the outer diameters of the process fluid pipe and the steam tracer pipe, respectively.

    Lai = (r1 r2 ) tan

    Figure 3a: Heat loss from a bare tracer

    Figure 3b: Heat loss from a cemented tracer

    Q in with tracer = Q lost in total


    Qta + Qtl = Qtl + Qal + Q pl
    Qta = Qal + Q pl
    Area of tracer in contact with annulus (top part in following diagram)

    Figure 4: Schematic diagram of tracer

    Circumference = 2r2

    Exposed area ( S ) = (2 2 )r2 S = 2r2 ( )

    If there are n tracers and pipe length = L : Ata = nSL = 2nr2 ( )

    Area of annulus in contact with insulation

    Aal = 2nL Lai


    Area of process fluid pipe in contact with insulation (top part of following figure) A log-mean area with correction factor was used, since the insulation will still touch the pipe between tracers.

    r r A pl = (2 (1.25 + 0.75n ) ) insulation inner ln rinsulation r inner


    Figure 5: Schematic diagram of process fluid pipe

    Qta = hc Ata (Ts Tann )


    T Tann hc = 1.18 s (W/mK) with T in K and r2 in m. 2r2
    T Tann hc = 0.377922 s (BTU/h ft F) with T in F and r2 in inches. r2
    Qal = Aal (Tann Tsurf ) Insthick 1 + k ins ho
    1 4

    1 4

    Q pl =

    L(T p Tamb )(2 (1.25 + 0.75n ) ) r1 ln r1( inner ) kw ln rins + r1 k ins + 1 ho rins

    ho =

    q Alog mean (Tsurf Tamb )


    4 Tsurf 4 T amb 55.55 55.55 + 1.957 Tsurf Tamb

    q = 0.548
    W/m2

    ) (
    5 4

    2.85Vm + 1

    (was also adapted in the spreadsheet to be in units of BTU/hft2

    Tsurf
    low

    rins rins ln r 1 = Tp q k ins

    Tsurf
    hottest

    r2 + insthick (r2 + insthick ) ln r2 = Tst q k ins

    It was assumed that 80% of the surface temperature is contributed by Tsurf, low, the surface temperature on the process fluid side of the pipe, and 20% of the surface temperature is contributed by the surface temperature on the steam line side of the pipe, which is also the hottest surface temperature.

    Calculating steam consumption


    Q = mCpT = mH latent m = Q / H latent

    Q = Qtl + Qal + Q pl

    References Le Roux, D.F. (1997) Thermal Insulation and Heat Tracing, Guideline presented by line manager D.F. le Roux, Secunda. Foo, K.W. (1994) Sizing tracers quickly (Part 1). Hydrocarbon Processing, p93-97. January. Sizing tracers quickly (Part 2). Hydrocarbon Processing, p93-97. February. Fisch, E. (1984) Winterising process plants. Chemical Engineering, p128-143, 20 August. Kenny, T.M. (1992) Steam tracing: do it right. Chemical Engineering Progress, p40-44, August. Coulson, J.M and Richardson, J.F. (1999) Chemical Engineering, R.K. Sinnot, London. Le Roux, D.F. (2005) Theoretical discussion and problem description, Sasol Limited, Secunda. Van der Spuy, E. (2005) Theoretical advice, and steam traps, Spirax Sarco, Secunda. Smit, J. (2005) Practical information, Sasol Limited, Secunda. Technical committee of specification SP 50-4, (2004) Specification SP 50-4 Revision 2 for Steam and Hot Water Tracing, Sasol Limited, Secunda.

    By: Andre de Lange, Winner of the 2005 spreadsheet competition delange.a@gmail.com

    Appendix 1 : Derivations of Formulas


    Bare Tracing Geometry Fig A1 Geometric analysis of bare tracer

    = cos 1
    tan =

    r1 r2 r1 + r2

    Lai (r1 r2 )

    Lai = (r1 r2 )(tan )

    21

    Energy added by tracer Energy lost by system: Refer again to Fig. 3a.

    The movement of energy across the system boundaries may be explained as follows: Qta + Qtp + Qtl = Qtl + Qal + Q pl

    Qtp 0

    (Insufficient area for heat transfer)

    Qta = Qal + Q pl

    22

    Fig. A2 Different areas of the system

    Ata = 2nLr2 ( ) Aal = 2nL(r1 r2 )(tan )


    rins r1 A pl = (2 (1.25 + 0.75n ) )L ln rins r 1 The term (1.25+0.75n) is used as a correction factor for the angle that the process fluid has that corresponds to the ambient air, when more than 1 tracer is used, otherwise the area would soon decrease to 0 with 3 or more tracers. This is a valid correction, when one keeps in mind that the insulation lagging folds over the pipes. See figure A3.

    23

    Fig A3: Validation for correction factor

    The convection heat transfer coefficient for the annulus (still air) is given by

    T Tann 4 hc = 1.18 s Le Roux, D.F. (1997) Thermal Insulation and Heat 2r2 Tracing, Guideline presented by line manager D.F. le Roux, Secunda.
    The remaining design equations for bare tracing are given in section 4.:Results and Discussion.

    24

    Cemented Tracing Geometry Fig A4: Geometric Analysis of Cemented Tracer

    = cos 1
    sin =

    r1 r2 r1 + r2

    Lai Lact L tan = ai ct

    Lact =

    tan (r1 r2 ) sin tan 1 ct

    tan (r1 r2 )

    25

    Energy added by tracer Energy lost by system: Refer again to Fig. 3b.

    The energy movement across the system boundaries may be explained as follows: Qca + Qcp + Qcl = Qcl + Qal + Q pl

    Qca + Qcp = Qal + Q pl Aca = nL(0.2357 Dt 2 (r2 + ct )) Acp = 4nLr2 Molloy, J.F., Fundamentals of Heat Transfer, p72 101 rins r1 A pl = L (2 (1.25 + 0.75n ) ) rins ln r 1 The remaining equations are given in section 4:Results and Discussions.

    26

    Appendix 2 : Table of qt vs. NPS


    Table 3. qt vs. NPS
    NPS " " 1" 1" 2" 2" 3" 3" 4" 6" 8" 10" 12" 14" 16" 18" 20" 24" 30" 36" 42" 48" qt (W/mK) 60.2879608 60.2879608 60.1127051 60.1127051 57.1333582 54.329267 50.9994087 48.7210846 47.1437833 41.7108566 37.6799755 32.2470488 25.5873322 21.3811954 17.1750586 17.1750586 17.1750586 16.9998029 16.9998029 16.9998029 16.8245472 16.8245472

    27

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