Download Technical Reference Guide on Steam Distribution PDF

TitleTechnical Reference Guide on Steam Distribution
TagsBoiler Pipe (Fluid Conveyance) Flow Measurement Steam Drop (Liquid)
File Size1.0 MB
Total Pages66
Document Text Contents
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Steam distribution

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1

Contents

Introduction 2
Steam distribution 2
Steam system basics 2

Working pressure 4
Determining the working pressure 4
Pressure reduction 6

Pipeline sizing 7
Effects of oversizing and undersizing pipework 7
Pipeline standards and wall thickness 8
Pipeline sizing on steam velocity 9
Pipeline sizing on pressure drop 11
Pipeline sizing for larger and longer steam mains 12

Steam mains and drainage 17
Drain points 18
Waterhammer and its effects 19
Branchlines 21
Branch connections 22
Drop leg 23
Rising ground and drainage 23
Steam separators 24
Strainers 26
Mains drainage method 27
Steam trap selection 28
Steam leaks 29
Summary 30

Pipe expansion and support 32
Allowance for expansion 32
Pipework flexibility 33
Expansion fittings 36
Pipe support spacing 40

Air venting 44

Reduction of heat losses 46
Calculation of heat transfer 47

Relevant UK and international standards 49

Summary 51

Appendix 1 - Sizing on pipeline capacity and pressure drop 52

Further information 57

Appendix 2 - Steam tables 58

Appendix 3 - Conversion tables 60

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32

Material
Temperature range °C

< 0 0 - 100 0 - 200 0 - 315 0 - 400 0 - 485 0 - 600 0 - 700

Mild steel 0.1-0.2 % C 12.8 14.0 15.0 15.6 16.2 17.8 17.5 —

Alloy steel 1 % Cr 0.5 % Mo 13.8 14.4 15.1 15.8 16.6 17.3 17.6 —

Stainless steel 18 % Cr 8 % Ni 9.4 20.0 20.9 21.2 21.8 22.3 22.7 23.0

Pipe expansion and support

Allowance for
expansion

All pipes will be installed at ambient temperature. Pipes carrying
hot fluids, whether water, or steam, operate at higher temperatures.
It follows that they expand, especially in length, with an increase
from ambient to working temperatures. This may create stresses
upon certain areas within the distribution system, such as a pipe
joints which could be fractured. The amount of the expansion is
readily calculated using the following equation, or read from
appropriate charts.

Expansion = L x D
t
x a (mm)

where: L = Length of pipe between anchors (m)
D

t
= Temperature difference °C

a = Expansion coefficient (mm/m°C) x 10-³
Table 4 Expansion coefficients (a)

Find the expansion of 30 m of pipe from ambient (10°C) to 152°C
(steam at 4 bar g)

L = 30 m
D

t
= 152°C - 10°C = 142°C

a = 15.0 x 10-³ mm/m°C

\ Expansion = 30 x 142 x 15.0 x 10-³ mm

i.e. expansion = 64 mm

Alternatively, the amount of pipe expansion can be determined by
using Table 6 (page 40) to calculate the amount of expansion
over 10 m of pipe for the different pipe materials. Expansion
charts like Figure 34 (page 41) are also an easy method for
determining the amount of expansion.

Example

Page 34

33

The pipework must be sufficiently flexible to accommodate the
movements of the components as it heats up. In most cases the
pipework has enough natural flexibility, by virtue of having
reasonable lengths and plenty of bends, that no undue stresses
are set up. In other installations, it will be necessary to build in
some means of achieving the required flexibility. An example of
building in flexibility is when condensate is drained from a steam
main drain trap to a condensate main. In this case, the difference
between the expansion of the two mains due to the change in
temperature or the pipes' material expansion rates must be
remembered.

The steam main may be at a temperature very much above that of
the return main, and the two connection points can move in
relation to each other during system warm up. Some flexibility
should be incorporated in the steam trap piping so that branch
connections do not become over stressed. (See Figure 24).

The amount of movement to be taken up by the piping and any
device incorporated in it can be reduced by the use of 'cold draw'.
The total amount of expansion is first calculated for each section
between fixed anchor points. The pipes are left short by half this
amount, and stretched cold, as by pulling up bolts at a flanged
joint, so that at ambient temperature, the system is stressed in one
direction. When warmed through half the total temperature rise,
the piping is unstressed. At working temperature and having fully
expanded, the piping is stressed in the opposite direction. The
effect is that instead of being stressed from 0 F to +1 F units of
force, the piping is stressed from -½ F to + ½ F units of force.

Pipework flexibility

Fig. 24 Flexibility in connection to condensate return line

Steam main

Condensate main

Steam Steam

Condensate

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64

Page 66

CM Issue 2TR-GCM-03

Spirax-Sarco Limited, Charlton House,
Cheltenham, Gloucestershire, GL53 8ER UK.

Tel: +44 (0)1242 521361 Fax: +44 (0)1242 573342
E-mail: [email protected]

Internet: www.spirax-sarco.com
' Copyright 1999 Spirax Sarco is a registered trademark of Spirax-Sarco Limited

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