Steam pipework

 
General Information on Steam
 
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Steam Pipework

Steam may be thought of as a medium to convey heat from the boiler to the point where it is needed.
As the temperature of saturated steam is fixed in relation to the pressure, the required temperature in any process can be controlled by the steam pressure.
But, for instance, a 20% reduction in designed steam pressure to a calorifier may result in a 15% drop in output.
Therefore while producing steam at the correct pressure and quantity in the boiler house is important, it is just as important that the designed steam properties are delivered efficiently at the plant maybe hundreds of metres away.
While distribution pipework can not be too big, the extra capital cost would not be acceptable.
If pipework is too small, then the increased steam velocity will cause noise and erosion and the excessive pressure drop may starve the equipment of steam.
Velocity should be designed to be below 15 metres/sec or 50 ft/sec.
In practice, sizing the pipework to produce a known pressure drop works best.
There are many programs, graphs and tables available that make use of the simplified formula:-

F = 
( P1 - P2 )
L

Where:-
P1 is the initial pressure
P2 is the final pressure
L is the equivalent length of pipework, adjusted for bends, valves, strainers..
F is the pressure drop.

For each branch in the steam main a theoretical pressure is calculated, and each branch can be designed using that figure as P1

Expansion

Pipework installed cold will expand at operating temperatures. While branches to equipment may well have enough bends to take up the expansion, mains pipework usually has to have bellows fitted. The pipe needs to be anchored mid-way between bellows and the pipe supports and insulation thereafter must allow for movement, obviously more movement the closer to the bellows. The pipe supports each side of the bellows must allow for axial movement only to avoid off-setting the bellows. The anchors must be strong to resist the substantial forces involved.
The table below gives the approximate expansion of ordinary steel steam pipe from a fitted temperature of 16C 60F.

Operating Temp
==
Expansion per
30m/ 100ft
C
F
mm
inch
66
150
 
19
0.75
93
200
 
29
1.15
121
250
 
41
1.60
149
300
 
50
2.0
177
350
 
61
2.4
204
400
 
74
2.9
232
450
 
84
3.3
260
500
 
97
3.8

Boiler Capacity

The output of a steam generating plant is often expressed in pounds of steam delivered per hour. Since this value may vary in temperature and pressure over time, a more accurate and complete expression is that of heat transferred over time, expressed as British thermal units per hour. Boiler capacity is usually expressed as kBtu/hour (1000 Btu/hour) and is given by the equation:

W = 
( hg - hf )
1000

 

where hg - hf is the change in enthalpy in Btu/lb.

An older expression of boiler capacity called "boiler horsepower" may sometimes be found. Use of this unit is discouraged as it is irrational, over thirteen times larger than regular horsepower and not widely accepted. If encountered, however, it is defined as:

boiler horsepower = horsepower 13.1547
1 boiler horsepower = 33475 Btu/hour
1 horsepower = 550 ft-lb/sec
1 horsepower = 746 watt

Horsepower of an Engine

Horsepower of an engine can be expressed using a simple and easy to remember mnemonic equation. Just think of the word "plan":

Horsepower =  
PLAN
33,000

where:-
P is the mean effective pressure per square inch on the piston,
L is the length of stroke in feet,
A is the area of the piston in square inches, and
N is the number of strokes per minute.

Mean effective pressure

The approximate mean effective pressure in the cylinder when the valve cuts off at:

1/4 stroke, equals steam pressure 0.597
1/3 stroke, equals steam pressure 0.670
3/8 stroke, equals steam pressure 0.743
1/2 stroke, equals steam pressure 0.847
5/8 stroke, equals steam pressure 0.919
2/3 stroke, equals steam pressure 0.937
3/4 stroke, equals steam pressure 0.966
7/8 stroke, equals steam pressure 0.992

Approximate Ranges in Steam Consumption by Prime Movers
(for Estimating Purposes)

Simple Non-Condensing Engines 29 to 45 pounds per hp-hour
Simple Non-Condensing Automatic Engines 26 to 40 pounds per hp-hour
Simple Non-Condensing Corliss Engines 26 to 35 pounds per hp-hour
Compound Non-Condensing Engines 19 to 28 pounds per hp-hour
Compound Condensing Engines 12 to 22 pounds per hp-hour
Simple Duplex Steam Pumps 120 to 200 pounds per hp-hour
Turbines, Non-Condensing 21 to 45 pounds per hp-hour
Turbines, Condensing 9 to 32 pounds per hp-hour

     Quality of Steam

The term Dry Saturated Steam is often used to differentiate from superheated steam. But steam is far from dry. Moisture particles entrained in the steam carry no latent heat, they add to the wet wall layer and reduce heat transfer in the plant and increase the amount of condensate to be returned so the dryer the steam the better.
Preserve the integrity of insulation by protecting from weather and maintenance traffic and replace valve boxes after maintenance, for instance, to reduce condensation. Fit a separator to help dry the steam.
When a plant is shut down, all the steam condenses in the pipework. When steam returns, air and water is pushed ahead of it and provision must be made for its removal.
Automatic air vents should be fitted at high points and condensate trap sets fitted at low points. Pipework should be laid to fall from/to the fittings to facilitate air/water removal.

The quality of steam (percentage) x, is given by the expression:

x =  
( hg - hf ) 100
hfg

 

where:-
hf is the heat of the liquid in Btu/lb,
hfg is the latent heat of evaporation in Btu/lb, and
hg is the total heat of steam in Btu/lb.


Related pages
" Home page
" Info on SI Units
" SI Unit Descriptions
" SI Derived Units
" SI Prefixes (mega, giga)
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" Scientific Notation
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Detail pages
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"
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" Density of bulk materials
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" Steam tables (4 pages)
" Watts, amps, volts, ohms
" M Threads & spanners
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last modified : 27th. October 2003


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