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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:-
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 16°C
60°F.
|
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
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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:
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":
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:
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.
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