volume vs. Pressure
can see below, as the steam pressure increases from 1atm to 4 atm,
the density of the steam molecules is increasing. As the specific
volume is inversely related to the density, the specific volume
will decrease with increasing pressure. We can see the reduced
volume in the last jar.
diagram clearly shows that the greatest change in specific volume
occurs at lower pressures, whereas at the higher end of the
pressure scale there is much less change in specific volume.
extract from the steam tables below, shows specific volume, and
other data related to saturated steam.
7 kg/cm2g, the saturation temperature of water is 170°C. More
heat energy 'hf' is required to raise its temperature to
saturation point at 7 bar g than would be needed if the water were
at atmospheric pressure. The table gives a value of 171.96 kcals
to raise 1 kg of water from 0°C to its saturation temperature
The heat energy (enthalpy of evaporation
'hfg') needed by the water at 7 bar g to change it into steam is
actually less than the heat energy required at atmospheric
pressure. This is because the specific enthalpy of evaporation
decreases as the steam pressure increases.
However, as the
specific volume also decreases with increasing pressure, the
amount of heat energy transferred in the same volume actually
increases with steam pressure.
steam requirements – m cp ΔT.
process needs heat at
correct temperature and
• the correct rate of heat
is being generated in the boiler in the form of steam. This heat
is being distributed by steam lines to the process. Steam pressure
determines the temperature at which heat is supplied, as saturated
steam temperature is directly proportional to pressure. We need a
ΔT of minimum 15-30°C to have efficient heat transfer
(rate of heat transfer).
Consider a heat exchange process.
The primary side is the steam space, and the secondary side is the
process. Steam is condensing on the primary side into water. It is
changing phase into liquid and giving off its latent heat to the
process. This is Primary Heat (Q).
Q = m x hfg
Q = Quantity of heat energy released (in kcals)
m = Mass of
steam releasing the heat (in kgs)
hfg = Specific enthalpy of
evaporation of steam (in kcals/kg)
On the secondary side,
this heat is being used for two things:
up' heat - to increase the product temperature to the degree
• 'maintainance' heat - to maintain the product
temperature as heat is lost by radiation, etc
Q = m x cp x ΔT
Q = Quantity of heat energy absorbed (in kcals)
m = Mass of the
substance absorbing the heat (in kgs)
cp = Specific heat
capacity of the substance (in kcals / kg °C )
Temperature rise of the substance (in °C)
is also modified and used to establish the amount of heat required
to raise the temperature of a substance, for a range of different
heat transfer processes.
The above equations are very
important. As Heat energy is being transferred from the primary to
the secondary side, in an ideal condition,
Q = Secondary Q
this is the equation to calculate the theoretical heat balance of
the entire system.
Example 1. Calculate steam flow
rate for an autoclave which is heating 10,000 bottles of 1 litre
each to a temperature of 120°C in 30 minutes. Steam supply is
at 3 kg/cm2g.
What we are asking for is - what is the mass of steam that is
supplied to the autoclave to heat these 10 bottles. This is 'm' on
the primary side. First we will calculate the heat absorbed by the
bottles (process), ie, secondary Q.
Q = m x cp x ΔT
Q = Quantity of heat absorbed by the bottles (in
= Mass of water in the bottles which is absorbing the heat (in
10,000 bottles X 1lt = 10,000 lt = 10,000 kg
Specific heat capacity of water (in kJ/kg °C ) = 1 kcal/kg °C
= Temperature rise of water (°C) assuming ambient is
120°C – 30°C = 90°C
Secondary Q =
10,000 kg x 1 kcal/kg °C x 90°C = 9,00,000 kcal
9,00,000 kcal is the heat energy absorbed by this autoclave on the
secondary (process) side in 30 minutes. Steam at 3 kg/cm2g has 510
kcal/kg latent heat hfg (from steam tables).
Sec Q = Pri Q,
9,00,000 kcal = m x 510 Kcal/kg
9,00,000 / 510 = 1765 kgs
1765 kgs is the steam required in
30 mins. So, steam flowrate is 1765 X 60/30 = 3530 kgs/hr
for this autoclave.
Suppose steam is supplied to a heat
exchanger at 3 kg/cm2g - hg 630 kcal/kg. Condensate is coming out
of the traps at 3 kg/cm2g hfg 130 kcal/kg. Ideally, the product
should absorb 511 kcal/kg. But, it doesnt. Heat gets absorbed by
the heat transfer barriers and is also lost via radiation. So, the
actual heat absorbed is less than 511 kcal/kg.
us understand what these heat transfer barriers are.
Heat transfer - U A ΔT.
general heat transfer equation .
= U x A x ΔT
= Heat transferred per unit time (kcals/hr)
U = Overall heat
transfer coefficient (kcals/hr / m²°C)
A = Heat
transfer area (m²)
ΔT = Temperature difference
between the primary and secondary fluid (°C)
will be a mean heat transfer rate if ΔT is a mean
temperature difference LMTD. The highest rate of heat transfer is
at steam inlet as the temp diff is highest here, and the outlet
has the lowest temp difference, therefore the lowest rate of heat
heat transfer coefficient (U)
The heat transfer coefficient
basically takes into account all the barriers to effective heat
transfer. This can be the deposits of scale, condensate , air
film, etc. It can be rust on the steel wall, or chemical reactions
between the process and/or steam with the wall. It could be fluid
flowrates, the physical nature of fluids, or the orientation of
the heat transfer surface itself. All the above play a vital role
in transferring heat to the medium, and are summed up in the heat
transfer coefficient, U.
Barriers that reduce the rate of heat transfer: Metal of the pipe
or jacketed pan; air, condensate and scale on the steam side;
stagnant product and burn-on on the product side
may be between 1 500 and 3 000 times more resistant to heat flow
Condensate film may be between 100 and 150 times
more resistant to heat transfer than a steel heating surface.
Steam Pipe sizing.
Line sizing is based on either
pressure drop per 100 m or velocity in m/s. Design parameters will
sometime vary from plant to plant but, as a rule, allowable
pressure drop is 0.115 kg/cm2g per 100 m for runs and less. For
pipe runs over 100 m long, 0.03 kg/cm2g per 100 is acceptable.
Velocity is normally held at 30 m/s for saturated steam.
most processes, the warm up loads will be much higher than the
running loads. The calculated steam consumption should have a
factor of at least 25% extra for the purposes of line sizing.
have the standard formula,
= Line size in mm
m = Mass flowrate of steam in kg/h
Specific volume in m3/kg
π = a constant 3.14
= velocity m/s
- Saturated Steam line sizing
Find out the
line size for mass flowrate of steam 3000 kg/h, at a working
pressure of 10.5 kg/cm2g.
We have the formula,
Working pressure is 10.5 kg/cm2 g
steam tables, we go down to 10.5 kg/cm2 g, and across
Saturation temperature: 185.59 0C
volume: 0.17 m3/kg
Determination of line size at the
three different velocities
Wet or flash steam : 15 - 25
Saturated steam : 25 - 40 m/s
Superheated steam : 40 +
velocity is is in the range of 25 m/s hence, actual line size will
= 1000 x √(4 x 3000 x 0.17) / (3600 x 3.14 x 25)
mm say 85 mm
velocity is 40 m/s, hence, actual line size should be
= 1000 x √(4 x 3000 x 0.17) / (3600 x 3.14 x 40)
the above conditions, the right selection is 80 NB pipe.
checking for 80 NB (77.93 mm ID pipe), the velocity through it is:
= (1000)2 x 4 x m x V
x 3.14 x D2
x 4 x 3000 x 0.17
x 3.14 x (77.93)2
is in the permissible range for saturated steam.
steam permisible velocity range is 25 – 40 m/s.
Condensate line sizing.
lets understand the condensate loop from the drawing . Where is it
coming from, how it is collected and where is it going?
logically, we start with the boiler. It is supplying steam to the
process. Process A and B are getting steam at a high pressure and
processes C and D are supplied steam after a PRS at a lower
The steam transfers its heat energy to the
process and condenses. The condensate flows from the drain of the
process to a trap. The steam space of the plant (like the inside
of the pressurized jacketed vessel) and the inside of the trap are
at the same pressure. Therefore, the trap must be lower than the
process so that condensate flows by gravity to the trap.
don”t want to lose pressure so this line from drain to trap
has to be sized correctly. Each process in the plant may be
designed for differing pressures and condensate flow rates, so,
the drain connection will not necessarily be the correct
Understanding start -up
When a plant is cold and steam switched on all
the processes are at ambient temperature. So, the material to be
heated needs a lot of steam at start -up to come to its working
temperature or running load. More steam translates to more
condensate and the lines to the trap must be sized properly to be
able to cope as, the condensing rate of steam is very high. This
is start-up load.
Also, the lines had air in them before
start -up. The incoming rush of steam carries this air to the
trap as well, again loading the trap.
So, pipe sizing
and subsequent trap sizing is done based on steam load multiplied
by a factor (2 or 3) times running load plus based on process
equipment and experience. In some plants the equipment is not used
all together , but in phases, depending on process requirement and
this needs to be taken into account while sizing return lines.
also take into account a frictional resistance of 1.4 m bar /
table has already taken into consideration start-up loads.
it is important to size pipes correctly?
go up hugely (disproportionately) as the size increases. So we
want to use pipes that can accommodate capacity comfortably, and
are not over sized for economic reasons.
Now we move forward in
our condensate loop- from the trap outlet to the discharge lines
What is our line carrying now? Besides
condensate and air & other gases (on start-up) these discharge
lines now also contain flash steam. This is because discharge
lines are at (ambient / lower pressure) than the pressurized trap
Therefore, in an ideal condition we must discharge
first to a flash separator (to recover energy from flash
In the absence of a flash separator, we must, in
any case recover the condensate, so the trap discharge lines
should go to the receiver of a CRPS or directly to the boiler
feedwater tank / deaerator.
At this end of the trap, we
again have to consider start up and running load conditions. When
the plant starts up the condensate is relatively cool (as the
steam discharges much more heat into a cold process) and there
will be little or no flash steam (flashing occurs closer to
But, condensing rate is very high and
so is the air content in the pipes. So, the pipes have to be at
least the same size as the trap inlet.
As the plant comes
to a normal running load, the condensate flow decreases to average
running load conditions. But, it's temperature is much higher than
at start-up. Flash steam is created as the hot condensate flows
from trap at high pressure to discharge lines at low pressures.
What will be the percentage of flash steam from
condensate at 3 kg/cm2g when released to atmosphere ? From the
graph above, we go up on the 3 kg/cm2g line to the 0 kg/cm2g red
line. This corresponds to a flash steam percentage of about
What will be the percentage of
flash steam from condensate at 7 kg/cm2g when released to
discharge lines with a pressure of 0.5 kg/cm2g ? Again, from the
graph above, we go up on the 7 kg/cm2g line till we touch the blue
0.5 kg/cm2g line. Here the flash percentage is 12.5%.
Flash steam percentage of 8.5% in example 1 and 12.5% in example 2
seems very trivial. Why are we doing this exercise of finding
flash steam percentage ?
This is because the volume of
flash steam can be up to 400 times the volume of condensate.
Especially, if the pressure difference between the trap body and
the condensate return line is high and the condensate is hot.
Therefore, it is clear - size your condensate lines on the flash
(steam) volume and not on condensate volume.
Let us taken an
The jacketed vessel
has a condensate load of 1000 kg/h. Pressure at trap is 4 kg/cm2g.
The discharge line is at atmospheric pressure. Calculate volume of
Looking at the graph we see that 10% of
the condensate will flash off. Therefore, 1 kg of condensate
discharged via the trap turns into 0.9 kg of water and 0.1 kg of
steam in the discharge line.
To calculate volume, of steam,
we look at the steam tables and see that specific volume of
saturated steam at 0 kg/cm2g is 1.66 m³ / kg.
of water =
900 liters = 0.9 m³/h
Volume of steam =
100 kg/h X 1.66 m³/
166 m³/ hr
Total Volume =
% volume of water =
0.9/166.9 X 100 = 0.54 %
% volume of steam
= 166 /166.9 X 100 = 99.46 %
As we can see, the flash steam
is taking up all the volume of the pipe. (In fact, the effect of
releasing this huge amount of steam into a small space like the
discharge line will be increased pressure - more than atmospheric.
Increasing pressure will reduce the pressure differential between
trap body and line which will reduce flash steam
Condensate collects at the bottom of the pipe.
It grows in thickness and moves at a lower velocity than flash
steam. The total mass flow through the line is calculate by adding
these two different rates of flow.
So, we have concluded
that the sizing of condensate discharge lines is to be done based
on the mass flow rate of flash steam.
In our example,
flow rate of flash steam = 0.1 x 1000kg/h = 100kg/h.
the pipe capacity chart on page 4.15, we see the size of pipe to
take 100 kg/h at discharge line pressure of 0 bar g is 65mm.
(Condensate flowing at the bottom of pipe can cause water
hammer so flash steam velocity should be lower than 15
Other factors to take into
account white pipe sizing return lines:
Separators & Deaerators in the system translates to a higher
pressure in the condensate discharge lines. The build up of
pressure in the line is because of flash steam in the flash
separators and deaerator. For proper functioning of traps be
careful to maintain a differential pressure between inlet (process
pressure) and outlet (discharge).
It is only if the pipe is
undersized for the flow of flash steam at return line pressure,
will the back pressure rise so much as to hinder trap
Each section of the pipe must be correctly
sized to carry condensate loads and flash steam at acceptable
velocities. This will mean little extra pressure is involved (?)
and that the discharge from a high pressure trap will not
interfere with that from a low pressure trap.
Back pressure on a trap reduces trap
capacities, although this becomes noticeable at fairly low
upstream pressures. More importantly , it makes air venting and
condensate removal tougher at start-up , which can lead to erratic
control or waterhammer with temperature controlled
Causes of backpressure:
pressure at the end of the line- either atmospheric or the
pressure of the vessel / receiver into which the condensate
• A trap at low level has to work against
“hydrostatic head” to push condensate to an overhead
return line. A lift of 1m =0.1 bar increase in back pressure.
Similarly, a lift of 5m is 0.5 Bar head the trap has to discharge
• Frictional resistance of pipe , bends, etc to
the flue flow of condensate , steam and air.
to be followed with common return lines.
A plant with
carefully sized lines has little to worry about, as far as a
common return line for traps goes. A little thought can go a long
way to maintain a return line.
(a) The actual connections ,
for eg, can be swept tees instead of square tees. (This will avoid
erosion from high velocity flash steam and blasts of water from
blast discharge trap- inverted bucket or thermodynamic trap).
Condensate must not be discharged into a flooded return main. A
pumped condensate return main often follows the same route as a
steam line and sometimes , this proximity cures plant crew and the
discharge from mains drain traps are simply connected to it.
the condensate, or else..
Condensate from mains traps are
at saturation temperature. At this temperature, the armount of
flash steam released into the low pressure main has huge
This pushes the water already present violently out
of the way. Bubbles of flash steam go along the pipe, contact
cooler condensate or pipe walls and collapse . This leads to
Therefore, all condensate must be given a
chance to cool a little and then released into the return lines.
a generous condensate collection pocket ( like in a thermostatic
balanced pressure trap) which holds back condensate till it is sub
• A continuous discharge float trap also
does the job as a steady flow can be more easily absorbed into a
• An un- lagged cooling leg of 2-3 m
upstream of the trap is another option. It can provide storage
volume for condensate so that it can cool down sufficiently before
• Collecting all condensate from traps into a
receiver and then pumping it back is the ideal solutions. It must
be remembered that these are only compromises and a gravity fall
from trap to receiver is always the aim.
Piping Practice Prevents Water Hammer in Steam Systems
of the most common complaints against steam heat is that a system
sometimes develops a hammer-like noise commonly referred to as
water hammer. It can be very annoying. However, it may indicate a
condition which could produce serious consequences including
damaged vents, traps, regulators and piping.
There are two
types of water hammer that can occur in steam systems.
type is usually caused by the accumulation of condensate (water)
trapped in a portion of horizontal steam piping. The velocity of
the steam flowing over the condensate causes ripples in the water.
Turbulence builds up until the water forms a solid mass, or slug,
filling the pipe. This slug of condensate can travel at the speed
of the steam and will strike the first elbow in its path with a
force comparable to a hammer blow. In fact, the force can be great
enough to break the back of the elbow. Steam flowing in a system
at 10,000 feet per minute is traveling more than 100 miles per
hour. The slug of condensate is carried along by the steam flow.
• The second type of water hammer is actually
cavitation. This is caused by a steam bubble forming or being
pushed into a pipe completely filled with water. As the trapped
steam bubble looses its latent heat, the bubble implodes, the wall
of water comes back together and the force created can be severe.
This condition can crush float balls and destroy thermostatic
elements in steam traps. Cavitation is the type of water hammer
that usually occurs in wet return lines or pump discharge piping.
properly piped steam system should not produce water hammer of
Water hammer in steam lines is
normally caused by the accumulation of condensate.
installation details to prevent water hammer in steam lines
include the following:
pipes must be pitched away from the boiler toward a drip trap
station. Drip trap stations must be installed ahead of any risers,
at the end of the main and every 300 to 500 feet along the steam
traps must be installed ahead of all steam regulator valves to
prevent the accumulation of condensate when the valve is in a
• "Y" Strainers installed
in steam lines should have the screen and dirt pocket mounted
horizontally to prevent condensate from collecting in the screen
area and being carried along in slugs when steam flow occurs.
All equipment using a modulating steam regulator on the steam
supply must provide gravity condensate drainage from the steam
traps. Lifts in the return line must be avoided.
Hammer in Condensate Return Lines
installations, water hammer in condensate return lines is caused
by steam pockets forming and imploding. Frequently, the cause is a
rise in the discharge line from a trap or a high pressure trap
discharging into a low temperature wet return line.
in the return line after the trap will cause water hammer because
the temperature of the condensate leaving the trap exceeds 100°C.
The high temperature condensate flashes, causing steam bubbles to
form. As these steam bubbles are pushed into colder condensate in
the return piping, they implode and cause water hammer. The water
hammer will normally be worse during start up due to the cold
condensate lying in the return piping. As the temperature of the
return line increases above 100°C the water hammer often
stops. Many industrial applications install lifts to avoid
installing additional condensate return systems. When installing a
lift, the most commonly used trap is an Inverted Bucket Trap since
the open bucket design tolerates moderate water hammer check valve
helps isolate the trap from the water hammer forces and prevents
back flow of condensate when the steam supply is secured.
a trap discharges into a wet return line, flashing will occur.
Again, these steam bubbles implode causing water hammer. This
condition is often found where a high pressure drip trap is
connected into a pumped return line with lower temperature
condensate. Old steam guides showed the use of a diffuser pipe to
break up the high temperature condensate to reduce the size of
steam bubbles that occur. The guide showed welding a pipe
tangentially in the return line and drilling 1/8 inch holes at
least 1 inch apart. Other methods include using a heat exchanger
to blend the two temperatures or the use of fin tube radiation to
cool the trap discharge.
most common method used is to install a flash tank on the drip
trap discharge allowing the condensate to flash to 100°C and
then pumping the cooled condensate into the common return line.
installation details to prevent this type of water hammer are
possible, use gravity return lines. Properly sized return lines
allow condensate to flow in the bottom portion of the pipe and
flash steam to flow in the top portion of the pipe. The top
portion also allows efficient air venting during start up of the
system. • • • Water hammer can occur in pumped
discharge lines. A condensate unit is pumping condensate near
saturation temperature to an overhead horizontal run and then
drops down into a vented boiler feed tank. A negative pressure
develops in the horizontal pipe due to the piping drop into the
vented receiver. When the pressure falls below saturation
temperature, water hammer can occur. A 4 metre vertical drop can
allow 88°C condensate to flash and cause water hammer. This
condition can be remedied by either creating a back pressure at
the low point or by installing a swing check valve open to
atmosphere in the horizontal pipe. The swing check will open,
allowing air to enter and the vertical water column to drain away.
• This condition can also occur in the boiler feed pump
discharge line from a deaerator or pre-heat unit. In many
installations, the discharge lines run overhead, a check valve or
regulator valve is installed near the boiler, and a check valve is
installed at the pump discharge. If the check valve at the pump
discharge does not hold tight, condensate drains back to the
boiler feed unit, allowing the condensate in the discharge to
flash. A steam pocket forms at the high point. The result is water
hammer when the pump starts. This can be corrected by replacing
the check valve.
condensing pressures at the point of use tend to save energy. and
also reduce the amount of flash steam generated when condensate
from drain traps is discharging into vented condensate collecting
It is worth noting that if condensate is
continuously dumped to waste, perhaps because of the risk of
contamination, less energy will be lost if the condensing pressure
condensate. In determining how the condensate is going to be
returned there are basically two considerations:
Can the condensate return headers be run below (on a lower floor)
the coil outlet trap for gravity drainage and
Does the condensate have to be lifted to overhead condensate
it is possible to run the return header below the trap outlet this
would be the more practical method in regard to the flow of
condensate. Regarding other concerns, dropping the trap discharge
piping down normally necessitates floor penetrations.
the condensate dropping down from the trap discharge to the return
header the designer doesn't have to be concerned with the lack of
lift pressure. However, if there is no alternative but to return
the condensate overhead then the designer is going to require a
Steamline CRPS. condensate pump. If it is at all possible combine
the flow of two or more traps by routing a collection header and
running it to the condensate pump.
condensate pump is an equipment item that allows condensate or
other liquids to accumulate, by gravity, in the pump chamber under
low pressure. The condensate then gets pumped to its destination
by steam, air or inert gas pressure.
to a higher plane
There are forces the CRPS has to work
against, to lift condensate. First, a few terms.
The potential energy of condensate at a given point is called
• Pressure Head. The pressure the condensate in a
pipe exerts at the point.
• Static head. This is the
vertical height of condensate from the reference point.
A static head of 10 metres water column = pressure head of 1 bar g
or 1 kg/cm2g.
the CRPS is required to pump to a receiver against a static
Delivery Head of 20 metres, or, 2 bar g. It is filling from a head
of 1 metre, or, 0.1 bar g. This head of water above inlet
connection provides the energy to fill the pump chamber during the
Steamline CRPS has to work against the delivery head of 20 m. This
is because the Suction head pressure is not present in the pump
body during pumping and has no effect on the delivery head against
which the pump has to operate.
Friction head loss. The
energy lost in just trying to move the condensate through the
We have friction losses through the pipe and the various
pipe fittings. So, we take an extra "equivalent length"
of pipe fittings. This is added to the actual pipe length, to give
total equivalent length.
equivalent length = Actual length of pipe + equivalent lenth of
practice, pipe fittings are not more than more than an additional
10% of the actual pipe length.
equivalent length = Actual length + 10%
Pressure drop calculations
Please refer to Steamline
Quality Control file, SQC 202R1.
Considerations in steam piping.
used for steam or condensate is generally of two types, ERW
(Electric Resistance Weld) or Seamless. Generally, ERW class C is
used for condensate, and seamless Sch40 pipes are used for steam
When we cycle the pressures in the
boiler because of variations in load (the number of machines using
steam), the temperature fluctuates. When the temperature reduces,
the steam condenses and becomes wet steam. U-bends help by
trapping condensate to prevent water hammer.
the varying temperatures in the steam pipes expand or contract the
lines. U-bends help absorb this variation in pipe lengths.
Unfortunately, U-bends also reduce pressure. So, we have to make
an intelligent compromise between pressure and dryness.
is a subject on its own, and will be covered in more detail in