Spirax Sarco, Inc.
Summary: The following article is a part of National Board
Classic Series and it was published in the National Board BULLETIN. (6 printed pages)
Boiler efficiency measures how much combustion energy is converted into steam
energy, while steam quality measures how much liquid water is present in the
A major benefit of using steam as a heat transfer medium is the large amount of
heat released when it condenses into water. With a latent heat of vaporization
(or condensation) as high as 1,000 BTU per pound, it takes very little steam to
carry a large amount of energy. Other advantages include the safe, nontoxic and
nonflammable characteristics of steam plus its ability to deliver heat at a
constant, controlled temperature. Steam can also be delivered to users with
conventional piping and valve equipment that is inexpensive, is readily
available, requires little maintenance, and has a long service life. Compared
to other heat delivery and distribution systems, steam is less expensive to
operate and is 100% recyclable.
In spite of these advantages, many steam users experience system safety
problems, premature equipment failures, and poor steam system efficiency.
Specific problems can include frequent boiler shutdowns from low-water level,
damaged steam pipes and valves due to water hammer, vibration, corrosion,
erosion, reduced capacity of steam heaters, and overloaded steam traps. These
problems are most frequently caused by low steam quality, often called "wet
steam" or "carry-over."
Steam quality is a measure of the amount of liquid water contaminating the
steam. (For example, steam at 100% quality contains no liquid water and appears
as a 100% clear gas, while steam at 90% quality contains 90% steam by weight
and 10% water by weight in the form of a fog, cloud, or droplets.) Water
droplets in high-velocity steam can be as abrasive as sand particles. They can
erode pipe fittings and rapidly eat away at valve seats. And if a puddle of
water is allowed to accumulate in steam pipes, it will eventually be picked up
by the high-velocity steam and accelerated to near-steam velocity, increasing
chances of it crashing into elbows, tees, and valves. This can lead to erosion,
vibration, and water hammer. This water hammer will gradually - and sometimes
catastrophically - loosen pipe fittings and supports.
Since steam is produced by the rapid boiling of water in high-heat flux
boilers, it can entrain (or draw in and transport) water as it escapes from the
water surface. This entrainment, while damaging to the steam system, is
independent of boiler efficiency. Basically, both high- and low-efficiency
boiler operation can produce - or not produce - excessive entrainment. While
entrainment cannot be completely prevented, it can be minimized by proper
boiler and steam system operation.
Case I, On-Off Boiler Feed
In a simplified explanation of boiler operation, a hot heat-transfer surface is
covered with water. Steam bubbles are produced at the heat-transfer surface,
rising through the water and then leaving the water surface to enter the steam
system. Because of the heat of water, the pressure at the heat-transfer surface
is slightly higher than the pressure at the surface of the water. Because of
this higher pressure, the steam bubbles produced at the heat-transfer surface
will either leave the boiler slightly superheated or be cooled to the
saturation temperature of the water as it rises through the water. Under normal
conditions, the steam bubbles tend to be cooled to saturation temperature as
they rise through the water.
When feedwater enters the boiler, it enters between the heat-transfer surface
and the surface of the boiling water. Even though the feedwater is pre-heated,
it is still necessarily colder than the water in the boiler and creates a cold
layer within the boiler water. As steam bubbles rise from the heat-transfer
surface through this cold layer, they cool and some of the steam in the bubbles
will condense. This causes two serious problems.
First, the steam bubbles leaving the surface of the water and entering the
steam system will contain a mist of water. When a large amount of feedwater
enters the boiler, the steam space above the water level becomes foggy. This
fog and the resultant water-contaminated, low-quality steam continue until the
water in the boiler becomes reasonably isothermal.
The second problem is the suppression of the rate of steam production. The
addition of a large amount of cooler water slows steam production until the
water reaches saturation temperature.
These problems can be prevented by using continuous boiler feed rather than
on-off feed. Since modulating feed adds water at a very low rate compared to an
on-off feed, the water in the boiler will remain relatively isothermal and no
cloud will be formed.
Case II, Reduced Operating Pressure
"Operate the boiler at its maximum design pressure" is a common saying among
boiler designers. But too often, this rule is not followed when energy cost
reductions are needed. During periods of low steam demand, or when all the use
points require pressure-reducing stations, boilers are often operated at
substantially less than design pressure.
While operating at lower pressure can, in some boilers, provide slightly higher
energy efficiency, low-pressure operation also reduces steam quality. This
reduced steam quality can be demonstrated from basic engineering principles.
Lower Pressure Increases Entrainment
As a steam bubble rises through the water and reaches the surface, it finally
breaks through the final layer of water and enters the steam space. This final
act of leaving the water causes water entrainment in several ways.
Initially, the bursting of the steam bubble or the rupture of the thin layer of
water surrounding it produces an initial rush of high-velocity steam that
carries a small amount of that thin water layer into the steam space. Then, the
loss of the steam bubble from the water surface briefly creates a crater on the
water surface. Water rushes in to fill this crater, colliding with water
rushing from the other sides of the crater, and produces a tiny splash near the
center of the crater. The water droplets from these splashes are then easily
entrained in the rising steam.
The size of the bubbles is directly related to steam pressure. Low-pressure
operation requires a larger volume of steam to carry the required heat energy.
This low-pressure operation produces more and larger steam bubbles and creates
greater turbulence on the water surface. These bubbles produce more craters and
larger craters, as well as more and larger splashes as they leave the water
surface. In addition, low-pressure operation results in a higher vapor velocity
which, when combined with the high turbulence of low-pressure operation, tends
to carry water droplets into the steam systems rather than allowing them to
fall out by gravity.
The solution is to operate the boiler at its maximum design pressure and use
pressure-reducing valves at the point of use where required.
Case III, Rapidly Fluctuating Demand
In most industrial steam systems, steam demand fluctuates over a wide range.
The rate at which these fluctuations occur can seriously affect steam quality.
A rapid, short-term steam demand increase of only 15% can cause high
entrainment of water in the boiler. Demand increases of 15% or more can occur
quite frequently in industrial plants when steam valves are opened all at once
at shift changes and as batch processes come online. For example, if a process
of which steam consumption is only 5% of boiler output is turned on rapidly
(such as with an on-off valve), the system demand can easily increase by 15% or
more until the process reaches a steady state of operation.
When a steam valve opens, two problems occur in the boiler. First, steam
pressure drops rapidly. The drop in steam pressure itself causes additional
entrainment as explained in Case II above. Second, the interface between water
and steam rises. This occurs because at the instantaneous lower pressure
operation, the rapid production of high-volume steam bubbles can literally
fluidize the water. (This phenomena is often called "swell.") The water level
can easily rise so high that water is literally sucked into the steam line.
Eventually, the loss of boiler water can cause the low-water level alarm to
sound. In some cases, this water loss can be so rapid that the boiler will shut
down upon producing a low-level water alarm. In the meantime, the steam lines
get filled with water.
Compact Boilers Can Magnify the Problem
Modern boilers are highly efficient and very compact. While this design has
advantages, these compact boilers have little steam space to dampen changes in
steam demand. If steam use increases only slightly, the pressure in the boiler
can drop significantly. This lower pressure operation, combined with the
shorter distance between the water/steam interface and the steam outlet pipe,
further increase entrainment. Older boilers, while much larger, have a larger
steam space which can tolerate greater changes in steam demand without severe
changes in steam pressure or water level.
High Entrainment Fools Low-Water Level Alarm
In some circumstances, steam demand increases are so disruptive to boiler
operation that boiler life as well as steam quality suffers. In some cases, the
external water level indicator shows the water level as satisfactory, yet the
actual level of the water/steam mixture in the boiler may be filling the steam
space and water may be literally pouring into the steam lines by steam demand
As water is lost rapidly and the steam/water mixture contains more and more
steam, tubes may overheat. By the time the external water level detector
eventually identifies a low-water level and shuts down the boiler, the steam
distribution system will be laden with water and boiler tubes may have been
damaged. Of course, the plant will now be without steam until the boiler is
The key to reducing this cause of poor steam quality is to prevent rapid
increases in steam demand. Modern computerized control systems can accommodate
this solution by measuring instantaneous steam flow or modulating demand based
on a maximum allowable change in steam flow.
Case IV: High TDS
Conventional wisdom teaches that high total dissolved solids in boiler water
increase tube corrosion and/or fouling. Indeed, that is true. High or highly
fluctuating TDS will result in low heat transfer, reduced boiler capacity and
efficiency, and shortened tube life. But it can also affect steam quality.
Increased TDS in the boiler water causes increased foam production on top of
the water. This low-density, two-phase system's foam is produced and easily
entrained by the steam rising out of the water. As rapid drops in steam
pressure caused by demand increases, this foam can be drawn into the steam
system, depleting the boiler of water before the level detector can identify
the problem, while filling the steam lines with corrosive water.
The solution is obvious - keep TDS at least as low as that recommended by the
boiler manufacturer. Unlike boiler feedwater, there is no definitive evidence
indicating a steam quality difference between on-off or modulating blowdown to
control TDS. However, given the adverse effect of rapid and intermittent
inflows of make-up water, modulated blowdown would be preferred.
Steam quality is a measurement of the amount of water entrained in the steam.
It depends not on the efficiency of the boiler but on the ability of the steam
to separate from boiling water, without carrying liquid water particles with it
throughout the entire range of boiler operations. Video camera studies of
internal boiler operation indicate the following operating recommendations for
preventing poor quality steam:
Control steam usage to ensure that steam demand does not exceed boiler
Control changes in steam usage to ensure that rapid changes in steam demand
will not reduce steam quality.
To affect either of the above, use modulating versus on-off valves at steam-use
Add boiler feedwater with modulating versus on-off controls.
Use TDS controls rather than time-based blowdown.
Operate the boiler near its maximum design pressure.
When any of these recommendations are not followed, reductions in steam quality
can be dramatic. Low steam quality can damage steam equipment, control valves,
and heat exchangers by water hammer, erosion, and corrosion, resulting in
shortened equipment service life, steam loss, low operating efficiency, and
even safety problems.
Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition and addenda of the ASME Boiler and Pressure Vessel Code for current requirements.