1. Correct Venting from a Boiler Feedwater Deaerator
Operators of steam plants must understand the intended operating modes of all the equipment and this is true
for the deaerator as it is for any other part of the steam plant. Unfortunately, in plants with relatively low to
moderate operating pressures, it is not uncommon for the importance of proper operation, monitoring and
efficiency of the deaerator to be overlooked. The deaerator often doesn’t receive the attention it deserves
because with relatively low operating pressure steam plants, say 200 psi and below, the levels of dissolved
oxygen is not as crucial as it is with higher pressure systems. In addition, chemical oxygen scavengers are
supplemented to enhance dissolved oxygen removal and this additional measure of chemical water treatment
provides confidence that the system is protected sufficiently. As a consequence, this supplemental chemical
treatment and added confidence sometimes induces complacency regarding the importance of deaerator
operation and efficiency and the implications of this can be reduced efficiency of the steam plant as a whole.
The deaerator heats water and removes oxygen to
acceptable concentrations. Because deaerators
generally perform very well, they often receive very
little attention. This is particularly true of systems
200 psi and below. Even if the oxygen removal
becomes less efficient than required, oxygen
scavenger dosing can simply be increased to
compensate for deficiencies in deaerator
performance.
Makeup water introduces considerable amounts of
potentially damaging oxygen into boiler and boiler
feed water systems. Oxygen can also enter through
the feed water and condensate return system. The
amount of oxygen and entrained air in systems and
the degree of mechanical and/or chemical
deaeration required can vary greatly. For example,
one rule of thumb in the deaerator industry is that
the vent valve passes 1/10 of 1% of the rated
deaerator capacity. This of course can vary greatly
for each specific system and the amount of non-
condensable gasses that need to be removed.
Pressure deaerators are commonly designed and
specified with removal efficiencies for dissolved
oxygen down to 7 ppb and carbon dioxide down to 0
ppb. When mechanical deaerators are
supplemented with chemical oxygen scavengers the
dissolved oxygen can be reduced further. A relatively
common rule of thumb is that a dissolved oxygen
level of 5 ppb or less is typically strongly
recommended for systems operating at 200 psi and
above while lower pressure systems, under 200 psi,
can generally tolerate levels up to about 40 ppb
dissolved oxygen. Equipment life can be extended at
little cost by limiting oxygen concentration in either
system to ≤ 5 ppb. Residual levels of chemical
oxygen scavenger should be maintained in order to
account for excursions in system operation and/or
equipment issues that may result in unexpected
elevated levels of dissolved oxygen.
With the high cost of steam, the deaerator vent
must be investigated to ensure excessive venting of
steam is not occurring yet still assure adequate
deaeration. Experience in evaluating mechanical
deaerator systems have revealed steam losses of
10’s of thousands of dollars and in some cases
losses that have exceeded $100,000.00 a year
because of unnecessary, excessive venting from the
deaerator. In individual cases, where the excessive
venting was addressed, water savings of greater
than 1,000,000 gallons per year have been realized.
Don’t just think about it as some wasted steam. The
system as a whole needs to be considered. Once that
is appreciated it is easy to see that the benefits of
minimizing excessive venting begin to multiply
quickly. Any water that is wasted thru excessive
venting of steam must be replaced. That means,
additional fuel, wasted valuable btu’s in the steam,
wasted water, reduced ratio of steam condensate
Adequate venting is required to assure protection of the system but excessive
venting may be costing you thousands of gallons of water and thousands of dollars.
Ask yourself these questions...
1. What type of deaerator do you have?
2. At what temperature and pressure does
your deaerator operate?
3. What is your deaerators design removal
efficiency for oxygen and carbon
dioxide?
4. What is the current oxygen
concentration of the water in your
deaerator storage section with and
without chemical oxygen scavenger
addition?
5. How much steam is your deaerator
venting?
If you cannot readily answer any one of these
questions, further educating yourself regarding
the specifics of your particular system may prove
to be very beneficial.

2. Correct Venting from a Boiler Feedwater Deaerator
returned which increases the cycles of concentration
in the boiler and required surface bleed to control
conductivity, increased chemical treatment
requirements, increased pretreatment of water for
makeup water to the system such as water softening
requirements, and so on. This also increases the
amount of wastewater that must be dealt with due
to increased surface bleed for conductivity control
and increased water softener regeneration
frequency, and so forth.
A balance between assuring adequate deaeration
and minimizing wasteful excessive venting should be
maintained. The benefits of minimizing excessive
venting beyond which is required is cumulative.
One relatively crude method of measuring adequate
vent plume and deaeration consists of observing the
height of the vent plume. Recommendations for
plume height vary from between 18 and 36 inches.
The question is often asked, “How big does the
deaerator plume need to be to achieve proper
deaeration”. The short answer is, proper deaeration
cannot be confirmed by the height of the plume
alone, yet frequently this is the only monitoring
method utilized to gauge deaerator operation.
Methods of measurement of dissolved oxygen are
often thought to be complicated, costly and include
instrumentation that can be hard for relatively small
systems to justify. There are in fact several methods
of measuring dissolved oxygen, some of which are
very economical and are relatively simple to use.
These include,
The AmpuImetric Method test offers ease
of operation and minimum time in
collecting reliable data. Ampoules are
available in the 0-100 ppb range, are
relatively easy to use, inexpensive and ideal
for small to moderately sized systems.
The Indigo Carmine Method is a
colorimetric procedure for determining
dissolved oxygen in the 0 to 100 ppb range.
Oxygen Analyzers offer accurate reliable
direct measurement in liquid streams. They
are used to monitor dissolved oxygen
continuously or intermittently at various
points in the condensate and feedwater
systems and are often used for more
critical, high pressure or larger, more
complex systems.
In summary, deaerators perform a vital function in
the safe, effective and efficient production of steam
and assuring the longevity of steam plant
equipment. However, knowledge of how they work,
and how they should be operated, maintained and
monitored is necessary if they are to perform
effectively and efficiently. Inefficient deaeration can
increase the cost of operations and drastically
increase water usage and wastewater discharged.
Quantifying dissolved oxygen levels and removal
efficiencies of the deaerator while also assuring
adequate yet not wasteful venting is imperative to
safe and effective operation as well as managing the
costs of operation and water usage.
By Robert Jewell Robert has 30 years of career experience in industrial utilities. He specializes in and has extensive
experience in the operation and management of steam plants, central plants, cooling and
refrigeration systems, potable (drinking) water systems, water treatment, wastewater treatment,
and facility management.
He is an advocate of water conservation, energy conservation and protecting our environment while
also incorporating those advocacies into sustainable water and energy efficiency activities and
programs that positively impact the organization and local community by finding efficiency and
savings in existing processes and procedures through creative solutions.
Connect with Robert on Linkedin at http://www.linkedin.com/pub/bob-jewell/26/810/32a