SUBJECT: Some more about condensate
What is condensate?
- Steam that has been condensed back into water by either
raising its pressure or lowering its temperature. Not to be
confused with demineralized, de-ionized, make up, or softened
water. When the condensate enters the boiler feed pump additional
chemicals are added and the product is now called boiler feed
Where does condensate come from?
- Condenser hotwells, the bottom part of the condenser
- Steam traps. They trap steam in the lines and let the
condensate drain through.
- Heat exchangers. Condensate must be removed to allow the heat
transfer. The condensate flows to the bottom where the steam trap
will open and allow the condensate to flow to the receiver. There
must be a positive differential pressure between the heat
exchanger and the condensate line so that the condensate will flow
out of the heat exchanger. If the differential pressure is not
there, a pump will have to be installed to remove the
- Or any other place that you are using steam.
We want to
keep dissolved oxygen out of condensate. Why?
- It will contribute to corrosion problems in the system.
Especially the boiler.
- Boilers like a ph of somewhere between 10 and 11.
- Hot water is almost the perfect solvent. Give it enough
time and it will dissolve anything. Remember that boilers have
to last thirty years or more. This means that water has plenty
of time to do its damage. It is the oxygen in the condensate
that makes condensate a strong oxidizing agent that can attack
- Some carbon seal faces can be attacked by high oxygen levels
in the condensate.
- The more gases entrained in the condensate the more likely the
pump will experience cavitation problems.
- The condensate temperature determines the amount of dissolved
oxygen. You are trying to conserve the energy (temperature) that
was added to the steam to keep the amount of dissolved oxygen
PPM DISSOLVED OXYGEN
The average level detected in condensate receivers is three parts
per million. This is almost one thousand times greater than the five
parts per billion level that can induce pitting corrosion.
How does oxygen get into the condensate
- Through the packing of condensate pumps. The stuffing box is
under a negative pressure, and air that is one third oxygen, leaks
- Valves above the water line can introduce oxygen as the
velocity of the water lowers the pressure at the valve stem.
- Flanges can have the same problem as valves.
- Oxygen is dissolved in makeup water that was added to the
boiler because of condensate leaks.
- Pumps with built in repellers that create a negative pressure
in the pump stuffing box.
How do you get rid of the dissolved
- Add chemicals to convert it. Hydrazine is an example. You are
adding hydrogen that will combine with the oxygen to form
- In nuclear applications it is common to add hydrogen to the
system for the same reason. Hydrogen and oxygen will combine to
form water in a neutron flux.
- Deaerate the condensate. This is normally done by heating the
condensate with steam in a deaerating tank that is located close
to the suction of the boiler feed pump.
- Convert to balanced, o-ring mechanical seals that will prevent
air from coming into the stuffing boxes of condensate pumps
- Seal valves above the water line and pipe flanges to prevent
air from entering the system.
Why are we
concerned about carbon dioxide in condenste systems?
- Carbon dioxide (CO2) will combine with water (H2O)
to form Carbolic acid (H2CO3) that will
lower boiler ph.
Where does the CO2 come
- Mammals exhale CO2. It enters
condensate and feed water:
- Through the packing in condnsate
pumps that take a suction on hot wells
- Valves above the water
Why do we have to use so much "make up"
water in our boiler ?
- Because we lose so much of it.
- Condensate pump discharge recirculation lines that are
trying to put a positive pressure on packing are a common
source of condensate loss.
- Boiler blow down is a major problem. Some boilers run with
a constant blow down because air that is entering the system is
changing the pH of the water, causing chemical addition that
increases the total solids, causing the need for additional
- Steam tools.
- Air ejectors that are used to create a vacuum in receivers
- Steam traps that drain to the ground.
What are some methods for conserving
- The discharge recirculation line used with packed pumps is a
big waste. Convert to a balanced o-ring seal and save a pile of
- Stop air from entering the system. The air is causing frequent
boiler blowdowns. You can easily seal flanges, valves and rotating
- Do not drain steam traps to the ground. Collect it in a tank
that can be pumped back into the system.
- If condensate is being circulated through the cooling jacket
on a pump, make sure it is not being discharged to a drain. There
is no reason it cannot be returned to the condensate system.
- If condensate is being circulated between dual mechanical
seals, it is not a good idea to return it to the condensate
system. There is too a high probability of contaminating the
condensate with product leakage.
For information about my CD
with over 600 Seal & Pump Subjects
Link to Mc Nally home page