Boiler feed water
Water changes its name throughout a steam system:
- It exists as both water and steam inside the boiler.
- It is called steam in the piping
- When it leaves the turbines and those pieces of equipment that uses the steam. the name changes to condensate.
- The name again changes to boiler feed water when the condensate leaves the boiler feed pump
- See the following diagram:
The above drawing describes a very basic steam system with the following components:
- B = The boiler where the steam is made
- T = The turbine connected to a generator where we turn the energy in the steam into electricity.
- C = The condenser where the steam is converted into hot water (condensate). Usually cold water circulates through the tubes to assist in the condensing.
- HW = The condenser hotwell where the condensate is collected. The hotwell is usually in a vacuum created by condensing the steam and ejectors installed on the condenser.
- CP = The condensate pump that will take a suction on the hotwell and send the condensate to the boiler feed pump.
- BF = The boiler feed pump that will discharge condensate and make up water into the boiler.
- ST = The surge tank that will compensate for the differences in volume between the boiler feed pump and the condensate pump. Sometimes steam is added to a tank like this and then the tank it is called a de-aerator.
- Pot = The chemical pot allows you to add chemicals to the boiler to control the pH, water hardness, oxygen level and anything else that needs controlling by the addition of chemicals
As described in the above drawing, condensate is steam that has been condensed back into water. Condensate should not 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 water.
Where does condensate come from?
- Condenser hotwells located at the bottom part of the condenser
- Steam traps. They trap steam and let the condensate drain through.
- Heat exchangers. Condensate must be removed to allow the heat transfer. The condensate flows to the bottom where a 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 condensate.
- Or any other place that you are using steam.
We want to keep air out of condensate. Why?
- Air contains both carbon dioxide and oxygen along with other gases. Carbon dioxide will lower the pH of the water. The CO2 combines with H2O (water) to form H2CO3 (Carbonic acid) that will lower the pH and contribute to corrosion problems in the system. Especially the boiler.
- Stainless steel boilers like a pH of somewhere between 9.0 and 11. Check for the pH requirement of your boiler
- 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 metals.
- 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 down
- 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 do oxygen and other gases get into the condensate system?
- Through the packing of condensate pumps. The stuffing box is under a negative pressure and air that is one third oxygen, leaks in.
- Valves located above the water line can introduce oxygen and carbon dioxide 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 make up 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 oxygen?
- Add chemicals to convert it into something else. Hydrazine is an example. You are adding hydrogen that will combine with the oxygen to form water.
- 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.
- De-aerate the condensate. This is normally done by heating the condensate with steam in a de-aerating tank that is located close to the suction of the boiler feed pump.
- Use balanced, O-ring mechanical seals that will prevent air from coming into the stuffing boxes of condensate pumps. Balanced seals can seal both pressure and vacuum.
- Seal valves and flanges to prevent air from entering the system.
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 blow down..
- Steam tools.
- Air ejectors that are used to create a vacuum in receivers etc.
- Steam traps that drain to the ground.
What are some methods for conserving condensate?
- The discharge recirculation line used with packed pumps is a big waste. Convert to a balanced o-ring seal and save a pile of condensate.
- Stop air from entering the system. The air is causing frequent boiler blowdowns. You can easily seal flanges, valves and rotating shafts.
- 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.
- On February 14, 2018