Water Chemistry Flashcards
What are cations, What are Anions
Cations are positively charged ions and Anions are negatively charged ions
What is pH
pH is defined as the logarithm of the reciprocal of the hydrogen ion activity. Or a measure of a substances acidity or alkalinity
What is sodium phosphate used for.
Sodium phosphate is a sludge conditioner that allows impurities to floc together and settle out of a solution
Name 3 conditions in a boiler that could cause foaming
High pH, high dissolved solids and colloidal substances in the boiler water could lead to foaming
Why do we treat feed water
To eliminate or reduce contaminates that cause scale, corrosion, and caustic embrittlement on boiler heating surfaces and unacceptable conditions in boiler water such as priming, foaming, and carryover
What are the two (2) sources of water?
Surface water is water that comes from lakes, ponds and rivers and Ground water from wells
What are the impurities in water
Suspended solids are organic materials such as mud silt and clay that you can see, they are removed by filtration.
Dissolved solids are mineral salts that can’t be removed by filtration. Examples include Calcium, Magnesium, and Sodium
Dissoved gases are entrained gases that exist in the water that are removed by dearation/degasification such as O2, CO2, and H2S
What pH level is maintained in boilers
We try to maintain the boiler pH between 9.5 to 10.5
What is foaming? What causes foaming in a boiler
Foaming is a layer of foam on the surface of the boiler water due to high alkalinity, high dissolved / suspended solids or the presence of oil or other colloidal solids
What is priming? What is carryover
Priming is the carryover of large slugs of water with the steam from the boiler. Carryover is the carryover of fine droplets of water and/or suspended/dissolved solids with the steam from the boiler
Where in a boiler will scale form
Scale will form in a boiler below the waterline
How are boilers laid up?
Boiler(s) are laid up using either the wet method (available on short notice) or the dry method (for longer periods).
How often should boilers be blown down
The bottom blowdown valves should be operated at least once per day but it is better to base the frequency on water testing. The use of surface/continuous blowdown will limit the use of bottom blowdown valves
What is galvanic corrosin
Galvanic corrosion ( happens in condensers) is an electrochemical process that occurs when two dissimilar metals are in contact in an electrolyte solution (water). the weaker metal gets attacked (higher corrosion rate). Cathode protection uses a small electrical current on the softer metal to reduce galvanic corrosion or use a sacrificial metal to let it get attacked rather than the tubes in a condenser per se
Name some common chemicals used in boiler feedwater treatment
- For O2 scavengers, Sodium sulfite, Na2SO3 and Hydrazine, N2H4
- For pH control, Sodium Hydroxide, NaOH and Ammonia, NH3
- For sludge conditioning Phosphates, NaPO4,
Polyphosphonates, and Chelants - For return line corrosion, Amines
Why do we treat feedwater
1 prevent sludge from depositing on boiler surfaces
2 prevent scale from forming on boiler surfaces
3 prevent corrosion of boiler metal
4 prevent carryover with the steam leaving the boiler
What is a common test for corrosion
pH
What is meant by phosphate hideout
Loss of phosphate residual on load upswing. Residual recovery on downswing due to resolubilizing. Temporary upset in the ability to neutralize caustic.
On high loads phosphate “hides out” on boiler surfaces and indicates no phosphate present during testing. You add more phosphate to the boiler and when you decrease load phosphate will resolubilize in the water. Now you have too much phosphate
What is caustic embrittlement
- Requires stressed condition such as cracked plate, riveted joint, porous condition, defects such as steam leaks
- Free caustic in system (NaOH) becomes concentrated and leads to crystalline cracking of the plate/ shell
What is the corrosion caused by Oxygen
Pitting
- main corrosion caused by Oxygen, happens around the NOWL.
- Active pitting has active pits indicated by a shiny surface
- inactive pitting has a layer of magnetite (orange brownish color)
Explain the corrosion caused by CO2
This is the main corrosion that happens in condensate return lines. CO2 in the lines can lead to the formation of carbonic acid which attacks the thickness of the pipe, especially at the threads
What are chelants
Chelants - EDTA, NTA, HEDTA
No precipitation
Work differently than phosphates by keeping impurities soluble in the water and allow impurities to be removed more easily by surface blowdown
How are polymers used in boiler water treatment
Polymers - Polyacrylite, Polymethacrylite, HTP-2/ULTRASPERSE
Keeps impurities in solution, allows for removal with surface blow down
Describe what is meant by a co ordinated phosphate program
- Primarily a solubilizing program with precipitation as upset control.
- Uses mono, di, and trisodium phosphate.
- Adjusts pH and provides phosphates to the boiler
- Requires some bottom blowdown
Describe what is meant by a phosphate program
Use just phosphate
- precipitating program (high bottom blowdown rates required)
- The phosphates cause impurities in the boiler to form a soft sludge. Impurities fall to the bottom of the boiler and is periodically removed by the botttom blowdown
How do we lay up a boiler wet
- Clean fire side and steam and water side of the boiler
- Close up steam and water side
- Fill the boiler with warm treated water until water comes out the vent
- close the vent
- maintain a pressure in the boiler slightly higher than atmospheric
How do we lay up a boiler dry
- Clean fire side and steam and water side of the boiler
- Dry out boiler completely
- Close all valves on the boiler to prevent moisture from entering drums
- Lay trays of moisture absorbing chemicals in boiler drums
- Close steam and water side
- Check moisture absorbing chemicals periodically and replace when required
What is meant by cycles of concentration
It is the concentration of dissolved solids in water divided by the dissolved solids in the make up water to the boiler. This indicates the number of times the water in the boiler has cycled up. ASME CODE Section 7 provides guidance for cycles of concentration
What is scale
Scale is an inorganic mineral matter precipitated from the boiler directly onto boiler surfaces. It is usually crystalline , dense, and laminated. It interferes with heat transfer
What is sludge
A sedimentary deposit formed by suspended matter or by precipitation due to chemical treatment (phosphates)
What are chlorides
Chloride ions( chlorides) are mineral salts that can cause scale to form on boiler surfaces and are used to monitor cycles of concentration and control blowdown rates. The chlorides test is most conveniently done on the P alkalinity test done on boiler water. This is because the chlorides test will not work on high pH boiler water. P alkalinity neutralizes the pH in that test
