Flashcards in Objective 3: Carryover Deck (14)
Solid, liquid, or vaporous contaminants that leave the boiler with the steam
Lack of carryover with a particular volume of steam
Effect of Carryover
When the water contacts heating surfaces or other surfaces in piping and equipment external to the boiler the solids will deposit out of the water.
What are 4 problems caused by Carryover?
1. Solid can deposit on Superheaters from entrained water boiling forming scale that restricts heat transfer and allowing corrosive activity to develop underneath the scales
2. Steam turbine blades can become fouled by solids, esp. silica, and water may erode the turbine blades.
3. Large slugs of water can cause thermal shock
4. Loss of production due to downtime for carryover repairs
What are the 2 Broad Categories of Carryover Causes?
What are Mechanical Causes of Carryover?
1. Poorly designed steam drum internals with poor steam/water separation
2. Operating with excessively high water level
3. Operating at higher than design steam rates
4. Priming: sudden slug of water entrained with the steam due to a sudden, drastic increase in firing rate
5. Misting: fog-like mist caused by steam bubbles bursting
What is the Chemical Cause of Carryover?
Foaming: formation of foam on the surface of the water that is easily carried by the steam as it is lighter than water.
What Boiler Water Conditions contribute to Foaming?
1. Presence of highly organic substances
2. Presence of high concentration of dissolved and suspended solids
How does Foaming occur?
Organic substances, e.g. oil, combine with the alkalinity in the boiler water to produce a soap-like substance. The solids strengthen the steam bubbles at the surface of the water, preventing them from bursting, and causing them to accumulate as foam.
What is a Miscellaneous Effect of Foaming?
Inaccurate indication of boiler water level
What are ways to prevent Mechanical Carryover?
1. Careful boiler design
2. Operating the boiler within the design parameters for water level and firing rates
3. Avoiding large and sudden swings in boiler load
4. Careful inspection and maintenance of steam separation equipment during boiler overhauls
What are the 3 main ways to prevent Chemical Carryover?
1. Eliminate all sources of oil and other high organics that might contaminate the feedwater
2. Inject antifoam agents into the boiler that will act upon the foam to weaken the bubble film to cause the bubbles to burst more easily or cause fewer but larger bubbles that readily collapse
3. Blowdown to control the concentration of solids in the boiler water so that they don't reach an amount that would contribute to carryover
What are the 2 Types of Blowdown?
1. Continuous Blowdown: removes a continuous flow of highly concentrated water from the steam drum
2. Intermittent Blowdown: removes suspended solids by regular, manual blowdown of water from the mud drum and/or lower water headers