Objective 3: Carryover

Question 1

Carryover

Answer 1

Solid, liquid, or vaporous contaminants that leave the boiler with the steam

Question 2

Steam Purity

Answer 2

Lack of carryover with a particular volume of steam

Question 3

Effect of Carryover

Answer 3

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.

Question 4

What are 4 problems caused by Carryover?

Answer 4

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

Question 5

What are the 2 Broad Categories of Carryover Causes?

Answer 5

1. Mechanical

2. Chemical

Question 6

What are Mechanical Causes of Carryover?

Answer 6

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

Question 7

What is the Chemical Cause of Carryover?

Answer 7

Foaming: formation of foam on the surface of the water that is easily carried by the steam as it is lighter than water.

Question 8

What Boiler Water Conditions contribute to Foaming?

Answer 8

1. Presence of highly organic substances

2. Presence of high concentration of dissolved and suspended solids

Question 9

How does Foaming occur?

Answer 9

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.

Question 10

What is a Miscellaneous Effect of Foaming?

Answer 10

Inaccurate indication of boiler water level

Question 11

What are ways to prevent Mechanical Carryover?

Answer 11

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

Question 12

What are the 3 main ways to prevent Chemical Carryover?

Answer 12

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

Question 13

What are the 2 Types of Blowdown?

Answer 13

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

Question 14

How is the amount of Blowdown to be performed determined?

Answer 14

By taking regular tests of the solids concentration and adjusting blowdown to keep the concentration within a specified range or below a specified maximum.