Objective 11: Deaerator

Question 1

What 4 sections does a deaerator usually contain?

Answer 1

1. Heater Compartment: water is heated to saturation temperature
2. Scrubber Compartment: released gases are scrubbed from the water
3. Storage Section: stores the deaerated water
4. Condensing Section: For condensing any steam that is being carried to the vent along with the released gases

Question 2

What are the 3 types of deaerators?

Answer 2

1. Spray type
2. Tray type
3. Bubbler type

Question 3

Describe how a Spray Type Deaerator works

Answer 3

1. The entering water passes through nozzles, which divide it into a fine spray
2. In the heater compartment, the water is heated by direct contact with steam
3. It then passes to the scrubber compartment where it is scrubbed and further heated by entering steam
4. From the scrubber, the deaerated water flows to the storage section
5. The steam flows to the heater compartment where it heats the incoming water spray and most of the steam condenses
6. The released gases flow to the vent condensing section where most of the steam present is condensed and the gases pass out of the vent to atmosphere

Question 4

What is the purpose of heating the water in deaerators?

Answer 4

This causes any free oxygen and carbon dioxide to be released from the solution.

Question 5

Describe how Tray Type Deaerators work

Answer 5

1. The water flow is broken up by trickling down over a series of trays
2. The entering steam scrubs the water in the lower trays and heats the water in the upper tray section
3. The released gases and remaining steam pass to the internal vent condenser, where most of the steam is condensed and the gases are vented to the atmosphere

Question 6

How do some Deaerators combine the Spray and Tray principles?

Answer 6

Water is first sprayed into the upper heating section of the deaerator and then trickled over a series of steam-swept trays

Question 7

When the Steam and Tray principles are combined in a Deaerator what happens to the released gases and remaining steam?

Answer 7

They pass through the incoming water spray on their way to the vent opening. This causes most of the steam that is being carried with the gases to condense

Question 8

How do Bubbling Deaerators work?

Answer 8

Steam is forced up through perforated panels upon which water flow. The steam bubbling up through the water causes agitation and intimate contact between the steam and water, thus effecting deaeration. There may be several trays in a stack and the steam rises up through each of the trays as the water descends.

Question 9

At what pressures are deaerators designed to operate at?

Answer 9

35-70 kPa

Question 10

What are the 2 conditions that cause oxygen to escape from the water?

Answer 10

1. Low Pressure

2. High Temperature

Question 11

Why is the vent to atmosphere always open in deaerators?

Answer 11

1. To remove oxygen

2. To reduce internal pressure

Question 12

What is the function of the pressure control valve in deaerators?

Answer 12

They control the steam entering the deaerator by reacting to the pressure in the deaerator

Question 13

Why do many deaerators have a second steam valve?

Answer 13

To remove steam from the deaerator if there is excess pressure

Question 14

How is water level in the storage section of deaerators controlled?

Answer 14

1. Float-operated level control valve

2. Overflow valve

Question 15

How is a controlled level of water in the storage section of deaerators maintained during normal operation?

Answer 15

Adding water from the treated water storage tank

Question 16

Why is it important to maintain water level in deaerators?

Answer 16

To ensure sufficient level for the feedwater pumps which take suction from the deaerator

Question 17

What can happen during periods of low flow in deaerators?

Answer 17

Steam can condense and cause the level to slowly increase

Question 18

What is problematic about the water level getting too high in deaerators?

Answer 18

Efficient deaeration is only possible up to a certain level

Question 19

How is overflow of water in deaerators prevented?

Answer 19

An overflow valve will open

Question 20

Why are deaerators normally elevated several meters above the inlet to the boiler feed pumps?

Answer 20

To ensure sufficient head pressure at the pump inlet to prevent cavitation and subsequent damage to the pump housing and impeller

Question 21

What concentration of oxygen in water is aimed for to reduce corrosion?

Answer 21

5-7 ppb

Question 22

What chemical can be added to deaerators to aid in oxygen removal?

Answer 22

Oxygen scavengers

Question 23

How do tests determine the effectiveness of oxygen removal in deaerators?

Answer 23

If there is a residual of oxygen scavengers