GB2142420A

GB2142420A - De-aeration of water

Info

Publication number: GB2142420A
Application number: GB08413785A
Authority: GB
Inventors: Frederick Egglestone
Original assignee: BP PLC
Current assignee: BP PLC
Priority date: 1983-06-01
Filing date: 1984-05-30
Publication date: 1985-01-16
Also published as: GB8413785D0

Abstract

A steam generating system comprising a boiler and a de-aeration unit, the de-aeration unit comprising a reservoir 15 for de-aerated water adapted for connection at 19 to the boiler, a contact zone 16 connected to the reservoir and having an inlet 20 for a stripping gas and means 23 for dispersing water onto the contact zone whereby stripping gas and water flow countercurrently in the contact zone. <IMAGE>

Description

SPECIFICATION De-aeration of water The present invention relates to boilers and more particularly relates to de-aeration of boiler feed water.

Atypical steam generating plant for producing steam for use in process plant or for electricity generation uses the heat from a fuel e.g. coal, oil, nuclear, to boil water circulating in boiler tubes to produce high pressure steam. It is well known that de-eration of the water is required to reduce the problem of internal corrosion of the boiler.

A conventional de-aeration technique involves the raising of the temperature of the feed water so as to reduce the solubility of the dissolved gases such as oxygen and carbon dioxide. This may be achieved by spraying the feed water into a column so that it passes downwardly and countercurrentto heating steam, the de-aerated feedwater emerging at the base of the column. This technique requires the feedwaterto be heated to near its boiling point to sufficiently reduce the amount of dissolved gases.

The residual oxygen can then be removed by scavenging chemicals.

The present invention relates to a means for de-aeration of boiler feedwater which operates at lower feedwater temperatures allowing gains in process efficiency to be achieved.

Thus according to the present invention there is provided a method of de-aerating water suitable for use in a steam generating system comprising dispersing water into a contact zone and passing a stripping gas countercurrently to the water in the contact zone, the water being at a temperature substantially less than its boiling point whereby the stripping gas displaces dissolved gases from the water, the water then being passed to the steam generating system.

The method is advantageously used in a boiler system operating on substantially sulphur free fuels as otherwise if the flue gas temperature is significantly below 110"C then condensation of sulphuric acid may occur leading to boiler pipe corrosion.

The invention also includes a steam generating system comprising a boiler and a de-aeration unit, the de-aeration unit comprising a reservoir for de-aerated water adapted for connection to the boiler, a contact zone connected to the reservoir and having an inlet for a stripping gas and means for dispersing water onto the contact zone whereby stripping gas and water flow counter currently in the contact zone. The means for dispersing water onto the contact zone is preferably a spray head, the spray head being most preferably supplied by steam condensate from the boiler and make up water.

Preferably the steam generating system also has a steam inlet for de-aerating water when using sulphur containing fuels.

The stripping gas is preferably a hydrocarbon gas, for example, methane. The contact zone is preferably in the form of a packed column of inert material having a large surface area such as Raschig rings.

The invention will now be described by way of example only and with reference to Figures 1 to 3 of the accompanying drawings.

Figure 1 shows a schematic diagram of a simplified boiler system comprising a chamber 10 containing a burner 11 which heats pipes 12 containing circulating feedwaterto produce steam and has an outlet 13 to a chimney (not shown), the downstream portion of the chamber having a heat exchanger 14 for flue gas and inlet feed water.

Figure 2 shows a schematic diagram of a cold de-aerator system for the boiler system of Figure 1.

A containment vessel comprises a lower reservoir zone 15 having a cylindrical form leading to an upper zone 16 also of cylindrical form but of narrower cross sectional area.

The lower reservoir zone 15 contains boiler feedwater 17 and has an outlet 19 in its base to supply water to the boiler. There is also an inlet for a stripping gas such as hydrocarbon gas, the inlet of gas being controlled by valve 20. Inlet valve 21 allows the stripper gas to be replaced by steam in certain circumstances for example to prevent corrosion of the boiler occurring when the fuel gas contains sulphur. A level detector 22 enables the make up water supply to be opened and closed by means of line 18 so as to maintain the feedwater at an optimum level. The lower reservoir zone 15 also contains means for introducing a chemical oxygen scavenger.

The upper zone 16 has a spray head 23 connected to the water supply which is the steam condensate return 24 and make up water lines 25. Below the spray head 23 is a surface assisted contact zone 24 comprising a column of packed Raschig rings. A vent 25 allows the process to take place at a gas pressure slightly greater than that of the local air pressure so as to prevent air entering the containment vessel.

During use, hydrocarbon stripping gas is passed into the lower reservoir zone 15 through valve 20 so that it passes upwardly through the surface assisted contact zone 24. Feedwater is sprayed from the head 23, the spray falling onto and passing downwardly through the column of Raschig rings. Thus the stripping gas and feedwater undergo a direct contact continuous countercurrent flow process, the stripping gas displacing dissolved oxygen from the feedwater.

After the stripping gas has passed through the packed column it is passed to vent 25 for disposal.

The stripping gas used is generally the same as that used to fuel the boiler.

Figure 3 shows the temperature profile through the 'economiser' of two conventional boilers each of which has been designed to accept a different water inlet temperature. They each have the same minimum temperature difference between the water and the flue gas as this is the main factor determining the amount of heat transfer surface required. The difference in temperature and weight of flue gas rejected to the chimney represents the additional heat recovered.

Thus the above system can enable greater heat recovery to be obtained by feeding colder water to the 'economiser' section of a boiler.

Claims

1. A steam generating system comprising a boiler and a de-aeration unit, the de-aeration unit comprising a reservoir for de-aerated water adapted for connection to the boiler, a contact zone connected to the reservoir and having an inlet for a stripping gas and means for dispersing water onto the contact zone whereby stripping gas and water flow countercurrently in the contact zone.

2. A steam generating system according to claim 1 in which the water dispersing means is a spray head.

3. A steam generating system according to claim 2 in which the spray head is supplied from the boiler and make up water.

4. A steam generating system according to any of claims 1 to 3 and having a steam inlet for de-aerating water.

5. A steam generating system according to any of the preceding claims in which the stripping gas is a hydrocarbon gas.

6. A steam generating system according to any of the preceding claims in which the contact zone is in the form of a packed column of inert material having a large surface area.

7. A steam generating system as hereinbefore described and with reference to the accompanying drawings.

8. A method of de-aerating water suitable for use in a steam generating system comprising dispersing water into a contact zone and passing a stripping gas countercurrently to the water in the contact zone, the water being at a temperature substantially less than its boiling point whereby the stripping gas displaces dissolved gases from the water, the water then being passed to the steam generating system.

9. A method of de-aerating water suitable for use in a steam generating system as hereinbefore described and with reference to the accompanying drawings.