GB2263330A

GB2263330A - Superheater tube protection device

Info

Publication number: GB2263330A
Application number: GB9200484A
Authority: GB
Inventors: Robert Geoffrey Ambler
Original assignee: Individual
Current assignee: Individual
Priority date: 1992-01-10
Filing date: 1992-01-10
Publication date: 1993-07-21
Also published as: GB9200484D0

Abstract

A steam superheater tube 1 is protected from erosion by flue gases In a boiler by a parallel sacrificial surrounding tube 9 and flow of air or other cooling fluid through the space between the tubes. As shown the pressure differential between the boiler interior and boiler house causes the cooling air flow. The protective tube may be split longitudinally, have a replaceable portion or have an obliquely cut lower end and both tubes may have a protective coating. <IMAGE>

Description

Superheater Tubes, An Active Failure Protection evince.

This invention relates to an active method of protection of superheater tubes in water tube boilers. It refers to installations of tubes where a repetative failure point(s) has been established. Reheaters, a particular form of superheater, are also covered by the following remarks.

Water tube boilers used for raising steam frequently employ superheaters.

These are used to heat steam to temperatures above the steam saturation temperature. The design of superheaters is a complicated balance of many factors. Some of these factors cannot be determined accurately under all operating conditions by the designer. In use superheater tubes tend to fail individually often involving a short part of a tube.

Superheater tube failures occur when external wastage of the metal thins the tube wall. When this cannot withstand the internal steam pressure the wall ruptures. After this occurs the boiler involved is rapidly shut down when this is possible. A rapid shut down limits secondary damage to the superheater and a small repair can often be made. However small an unscheduled repair of a boiler superheater tube is the total consequences are invariably expensive and very disruptive.

Tube wastage is not uniform in a superheater. Patterns of wastage, where there are consistently higher than average wastage rates, can often be seen. While redesign and modification of the superheaters could reduce some of the higher wastage rates it is rarely practicable. For this reason boiler plant operators employ an array of measures intended to control wastage rates on the tubes where this occurs at higher than average rates.

According to the present invention there is provided a sacrificial surface as a protective tube on a common or parallel axis with, and surrounding, a length of superheater tube requiring protection, the inner diameter of the protective tube being larger than the outer diameter of the superheater tube such that there is a continuous space, this space being cooled by a supply of a cooling medium, the protective tube providing the sacrificial outer surface where this outer surface material loss is controlled by the active regulation of the wall temperature.

A specific embodiment of the invention in a simple form will now be described by way of example with reference to the accompanying drawing in which : Figure 1 shows in perspective, a simple superheater and a zone of heavy wastage.

Figure 2 illustrates the protective tube fitted in the zone of heavy wastage.

Figure 3 shows the set screws used to set the position of the the protective tube.

Referring to the drawing figure 1 shows where the superheater tube 1, one of many similar tubes, is connected to the saturated steam supply header 2 and a super heated steam return header 3. The connections to both headers pass through the boiler casing 4. A typical zone of the superheater tube arrangement subject to a higher than average wastage is shown between 5, 6, 7, & 8. The zone contains superheater tubes at the highest metal temperatures due to the patterns and temperatures of gas flows and the direction of the steam flow in the superheater tube. The direction of the gas flow is shown by the arrow 10.

Figure 2. shows a superheater tube 1 connected to a saturated steam supply header 2 and a super heated steam return header 3. The connections to both headers pass through the boiler casing 4. The section of the superheater tube subject to higher than average wastage is a length shown between 5 and 6. The gas flow direction is shown by the arrows 10. A tube 9, with an internal diameter sufficiently larger than the outside diameter of the superheater tube, is fitted to form a continuous annular space 11. In this case the two tubes are fitted on a common axis. The relative positions of the two tubes are determined by set screws 12. The outer tube 9 is terminated just below 6 and is cut obliquely, the longer side facing the flow of gases in the boiler. The other end of the outer tube 9 is terminated outside the boiler casing to provide an entry for air from the boiler house 13.The gas flow shown by the arrows 10 has a maximum velocity normally found between them. Where the static pressure differential between the boiler house and that in the gas flow in the superheater section provides an air flow adequate to cool the outer tube no additional means are applied to promote the flow of cooling air.

Figure 3 shows an installation of set screws 12, at the top as an end view and below as a part of a cross section. These are adjusted once only when the protective tube is fitted. They are used to establish the shape and position of the space between the superheater and protective tubes. The number fitted and their positions are sufficient to achieve this purpose.

If the set screws are inadequate to maintain the vertical position of the protective tube an external collar may be added such that the boiler case or other external parts of the boiler carry the added weight of the tube 9 and any adhering solids.

Some of the variables which affect the above design are as follows: The required life of the protective tube 9, the length of this tube, this tube material, the wall thickness of this tube, the cross sectional area and shape between the two tubes, the temperature range of the boiler gases at this point, the range of the various boiler gas component concentrations at this point, the design constraints imposed by the size of the gaps between superheater tubes at right angles to the gas flow, variable rates of erosion, corrosion and fouling, and others.

Where dimensional constraints will allow it, the protected superheater tube and the protective tube external surfaces should have a thin thermally resistant coating applied. These coatings improve the performance of the invention in the following way. The heat flux to the protective tube from the boiler gases can be reduced. The heat flux to the cooling air between the protected and protective tubes can also be reduced. Less cooling air might then be needed to achieve a given protective tube wall temperature with the coatings than without them.

The outer surface area of the protective tube available for heat transfer is increased by the coating. Also where two or more adjacent tubes are treated in the manner described and the line between their centres is at right angles to the gas flow there is a reduction in the area available for gas flow. This will increase the gas velocity between the protective tubes and possibly the gas to tube heat transfer rate. This could increase the air flow needed to keep the protective tube wall temperature at an acceptable level.

Any reduction in the required cooling air flow rate reduces the thermal losses that this air flow creates. The most important of these losses is the heat flux from the superheater tube. As this is reduced a higher super heated steam temperature is achieved.

The preceding factors should be considered in the light of the conditions applying to any proposed application.

Claims

1. A sacrificial surface as a protective tube on a common or parallel axis with a length of superheater tube requiring protection, the inner diameter of the protective tube being such that the inner surface of this protective tube and the outer surface of the superheater tube covered by it form a continuous space, this space being cooled by a continuous supply of a cooling medium, the protective tube providing a sacrificial outer surface where this outer surface loss is controlled by the active regulation of the temperature of the protective tube. The protective tube would be constructed from metal although other materials are not excluded from this use. The material used would be selected with reference to the gas temperature and analysis at the projected point of use.

2. A sacrificial surface as a protective tube as claimed in Claim 1 wherein both the outer surfaces of the protective tube and the protected superheater tube are provided with a thermally resistant coating, or either singly.

3. A sacrificial surface as a protective tube as claimed in Claim 1 or Claim 2 wherein the axis of the protective tube might be moved with respect to the axis of the superheater tube to accommodate differences in the heat flux to the periphery of the protective tube.

4. A sacrificial surface as a protective tube as claimed in Claim 1 or Claim 2 or Claim 3 wherein the flow of the cooling medium is increased by mechanical means.

5. A sacrificial surface as a protective tube as claimed in Claim 1 or Claim 2 or Claim 3 or Claim 4 wherein the protective tube wall temperature is measured using a thermocouple or similar device sensing from a position close to a point where previous superheater tube failures have occurred.

6. A sacrificial surface as a protective tube as claimed in Claim 1 or Claim 2 or Claim 3 or Claim 4 or Claim 5 wherein the thermocouple output is used with other equipment to regulate the flow of the cooling medium.

7. A sacrificial surface as a protective tube as claimed in Claim 1 or Claim 2 or Claim 3 or Claim 4 or Claim 5 or Claim 6 wherein a part of the sacrificial tube might be a replaceable item.

8. A sacrificial surface as a protective tube as claimed in Claim 1 or Claim 2 or Claim 3 or Claim 4 or Claim 5 or Claim 6 or Claim 7 wherein the cooling medium might contain more than one component.

9. A sacrificial surface as a protective tube as claimed in Claim 1 or Claim 2 or Claim 3 or Claim 4 or Claim 5 or Claim 6 or Claim 7 or Claim 8 wherein the protective tube is formed from two or more parts longitudinally joined to simplify the installation over existing superheater tubes.

10. A sacrificial surface as a protective tube as claimed in Claim 1 or Claim 2 or Claim 3 or Claim 4 or Claim 5 or Claim 6 or Claim 7 or Claim 8 or Claim 9 wherein the inner surface of the protective tube might be extended to increase the quantity of heat transmitted to the cooling medium per unit of area of the outer surface of the protective tube.

ll.A sacrificial surface as a protective tube substantially as described herein with reference to Figures 1 and 2 of the accompanying drawing.