CA1037790A - Method of inhibiting deposition of internal corrosion products in tubes - Google Patents

Abstract

METHOD OF INHIBITING DEPOSITION OF
INTERNAL CORROSION PRODUCTS IN TUBES

ABSTRACT OF THE DISCLOSURE

A method for inhibiting the deposition of porous corrosion products on the internal surface of fluid conducting tubes in a high heat absorbing zone of a boiler furnace by chrome plating the tubes internally.

Description

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BACKGROUND OF THE INVENTION
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Since the development of high-pressure boilers, operating at both sub~critical and super-critical pres-sures, the origin, formation and effects of internal corrosion product deposits have been a vital concern to the designer. Although, for sustained operation, an extremely limited degree of internal scale forma-tion is desired to protect the base metal from ~xida-tion, the deposition of excess corrosion products in porous form and in a unique wave-like pattern can re-sult in increased frictional and heat transfer resis-tances and the requirement for frequent chemical cleaning. This requirement of chemical cleaning re-sults in lower availability and costly outage time but is necessary if tube overheating, particularly in `
the high heat absorbing zones of the furnace, and forced outages are to be prevented. Efforts have been concentrated in the areas of cycle design and water treatment in an attempt to minimize the deposition of water contaminants. Presently5 condensate polishing -demineralizers are used for total solids control, and dissolved oxygen and carbon dioxide are controlled by deaeration and hydrazine treatment, but the gradual build-up of internal deposits persists. It has been found that even with optimum boiler water conditions, the basic iron and water reaction takes place continu-ously within a closed cycle. This reaction is a con-tinuous process of deposition, release, transport and ~ .

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; redeposition of corrosion products generated within the boiler, in addition to the corrosion produc-ts and water contaminants carried into the boiler from ex--ternal sources.

SHMMARY OF THE INVENTION

As a r~sult o~ extensi~,- field and la~oratory testing on operating units it has been found that, in accordance with the invention, chrome plating the in-ternal surface of the tubes, particularly in the burner or high heat absorbing zones of the furnace, markedly reduces the deposition of internal corrosion products.
In this regard it is important to differentiate between chrome plating to provide corrosion resistance to a base metal and chrome plating to inhibit the deposition of corrosion products from the fluid flowing within the tubes. Chrome plating has been used to provide corro-sion resistance to such items as autGmobile bumpers and trim and noble metals such as gold and silver have been plated on base metals for oxidation resistance and decorative effects. However, in accordance with the present invention, a novel and unexpected effect has been determined when chrome plating is applied to the internal surface of tubes conducting a fluid in a high heat absorption zone of a furnace which is manifested by a great reduction in deposition of corrosion products from the fluid flowing within the tube as compared to an adjacent unplated portion of the same tube.

.-BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a sectional side elevation of a forced -: ~ ., ~. ,. . :

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flow once-through vapor generator embodying the inven-tion.
Fig. 2 is a plan view in section of a portion of a furnace division wall taken along the line 2-2 - of Fig. 1.
Fig. 3 is a sectional view of a test section installed in the division wall.

DESCRIPTION OF THE PREFERRED EMBODIMENT
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In the drawings, the invention has been illus-trated as residing in a forced flow once-through vapor generator intended for central station use and speci-fically in the high heat absorbing zone of the furnace.
The unit, as depicted in Fig. 1, comprises a setting 10 having a gas passageway situated therein and including a furnace chamber 12 whose upper end opens for discharge into a horizontal gas passage 14 with the latter, in turn, discharging into a down flow or convection gas passage 16. The setting 10 includes -an upper chamber or penthouse 18 disposed above the furnace chamber 12 and the gas passages 14 and 16~ and isolated therefrom by a substantially gastight parti-tion wall or furnace roof 20. The penthouse 18 ex-tends upwardly from the partition wall 20 and laterally beyond the furnace and gas passage boundary walls to form an insulated chamber for housing headers, tubes and piping associated with the vapor generator. The roof 22 over the setting 10, which also serves as the penthouse roof, accommodates the passage therethrough of a plurality of upright members or rods 24 used to supportingly connect the vapor generator to cross beams , .. : - , .... . . , , . ,, . . :

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26, the latter being part of the structural steel work which includes upright columns 28 transmitting the weight load of the vapor generator to ground G.
The furnace chamber 12 is formed with a sloped bottom 13 which includes a discharge opening 30 com-municating with an ash hopper 32 disposed thereunder and ground supported by a plurality of stanchions 34.
The fuel firing equipment comprises independently operable fuel burners ~not shown) extending horizon-tally along the center line of respective burner ports36 located on the rear and front walls at the lower portion of furnace chamber 12. The furnace chamber 12 is bounded by high heat absorbing surfaces including the sloped bottom 13, the front wall 15, the rear wall 17, the secondary superheater 38, the partition wall or furnace roof 20, and side walls 19. Within this high heat absorbing zone are contained a multiplicity of division walls 21 spaced between side walls 19.
The gas passage 14 contains a secondary super-heater 38 and a reheater 40 arranged in series withrespect to gas flow. The gas passage 16 contains a primary superheater 42 having two banks also arranged in series with respect to gas flow.
The penthouse 18 is shown as housing the fur-nace front wall upper header 44, the secondary super-heat inlet and outlet headers 46 and 48, respectively, the reheater outlet and inlet headers 50 and 52, re-: spectively, the primary superheater outlet header 54 and the convection gas pass rear wall header 56. It ;::
should be understood that the penthouse normally houses numerous other headers as well as tube sections and _L~_ .. .
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-~ CASE 4007 ~ Q37790 piping. These were omitted from Fig. 1 so as not to encumber the illustration.
In Fig. 2 there is shown a plan sectional view along the line 2-2 of Fig. 1 of a portion of division wall tubes 21. Individual division wall tubes 23 are shown in tangent relationship and with interior plating 25.
Fig. 3 shows a test section of a division wall tube 23 approximately 8 feet long which was installed in an operating unit and removed after approximately 1-1/2 years in service. The test section consisted of four different internal tube surface materials which are tabulated in Table 1 with the weight of deposit on each section after this period of service.
Table 1 Piece Number ~aterial Deposit Wei~ht (Fig. 3) grams/ft 1 Croloy 5 - ITnplated 27.4

2 Croloy 1/2 - Chromium Plated 3.3

3 Croloy 1/2 - Nickel Plated 12.7 ;

4 Croloy 1/2 - IJnplated 18.0 This clearly shows the marked decrease in depo-sition of corrosion products on the plated sections with the chrome plated section superior to the nickel plated section although this also shows a decrease over the unplated sections. The deposit is porous and con-sists principally of magnetite, Fe304, in crystalline ~-form with a well-defined regular rippled or wave-like surface pattern with the wave crests at right angles to the longitudinal axis of the tube.

An additional test program was initiated on a _5_ . .:
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' CASE 4007 second operating unit in a different plant. A 10-foot section o~ unplated Croloy 1/2 tubing was removed from the division wall after 6 months service and a magnet-ite deposit weighing 20 grams per sq. ft. was found.
This section o~ tubing was replaced with a 10-foot section o~ chromium plated Croloy 1/2 tubing and after 6 months service under similar conditions the plated section was removed and a deposit weighing 1 gram per sq. ft. was determined, confirming the earlier results on the effectiveness of chrome plating in inhibiting internal deposits.
It will be understood that the invention may ' - be generally applied to the internal surface of all the high heat absorbing tubes of the furnace or wher-ever the deposition of porous corrosion products on ~ the internal surface of the tubes presents a problem.
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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. The method of inhibiting the deposition in solid form on the inside surfaces of heat absorbing vaporizable liquid conducting tubes of suspended solids, consisting principally of magnetite, Fe3O4, transported by the liquid in the tubes which have their out-side surfaces exposed to hot gases and are embodied in a closed cycle vapor generating system wherein almost all of the vapor generated is recovered as condensate and returned to the tubes, which comprises using ferrous metal tubes having a film of chromium or nickel on the inside surfaces of the tubes.