Liquid heaters and vaporizers – Cleaning
Reexamination Certificate
2001-03-22
2003-02-25
Lu, Jiping (Department: 3749)
Liquid heaters and vaporizers
Cleaning
C122S380000, C122S390000
Reexamination Certificate
active
06523502
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
THIS INVENTION relates to a method of, and apparatus for, purging loose magnetite from boilers. The invention is particularly suitable for, but not limited to, removing exfoliated magnetite from the bends and inner bores of chromium stainless steel boiler tubes.
2. Prior Art
Chromium stainless steel superheater and reheater tubes, which operate in a high steam temperature environment for extended periods, develop a two-part oxide layer on the inner bore of the tubes. The oxide, which is given the general name “magnetite”, is characterised by two distinct phases—an outer layer (closest to the tube centre) which is iron rich, and an inner layer which is chrome rich. The two layers have very different coefficients of expansion. The process of cooling out the boiler induces large stresses between the two oxides. At metal temperatures of approximately 90°-150° C., the outer layer of magnetite oxide tends to delaminate from the tightly adhering inner layer and parent metal.
Exfoliation of this outer layer and a small amount of inner layer causes magnetite to fall and partially or totally block the bottom of vertical or pendant superheaters and reheaters. Once a total blockage occurs in a tube, the steam flow paths being established in a boiler as it builds steam pressure bypass this blocked tube (due to small differential pressures within parallel paths of the superheater/reheater pendants). The lack of cooling steam causes overheating of this blocked tube leg and results in failure of the tube through short term overheating. The time and point of failure is often not detected.
Rupture of a single tube results in that tube moving violently amongst neighbouring tubes. This permits other tubes to become damaged and potentially rupture. Steam impingement on nearby tube walls becomes another mechanism of failure. Eventually, so much steam will be lost to the gas path side of the boiler that a gas side pressure excursion will remove the boiler from service, or the boiler feed pumps will not maintain the condensate feed. Several weeks may be required to repair the damage from several hours of damage caused by a ruptured tube.
Callide, Tarong and Stanwell Power Stations in the State of Queensland, Australia, have superheaters that are vertical pendants made of 321 stainless steel, and operate at high metal temperatures (believed to be 580°-640° C.) for extended periods. The combination causes a large amount of magnetite exfoliation to occur during boiler cool downs.
To ensure that a tube does not rupture on the boiler's return to service, the traditional practice is to:
(a) allow the boiler to cool out using the draft fan groups;
(b) build a scaffold access to the superheater bends;
(c) x-ray all superheater loops;
(d) cut and clean all tubes which have more than a 50% section area blockage;
(e) re-weld all cut tubes and check the weld quality with x-rays; and
(f) remove the scaffold and return the boiler to service.
There still remains some risk that further falls of magnetite and possible tube blockage will occur in the loops after the initial x-rays and scope of work have been completed. This phenomenon can possibly occur due to high stresses continuing to undergo relaxation in the outer layer magnetite, the initially wet tube drying and the magnetite flakes losing their adhesion sites on the tube bore, and the action of pumping water into the superheater loops for a boiler pressure test. Case studies of Japanese boiler plants show the need to x-ray, cut and clean tubes up to three times in the same boiler shutdown.
Options suggested to stop or significantly reduce the effects of magnetite exfoliation include:
(a) replace the stainless steel boiler tube with material that exhibits a lower tendency to produce exfoliating magnetite. Retrofitting a 350MW boiler with new tube is estimated to cost as least $(AU)5M in materials and labour. A significant additional cost may be the loss of availability of the boiler;
(b) operate the boiler at a lower steam temperature (eg, below 530° C. main steam temperature). The rate of formation of magnetite at this temperature is very much slower. The trade-off is the resulting efficiency loss;
(c) cycle the boiler so that only small amounts of magnetite exfoliation occur between boiler cool downs. This is a very costly exercise due to the significant start-up fuel costs and life expended on metal components. (The boiler materials have a finite life due to stress cycles induced by the cooling down and heating up process);
(d) perform a chemical clean on the superheaters so as to strip off the outer layer of magnetite and leave intact the inner layer to act as an impediment to the further migration of iron and the resulting formation of the outer layer of magnetite. Performing a chemical clean costs approximately $(AU)300,000 and requires at least fifteen lost generation days. Callide and Tarong units have undergone chemical cleans on the superheaters. Magnetite shredding after these chemical cleans at Callide were greater in quantity than that seen prior to the chemical cleans. Doubt exists as to the effectiveness of this method of controlling exfoliating magnetite;
(e) create a flow path for the magnetite to be expelled from the superheater loops. Devising a flow path exit for exfoliating magnetite is attractive due to its minimal once off capital cost, low technology, simplicity, speed of performance and repeatability.
One prior art suggestion proposed to generate a flow path for exfoliated magnetite is by “backwashing” the superheater loops using water filled loops and compressed air to drive water and magnetite from the loops. This process has the disadvantage that the exfoliated magnetite is distributed upstream of the superheater loops, and is not removed from the boiler. It is suspected that when the boiler is recommissioned, the magnetite distributed in the boiler is not completely removed during the initial steam purges and can later cause impingement damage to the turbine components and control valves.
The prior art suggestion referred above utilises a reservoir for the compressed air/steam, introduces the compressed media to the water/magnetite solution and drives the water/magnetite slug from the superheater loops. The water/magnetite slug is initially at atmospheric pressure. Upon opening the compressed media reservoir, the water interface nearest the reservoir experiences a sudden pressure rise. This transient wave propagates through the water to the magnetite slug. The slug is compressed and radial frictional forces are significantly increased. The removal of a magnetite slug completely filling a tube bore can only happen if the pressure wave can overcome the frictional resistive forces. A critical length of magnetite slug exists whereby a nominal pressure transient will not move the slug of magnetite. A boiler tube failure at Tarong Power Station (Unit 2, February 1999), soon after a boiler restart, has been attributed to the inability of the prior art suggestion's water/air purging technique.
The cooling down of boilers, for maintenance/repairs is another major problem.
Large capacity, high-pressure boilers found in power generation plants have numerous headers to mix and redirect the steam path. Additionally, these boilers can have one or more drums or separators. These vessels are typically made of carbon steel or alloy materials and have 40 mm-140 mm thick walls. This produces a high metal volume to surface ratio that makes cooling of the vessels difficult.
Forced outages or planned outages (shut downs) occur on boilers to enable rectification work to be performed or statutory inspections to be fulfilled. Some of this work is conducted on, in, or near the above vessels. It is typical to require a waiting time of 4-8 days for thick walled vessels to be cool enough for personnel to have skin contact. The 350 MW units at Callide Power Station, Queensland, Australia, require 4-6 days before the tertiary superheater header metal temperatures fall below 100° C. when employing a
C S Energy Ltd
Hudak, Shunk & Farine Co. LPA
Lu Jiping
LandOfFree
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