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Advanced Erosion Protection Technology for Steam Boiler Superheat, Reheat and Evaporator Tubes

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Abstract
Faced with deregulation, increasing retail competition, and pressures to keep boilers on-line, many coal fired power generating stations have adopted business strategies centered on increasing unit availability, reliability and increasing the operational life of critical equipment. However, boiler tubes failures continue to be the number one cause of forced outages in fossil plants today. These costly forced outages are responsible for an estimated 6% overall loss of unit availability. One of the major causes for premature tube failure is excessive fireside boiler tube erosion caused by the impact, cutting action, and abrasive wear of fly ash entrained flue gases undercutting the area they strike. This report will discuss the results of several comprehensive laboratory analyses comparing a wide variety of wear resistant materials for the protection of high erosion prone fireside boiler tubes as well as verification of laboratory analyses in actual field trials. The report also includes: the primary instigators of boiler tube failure by erosion, a discussion of erosion and its variables, and high temperature erosion testing procedures and results.

Introduction
Power generation utilities and holding company goals are to extend times between major planned boiler outages. Systems types and configurations, the age of the plant, their specific plant operating demands and both preventative and general maintenance philosophies can dictate the accomplishment of these goals. Extending time between major outages two, four, and even five years is resulting in increased forced outages due to tube failures. An estimated seventeen causes of tube leaks have been sited. However, one of the most problematic, hardest to predict and seemingly increasing is erosion caused failures.

Electric Power Institute has generated an in-depth report titled Tube Repair and Protection from Damage Caused by Sootblower Erosion 10080837 March 2004 which will be summarized in the
following pages. However, the focus of this paper is to qualify by actual field tests the hot erosion lab tests conducted in a variety of highly erosive boiler environments.

Erosion
Erosion is caused by the impact, cutting action, or abrasive wear of small solid particles freely immersed in the direction of fluid flow that frequently undercut portions of the material they strike [1]. Erosion is the progressive loss of original material from a solid surface due to mechanical interaction between that surface and the impinging fluid or solid particles [2]. If high erosion-resistant particles such as Tungsten carbide exist in low erosion resistant or soft matrix, the impacting particles can undercut and remove portions of the material (Figure 1). However, if the high erosion resistant particles are densely packed in a matrix material that causes the impacting particles to impinge on a greater percent of the hard particle, the erosion resistance increases dramatically (Figure 2).

Figure 1

Figure 2

When evaluating the relative erosion resistance of materials, a number of factors must be considered. The obvious factors are temperature, velocity of the impacting particles, their size and shape, and the impacting or impinging angle. These factors can be controlled in standardized testing but combining their range of variability to comprehensively evaluate performance is limited.

Standardized testing procedures, such as ASTM G76 Figure 3, reduce a number of the variables with the intent of providing a common baseline for comparison. This test method utilizes a repeated impact erosion approach involving a small nozzle delivering a stream of gas containing abrasive particles which impact the surface of the test specimen. A Standard set of test conditions is described. However, deviations from some of the standard conditions are permitted if described thoroughly. These test methods can be used to rank the erosion resistance of materials under the specified conditions.

Figure 3

Additional testing methods both ASTM standardized and mathematical erosion models are described in detail in Tube Repair and Protection from Damage Caused by Sootblower Erosion 10080837 March 2004.

Erosion Test Setup

Erodent:
High temperature erosion tests were carried out using the bed ash from an operating boiler as the erodent material. The particle morphology was a mixture of both round and angular with a mean particle size of 556 microns and mean particle density of 164.4 LBm/ft3. The erodent material particles were comprised of high concentrations of silicon and calcium with minor concentrations of aluminum, magnesium, sulfur, iron, phosphorus, titanium and chlorine.

Test Conditions:
Particle Velocity	141.2 ft/s (40m/s)
Temperatures	900°F (482°C), 1100°F (593°C)
Impact Angles	30°, 90°
Test Duration	3 hours
Loading	0.441 Lbm (0.2 kg)

Tests focused on elevated temperature solid-particle erosion under generally oxidizing conditions. Ref***

Reporting:
Test results are typically reported as both a weight loss and a thickness loss for each of the tested specimens. However, since the weight measurements included the material erosion wastage (-), oxide scale (+), ash deposit (+), and different densities, the weight loss scheme was not a desirable approach for predicting the erosion rate. Therefore, the thickness loss was determined to be a more valid method for determining the erosion rates of the tested alloys.

Materials Tested:
Twelve alloys were selected for high temperature erosion testing. The list was generated through a combination of those applied in the industry and those found from erosion and/or erosion data of carbon steels, stainless steel alloys, nickel-based alloys, tungsten carbide claddings and thermal spray coatings, as presented in Section 3.3.4 Tube Repair and Protection from Damage Caused by Sootblower Erosion 10080837 March 2004. Details regarding the selected material descriptions, specifications, chemical composition, thermal properties, application processes, and cost estimates are also available in the aforementioned Electric Power Institute report.

SA387 Grade 11 alloy steel Nickel alloy 52 — GMAW Nickel alloy 622 — GMAW Nickel alloy 602CA — GMAW WC200 braze alloy — infiltration brazed Duocor coating — TWAS

309L stainless steel — GMAW Nickel alloy 72 — GMAW Nickel alloy 625 — GMAW 312 stainless steel — GMAW Cr3C-NiCr coating — HVOF LMC-M WC blend coating — HVOF

The base material for all test samples was SA387 grade 11 alloy.

High Temperature Test Results
Table 1-1 shows test results of 12 materials tested in order of erosion resistance. Chart 1-1 shows
graphically the results omitting the Duocor coating.

		Thickness Loss
No.	Target material	At 900°F (482°C)		At 1100°F (593°C)
		30°	90°	30°	90°
1.	Cr3C2 - NiCr coating	5	19	11	38
2.	Wc200 Cladding	6	23	13	56
3.	LMC-M+WC coating	20	28	25	99
4.	Nickel alloy 625	54	51	74	90
5.	Nickel alloy 622	56	54	71	104
6.	Nickel apply 602CA	63	72	75	84
7.	Nickel alloy 52	65	62	68	83
8.	Nickel alloy 72	66	58	73	94
9.	312 Stainless steel	67	64	70	74
10	309L Stainless steel	71	65	74	85
11.	SA387 steel	76	65	90	97
12.	Duocor coating	187	752*	225	825*

* indicates coating worn through
Table 1-1

 
Chart 1-1

Hot Erosion Test Summary
The results indicate that among the twelve alloys tested, the materials with the highest density of erosion-resistant particles i.e., Tungsten carbide and Chrome carbide showed the highest erosion resistance. The Cr₃C₂-NiCr HVOF - applied coating showed the highest erosion resistance followed closely by the infiltration brazed WC 200 material both with erosion resistance particle percentages of close to 70%.

Additional detailed information regarding the lab test summarized above can be found in Electric
Power Institute’s technical report - Tube Repair and Protection from Damage Caused by
Sootblower Erosion 10080837 March 2004.

Continuing Tests — Field Application
Erosion resistance is complex, combining the many variables to actually duplicate, recreate, field environments is next to impossible in laboratory tests. Additional environmental factors such as thermal shock, erosion resistant material bond strength, as well as many others come into play. The following field tests will compare the laboratory qualified high density erosion resistant particle materials to other industry accepted methods of erosion protection.

Field Test 1
Tennessee Valley Authority, Shawnee Fossil Plant
7900 Metropolis Lake Road, Paducah, KY 42086

Plant overview
The Shawnee Plant generates its 1750 MW with 10 boilers supplying over 580,000 area homes with power. The plant’s unit 10 is the nation’s first commercial scale atmospheric bubbling fluidized bed unit designed in the early 1980’s as a test unit for advanced coal firing technologies. This unit started up in October of 1988 and began a demonstration period until May 1991 when the unit was turned over to the TVA generating group for normal commercial operations.

Unit 10 atmospheric bubbling fluidized bed
There are 3 evaporator sections in the boiler fed in parallel from the boiler feed pumps. Each section is a vertical 4 pass arrangement with the bottom tube being pass 1. The in-bed tubes are submerged in a mixture of coal, limestone and recycled ash in the bottom of the furnace where combustion takes place reaching maintained temperatures of 1450°F to 1600°F for optimum sulfur capture. Combustion air enters the bed through air nozzles located in the furnace floor causing the materials to become fluidized and completely cover the in-bed tubes to a depth of 40”-42”. Large bubbles of air form and carry the burning bed material up through the tube matrix at a velocity of 7.5 ft/second. The bed material, with less than 3% combustible material, is made up of calcium sulfate, calcium oxide and coal ash.

Evaporator tube history - early erosion rates
The original evaporator tubes were 2.25” OD x .220” SA178C rifled tubes. The bottom row of tubes (pass 1) and all evaporator bends were protected with Extendalloy flame sprayed and fused 45% tungsten in a nickel matrix.

From December 1988 — October 1991 the maximum erosion rates ranged from .001 - .002”/1000
hours near the recycle feed nozzles. During the Mid 90’s changes in fuel and operating conditions increased tube erosion resulting in numerous failures (Figure 4). Due to the cost and availability, a mixture of pet coke and coal was burned in the unit. The pet coke reduced the amount of ash the unit produced resulting in the need to add additional limestone to the bed to control temperature.

Figure 4

By 1996 tubes leaks had become a serious problem resulting in replacements of tubes in
evaporator 2 and half of evaporator 1. The replacement tubes were rifled 2.25” x .220” SA210A1
coated 360 degrees all passes with Extendalloy. The remaining tubes in evaporator section 1 and
3 were flame sprayed on-site with the same Extendalloy material. However, continuing erosion
caused tube failures resulted in all evaporator tube replacements during December 1999 outage.

Evaporator tests
Replacement in-bed tubes installed during the 1999-2000 outage were coated 100% - 360
degrees with another spray and fuse material of similar composition as Extendalloy. However,
due to the past poor performance of this type of coating, boiler engineers from TVA installed test
materials in high erosion areas for evaluation. Each test area consisted of an approximately 6”
section coated with the test material separated by the current spray and fuse material. The
materials tested were a Stoody 140 weld overlay, NiCr-3 and NiCrMo-3 HVOF, 312 and 309
stainless steel weld overlay, and a Chrome carbide weld overlay. In January 2002, both stainless
steel weld overlay test samples had worn through and resulting in a tube failure. After
inspections and noticeable erosion on the remaining test sections, all segments were removed.

Evaporator tube continued testing
With spray and fuse tube failures continuing to plague the unit, TVA boiler engineers continued
the search for a suitable erosion resistant material for tube protection. Due to successful erosion
prevention applications in other areas of TVA’s fleet, Conforma Clad was contacted. Conforma
Clad supplied in-bed testing, infiltration brazed 70% tungsten carbide coating applied to the high
erosion areas of tube bends (Figure 5). The Conforma Clad coated test sections were installed
during a November 2003 outage. Arrangements were made between TVA engineers and
Conforma Clad to perform inspections during unit availability to track erosion. Due to the nonmagnetic characteristics of the infiltration brazed NiCr matrix coating, eddy-current
measurements could be taken for accurate measurements (Figure 6).

Figure 5

Figure 6

Evaporator tube inspection results

Evaporator 2 Tube inspections Conforma Clad
Date	Thickness	Material Loss
Nov-03	.036"	as supplied
Nov-04	.036"	NA
Apr-05	.036"	NA
Sep-05	.0348"	.0012"*
Apr-06	.0342"	.0018"

* Material loss measured 1.5" x .750 area directly in-line with nozzle.

Field Test 1
Tennessee Valley Authority, Shawnee Fossil Plant

Summary
Although there are differences in materials from the lab test to actual field tests, high erosion
resistant particles densely packed in a matrix material characteristic of infiltration brazed
Conforma Clad coating, withstand the impinging particles of these environments substantially
better than other materials. Additional factors, as mentioned in these discussions, were thought
by Tennessee Valley Authority engineering to play a role in the success or failure of erosion
resistant coatings. Material bond strengths to the base substrate were thought to play a role in the
failure of the spray method coatings. Bond strengths of only approximately 40MPa for the spray
methods were unable to withstand thermal cycling along with the simple handling and
installations. However, the bond strength of the infiltration brazing is 483MPa and easily
withstands the environmental requirements. Due to the low erosion rate of the Conforma Clad
cladding over the past 29 months, and the extrapolated life resulting from these tests, Tennessee
Valley Authority will be replacing all 3 sections of the evaporator with Conforma Clad
infiltration brazed cladding in the scheduled 2007 outage.

Field Test 2
American Electric Power, Philip Sporn Generating Station
Route 33 West, New Haven, West Virginia 25265

Plant overview
The Philip Sporn Plant generates 1050 MW of power. This plant is part of the largest electricity
generator in the United States who owns and operates more than 42,000 megawatts of generating
capacity in the U.S. and abroad, and serves 5 million customers in eleven states.

Unit 1 front fired boiler
Philip Sporn’s unit one is a Babcock & Wilcox front fired boiler with a name plate rating of
153MW, burning Bituminous coal. The unit went on line in 1950, utilizing two evaluations of
burners supplied by five B&W EL70 pulverisers.

Unit 1 primary superheat tube legs
Superheat tube legs 2.750” x 2.80” “S” shape 60” long SA210 grade 1A experience high velocity
fly ash entrained flue gas. The erosive attack is accelerated by increased fly ash concentration
during periods of soot blowing. The flue gas temperature in the area of the superheat tubes is
approximately 700 degrees F during full load. The steam condition inside the tubes as 2450 PSIG
and 550 degrees F. Prior to major re-tubing in the Spring of 2004, the tube legs required weld
pad and weld repair every two — four years.

Superheat tube leg replacement
During the Spring 2004 outage, the superheat area underwent re-tubing. For the superheat tube
legs, plant engineers installed tubes protected 200° on the flow side of the tubes with erosion
resistant infiltration brazed cladding (Figure 7). Successful applications within the system and
the Sporn plant prompted engineers to include this type of wear protection to prevent reoccurring
damage and eventual area tube leaks.

Figure 7

For added erosion protection, the plant applied a trowel applied ceramic packing in the highest
erosion area over the infiltration brazed tubes (Figure 8) approximately eight inches thick.

Figure 8

Unit 1 primary superheat tube leg inspection results
Inspections were performed in March of 2006. After a run of over three years, all eight inches of
the trowel applied ceramic material had eroded away exposing the infiltration brazed tubes to the
environment for an undetermined length of time. Eddy-current measurements taken during the
inspection of the infiltration brazed cladding showed no measurable loss during the time of
exposure (Figure 9).

Figure 9

Field Test 2
American Electric Power, Philip Sporn Generating Station

Summary
Due to the undetermined exposure of the infiltration brazed material to the erosive environment,
the results of this field test were to some degree inconclusive. However, referring back to the
statement that if high erosion-resistant particles, in this case Alumina, exist in low erosion
resistant or soft matrix, the impacting particles can undercut and remove portions of the material.
This would explain the high volume loss of the trowel in ceramic. Arrangements are in place to
continue monitoring this application.

Conclusion:
Various maintenance programs have been initiated over the years to increase unit availability,
but boiler tube failures continue to be the number one cause of forced outages in fossil plants
today. Tube failures are reportedly responsible for an estimated 6% loss per year costing owners
hundreds of thousands of dollars for each occurrence and maintenance teams precious man
hours. While erosion caused failures are only one of the many reasons for tube failures, returns
on the initial investment of preventative maintenance programs involving high erosion prone
boiler tubes have had a payback in as little as one forced outage avoidance. Utilizing today’s
modern erosion technologies for boiler tubes protection is getting plants one step closer to
achieving the new outage-to-outage goals.

References:

Tube Repair and Protection for Damage Caused by Sootblower Erosion, K Colman, D.Overcash
Fossil Repair Applications Center (FRAC), EPRI, Charlotte, NC

B. Wang, A Comparison of Erosion Resistance of Twelve Different Materials. Technical and
Research Memorandum, Nov 13, 2003. LBW001-1

[1] Metals Handbook. H. Boyer and T. Gall, eds. American Society for Metals, Metals Park, OH,
1992

[2] Annual Book of ASTM Standards,Vol. 03.02 Wear and Erosion; Metal Corrosion. American
Society for Testing and Materials, Philadelphia, PA 2002

Written By:

Chris Harley, Senior Applications Engineer
Conforma Clad Inc.
501 Park East Boulevard, New Albany, IN 47150

Andrew McGee, P.E.
EPRI RRAC
1300 Harris Blvd., Charlotte, NC 28262

Richard J. Stangarone, System Engineer
Combustion Process
1101 Market Street LP 2L-C, Chattanooga, TN 37402

Mike Palmer
American Electric Power Company
Philip Sporn Generating Station
Route 33 West, New Haven, West Virginia 25265