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ExxonMobil ProprietaryFIRED HEATERS Section Page

CONVECTION SECTION CLEANING VIII-H 1 of 10

DESIGN PRACTICES November, 2003

ExxonMobil Research and Engineering Company – Fairfax, VA

CONTENTSSection Page

SCOPE ............................................................................................................................................................2

REFERENCES ................................................................................................................................................2GLOBAL PRACTICES.............................................................................................................................2

CLEANING REQUIREMENTS ........................................................................................................................2LIQUID FUELS - HIGH ASH CONTENT .................................................................................................2DIRTY GASEOUS FUELS ......................................................................................................................2CLEAN FUELS........................................................................................................................................2BORDERLINE CASES............................................................................................................................3

CLEANING METHODS ...................................................................................................................................3RETRACTABLE SOOTBLOWERS .........................................................................................................3ROTARY SOOTBLOWERS ....................................................................................................................6MANUAL STEAM LANCING ...................................................................................................................6WATER WASHING .................................................................................................................................6ADDITIVES .............................................................................................................................................7SONIC SOOTBLOWERS........................................................................................................................7

Table

Table 1 Retractable Sootblower Steam Requirements..........................................................................8

Figures

Figure 1 Retractable Sootblower ...........................................................................................................9

Figure 2 Rotary Sootblower...................................................................................................................9

Figure 3 Typical Sootblower Positioning .............................................................................................10

Revision Memo

11/03 The highlights of this revision are:1. References to International Practices changed to Global Practices2. More informaion on Additives and Sonic Sootblowers added3. Editorial changes made.

Changes shown by ➧

DESIGN PRACTICES FIRED HEATERS

SectionVIII-H

Page2 of 10

CONVECTION SECTION CLEANING

DateNovember, 2003 PROPRIETARY INFORMATION - For Authorized Company Use Only

ExxonMobil RESEARCH AND ENGINEERING COMPANY - FAIRFAX, VA.

DESIGN PRACTICES

SCOPEThis section covers automatic and manual means of onstream cleaning convection sections of fired heaters.

REFERENCES

GLOBAL PRACTICES➧

GP 3-4-1, Piping for Fired EquipmentGP 7-1-1, Fired HeatersGP 7-3-1, Sootblowers for Fired Equipment

CLEANING REQUIREMENTS

LIQUID FUELS - HIGH ASH CONTENTFired heaters burning residual fuels will encounter build-up of soot or ash deposits on all convection section tube surfaces. Themost severe convection section fouling problems occur when high ash liquid fuels which contain a considerable amount ofvanadium, sodium and sulfur compounds are fired.Convection section fouling occurs in two ways. First, in the area of the convection section where the flue gas temperature isabove the ash melting point [about 1200°F (650°C)], fouling will take place when the partially molten ash solidifies upon contactwith the relatively cool tubes. Secondly, when the flue gas temperature is below the ash melting point, fouling will occur from thedry ash particles settling on tube surfaces and becoming trapped between studs or fins. In both cases, the result is an insulatinglayer of deposits forming on the tube surface, reducing the heat transfer rate and increasing the flue gas pressure drop. In aheater whose stack height is set by draft requirements and not pollution, the available draft in the heater will soon cause alimitation on the firing rate if cleaning methods are not employed. Even in heaters with tall stacks designed for pollution control,convection section fouling will significantly reduce efficiency and the flue gas pressure drop will eventually limit throughput.When a residual fuel containing greater than 0.01 wt% ash or heavier than 25° API is to be fired, some method of onstreamconvection section cleaning must be specified to cover the entire tube bank. In addition, convection section tubes should bedesigned to facilitate cleaning. Acceptable outside surfaces are bare tubes, cylindrical studded type tubes and serrated or solidfinned tubes containing no more than 3 fins per in. (118 fins per m), 3/4 in. (19 mm) maximum height, and with 0.105 in. (2.67mm) fin thicknesses. Serrated fins are slightly easier to clean than solid fins but have lower mechanical rigidity and so are usedinfrequently. For new designs, solid thick fins should be chosen in preference to studs as the availability of studdingmanufacturers has declined substantially over the years. Very often the cost of studded tubes exceeds that for finned tubes. Theconvection section walls must have an erosion resistant lining, usually a dense refractory facing or when significant erosion hasbeen observed, stainless steel shrouds.

DIRTY GASEOUS FUELSDirty gaseous fuels producing more than 5 wppm (5 mg/kg) of particulates in flue gas should also be provided with onstreamconvection section cleaning facilities throughout the tube bank. Fired heaters which combust overhead gases from catalyticcracker regenerators and/or fluid coker burners usually fall in this category. For these applications, which usually includeparticulate loads over 50 wppm (50 mg/kg) in flue gas studded tubes have generally been specified over thick fins to ensuredeposit removal. The level of particulate matter in the flue gas will determine final choice of extended surface.

CLEAN FUELSHeaters firing a gas fuel or a clean liquid distillate fuel, such as naphtha or diesel oil, will normally encounter little convectionsection fouling. In these cases, any small amount of deposit build-up can simply be washed away during turnaround periods, andtherefore, facilities for onstream cleaning need not be considered.





CLEANING REQUIREMENTS

BORDERLINE CASESThere will be cases where the amount of convection section fouling cannot be determined during heater design work. Examplesof such cases are:1. Where residual fuel containing greater than 0.01 wt% ash or heavier than 25° API, is used for start-up purposes and

infrequently after that.2. Where residual fuel may be fired in the future due to changing economics.3. Where small percentages (less than 5%) of residual fuel are fired on a continuous basis.4. Where borderline fuel oils containing less than 0.01 wt% ash or lighter than 25° API, will be fired on a continuous bases.5. Where ammonia and sulphur compounds are present.In these cases, onstream sootblowing facilities need not be specified. However, the design should include provisions for possiblefuture installation of sootblowers. These provisions should include space in the convection section for the sootblower lances,appropriate convection section extended surface tubes, and erosion resistant lining for the convection section walls. Dependingupon the specific situation, consideration should also be given to providing access doors for possible interim onstream tubecleaning by manual lancing. While these provisions are relatively inexpensive in the initial design, the later addition ofsootblowers where no provisions have been made can be very costly.

CLEANING METHODSIn general, the recommended cleaning system for nearly all new heater designs where fouling is expected is the use ofretractable steam sootblowers. Other cleaning systems as listed below are only used in special circumstances.1. Rotary sootblowers2. Manual steam lancing3. Water washing4. Additives5. Sonic sootblowers

RETRACTABLE SOOTBLOWERSThe most successful method for onstream cleaning convection section tubes is the use of retractable steam sootblowers.Operating experience with this system has shown that heater efficiencies can be continuously maintained at the clean tube levelwith proper system design. Sootblower design improvements over the years have eliminated many of the mechanical problemsexperienced with older models.Sootblower Design - A retractable sootblower consists of a long, telescoping lance fitted with two diametrically opposed steamnozzles at the end as shown in Figure 1. The lance enters the convection section through the sidewall and travels perpendicularto the tubes while rotating, which results in a helical steam blowing pattern for maximum coverage. A 3/4 to 1-1/2 hp (0.6-1.1kW) electric motor, depending on the model, is supplied on each sootblower to translate and rotate the lance. The steam entersthe lance through an isolation valve actuated automatically by linkage from the movement of the blower carriage. The lance,which is cooled by the steam during its travel, is fully retractable and exposed to the high temperature flue gases only during thecleaning operation.The moving lance slides over a stationary inner steam feed tube with the pressure seal maintained by a packing box. Allgraphite-type packings are recommended for this application since poor service has generally been experienced withconventional packings and chevron rings.Upon completion of the blowing cycle, purge air enters the lance through a spring or gravity actuated check valve. The purge airdrawn in by the draft in the heater prevents corrosive flue gases from entering the lance as the remaining steam condenses.However, on positive pressure convection sections, more typically encountered in boilers, a pressurized supply of purge air mustbe provided.


SectionVIII-H

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DESIGN PRACTICES

CLEANING METHODS (CONT)Controls - Electric motor drives and controls should be specified for the sootblowers. The electric components are required tobe designed for the appropriate electrical area classification, usually Class I, Division 2 or as dictated by local codes. Pneumaticsystems commonly used in the past, though intrinsically safer than electrical systems, are no longer specified because of poorservice and high maintenance requirements.The control panel should be located at grade and provide the following features:1. Automatic warm-up interval for the steam distribution piping with automatic condensate drainage.2. Pre-selected automatic sequencing of sootblowers with manual override capability.3. Status indication of individual blower operation including automatic retract upon low steam flow.4. Automatic steam shut-off at the conclusion of the cleaning operation.

➧ Steam Piping - Sootblower piping shall be designed to prevent condensate from entering the sootblower during the entireblowing cycle.In general, steam should normally be isolated from the distribution piping when not in use. A warm-up period prior to blowing isthen required to heat the cold distribution lines to prevent condensate from entering the blowers which could cause tube erosion.A drain valve at the end of the distribution piping is required to rapidly purge condensate formed as the lines are heated. Thecontrol system should be designed to perform this warm-up automatically.Maintaining the steam distribution piping hot at all times, as was the common practice in the past, is not recommended. Besideswasting steam, the steam isolation valve on each sootblower develops leaks sooner resulting in condensate leakage into theheater.In order to promote condensate drainage while sootblowing, the steam distribution piping should be sloped downward at a pitchof 1:24 in the direction of steam flow and all low points fitted with steam traps. In addition, the sootblower lance should have afew drain holes located at the lance tip and the lance should be slightly sloped towards the tip to prevent condensate buildup ifthe individual sootblower isolation valves develop leaks during the blowing cycle.

Cleaning Effectiveness - The cleaning effectiveness of sootblowers is a function of the blowing media, nozzle design,sootblower spacing, translational and rotational lance speed, type of deposit, and type and arrangement of tube surface to becleaned. In general, the recommended cleaning medium available to the sootblowers should be dry and slightly superheatedsteam. If superheated steam is not available the supply must be properly trapped to minimize condensate carryovers to thesootblowers. Air could also be used but is normally uneconomical since refinery air systems are usually not capable of supplyingthe sootblower requirements without an additional air compressor.The cleaning ability of a sootblower is related to the thrust produced by the nozzles. Thrust is calculated by the followingequation:

)pp(AVWkT aeee −+= Eq. (1)

where: T = Thrust, lbf (N)W = Mass flow rate through each nozzle, lb/sec (kg/s)Ve = Nozzle exit velocity, ft/sec (m/s)Ae = Nozzle exit area, in.2 (m2)pe = Nozzle exit pressure, psia (Pa)pa = Pressure at nozzle, psia (Pa)k = Constant: 0.031 Customary units (1 Metric unit)

In general, converging/diverging nozzles are used on sootblowers since they produce about 15% more thrust than converging orstraight bore nozzles. With the converging/diverging nozzles typically used in sootblowers, the steam is almost fully expanded inthe diverging portion. For simplification, pe can be assumed to equal pa. The thrust then is simply a function of the flow rate andthe nozzle exit velocity which is supersonic. For steam flow the equation can be further simplified to:

ei hhWCT −= Eq. (2)

where: hi = Inlet enthalpy, Btu/lb (MJ/kg)he = Exit enthalpy, Btu/lb (MJ/kg)

(determined from steam tables assuming isentropic expansion through the nozzlewith pressure equal to atmosphere)

C = Constant 6.95 Customary units (70.9 Metric units)





CLEANING METHODS (CONT)Based upon vendor test data and field experience, effective cleaning of most deposits can be achieved with a thrust of about 130lbf (580 N) per nozzle for the recommended sootblower spacings discussed in the next section. Since the available steam supplypressure will vary from location to location, Table 1 presents recommended nozzle sizes, blowing pressures, and resulting steamrates which provide the required thrust. The blowing pressure is the pressure measured downstream of the individual sootblowersteam isolation valve. The calculated sootblower steam rate is based on feed and lance tube pressure drop of 10 psi (70 kPa)and a nozzle discharge coefficient of 0.85. In general, a high steam supply pressure [about 250 psig (1725 kPa-g)] should beused consistent with high blowing pressures and small nozzles to minimize steam requirements. Additional steam flow isrequired for lower supply pressures to compensate for the reduced nozzle exit velocities attainable. The use of blowingpressures below approximately 150 psig (1035 kPa-g) will often require the manufacturer to design larger diameter steam lancethan standard to maintain the higher flow rate necessary to achieve the required thrust. Use of blowing pressures higher thanabout 300 psig (2070 kPa-g) are not recommended because of the increased tube erosion risk due to high nozzle velocities.

Since the sootblowers are operated in sequence, the maximum steam demand is equal to the individual sootblower steamdemand. When sootblowers are installed on small natural draft heaters, the stack should be sized to provide adequate draftconsidering the additional steam flow which combines with the flue gas. For sootblowers being retrofitted on existing smallnatural draft heaters with short stacks, a steam eductor can be used in the stack to provide the additional draft required whensootblowing if a short reduction in firing cannot be tolerated.

Superheated steam is normally not specified or required for sootblowing; though a slight amount of superheat [about 25°F(14°C)] can be beneficial in minimizing condensate formation while blowing. Theoretically, superheated steam can reduce therequired steam rate since higher nozzle exit velocities can be developed. About an 8% reduction in steam rate can be achievedwith 200°F (110°C) of superheat assuming the same nozzle thrust. However, the sootblowers would have to be speciallydesigned with larger nozzles and could possibly require larger lances and improved lance materials.

Steam consumption can also be minimized by optimizing sootblower lance traverse speeds and blowing frequency. Lancetraverse speeds are typically 6 to 8 ft/min. (1.8 to 2.4 m/min.). Effective cleaning has been demonstrated by one vendor,Diamond Power, at traverse speeds up to 10 ft/min. (3 m/min.) and is preferred since less steam is used during a blowing cycle.However, not all vendors supply sootblowers with the high 10 ft/min. (3 m/min.) traverse speed. The optimum cleaningfrequency, from an overall operating cost standpoint, is a function of fuel, steam, labor costs, and environmental constraints forthe specific location. Typical cleaning cycles vary from once every day to once a week depending on the degree of fouling.

Positioning (see Figure 3). Sootblowers should be spaced to cover a maximum horizontal distance of 3-1/2 ft (1.05 m) to eitherside to obtain adequate cleaning [7 ft (2.1 m) maximum coverage per sootblower] for fuel oils lighter than 10 API (1.0 relativedensity) and gaseous fuels producing less than 50 wppm (50 mg/kg) particulates in the flue gases. For fuel oils 10 API andheavier and for gaseous fuels producing more than 50 wppm (50 mg/kg) in the flue gas, the sootblower shall be spaced closertogether to maintain adequate cleaning effectiveness. For these cases, the maximum radial distance between sootblowers shallbe reduced to 3 ft (900 mm), providing each sootblower with a maximum coverage of 6 ft (1.83 m).

For fuel oils lighter than 10 API or gaseous fuels producing less than 50 wppm (50 mg/kg) particulates in the fuel gas, theretractable sootblower will adequately clean a maximum of 4 tube rows, bare or extended surface, immediately above and belowthe lance position. Vertical spacing should, therefore be limited to a maximum 4 rows or 3-1/2 ft, whichever is less. For dirtyfuels (fuel oils heavier than 10 API or gaseous fuels producing more than 50 wppm (50 mg/kg) particulates in the fuel gas) thespacing between sootblower rows should be reduced to a maximum of 3 ft or 3 tube rows, whichever is less.

The locations of any intermediate tube supports in the convection section should be specified when the sootblowers are to beused. These supports should be positioned midway between adjacent sootblowers, to eliminate interference with the spraypattern.

To minimize tube erosion problems, the vertical space in the sootblower cavity should be at least 15-1/2 in. (395 mm). This willpermit a 6 in. (152 mm) clearance between the lance O.D. [typically 3-1/2 in. (90 mm)] and the tube O.D. (including extendedsurface). The following additional clearances should be added to cover the vertical deflection (“droop") of the sootblower lance intraversing wide convection sections:


SectionVIII-H

Page6 of 10




DESIGN PRACTICES

CLEANING METHODS (CONT)

CONVECTION SECTION WIDTH ADDITIONAL CLEARANCEft m in. mm10 3.05 0 015 4.57 2 5020 6.10 5 127

To minimize sidewall erosion problems, the entire height of the convection section should be protected with dense castablerefractory when sootblowers are used or when space is provided for future sootblowers.Specification - Most of the sootblowing system design requirements discussed above are adequately covered by GP 7-1-1,GP 7-3-1 and GP 3-4-1. The following information should, however, be included in the heater design specification:1. Fired heater specification sheet:

a. Number of sootblowers requiredb. Steam pressure and temperature available for sootblowing

[Normally saturated or slightly superheated and about 250 psig (1725 kPa-g)]2. Heater sketch:

a. Location of sootblowersb. Clearances in tube bank for sootblowersc. Location of intermediate tube sheets

ROTARY SOOTBLOWERS(see Figure 2)The rotary sootblower consists of a lance permanently positioned perpendicular to the tubes, across the width of the convectionsection, with several nozzles spaced along the lance. Although less expensive than the retractable type sootblower, the rotarysootblower should normally not be specified. Lance life is usually short and plugging typically occurs since the lance and nozzlesremain exposed to the flue gas. In addition, steam pressure in the range of 400 to 600 psig (2760 to 4140 kPa-g) must be used toobtain reasonable cleaning effectiveness, though not comparable to retractable sootblowers, since a large number of smallnozzles are used.

MANUAL STEAM LANCINGOnstream manual steam lancing of convection sections has been specified in past designs for low cost heaters in areas wherelabor and fuel costs were also low. However, high fuel and labor costs have now precluded the use of manual lancing in nearlyall new designs, except for the borderline cases outlined earlier. This method is highly dependent upon the skill of the operator,and in no case is it as effective as retractable sootblowers in maintaining a clean convection section.

The arrangement of the convection section for manual steam lancing is the same as provided for a retractable system. Platformsat least as wide as the lance length must be provided if routine cleaning is to be performed. In the case of a wide convectionsection [about 6 ft (2 m) or more], platforming on both sides of the heater should be provided to avoid the use of unwieldy lances.Tight closing lance doors must also be provided in the side(s) of the convection section located on the same basis assootblowers.

WATER WASHINGWater washing is commonly used for cleaning convection sections during shutdowns. Water is poured onto the top of theconvection section and deposits are dissolved and flushed away.

Onstream water washing of heaters is occasionally used for existing heaters where no other means of cleaning is possible. Withthis method, deposits are thermally shocked and broken loose. This contrasts with sootblowing, which depends upon a highsteam momentum and only a low thermal shock factor to remove deposits. Because of the inherent risk of thermally shockingand damaging refractory and tube supports, onstream water washing should not be included in new designs.





CLEANING METHODS (CONT)If onstream water washing is used, special attention must be given in the design or procedures to avoid spraying water on tubesupports and refractory. Also, the risks associated with the possible failure of a tube due to possibly high thermal stresses fromthe rapid quenching should be carefully considered by the refinery. The risks may be reasonable for a pipestill furnace when thisis the only alternative to a costly shutdown. However, the risks are much greater for a higher temperature/pressure service whichoperates close to allowable tube stress limits.The following two techniques have been used for water washing of heaters:1. Fixed Water Washing Systems (Two Types are Available):

a. One water washing system consists of fixed piping located above the convection section and designed to cascade waterdown through the tube bank.

b. Another water washing system consists of spray nozzles mounted on fixed piping located throughout the convectionsection.

Of the two systems, the first is preferred since, in the second system, the spray nozzles tend to plug between cleaning periodsand the tubes tend to erode due to high velocity water impingement.2. Manual Water Lancing - For cleaning convection section tubes lances are inserted through available ports and the water is

sprayed on the tubes with caution not to hit supports and refractory. Onstream manual water lancing has also beensuccessfully used by some refinery plants on radiant tubes to remove deposits developed from catalyst fines which aresometimes found in the fuel oil. The successful application of manual lancing is highly dependent upon operator skill.

ADDITIVES➧ Injection of additives into flue gas for continuous onstream convection section cleaning for new designs is not recommended. In

general, experience to date indicates continuous use of additives usually has little effect on fouling prevention and may evenaccelerate deposition in some applications. However, a few additives have been successfully applied on a one time basis inseveral instances to clean severely fouled convection section where sootblowers were not functioning correctly or not installed.There has been a few recent instances where alkali nitrate based dry powder has been sprayed onto convection section tubeswhile the heater is online and achieved satisfactory cleaning results. For such applications the Heat Transfer Equipment Sectionof EETD should be consulted for the latest experience with various additive brands.

SONIC SOOTBLOWERS➧ Sonic (acoustic) sootblowing is a technique where horns are used to generate sound waves at controlled frequency bands which

fluidize soot and ash deposits remove them by shearing them from tube surface. Thus, removal of sticky or moist deposits maynot be possible by sonic sootblowers. Though these systems have lower utility requirement (compared to steam sootblowing), butcan pose a noise problem at times.

Sonic sootblowers usually employ pneumatically operated sound generators. Compressed air enters the sootblowers plenumcausing a diaphragm to vibrate causing sound pressure variations. The shape of the housing amplifies the sound waves anddirects them in the appropriate direction. Thus, orientation of the section being cleaned is important. Sound waves must pass toall sections of the bank to work effectively, or multiple acoustic sootblowers can be employed. Sonic sootblowers may requirereplacement of the diaphragm regularly depending on the deposits. Some designs require a continuous compressed air bleed toprevent soot from clogging the sootblower or diaphragm. The heater internals should be in good condition. Sonic sootblowersmay cause deteriorating refractory to come loose and obstruct heat transfer and duct areas.

Based on currently available technology, these acoustic cleaning systems do not yet have enough results proven, and should notbe used for new designs. Sonic sootblowers developed to date are not capable of maintaining the convection section in a cleanstate. They have been successfully used in some installations to supplement steam sootblowing and thus reduce steam blowingfrequencies. Significantly high noise levels have been experienced with the high frequency horns currently available, 250 Hz and360 Hz. One manufacturer offers a low frequency horn, 20 Hz, which is below the audible range.

Sonic sootblowers can be considered only in 'retrofit' cases where there is no other practical means or available option is veryexpensive (eg. in a boiler or heater which does not have any sootblowing lane). It is important to note that sonic sootblowers maynot be able to provide similar cleaning effectiveness with various fuels, since soot characteristics change with fuel. Usually, sonicsootblower vendors too have difficulty in predicting cleaning effectiveness and usually grant a trial period for operation.For such applications the Heat Transfer Equipment Section of EETD should be consulted for the latest experience.


SectionVIII-H

Page8 of 10




DESIGN PRACTICES

CLEANING METHODS

Table 1 Retractable Sootblower Steam Requirements

[To Obtain 130 lbf (580 N) Thrust per Nozzle]

AVAILABLE STEAMSUPPLY PRESSURE(1)

RECOMMENDED THROATDIAMETER OF EACH NOZZLE

REQUIRED BLOWINGPRESSURE(2)

RESULTING STEAM RATETHROUGH 2 NOZZLES(3)

psig KPa-g in. mm psig kPa lb/hr kg/s265 - higher 1830 - higher 3/4 19 245 1690 9500 1.20210 - 265 1450 - 1830 7/8 22.2 190 1310 10000 1.26170 - 210 1175 - 1450 1 25.4 150(4) 1035 10500 1.32145 - 170 1000 - 1175 1-1/8 28.6 125 860 11100 1.40125 - 145 860 - 1000 1-1/4 31.8 105 725 11700 1.47

Notes:(1) Assumes dry saturated steam at sootblower level.(2) Measured just downstream of steam shutoff valve provided on sootblower.(3) Assumes a nozzle discharge coefficient of 0.85 and 10 psi pressure drop across steam feed tube and lance.(4) For blowing pressures below 150 psig (1035 kPa), the manufacturers standard lance O.D. [typically 2-1/2 in. (64 mm) O.D.] will

have to be increased to achieve the higher steam flow rates required.





Figure 1 Retractable Sootblower

Poppet Valve

Motor Drive

Stainless Steel Feed Tube

Drive Shaft

Steam Tube

Front Roller Supports

Figure 2 Rotary Sootblower

Rotor GearsMotor Drive

Steam


SectionVIII-H

Page10 of 10




DESIGN PRACTICES

Figure 3 Typical Sootblower Positioning

Typical Observation Doors toInspect Condition of Tubes

Show Locationsof Sootblowers

Show Location ofIntermediate Tube Sheet

7 ftmax.

(2100 mm)

3 ft min. (900 mm)3-1/2 ft max.

Typical Blowing Traverse

WcSteam Inlet

SootblowerLance

Sootblower

Typical End of Lancein Rest Position

Typical Position ofLance at Start and

End of Blowing Cycle

15-1

/2 in

.

(400

mm

)

Tube

O.D

. +

Exte

nded

Sur

face

+

15-1

/2 in

. (40

0 m

m)

(1050 mm)

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