Heat Transfer K. MALHOTRA, S & B Engineers and Constructors Ltd., Houston, Texas
Rethink specifications for fired heaters Fired heaters are essential and important major pieces of equipment in a process plant. Specifying fired heaters with correct requirements results in a reliable and safe heater design, fewer operational issues, longer run lengths and extended heater life. Fired heaters are used to heat process fluids from the given values of initial temperature and pressure to certain higher specified values based on the process requirements. Apart from the process duty requirements, other design parameters discussed here are either good design practices or lessons learned from past experiences. API 560 provides general guidelines on the heater design; however, the significance of some key parameters is mostly not realized. Tube configurations and heater operation. The main
components of a fired heater include: • Radiant section • Convection section • Stack section. Tubes are present in the radiant and convection sections to recover heat from the escaping flue gases. Horizontal and vertical radiant tube layouts are the most common. Radiant tubes are laid out in various configurations, while convection tubes are placed horizontally. Other radiant tube layouts include helical coil and arbor/U coil. Horizontal radiant tube layout offers self-draining tubes, but requires more space. Box layouts (horizontal or vertical tubes) are an obvious choice when process duties (heat requirements) are large. Vertical cylindrical layouts are preferred when real estate and heater costs are important issues. If the heater layout is not specified, the heater fabricator may choose a vertical cylindrical layout due to lower fabrication costs. Additional heat recovery mediums/mechanisms are also used to increase the heater efficiency. Forced-draft fans are used with air preheat for additional heat recovery or to achieve a tighter flame pattern. Stack or induced-draft fans compensate for additional draft needs resulting from resistance created by convection, damper, heat recovery mediums, emission reduction units and other stack losses. Fired heater design considerations. One of the key design parameters in the fired heater design is radiant heat flux, and it is calculated by dividing the heat absorbed by radiant tubes in the radiant section by the surface area of the radiant tubes. A typical fired heater recovers approximately 60%–70% of the total process duty in the radiant section. The heat released
by the burners is mainly transmitted by radiation in the radiant section. FIG. 1 is generated using the formula for calculating the radiant heat flux value. It shows the effect of radiant heat flux variation on various heater design parameters. Selection of the average radiant heat flux is dependent on the type of process, process conditions, critical film temperature limits, and whether the design is for a single- or double-fired heater. Critical film temperature identifies the maximum allowable temperature limit of the process film before degradation. The process film has a higher temperature than the process bulk temperature, since the film is in contact with the higher-temperature tube wall. Higher process film temperature leads to higher cracking. Maximum allowable pressure drop and fouling allowances should be specified. The increase in process pressure drop can be utilized to reduce the process film temperatures. Higher pressure drop increases the process turbulence inside the tube, thereby reducing the film and tube wall temperatures. For single-fired boxes, the peak radiant heat flux is approximately 1.8–1.9 times the average radiant heat flux. For doublefired boxes, the peak radiant heat flux is approximately 1.2–1.3 times the average radiant heat flux, with tubes spaced at two nominal diameters apart in a single row. Nearly 90%–91% efficiency can be achieved from a fired heater, with stack temperatures reaching close to sulfur dewpoint. A minimum approach temperature of approximately 100°F is recommended. FIG. 2 shows the estimated heat loss (% of heat input) from the heater as a function of stack temperature and excess oxygen (dry %). In one case study, a heater had a daily cyclic loading operation with a cold feed inlet temperature for a short duration, and then it cycled to a hot feed. Design calculations showed Low radiant flux Large Radiant box size Small
Low
Low
Low
Process film temperature
Tube wall temperature
Bridgewall temperature
High
High
High
High Cost Low
High radiant flux FIG. 1. Radiant heat flux vs. heater characteristics. Hydrocarbon Processing | OCTOBER 201567
Heat Transfer
Heat loss, % of heat input
that, with this cyclic loading condition, the process inlet tubes will witness flue gas acid dewpoint condensation during the cold feed, which is not desirable. To remedy this potentially undesirable condition, the process inlet was specified to be a co-current design in the convection section. The convection tube metallurgy was upgraded from carbon steel to stainless steel (SS), resulting in a projection of longer tube life. In addition, access doors were provided to periodically monitor tube conditions. Heater turndown requirements should be addressed at the initial stages of the design phase. Typically, burners witness carbon monoxide (CO) breakthrough (increased CO emissions) and an increase in volatile organic compound (VOC) emissions, with the bridgewall temperatures lower than 1,300°F. Bridgewall temperature is a function of the process outlet temperature and the radiant heat flux utilized. Every furnace should be evaluated to ensure that the emission guarantees are valid for the turndown conditions. Salt formation in selective catalytic reduction (SCR) catalyst and downstream equipment should also be considered when designing for heater turndown requirements. Depending on the sulfur content in the fuel, sulfur dewpoint issues should be addressed as well. Among plot plan considerations, provisions should be made for tube removal/retubing/repair activities in the design of the fired heaters. Crane access/rigging operations should also be considered. For continuously operated platforms, stack emissions of nearby continuously operated vents (i.e., heater stacks, thermal oxidizers, etc.) should be taken into consideration. This may require dispersion modeling to address environmental issues. Refractory provides insulation to reduce heat loss from the fired heaters. Refractory losses typically range between 1.5% and 3% of the firing rate. API 560 provides guidance on the design requirements. The refractory material selection should consider flue gas velocity, operating temperatures, cyclic loading, mechanical stress, fuel impurities, maintenance access and any other form of erosion anticipated. Heaters designed for cyclic loading induce additional thermal stresses. Ceramic fiber selection in this scenario can help reduce the cyclic stresses. 55 50 45 40 35 30 25 20 15 10 5
0
2
4
300°F flue gas 400°F flue gas 500°F flue gas
6 Excess O2, dry %
600°F flue gas 800°F flue gas
8
10
1,000°F flue gas 1,200°F flue gas
FIG. 2. Estimated heat loss, %, from the heater as a function of stack temperature and excess oxygen, % dry.
68OCTOBER 2015 | HydrocarbonProcessing.com
12
Radiant bull nose, convection, breaching, stacks and ducts should be specified with castable refractory; this is especially important when SCR is a consideration. Ceramic fiber flaking issues have been observed in the heaters. Use of a high-temperature rigidizer in the radiant section can reduce the effects of ceramic fiber erosion. For heaters where fuels have sulfur content, internal protective coatings should be specified behind the refractory materials to prevent skin corrosion. Higher sulfur content may require the use of vapor barriers. In addition, castable refractory dryout should be specified to prevent loss or damage to the refractory due to alkali hydrolysis. In one case study, bricks on a heater floor buckled up, burner tiles were bent, and flame shapes were distorted. Field inspection showed that the spacing between the bricks and burner tiles was inadequate, which restricted brick expansion. This resulted in buckling of the heater floor and displacement of the burner tiles. The situation was remedied by re-laying the heater floor tiles with proper expansion joints between the bricks and burner tiles. It is recommended that the heater floor be vacuum-cleaned after the construction work is complete, and before closing the heater. In another case study, the SCR catalyst was becoming plugged up often. Therefore, the SCR unit had to be shut down for regular cleaning. An investigation showed that the convection section and the flue gas ducts had ceramic fiber installed for its lighter weight and higher insulating properties. High flue gas velocities caused the ceramic fiber to flake off and plug up the SCR. The problem was remedied by replacing ceramic fiber with castable refractory. In special cases, a metal liner can also be used in existing heaters to address the ceramic fiber flaking issue. Tube metallurgy and support considerations. Selection of
tube size is typically a function of the type of process, process flow and pressure drop available. The selection of tube material and thickness depends on the type of process, process conditions (design process temperature, elastic/rupture pressure, etc.), oxidation, corrosion and deposition mechanisms. The use of seamless tubes is highly recommended. API 530 provides guidance on the design limits for metal temperatures for heater tubes and fin materials. Process tubes are typically designed for a 100,000-hr operational life span. If higher tube life design is required, it should be clearly specified. Temperature and corrosion allowances should be specified. Post-weld heat treatment and radiography requirements should also be specified if, and as, required. Positive material identification (PMI) criteria should be outlined as per the project specifications. For heaters requiring tube cleaning, generally removable spool pieces are specified at the heater inlet and outlet tubes with flanged crossovers. Steam air decoking, manual cleaning and online spalling access requirements should be addressed. Cleaning mechanisms like soot blowers (which can be fixed or retractable) are typically utilized for external tube cleaning in convection/downstream equipment, when fuel oil or another fouling medium is used. 25Cr-20Ni (HK-40 grade) material is typically used as tube support in the radiant section. However, high vanadium, sodium salts (in fuel oils) and chlorides may also require an upgrade of the tube support metallurgy. API 560 provides maximum de-
Heat Transfer
Stack design considerations. For natural-draft heaters, the
stack provides the draft needed to overcome resistance created by convection, damper and stack losses. However, apart from the draft requirements, the determination of stack height depends on its location inside the process plant. If located close to any equipment (i.e., a tower) with access to a platform, stack/ platform elevations may need to be adjusted to meet the minimum height requirement. Dispersion modeling requirements should also be addressed prior to finalizing elevations. API 560 recommends one takeoff stack for every 40 ft of the convection tube length. The draft system should be capable of providing at least –0.1 in. water column (inWC) pressure at the radiant arch, at 120% of normal heat release with design excess air and design stack temperature. Stack damper design should be such that it provides adequate draft control for all design scenarios, including the turndown condition. FIG. 3 shows the flue gas draft generation as a function of flue gas temperature and ambient air conditions at sea elevation.
TABLE 1 shows values for the draft correction factor for altitude variation with respect to the mean sea level. As a good design practice, the stack exit should be sized to ensure a minimum velocity of 10 ft/sec at heater turndowns to avoid inversion, which can cause draft instability. Typically, design stack exit velocities are approximately 25 ft/sec. For heaters with extended turndowns, higher design exit velocities should be considered. A stack cone is usually added to achieve increased velocities. Minimum stack exit velocity requirements should also be defined for the dispersion modeling.
Draft/ft, inWC
sign temperatures for various tube-support materials and guidelines on support lengths. The design of external piping supports, guides and clips (requiring support from the heater) should be checked using pipe stress analysis. Details of the external structural supports and clips should be communicated to the heater fabricator. Material procurement and fabrication locations can have a major impact on the heater cost.
0.014 0.013 0.012 0.011 0.010 0.009 0.008 0.007 0.006 0.005 0.004 0.003 0.002 0.001 20
40
60
80 100 Ambient air temperature, °F
Flue gas temp = 300°F Flue gas temp = 400°F Flue gas temp = 600°F Flue gas temp = 800°F Flue gas temp = 1,000°F
120
140
160
Flue gas temp = 1,200°F Flue gas temp = 1,400°F Flue gas temp = 1,600°F Flue gas temp = 1,800°F Flue gas temp = 2,000°F
FIG. 3. Flue gas draft at sea elevation.
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Heat Transfer FIG. 4 shows the stack area required as a function of flue gas temperature and flowrate for a 25-ft/sec stack exit velocity. Requirements for aircraft warning lights should be checked with the appropriate authorities. This will depend on the stack height, plot plan location and nearby structures. US Environmental Protection Agency (EPA) testing or continuous emissions monitoring systems (CEMS) nozzle requirements (quantity and size) should be checked and appropriately sized and/or specified. Purge fans or stack steam eductors should be specified for any purging needs (as required for natural draft startup). Using snuffing steam for purging in the radiant box during heater startup has been associated with reliability issues to the pilot flame, the ignition rod and the flame scanner. These issues are attributable to condensate buildup and possible refractory damage.
Burner design and emissions considerations. The burner is the heart of a fired heater, since this is where combustion occurs. Burners should be specified with continuous running pilots with flame and ignition rods. API 560 provides guidelines for determining maximum heat release based on the number of burners, and minimum clearance guidelines for placement. However, in general, the number of burners selected should be such that, when a burner is down for maintenance, the rest should TABLE 1. Draft correction factors vs. altitude variation 0 1,000 2,000 3,000 4,000 5,000 6,000 7,000
Altitude, ft
Correction factor 1 0.967 0.933 0.899 0.865
0.83 0.803 0.769
120
Stack flow area, ft2
100 80 60 40 20 0 300
400
500
600 700 Stack temperature, °F
25,000 lb/hr 50,000 lb/hr 100,000 lb/hr
800
150,000 lb/hr 200,000 lb/hr
900
1,000
250,000 lb/hr 300,000 lb/hr
Lb of flue gas/lb of fuel
FIG. 4. Stack area vs. flue gas flowrate. 38 36 34 32 30 28 26 24 22 20 18 16
10
20
30
40
50 60 Excess air, %
70
80
90
100
FIG. 5. Flue gas flowrate range for refinery fuel gases vs. excess air, %.
70OCTOBER 2015 | HydrocarbonProcessing.com
be able to provide full design duty. Inadequate burner-to-burner spacing can cause flames to coalesce, which can, in turn, cause increased flame lengths and NOx emissions. Burner turndown criteria should be checked according to the heater operation criteria. Radiant flue gas temperatures, fuel data (composition, pressure, temperature, heating value, etc.), combustion air temperature and design excess air requirements have direct effects on burner emissions. Higher air-side pressure drop can be used to generate a smaller flame profile. Acceptable amounts of corrosive elements in the fuel, such as sulfur, vanadium, chlorides, etc., should be clearly specified. High H2S content in fuels may necessitate the use of SS piping for fuel gas pipe and burner manifolds. The presence of chlorides can also have adverse effects on both the fuel gas and flue gas systems, and metallurgy selection should be evaluated. The use of SS metallurgy on burner damper blades with graphalloy bearings is recommended in environments such as the US Gulf Coast, as this metallurgy can increase burner operational reliability. Relative humidity requirements for heater design should be outlined as per the process design basis of a project. Available fuel gas pressures should be checked for proper design of the burners. Extra fuel gas pressure-drop elements like knockout drums, coalescers, filters and fuel gas skids should be considered. Typically, 15% excess air is used for the design of burners when using fuel gas. Burners should not be operated below 10% excess air. Noise abatement requirements should be addressed as per the project specifications. Apart from the need to aid burner combustion with excess air, other design elements worth examining are O2 analyzer reporting lag time, air leakage in the radiant section, variations in process needs, variations in fuel gas composition, and changes in the draft requirements. Several items should be considered when specifying flame scanners: • They are X-ray safe and will not result in spurious trips • They should be suitable for the flame type and fuel range being utilized • They should be purged to minimize debris accumulation • To set a correct scanner position, flame scanner mounting should be a lockable swivel type • If a flame scanner is being mounted on the burner, it should be tested for performance in the burner test for ideal orientation/position. FIG. 5 shows the flue gas flowrate range for typical refinery fuel gases vs. excess air. Burner emissions and remediation. Typical emissions and methods for addressing them include: • NOx: Mainly NO and NO2 emissions. Air staging, fuel staging and increased flue gas recirculation are common ways to reduce NOx emissions. In special circumstances, steam injection can also be used to reduce NOx ; however, steam injection is known to cause refractory damage and accelerate metal corrosion. Post-flame treatment methods to reduce the NOx are SCR and selective non-catalytic reduction (SNCR) technologies. Note: Some methods to reduce burner NOx emissions have a direct effect on the burner stability, which can result in increased carbon monoxide (CO) and VOC emissions. The burner stability and emissions should be checked in a burner test (i.e., either a single-burner or multiple-burner test).
Heat Transfer • CO: Result of incomplete combustion or improper mixing of fuel. • VOC: Typically caused by incomplete combustion. API 535 defines VOC as any compound of carbon that can participate in atmospheric photochemical reactions. • Particulate: According to API 535, all fuels contain or produce particulates. Some particulates can also result from eroded refractory, tube scales, etc. • SOx: Sulfur content in the fuel directly contributes to SOx emissions. SOx emissions react with water to form sulfuric acid. The best way to reduce SOx emissions is to reduce the sulfur content of the fuel. • NH3: Ammonia slip emissions are caused by the unreacted ammonia passing over the SCR catalyst. These are to be addressed when an SCR unit is installed with the heater. Typically, computational fluid dynamics is carried out to ensure proper ammonia distribution. • CO2: One of the byproducts of combustion. Higher heater efficiency reduces CO2 emissions. Burner test. Tests are conducted for fuels specified to test the burner for flame envelope, emissions and stability. A CO probe should be used to check on the flame envelope. CO with a ppm of 2,000 is typically considered a visible flame. However, lower CO levels may also be checked to account for an invisible flame envelope. Pilot and burner compatibility should be checked for startup/shutdown/operating conditions to ensure safe and reliable
operations. Final burner capacity curves should reflect the actual test tip pressures and orientation. In one case study, a vertical cylindrical natural draft heater was experiencing unequal air to the burners, resulting in unpredictable flue gas patterns. A field visit confirmed that the heater was located on one end of the process plant, and strong winds blowing across the burner intakes were disrupting air intake to the burners. A 12-ft-tall metal wall was built around the heater to reduce the wind effects, solving the problem. In another case study, existing raw gas burners for a reheat furnace with arbor coils (wicket type) were retrofitted with ultra-low-NOx burners to reduce NOx emissions. When the heater was fired up, all the flames leaned to one side, impinging on the tubes and resulting in disruption of heater operation. A field visit showed that the furnace created asymmetrical flue gas patterns because of the hot-end and cold-end tube walls. This problem was solved by redesigning the floor, erecting a center wall and installing new flat-flame burners. Stack damper design considerations. Stack dampers are
used to control the draft in the natural draft heaters. Dampers are also used in the flue gas and air ducts. Fan dampers are utilized to control forced draft and induced-draft fans. API 560 recommends a minimum of one blade for every 13 ft2 of internal crosssection area for butterfly dampers. The stack damper material should be 18Cr-8Ni material. A direct-mounted actuator should be specified for the operation of a stack damper.
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Heat Transfer Other criteria are correct bearing (for the temperature range and type of operation) selection, shaft surface finish and hardness requirements, shaft outside diameter to bearing inside diameter clearances, and adequate clearance between the damper blade and the refractory. Dampers should also be provided with a pointer to provide a visual on the indication of stack-damper position from the grade. In another case study, a vertical cylindrical heater had a stack damper with an actuator. To adjust the draft, the operator closed the damper, using an actuator from the control room. The stack damper became stuck, and the extra force from the actuator caused the damper to free up and close suddenly, which created a pressure surge in the box and put out the burner flames and the pilots. Since no flame scanners were installed, the heater witnessed flame-out with fuel leak. Flame-out was evident from the falling process outlet temperature and an increase in O2 in the radiant box. To remedy this issue, a stack damper was retrofitted with new bearings with adequate clearances. All linkages and blades were refurbished to ensure that the damper did not stick and cause another safety hazard. Flame scanners were also installed as safeguards. Air preheat considerations. There are two types of air preheaters: rotary (regenerative) and static (recuperative). A static air preheater offers the advantage of no leakage and no moving parts. When considering an air preheater, the burner design should conform to the new operating conditions. Acid dewpoint issues should be given consideration at the cold end. Cold-air bypass or air preheating are common methods to prevent cold-end dewpoint issues. In certain circumstances, a borosilicate glass tube design is used at the entry section to address the cold-end dewpoint issues. When utilizing an air preheater with an SCR unit, water wash or other cleaning means should be provided for salt cleaning. Heater instrumentation. API 556 provides guidance on heater instrumentation. However, several key parameters should also be given appropriate consideration during the design phase. Process flow imbalance in the process passes should be minimized, since this leads to tube overheating, increased coke formation and heat flux imbalance in the heater. Manifold designs should ensure low process pass imbalance. For conventional heaters, the installations of flow transmitters, individual pass flow controllers on the process inlet passes and temperature transmitters at the outlet passes are required. These transmitters help make required flow adjustments to avoid process heat flux imbalances. Process low-flow alarms and trips should be configured appropriately. Tube skins provide valuable information on tube conditions. A minimum of two tube-skin thermocouples per pass are recommended. Crossovers should also be specified with temperature connections. The provision of spare access ports at the radiant floor, bridgewall, upstream and downstream of the dampers is recommended. Similar access ports are recommended for monitoring duct pressures upstream and downstream of fans, heat recovery equipment and emission-reduction units. Several instruments are recommended for the flue gas side: • An O2 , temperature and draft transmitter at the radiant bridgewall 72OCTOBER 2015 | HydrocarbonProcessing.com
• An O2 and temperature transmitter in the stack (last flue gas exit) • Temperature transmitters are recommended upstream and downstream of heat recovery equipment with a differential pressure transmitter (dPT) to monitor pressure drop • Temperature transmitters are recommended upstream of the emissions reduction unit with a dPT to monitor pressure drop • When considering an air preheater, an O2 transmitter should be installed both upstream and downstream to monitor air leakage into the flue gas • Emissions monitoring equipment should be installed as per project requirements. The following instrumentation is recommended for main fuel gas and pilot gas lines: • Gas pressures should be monitored with pressure transmitters, and they should have high- and low-pressure trips based on burner curves • Independent trip and control valves should be provided for gas lines; generally, a pressure regulator is used for pilot operating pressures • Strainers should be provided upstream of the trip valves to ensure a tight seal for the trip valves during heater shutdown; this also prevents debris from plugging up the burner tips • Coalescers should be used if liquid issues are anticipated in the fuel gas; fuel gas heat tracing and insulation can be considered • When considering fuel gas line insulation, corrosion under insulation should also be evaluated • CEMS requirements should be checked; these generally depend on the heater duty and the state regulatory requirements • All instrument transmitters should be wired with control room indication • All hardware and software design of the instruments should be as per the required safety integrity level (SIL). In one case study, a heater had draft issues even with its stack damper in an open position. A site visit showed that the stack damper was stuck, and the position transmitter was mounted on the actuator with its linkage mechanism broken. The problem was remedied by making the stack damper functional again. The stack damper position transmitter was mounted on the damper shaft, with a functioning linkage to provide positive position of the damper. Other design considerations.5 The heater should be de-
signed to withstand seismic and wind load requirements for the site location. Snuffing steam connections are provided at the radiant, convection and header boxes. The heater should have access doors to provide access for the radiant, convection, stack damper, ducts and other heater components. Electrical area classification and requirements, instrument air requirements, etc., should be stated as per project design specifications. Fans (induced-draft or forced-draft) should be sized to provide a minimum of 15% margin to the design flow (lb/hr). A temperature margin is also recommended for the test block case. Summer air temperatures, relative humidity and altitude of the installation should be specified. If the fan operation is
Heat Transfer is critical and would result in a shutdown of the downstream units, then a spare fan should be specified. For low turndowns, a variable frequency drive or a combination inlet/outlet damper should be considered. A heater shall be provided with sufficient peep doors to provide visuals for all radiant tubes and burners. A sealed self-closing peep door with glass protection ensures that the heater is sealed against tramp air leakage. The glass protection also provides shielding against any positive pressure conditions. Floor view ports can be considered to provide an unrestricted view between the tubes and the burners. Any special requirements needed for infrared scanning should be discussed and investigated. Heaters should be specified with tube seals to seal up all of the tube penetrations and guides. All unwelded seams should be caulked. Ladders and platforms should be provided to access all peep doors, maintenance access doors, stack dampers, fans, emissions reduction equipment, EPA nozzles and heater instrumentation. Ladders and platforms are generally hot-dip galvanized. Requirements on safety gates (color designation and type) should be checked in the site-specific needs or project specifications. Consideration should be given for modularized construction to reduce field work and facilitate shipping and transportation, as well. Startup spares should be specified as a part of the heater package. Heater paint should be specified as per the project specifications. Fireproofing should be addressed in the field installation. Clips for fire protection installation should be provided by the
heater fabricator as per the specifications. Specific requirements for the grounding lugs should also be specified. Inspection and testing performed on the heater components should be witnessed and approved by the client representative/inspector. In one case study, it was reported that burners were leaking fuel gas in a heater. A field investigation identified that burners were plugged up due to inferior fuel gas quality. The fuel gas risers were connected by unions. Due to the regular plugging, a daily burner maintenance program was being performed. The constant removal and installation of the risers caused the unions to wear off. As a result, the seal integrity was compromised, which, in turn, caused the fuel gas to leak. This leak was remedied by adding a fuel gas filter and a coalescer, and by replacing the burner fuel riser connections. NOTE All case studies presented here have been developed solely for the purpose of illustrating typical problems in fired heaters, and their respective solutions. Any similarities found to actual problems in existing installations may be coincidental. LITERATURE CITED Complete literature cited available online at HydrocarbonProcessing.com. KAPIL MALHOTRA is a heat transfer engineer at S & B Engineers and Constructors Ltd. in Houston, Texas. He has more than 12 years of experience in the design, engineering and troubleshooting of fired heaters, combustion systems and thermal equipment. He holds a master’s degree in mechanical engineering from Oklahoma State University. Mr. Malhotra is a registered professional engineer in the state of Texas.
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