Electric actuators for industrial valve automation Safe, robust, precise.
For decades, AUMA actua-tors have excelled by their outstanding reliability throughout many market segments. Their capability for overall host system integration is of crucial importance for safe and economical operation in process technology.
For highest safety requirements
■ SIL 3 capability (1oo2)
■ Certification according to IEC 61508
■ Patented fail safe functionality
■ Available in fire-proof version
Select 166 at www.HydrocarbonProcessing.com/RS
Heat Transfer
Various design modifications were
evaluated, with the constraint of reusing original low-NOx burners. Reducing the burner circle diameter to increase the clearance between the burners and the ra-diant tubes was initially evaluated. These modifications showed some improvement in the heat flux distribution and some re-duction in the TMT. The additional mod-ification of inclining the burners toward the center of the heater was then evaluat-ed. The combined modifications showed substantial improvement in the heat flux distribution over the radiant tubes and some reduction in the TMT. Results for the final proposed modifications are dis-cussed to show the improvement.
FIG. 9 shows the temperature profile in the heater for the proposed inclined firing case. High flue gas temperatures move away from the tubes and toward the cen-ter of the heacen-ter, reducing flue gas tem-peratures by approximately 200°F around the radiant tubes. Inclining the burners keeps the high temperature region re-stricted only to the center of the heater.
FIG. 10 shows the flame profiles. The flames have moved away from the radi-ant tubes toward the center of the heater.
Inclining the burners helps to increase the clearance between the flames and the radiant tubes, which, in turn, reduces the heat flux and TMT. Reducing the burner circle diameter helps to reduce the veloc-ity of downward flue gas flow, keeping the flames in the central core of the heater.
FIGS. 11 and 12 show the heat flux and temperature profiles on the radiant tubes for the proposed case. The maximum heat flux in the bottom one-third sec-tion is reduced to 43,100 Btu/hr-ft2 from 55,670 Btu/hr-ft2. Also, the heat flux dis-tribution is almost uniform in the bottom two-thirds section. The heat absorption pattern for the bottom and middle one-thirds sections is 43% each, and the top one-third section is 14%. The tempera-ture profile also shows uniform TMT, reducing the maximum TMT to 1,010°F, showing a reduction of almost 70°F.
FIG. 14. Comparison of normalized heat flux distribution.
TABLE 1. Comparison of key process parameters for existing and proposed cases
Parameter Unit Existing case Proposed inclined case
Air-side pressure drop in. WC 1.1 1.1
Fuel pressure drop psi 19.2 19.2
Average radiant heat flux Btu/hr-ft2 20,560 19,430
Maximum radiant heat flux Btu/hr-ft2 55,670 43,100
Maximum TMT °F 1,080 1,010
Flue gas outlet temperature °F 1,350 1,416
Flame height ft 20.5 23.5
TABLE 2. Comparison of average radiant heat flux Section
Average radiant heat flux
Existing case Proposed inclined case
Bottom one-third 33,600 23,200
Middle one-third 18,800 24,800
Top one-third 9,200 10,200
FIG. 15. Comparison of CO mass fraction.
FIG. 13. Comparison of temperature profiles for existing and proposed cases.
FIG. 12. Tube metal temperature for the proposed inclined firing case.
Heat Transfer
Comparison of cases. The comparison of key process parameters for the existing and proposed cases is shown in TABLE 1.
The maximum heat flux value is reduced by 12,500 Btu/hr-ft2, and the maximum TMT is reduced by 70°F for the radiant tubes. For the inclined firing case, the flame height is increased by 3 ft. Overall, a significant improvement is observed in the flue gas flow pattern and temperature profile of the heater, as shown in FIG. 13.
The high-temperature flue gas flow-ing over the radiant tubes is completely eliminated in the proposed inclined fir-ing case. For the proposed case, the cen-tral region of the heater has a high tem-perature, helping to reduce the high heat flux and corresponding high-TMT spots in the heater.
FIG. 14 shows the comparison plot of the normalized heat flux distribution of radiant tubes for the existing and pro-posed inclined firing cases. The heat flux of the radiant tubes is normalized based on the average radiant heat flux of 20,000 Btu/hr-ft2 for both cases. A nor-malized heat flux distribution plot along the height of the radiant tube shows that, for the proposed inclined firing case, heat flux valves have been considerably re-duced for the bottom one-third section of the heater. The total height of the ra-diant tubes is divided into three sections:
the bottom third, the middle one-third and the top one-one-third. The average radiant heat flux for each of the three sec-tions is compared for both the existing and proposed cases, as shown in TABLE 2.
FIG. 15 shows the comparison of CO mass fraction contours at different eleva-tions in the heater. CO mass fraction con-tours are used to understand the flame profile along the height of the heater.
Horizontal planes at different elevations in the heater are used to visualize the flame profile for both cases. Regions with red color signify a CO mass fraction value of 2,000 ppm or more. For the proposed case, the flame profile is at considerable distance from the radiant tubes. These profiles also show that the diameter of the flame for the proposed case decreases along the heater height.
CFD simulation results provided de-tailed insights of the heater in terms of flame profiles, temperature distribution, regions with high TMT, and flue gas re-circulation patterns. Using CFD simula-tions, multiple design modifications were
evaluated to understand the extent of im-provement with respect to reduction in maximum TMT, flue gas circulation pat-terns and flame profiles. Based on these results, the design modifications that provide the most improvement in heater performance were selected.
Proposed modifications based on the CFD analysis presented in this study were implemented by the client, and the field results reported an approximate reduction of 150°F in TMT. Neither hot flue gas im-pingement nor flame leaning toward the radiant tubes was observed. CFD analysis was used alongside the process calcula-tions to evaluate the possible modifica-tions in the burner layout to eliminate the high-TMT issue in the heater, and also to improve the run length of the heater.
CFD is a viable and proven simulation tool that is widely used for the analysis of fired heaters and for troubleshooting heater performance issues. Any poten-tial design modification can be evaluated using CFD before implementation in the field to understand the extent of im-provement possible.
AMARVIR CHILKA works at Furnace Improvement Services' office in Pune, India, as a CFD engineer.
He has 14 years of experience in the field of computational fluid dynamic modeling. He has worked on a variety of process fired heaters, performed detailed combustion analyses, studied and improved air flow maldistribution across multiple burners, and improved flow and reduced system pressure losses for the induced-draft fan suction and discharge side. Mr. Chilka previously worked at the Fluent India office for six years, and Tridiagonal Solutions for seven years, working on various CFD consulting projects for the process, energy and oil and gas sector. He holds an MTech degree from the Indian Institute of Technology in Madras, India.
ASHUTOSH GARG has more than 40 years of practical experience in the design, engineering and troubleshooting of fired heaters.
He has provided fired heater training for more than 15 years, and has worked in the heater groups of KTI India, Engineers India and KTI Corp. for almost 20 years. Since 1996, he has being leading more than 20 engineers and designers at Furnace Improvements.
Mr. Garg is a registered professional engineer and a member of AIChE. He is also a member of the API subcommittee on heat transfer equipment.
He holds five patents on fired heater improvements, and he has also published several papers on fired heaters. Mr. Garg graduated from the Indian Institute of Technology in Kanpur, India in 1974.
InstruCalc 9.0 calculates the size of control valves, fl ow elements and relief devices and calculates fl uid properties, pipe pressure loss and liquid waterhammer fl ow. Easy to use and accurate, it is the only sizing program you need, enabling you to: Size more than 50 diff erent instruments; Calculate process data at fl ow conditions for 54 fl uids in either mixtures or single components and 66 gases, and; Calculate the orifi ce size, fl owrate or diff erential range, which enables the user to select the fl ow rate with optimum accuracy.
Updates and What’s New in InstruCalc Version 9.0
ENGINEERING STANDARD UPGRADES Control Valve Revisions:
• Updated to ANSII/ISA 75.011.01-2012
• Calculation accuracy changed for critical fl ows
• Viscosity correction factor changed
• Pressure drop calculation revised to agree with Crane Technical paper No 410.
• Option of Cv Units (English) or Kv units (Metric) added.
• Option of either aerodynamic noise calculation by ISA 75.17 method or InstruCalc method
• Calculation accuracy added (input data within acceptable limits) Relief Devices:
• Pressure Relief Devices Program follows API 520 Pt 1, 9th edition dated 7/14 OPERATIONAL IMPROVEMENTS The ability to have more than one calculation open at a time has been added.
Each instance of the program is framed in a diff erent color. The user can have multiple “what if” scenarios displayed for making engineering decisions.
Order Direct from the Publisher.
GulfPub.com/InstruCalc or call +1 (713) 520-4426.
InstruCalc
CONTROL VALVES • FLOW ELEMENTS • RELIEF DEVICES • PROCESS DATA NEW VERSION
REGISTER ONLINE AT WWW.ILTA.ORG HOUSTON, TEXAS
G E O R G E R . B R O W N C O N V E N T I O N C E N T E R