New Damper Design Improves Draft Control in Fired Heaters

Paper from the AFRC 2017 conference titled New Damper Design Improves Draft Control in Fired Heaters

Fired heaters are major consumers of energy in the Refining and Petrochemical Industries. Almost 40 to 70% of the total energy consumption in a refinery or petrochemical plants is in fired heaters. Refineries in the USA spend upward of $2.5 Billion per year on the fuel bill.; While most of the plant operators and engineers are aware of the importance of controlling Excess Oxygen in the fired heaters, the draft control in fired heaters is often over looked. Most of the heaters are operating with very high draft. The average draft in the fired heaters maintained is at almost 3-4 times the value recommended. ; This has happened over the years as the not much attention is paid to the design and operation of stack dampers. Most of the stacks are sized for summer ambient temperature and 115% load. The damper loss considered in design is 1.5 velocity heads although fully open damper consumes only 0.5 velocity heads. Stack heights are also controlled by the pollution considerations. Stack diameters are dictated by structural considerations. As a result of all the oversizing, the damper ends up consuming lot more pressure drop during the actual operation. The stack damper could be operating at 40-50% open at full loads. The heater gain draft in the night due to lower ambient temperatures and it could mean another 0.1 inch WC of draft or more. This means that the stack damper should be controlling draft accurately during the day and night cycle. Most of the dampers are kept fully open all the time. This type of operation causes considerable loss of energy and inefficiency. We have developed a new type of damper operation for controlling draft in the fired heaters. Our patent pending damper design has multiple pneumatic operators for controlling the damper. Typically two operators will cover most of the range but in some very large dampers, we may recommend installing 3 operators. By installing multiple operators, we are able to change the control characteristics of the damper itself and damper can work smoothly from 50 % to 115% load without any issues. We are going to talk about the draft control in firing heaters using the new patent pending control system . It is possible to save substantial amount of energy by improving the controls. We can take care of tramp air by operating the heater at correct draft. We have a developed a very Rugged and Reliable Damper Control System which can be used by itself or can be integrated with existing draft control system

New Damper Design Improves Draft Control in Fired Heaters Ashutosh Garg Furnace Improvements www.heatflux.com New Damper Design Saves Energy vWhy talk about draft control? vDraft and Excess O2 are the two parameters that are being monitored and controlled vLot of progress has been made in Oxygen Analyzers in the last 20 years vVery little attention has been paid to draft control vPoor design and quality of final control element- stack dampers vVery few dampers really operate properly in the Industry (most are left fully open) Fired Heater Operation vOperators need to adjust stack damper for draft control in fired heaters v90% of fired heaters in US are natural draft heaters vTramp air is directly dependent upon the draft inside the heater vHigh draft can even affect the flame patterns vTramp air leakage can misguide the operating personnel Tramp Air Leakage vAir can leak from all the openings § Peep Doors, Header Boxes, Tube Penetrations etc. vThis air does not mix with fuel and shows up in O2 analyzers vIt absorbs the heat that should be transferred to the heater tubes vAll air entering heater should be entering through the burners that are on Stacks are highly oversized vStacks are designed for 115-120% of the loads and maximum ambient temperatures of 95ºF to 105ºF vStack design use 1.5 velocity heads pressure drop across damper, we need only 0.5 in fully open position vStack diameters and height are often decided by structural stability and minimum height considerations vStacks are over designed and produce very high draft in fired heaters Stack Damper Operation vDraft in natural draft heaters varies with ambient air temperature vNight time cooler air temperature produces higher drafts, almost 0.1 inch extra draft is available vStack damper needs to be adjusted at least twice a day to optimize the heater operation vStack damper position for 115% load is around 55º-60º open vStack damper needs to be open only at 45º-50º for full load operation Stack Damper Operation vFor a manually operated damper, the stack damper operation becomes very cumbersome vFor a pneumatically operated dampers, the operators are concerned about closing the damper and tripping the heater vIn practice, most of the time damper blades are left fully open Conventional Damper Blade System vParallel or Opposed Blade Operation vAll the blades are operated with a single actuator vAll the damper blades move at the same angle vArea around the blades is available for flue gas leakage vNon-Linear damper flow characteristics makes the system less efficient for draft control Parallel Blade Dampers Opposed Blade Dampers Damper Flow Characteristics Damper Opening Vs Heater Load 100% 80% Heater Load, % vBoth parallel and opposed blade configurations deviates considerably from ideal operation of linear flow characteristics vFor the same heater load opposed blade configuration will have larger damper opening as compared to parallel blade configuration 60% 40% Parallel Blade Operation 20% Opposed Blade Operation Ideal Operation 0% 0% 20% 40% 60% Damper Opening, % 80% 100% Fired Heater Draft Profile vDamper is modulated to maintain 0.1"WC draft at arch vFlue Gas Pressure Drop consists of pressure losses along: § Convection Section & Stack Entry § Stack friction § Stack exit loss vStack effect: § Movement of ambient air into the heater resulting from buoyancy due to density difference in hot flue gas and ambient air resulting from temperature differences STACK EXIT LOSS (SE)a STACK EFFECT IN STACK Pc (SE)c NEGATIVE PRESSURE 0.05"- 0.1" W.G. AT TOP OF RADIANT SECTION Pb (SE)r Pa NEGATIVE POSITIVE PRESSURE PRESSURE 0 Stack damper needs to consume higher pressure drop as the heater load goes down 0.85 Flue Gas Pressure Drop and Stack Effect Vs Heater Load Pressure, inches WC 0.71 0.57 0.43 Flue Gas Pressure Drop Stack Effect at 30F Stack Effect at 85F 0.29 0.15 35% 49% 63% 77% Heater Load, % 91% 105% Safe & Reliable Draft Control System vArea surrounding the dampers is sealed to prevent flue gas leakage vMultiple pneumatic actuators provided to operate damper blades individually vIndependent control of damper blades changes the damper characteristics providing precise control at any load Seal Plate Seal Plate Multiple Actuators Salient Features vMultiple pneumatic operators with hand actuators at the panel vBetter controlling characteristics vOperator friendly vAvoid tramp air leakage vCan be controlled from the control room Stack Damper with 2 Blades Conventional Design with Single Control Driver Parallel Opposed SRDCS with Two Control Drivers Stack Damper with 4 Blades Conventional Design with Single Control Driver Parallel Opposed SRDCS with Three Control Drivers General SRDCS Operating Philosophy vOperating philosophy for a smaller diameter stack with 3 damper blades § 100-75% heater load: One Blade is kept closed and other two blades are operating § 75-50% heater load: Two blades are kept closed and only one is operating vThis allows us to make major adjustment with one actuator quickly as the load is adjusted and then smaller adjustments can be done with the other actuator Option-1 (Heater Load: 100-75%) Option-2 (Heater Load: 75-50%) Damper Characteristics Damper Opening Vs Heater Load 100% Heater Load, % 80% 60% 40% Parallel Blade Operation Opposed Blade Operation SRDCS-1 Operation (100-75%) SRDCS-2 Operation (75-50%) Ideal Operation 20% 0% 0% 20% 40% 60% Damper Opening, % 80% 100% Case Study Large Steam Super Heater Furnace Process Conditions Unit 100% Load 80% Load 65% Load 45.5% Load Heater Load MMBtu/hr 327.7 260.9 213.4 149 Flue gas flow rate per stack lb/hr 183,587 145,493 116,115 79,881 °F 529.3 478.3 435.6 383.2 lb/ft3 0.037 0.039 0.041 0.045 °F 85 85 85 85 Parameter Flue gas temperature Flue gas density Ambient temperature Existing Stack Details v2 Identical Stacks per Heater 6'-11 1/2ʺ 6'-11 1/2" I/S INS. I/S INS. 83' I/S INS. CL STACK & CONV. v Damper § Two blades- Parallel Operation EL. 130'-0" CL STACK & CONV. § Location- Top of Convection § Type- Self supporting § Stack Diameter- 7'-11 ½" (I/S Insulation) § Stack Length- 83 ft § Stack Elevation- 130 ft 7'-11 1/2ʺ Stack Rating Calculation STACK ________ CONVECTION SECTION RADIANT SECTION vStack rating calculation is performed at 100% heater load vDraft at arch: 0.200" WC vStack effect is considered at a maximum ambient temperature of 85ºF and average flue gas temperature along radiant, convection and stack section vFlue gas side pressure drop: § Along convection section tubes § Stack entry loss= 0.5 velocity head § Pressure drop across damper= 1.5 velocity head (60º Open) § Stack exit loss= 1 velocity head 0.05"- 0.1" W.G. DRAFT DRAFT AT RADIANT SECTION OUTLET, R0 BURNERS Draft Profile: Stack Rating Condition DRAFT PROFILE: RATING AT 100% LOAD, 85ºF AMBIENT TEMPERATURE FLUE GAS PRESSURE DROP (INCHES WC) CONVECTION+STACK ENTRY: 0.228 STACK EXIT= 0.0" WC ACROSS DAMPER : 0.126 STACK EXIT : 0.082 145 (+) PRESSURE (-) PRESSURE DAMPER = -0.388" WC 87 58 CONVECTION= -0.262" WC 29 ARCH = -0.200" WC BURNER= -0.384" WC -0.550 -0.410 -0.270 -0.130 PRESSURE, (INCHES WC) 0.010 0 0.150 ELEVATION, (FT) 116 STACK EFFECT (INCHES WC) RADIANT SECTION : 0.184 CONVECTION SECTION : 0.166 STACK HEIGHT : 0.470 Available Pressure Drop Across Damper Flue Gas Flowrate Flue Gas Pressure Drop Stack Effect Percentage, % 100 % 80 % 65 % 45.5 % inches WC 0.510 0.387 0.316 0.253 inches WC 0.636 0.623 0.602 0.565 Available Pressure Drop Across Damper inches WC 0.126 0.236 0.286 0.312 v Available Pressure Drop Across damper= Stack Effect - Flue gas pressure drop v Flue gas pressure drop= Flue gas pressure drop (Convection + Stack) + 0.2 inches WC (Draft at Arch) Flue Gas Pressure Drop and Stack Effect Vs Heater Load Flue Gas Pressure Drop Stack Effect ΔP across damper= 0.236 "WC 0.80 0.35 ΔP across damper= 0.286 "WC 0.50 ΔP across damper= 0.312 "WC Pressure, inches WC 0.65 ΔP across damper= 0.126 "WC Available Pressure Drop Across Damper 0.20 0.05 30% 46% 62% 78% Heater Load, % 94% 110% Available Pressure Drop Across Damper Pressure Drop Across Damper, inches WC Heater Load Ambient Temperature, 85ºF Ambient Temperature, 30ºF 100% 0.126 0.282 80% 0.236 0.391 65% 0.286 0.441 45.5% 0.312 0.467 Pressure Drop Comparison: 85ºF Vs 30ºF 0.55 Available Pressure Drop Across Damper Vs Heater Load Available Pressure Drop at 30F Available Pressure Drop at 85F Pressure, inches WC 0.45 0.35 0.25 0.15 0.05 30% 46% 62% Heater Load, % 78% 94% 110% Additional Draft Available Damper kept fully open Heater Load Ambient Temperature, 85ºF Ambient Temperature, 30ºF 100% 0.084 0.240 80% 0.210 0.366 65% 0.270 0.426 45.5% 0.302 0.460 Damper Operation vParallel and opposed blade operation § All the blades move at same angle (ϴ) v For SRDCS operation two options are evaluated: § 2 damper blade system • Each damper blade can be operated individually at angles ϴ1 and ϴ2 § 4 damper blade system • Centre two damper blades are operated together with a single actuator at angle ϴ2 • Extreme two damper blade are operated at angle ϴ1 and ϴ3 Parallel Blade Dampers SRDCS: Option 1 SRDCS: Option 2 Option 1 2 Blade Damper Operation Damper Operation vParallel and opposed blade operation, all the blades move at same angle (ϴ) vSRDCS operation, the dual damper blades are operated independently at ϴ1 and ϴ2 Conventional System SRDCS Damper Openings at Different Loads Comparison of Damper Openings Flue gas flow rate ΔP Across Damper lb/hr inches WC ϴ ϴ ϴ1 ϴ2 183,587 (100%) 0.126 58º 60º 67.5º 67.5º 145,493 (80%) 0.236 42º 49º 54º 54º 116,116 (65%) 0.286 33º 42º 0º 75º 79,881 (45.5%) 0.312 19º 30º 0º 52º Parallel Opposed SRDCS Impact of Ambient Temperature vTwo ambient conditions were considered to study the change in two blade damper angles for 100% heater load Comparison of Damper Angles Ambient Temperature ΔP Across Damper ºF inches WC ϴ ϴ ϴ1 ϴ2 85 0.126 58º 60º 67.5º 67.5º 30 0.282 46.5º 52.5º 57.5º 57.5º Parallel Opposed SRDCS Option 2 4 Blade Damper Operation Damper Angles Comparison of Damper Angles Flue gas flow rate ΔP Across Damper Parallel Opposed lb/hr inches WC ϴ1 ϴ1 ϴ1 ϴ2 ϴ3 183,587 (100%) 0.126 58º 60º 0º 87.5º 90º 145,493 (80%) 0.236 42º 49º 0º 65º 90º 116,116 (60%) 0.286 33º 42º 0º 73º 0º 79,881 (45.5%) 0.312 19º 30º 0º 60º 0º SRDCS Impact of Ambient Temperature vTwo ambient conditions were considered to study the change in damper angles Comparison of Damper Angles Ambient Temperature ΔP Across Damper ºF inches WC ϴ1 ϴ1 ϴ1 ϴ2 ϴ3 85 0.126 58º 60º 0º 87.5º 90º 30 0.282 46.5º 52.5º 0º 68.5º 90º Parallel Opposed SRDCS Summary vSRDCS provides flexibility of altering damper configuration for various heater loads vWith multiple actuators, one actuator could be used for major adjustments and other actuators for minor adjustments vSRDCS design always has damper angle in the range of 45-80 degrees