CFD Study of a Heater Stack Swing
The heater in one local refinery is used to preheat the distillate and hydrogen before the reactors. Recent operation of the heater has been challenging. The heater has exhibited swings exceeding 1 in WC but the target operating draft is 0.2 in WC.
The swings in draft are causing burner stability problems resulting in partially combusted fuel. Between these swings, it is believed the burner briefly operates at stable conditions. And recent research has found out that the main damper has been out of position.
In this paper, a CFD model was built by turbulence model and applied to operating conditions. The model was then validated, showing satisfactory agreement with provided field data.
Based on the information provided by the plant, assumptions and simplifications were conducted to do this study: 1. The CFD study will only focus on the downstream ductwork above the convection zone to the stack. 2. The geometry will include major parts of the ductwork as it is shown in Figure 1.
According to Figure 2, the velocity of fuel gas through the damper has been accelerated and determines significant vortices at the time of 2.5s to 3.5s.
Through field experiments and general operation devices detection, the pressure swing profile was obtained. The amplitude of pressure drop fluctuation is around 1 in WC (250 pascal), and the frequency with 0.25s-1 can match the field data satisfactory which is shown in Figure 3.
The fire heater had been well presented by using CFD method. After validation against field measurement data, the simulation results had identified the vortex shedding generated by damper as the cause of the pressure swing.
As requested by and with the guidance of the refinery plant, PUC generates series of cases to figure out the pressure fluctuation profile, about the amplitude and the frequency.
The relationship between the pressure drop and velocity has been identified as well using UDF method. The impact of geometry condition has been analyzed that the original design is the best of all the three test designs.