Nov-2023

Restoring fired heater furnace heat transfer efficiency

Convection section cleaning of process heaters at an Egyptian facility achieved more than 90% treating of all tube and surface areas. Inefficient heat transfer in the convection section of a process heater can be readily witnessed by an increase in stack temperature beyond design parameters. The timing and urgency for addressing this situation are related to not only the degree of overheating in the stack but also product demand, economics, and fuel costs.

Scott Donson
IGS Tube Tech

Article Summary

Inspection and technical evaluation
For many IGS Tube Tech projects, a site visit is made to complete an inspection of the convection section and collect current operating data so an evaluation can be conducted. The evaluation determines the baseline operating conditions and estimates the potential project scope and expected benefits.

The evaluation is based on calculating the external fouling resistance factor for each bundle in the convection section based on the process data. The factors will be relieved by cleaning, and a further evaluation would determine the post-cleaning performance, based on a constant processing duty, for example. The IGS Tube Tech cleaning technology allows for more than 90% of all the tube and fin surface area to be treated, unlike other more conventional methods, which reach only 20-40% of the tube surfaces, depending on the configuration.

Inefficient heat transfer in the convection section of a process heater can be readily witnessed by an increase in stack temperature beyond design parameters. The timing and urgency for addressing this situation are related to not only the degree of overheating in the stack but also product demand, economics, and fuel costs.

Inspection and technical evaluation
For many IGS Tube Tech projects, a site visit is made to complete an inspection of the convection section and collect current operating data so an evaluation can be conducted. The evaluation determines the baseline operating conditions and estimates the potential project scope and expected benefits.

The evaluation is based on calculating the external fouling resistance factor for each bundle in the convection section based on the process data. The factors will be relieved by cleaning, and a further evaluation would determine the post-cleaning performance, based on a constant processing duty, for example. The IGS Tube Tech cleaning technology allows for more than 90% of all the tube and fin surface area to be treated, unlike other more conventional methods, which reach only 20-40% of the tube surfaces, depending on the configuration.

Project planning and execution
To perform an effective cleaning using a remotely operated vehicle (ROV) and protect the existing equipment from water exposure, the following five steps are considered for each project (see Figure 1):
υ    Creating or using existing access openings (typically 350 x 500mm) to enable placing the ROV on top of each convection bundle (see Figure 2)
ϖ Isolating the radiant box
ω Assembling scaffold at the bottom of the convection section, allowing the installation of a thick plastic tarpaulin
ξ Protecting refractory walls around convection and radiant sections with the tarpaulin (see Figure 3)
ψ Collecting all effluent and debris after cleaning into tarpaulin and moving them safely to ground level in sediment containers.

The ROV (see Figure 4) is designed to fully clean convection bank coils by penetrating its lance deep between tube rows. The technology removes more than 90% of fouling from all convection bundles (see Figure 5). No refractory is damaged since the ROVs are programmed to direct a high-pressure medium to the tubes only. All activities typically can be completed in 72-120 hours (in three to five 12-hour shifts).
Post-project benefit analysis

In many cases, the stack temperature is determined to be a key performance indicator to identify and quantify the benefit after cleaning (see Figure 6). To illustrate the benefits, a recent project referred to as Project X is reviewed. On completion of Project X, a sizeable benefit of 40°C in stack temperature reduction was achieved. The plant also reported an average increase in overall fuel efficiency from 89.5% to 91.5%, leading to 2 MW less heat loss to the stack under the same operating capacity. The facility reported a payback period of less than four months.

New data two years after cleaning
A balanced heat distribution between sections is crucial in coking-sensitive services. Keeping the convection section surface clean not only helps to save fuel and increase steam generation but also positively influences the steam cracking process. The achieved benefit on Project X was 16,000 MWh/year fuel savings and 2,500 tons annual CO₂ reduction (fuel is CH₄/H₂ mixture).

After almost two years, the client reported a slight elevation in stack temperature of 10°C and a stable efficiency increase of 1.5%. The next cleaning is anticipated to take place after six years to reinstate the efficiency. To ensure the obtained benefit is long-lasting, IGS also recommends a combination of the Tube Tech ROV cleaning with Cetek’s proprietary ceramic refractory coatings to protect and encapsulate the ceramic fibre and stop refractory deterioration and new fouling formation on the outside surface of the convection tubes.

ROV cleaning: ethylene furnace
IGS was provided with furnace configuration and process parameters for each bank of the convection section of this referenced ethylene furnace. The objective was to keep the process duty and, thus, the coil outlet temperature the same and compare scenarios before and after (see Table 1).

It is worth mentioning that these furnaces were designed to have 68.3-71.0% process duty of total absorbed duty (depending on whether it is SOR or EOR). If, for example, steam superheating coils are fouled, cleaning may reduce this ratio process/total duty. An additional indicator of this potential fouling consequence is the fact that almost no boiler feed water (BFW) is being used for attemperation (desuperheating). For this scenario, constant flow rates for process streams are assumed, but it is also possible to consider all the factors and heat balance changes for the entire system.

ROV cleaning: catalytic reformer heater
In configurations where a steam generator is only located in the convection section, such as catalytic reformer heaters, it is crucial to adjust flow rates and reflect changes in absorbed duty for each bundle that may result from cleaning. Specifically, a steam drum should be included in the model to respond accurately to all changes in temperatures/pressures of inlet/outlet streams.

The convection section evaluation (see Table 2) reflects the most widespread design with the following bundles (from top to bottom): economiser, upper steam generation, steam superheating, and lower steam generation (example under the same firing rate).

Case study: revitalising HGU performance
An Egyptian oil refinery, in operation since 1999, has faced challenges with its hydrogen generation unit (HGU) since 2005. Issues such as hot spots on catalyst tubes, ageing reformer tubes and outlet systems, and reduced hydrogen demand have led to the unit operating at a reduced capacity. To address these bottlenecks and evaluate the unit’s status, a comprehensive assessment and debottleneck study was conducted by the OEM.