Careers Preventing Fired Heater Tube Overheating in Vacuum Units
Importance of Considering Phase Separation in Refinery Operations
One example is how phase separation in fired heater outlet tubes and transfer lines affects the predicted heater outlet temperature and the estimated liquid flow rates in the wash section. Neglecting this phenomenon can reduce the yield of gasoil products and lead to an excessively low wash oil flow rate. More critically, it may result in overheating of fired heater tubes in vapour-only regions with a low heat transfer coefficient, causing the actual tube metal temperature (TMT) to approach or exceed design limits. This is not just a matter of process efficiency but equipment safety and reliability.
Effects of Phase Separation on Fired Heater Performance
Most conventional modelling assumes liquid-vapour equilibrium at the heater outlet—even for large-diameter horizontal tubes. However, in practice, especially after lowering the vacuum column pressure, liquid and vapour phases separate before the transfer line.
Calculated phase regimes are either stratified or stratified wavy. Stratified phases cause the liquid and vapour to have poor mass and energy exchange across the interface, resulting in poor liquid and vapour contact. Thus, phase separation causes the vapour to flow through the top of the horizontal portion of roof tubes, and this vapour becomes superheated due to pressure reduction. The result of the roof tube overheating is shown in the image on the right. The upper part of the tubes doesn’t «see» flame, so it is not subject to any significant oxidation scale formation. Keeping that in mind, the sizeable gap between the actual process outlet temperature and tube metal temperature (>200°C) can be encountered, way exceeding the best design practices of no more than 80 °C or a maximum TMT of 500 °C [3-5].
IGS Ceramic Wrapping Solution for Fired Heater Tube Protection
The IGS Thermal Efficiency Team was contacted by one of the global oil and gas supermajors to develop a solution. After multiple unsuccessful operational adjustments, IGS proposed a non-invasive fix that would avoid costly revamps during a planned shutdown.
Due to the large outer diameter (OD) of the vacuum heater outlet roof tubes (8” and 10”), phase separation caused hot spots due to poor heat transfer. IGS recommended proprietary ceramic wrapping insulation technology, applied to the 180° circumferential surface of the roof tubes, and secured with high-temperature wrapping wires.
Project Goals: Reducing Heat Flux and Hot Spots
The objective of this project (Figures 4 & 5), according to the IGS evaluation, was to reduce heat flux from approximately 11000 to 1150 Btu/(ft2.hr) with a ca. 190oC tube wall temperature drop, entirely removing hot spots on the top part of these 8” and 10” tubes. The IGS operations team successfully completed all work within the heater over a three-shift period, finishing on time and within budget while prioritising safety as their foremost principle.
Measured Results: Performance Gains in Fired Heater Efficiency
IGS compared skin thermocouple readings before and after implementation. Results confirmed the success:
-
Average tube metal temperature dropped from 596 °C to 416 °C
-
Flue gas temperature entering the convection section rose by just 12 °C, without raising stack temperatures
This solution debottlenecked the vacuum heater, enabling increased throughput without TMT-related limitations or compromising thermal efficiency.
Image: Roof Tubes Skin Thermocouple Readings Before and After
Facing reliability issues with your fired heater tubes?
Talk to our IGS experts about implementing ceramic wrapping technology to optimize your heater performance and protect your assets.
Watch: How IGS’ Solutions Increase Refinery Efficiency and Reduce Co2 Emissions
Free consultation with an IGS Subject Matter Expert
IGS is here to provide information, answer questions and create an effective solution for your needs.