PTQ Q3 2026 Issue

Evaluating steam injection in vacuum unit fired heaters

Thorough and strategic evaluation of steam injection in vacuum unit-fired heaters can improve their performance

V acuum heaters process the reduced crude oil (RCO) from the bottom of the crude column. RCO is consid- ered to be a heavier component of the crude oil, given that the lighter ends are distilled or separated in various sec- tions of the crude column. Thus, arguably, the heavier com- ponent or RCO, which is further processed in the vacuum distillation unit, requires a higher degree of care as well as control of its sensitive operation owing to the fluid’s inherent nature to crack and form coke more easily as compared to crude oil. Vacuum heater designs are relatively standardised, with predominantly horizontal coil-box-type heaters employed worldwide. Indeed, there are a few vertical coil heaters in older installations or in smaller units where horizontal coil heaters may prove to be difficult to design. However, the industry trend has shifted largely to the use of horizontal coils. The horizontal coil configuration allows a more uni - form heat profile, especially considering that the variation in heat flux along the longitudinal axis of the tubes is less compared to vertical coil heaters. Apart from the coil configuration or design aspects of fur - naces handling heavier or viscous fluids, such as vacuum distillation unit (VDU) or delayed coker unit (DCU) feeds, another crucial aspect is the injection of steam into the fur- nace coil. This plays a crucial role in furnace operation and adds to its reliability and long-term health. The three Ws – ‘why, when, and where’ to introduce steam – all play a major role in achieving the desired outcome. Over the years, this ‘coil injection steam’ or ‘velocity steam’ requirement, both at the design and operation phases, has reached a certain level of maturity, with individual designers adopting distinct practices, often driven by thumb rules rather than analytical treatment. In this article, the aim is first to explain why steam injec - tion is needed and what benefits it brings to end users. To understand the first ‘W’ of ‘why,’ there must be a focus on understanding another ‘W’: namely, what happens when steam quantity and locations are varied across whole fur- nace coil section. A comparative analysis will help the oper- ator appreciate the crucial importance of steam injection. Impact of coking in fired heaters Fired heaters or furnaces, as they are interchangeably Ankur Saini, Rupam Mukherjee, and Shilpa Singh Engineers India Limited

called, have been a primary reason for many bottlenecks in operating units, preventing them from running at full capac- ity. Among the various factors affecting furnace capacity, accelerated coking is a frequent cause that often demands exploring opportunities for revamps across all existing fur- naces, triggering costly shutdowns and resulting in produc- tion losses. Furnaces in heavier services, such as VDU, delayed coker or visbreaker, particularly face the problem of coking and are the focus of the present discussion. Coking inside furnace tubes impacts the performance of the process fired heater in three ways: • Coking constricts the internal tube diameter, which increases process fluid mass velocity. Increased fluid mass velocity has a very strong, direct correlation with the pres- sure drop across the furnace. Thus, coking, indicative of tube constriction, promotes a higher pressure drop and can lead to a charge pump limit. • The coke formed, which leads to the term ‘coking,’ is a very poor conductor of heat owing to its bad thermal conductivity. Thus, even a thin layer of coke can introduce considerable resistance in the overall heat transfer mechanism, thereby calling for higher firing and fuel consumption. Thus, the cycle continues, with the situation worsening over time. • Corollary to the previous point is the aspect of increased tube metal temperature during coke formation, which leads to undesirable conditions, such as tube bowing, tube bulg- ing, hot spots, or even tube rupture in the worst case, which can be catastrophic. The previous three points are the reasons a refinery may call for a unit shutdown to periodically clean the furnace coil. The time between consecutive shutdowns taken to facilitate furnace coil decoking is termed the furnace run length. Thus, to sum up, coking reduces the furnace run length. It is important to note that improving run length not only reduces the effort and time spent on offline decoking but also adds significant productivity to the unit. Although cok - ing is a complex phenomenon extending to various reactions and assumptions, the following empirical yet established facts should also be noted: • Coking rate or rate of formation of coke has an inverse relationship to fluid mass velocity. Analytically, this can be explained by the concept of shear, preventing coke deposits.

PTQ Q3 2026