April 2025

Heat Exchange/Management

Heat exchanger modifications during hydrocracking and lube oil basestock units’ revamp at BPCL’s Mumbai refinery

This article provides insights into several possibilities for modifying heat exchangers that will help refinery owners/operators and consultants to plan similar revamp activities. 

IP: 3.139.99.145

FIG. 1. View of the exchangers. 

“It is not the most intellectual of the species that survives; it is not the strongest that survives; but the species that survives is the one that is able to adapt to and to adjust best to the changing environment in which it finds itself,” Leon C. Megginson, Petroleum Management, 1964 (with attribution of concept to Charles Darwin). 

This statement is true for any organization. To remain competitive and grow in an uncertain and volatile energy market, plant operators must find ways to increase operational efficiencies, maximize productivity and produce refined products at lower costs.  

In its quest to be sustainable, high performing and future ready, Bharat Petroleum Corp. Ltd. (BPCL) has initiated its Go-Green, Go-Digital and Go-Petrochemicals (Go-GDP) program, where the company is focusing on maximizing profitability by producing the right mix of on-specification products. In one effort to adapt to new requirements, existing hydrocracking and lube oil basestock (LOBS) units in BPCL’s Mumbai refinery were revamped, and a new de-aromatized solvent (DAS) unit was added. BPCL engaged the co-author’s company to provide engineering, procurement and construction management (EPCm) services for the revamp activity. As a part of the activity, the co-author’s company studied various modifications for heat exchangers in close collaboration with the licensor. This article provides insights into several possibilities for modifying heat exchangers that will help refinery owners/operators and consultants to plan similar revamp activities. 

BPCL’s Mumbai refinery. BPCL’s Mumbai refinery is one of the most versatile refineries in India and is capable of processing various types of crude oils. It is a compact refinery located in the densely populated city of Mumbai. It was commissioned in 1955 with a crude oil processing capacity of 2.2 MMtpy. The capacity has been augmented through various revamp activities and the addition of new units to its present level of 12 MMtpy. The refinery produces various products like liquefied petroleum gas (LPG), naphtha, motor spirit (petrol), benzene, toluene, hexane, kerosene, jet fuel, diesel, light diesel oil, fuel oil and bitumen, among others.  

Since the refinery is located within the city and options for additional land are limited, the revamp of the Mumbai refinery is a challenging task. In addition, the refinery is a strategic asset that provides petroleum products to nearby commercial and industrial zones. Thus, turnarounds (TARs) at the refinery must be planned well in advance with the emphasis to minimizing activities during these projects.  

The objectives of revamping the hydrocracking unit (HCU) and LOBS unit were primarily capacity augmentation and metallurgy upgradation based on the licensor’s recommendation. Various strategies were used specifically for heat exchangers based on revamp objectives and site constraints. Some heat exchangers were modified, replaced, augmented with an additional parallel unit and/or reconfigured for use in different service. The details of these modifications are discussed below.   

Reactor effluent air cooler (REAC) modifications. An REAC is the most important and critical piece of equipment for the HCU. Any problems with the equipment’s operation affects the entire unit, and therefore the refinery’s operation. The equipment operates under high pressure and severe corrosive conditions. Challenges are compounded due to relatively low temperatures that result in the precipitation of ammonium bisulfide (NH4HS) and ammonium chloride (NH4Cl) salts. These salts deposit in the tubes and affect heat transfer and cause corrosion. Wash water is injected into the process stream in the upstream REAC to dissolve and reduce the concentration of these salts. The earlier installed unit in the refinery was made of carbon steel (SA-516 GR 60). There were two main objectives for the REAC in the revamp project: 

1. Upgrade the metallurgy to nickel-iron-chromium alloy 825—based on the licensor’s recommendation. 
2. Increase the heat duty by 13% due to the reconfiguration of the HCU. 

It was thought that the entire unit required replacement to meet the objectives of the revamp project. However, during thermal calculations, it was noted that the increased duty could be achieved by increasing the number of tube rows. Tube length and bundle width could remain the same as that of the existing unit. The fin density (the number of fins per unit length of tube) and air flowrate could be reduced to compensate for a higher air-side pressure drop in the additional tube rows. The existing fans and motors were adequate to supply the required quantity of air. Reduced air flow resulted in a higher outlet air temperature; however, this was within an acceptable limit. Considering these aspects, the new tube bundle was designed with a greater number of tubes than the existing one and proved adequate for the duty with the existing fans and motors (TABLE 1). 

The new bundle was bigger than the existing one, so the weight of the new bundle was calculated and the suitability of the existing structure was evaluated. Alloy 825 is stronger than carbon steel at design temperature; therefore, the thickness required for the alloy 825 material was less vs. carbon steel. The licensor had specified a minimum thickness for tubes consisting of alloy 825 material. The thickness was less than the thickness of the existing carbon steel tubes and was suitable for the design pressure. The thickness of the tubesheet and other header components were less than that of the existing bundle. After a weight evaluation study, it was noted that the weight of the new bundle was 40% less than the existing bundle, even though it contained 20% more tubes. With the reduction in weight of the bundle, the existing structure was found suitable for new bundles. 

The height of the box header was greater than that of the existing one, so the nozzle locations could not be matched. Thus, the inlet piping required modification during shutdown. 

After an evaluation of the available options and constraints, a design was finalized in which only tube bundles were replaced. This significantly reduced the modification work during the TAR. 

The replacement of air fin cooler tube bundles to increase capacity. The purpose of side-cut lube coolers and product lube coolers is to cool the final products to 80°C so it can be stored in aboveground storage tanks. The flowrate for these products was anticipated to increase post revamp by 47% and 80%, respectively. There was a reduction in the temperature of the products at the inlet of the air coolers, and the increase in heat duty was marginal. Due to the reduction in inlet temperature, there was a reduction in the overall effective mean temperature difference, and the existing air coolers proved to be inadequate for revamp conditions. The outlet temperature was estimated to be in the range of 95°C–100°C (203°F–212°F) during peak summer. This was unsafe for storage and inadequate for revamping conditions. In addition, the pressure drop increased due to higher flowrates. The existing unit was unsuitable for revamp conditions and required modifications. 

Various options were studied: 

• Increasing air flow. The airflow required to meet the duty was very high and would have required the replacement of fans, motors, variable frequency drives, etc. Increased fan power was also a parameter that was unacceptable in terms of operating cost. 
• Existing tube bundles have tube inserts. The option of replacing the tube inserts was evaluated. The replacement of tube inserts was deemed insufficient to enhance the duty to the required level. There was an increase in tube-side pressure drop with this configuration. 

Finally, it was decided that the tube bundles would be replaced with a higher capacity. New tube bundles were designed with the same length and width to fit within the same plot space. The pass arrangement of the new bundles was designed to meet the pressure drop requirement for the revamped higher flowrate. The air flowrate was specified that the existing fans and motors could be used without any modifications. Finally, the replacement of the bundles was completed within a short TAR time and without requiring additional plot space. 

The addition of a new tube bundle in parallel to the existing air cooler. In the case of the vacuum bottoms lube product air cooler, the revamp led to a 250% increase in flowrate and a 180% increase in heat duty. The existing heat exchanger proved inadequate for the revamped duty—it required an additional similar sized unit. The existing configuration was one bundle and one bay mounted on a pipe rack structure floor. There was space available beside the existing air cooler to install an identical bay. It was decided to install an identical unit next to the existing one so it could work in parallel. The new bundle was designed to be identical to the existing one—in terms of flow hydraulics—and to have uniform flow distribution between the existing and new bundles. The exact model of the tube inserts was discontinued by the manufacturer and were not available in the market for the new bundle. It was decided that the tube inserts of the existing unit would be replaced to match the flow hydraulics with the new unit. A completely new unit—including bundle, fans, motors and structure—was procured and installed beside the existing unit. Equipment erection and installation work was completed during pre-shutdown activities, and only the piping tie-up was done during shutdown, minimizing TAR activities. 

Replacement of high-pressure screw plug exchangers. The licensor recommended changing the shell’s metallurgy from carbon steel to low-alloy steel due to the potential of high-temperature hydrogen (H2) attack and a marginal increase in heat duty. The tubesheet’s material was changed from 1.25Cr-0.5Mo to 2.25Cr-1Mo—these are high-pressure heat exchangers with proprietary, screw plug-type high-pressure closures.  

Many pressure-containing components are integral to the equipment. It is not possible to replace or modify individual components, so it was decided that the exchangers should be replaced. The new exchanger was designed to fit on the existing foundation, with the nozzles in the same location as the existing ones. The new exchangers with modified metallurgy and a marginal change in heat duty were procured and kept on-hand prior to the TAR. Replacement activities were performed during the TAR. 

Tube-side vibrations in the first-stage reactor’s feed-effluent screw plug exchanger. After the revamp, there was a marginal increase in the flowrate of the first-stage reactor’s feed-effluent exchanger. The exchanger was adequate for thermal duty and pressure drop; however, there were tube-side vibration issues. The unsupported tube span exceeded the Tubular Exchanger Manufacturers Association’s (TEMA’s) standard by 80%. Initially, the licensor recommended replacing the tube bundle to address the vibration issues—this is a high-pressure heat exchanger with a screw plug-type high-pressure closure. It was also a very large exchanger with 1,100 U-tubes and two shells in a series. 

Considering the complexity of replacing the bundle, an alternative method for stiffening the tubes to address vibration problems was studied. A previous reference for a successful similar tube stiffening arrangement was consulted. It involved the use of strip baffles between the tubes in the inlet and outlet regions. The strip baffles were 4-mm thick x 25-mm wide flat bars of SS321 material that were inserted between the tubes to fit tightly (FIG. 2).  

FIG. 2. Stiffening to tubes using strip baffles. 

The strips are welded at one end to the peripheral strip and to the longitudinal baffle at the other end. The original equipment manufacturer (OEM) was approached to conduct the bundle modification. The OEM agreed to supply the strip baffles and to perform the necessary modifications. The activity is planned to be executed in the next TAR in 2026 and is more cost-effective vs. a complete bundle replacement (FIG. 3). 

FIG. 3. View of the revamped exchanger. 

Reconfigured exchangers to be used for different service. The waxy-150N and waxy-500N product circuits have combinations of shell-and-tube heat exchangers and air-cooled heat exchangers for cooling the final products. Steam is generated in a kettle-type shell-and-tube heat exchanger. However, in the waxy-100N product circuit, there was only an air-cooled heat exchanger because the duty was small. In the revamp configuration, the licensor added a shell-and-tube heat exchanger for the waxy 100N product. The products’ flowrates (150N and 500N) would increase after the revamp, leading to an increase in heat duty. Therefore, three new heat exchangers were required to meet the revamped process conditions. 

When the heat duties were compared to the existing and revamped conditions, it was observed that the existing exchanger could be reused for another service. The existing waxy-500N product steam generator could be used for the waxy 150N product in the revamped case, and the waxy 150N product steam generator could be used for the waxy 100N product circuit. The exchangers are located close to each other, so it was decided that the exchangers would be used as-is, and only the piping would be modified to reconfigure the exchangers for the new service. These piping modifications were performed during the TAR. 

For the waxy 500N product cooler service, a new exchanger was designed and installed during pre-TAR activities. Only the tie-in with the existing piping was completed during the TAR. For the waxy 100N product steam generators, the licensor recommended that the tube thickness be 12 gauge (2.77 mm). Due to this requirement, the existing exchanger could not be used for this case. Therefore, a new exchanger was ordered for the waxy-100N product and installed during pre-TAR activities. Piping tie-ins were completed during the TAR. A duty comparison for the waxy product steam generators pre- and post-revamp is detailed in TABLE 2. 

Tube pass modifications for a DW vacuum column bottoms exchanger. The existing heat exchangers had four AES configured shells connected in a series, with each shell having 10 tube passes. The tube-side flowrate increased by two times, while the duty increased by 6% in the revamp case. The heat transfer area in the existing exchanger was adequate for the revised heat duty; however, the pressure drop on the tube-side increased significantly. The calculated pressure drop on the tube-side for the governing case was 7.79 kg/cm2, whereas the allowable value was 2.7 kg/cm2. The reduction in the number of tube passes was the best option to bring the pressure drop to within the allowable limit.  

Various configurations of modifying the tube pass were studied. A configuration in which the number of tube passes was reduced to four appeared to be feasible in terms of minimum modification efforts. There were 58 tubes in pass 1/3 and 87 tubes in pass 2/4, as per the revised pass arrangement. An unequal number of tubes in each pass is typically not recommended; however, it was not a problem for this case if the pressure drop was within an acceptable limit. The tube-side nozzle size was also changed from 4 in. to 6 in. to handle the higher tube-side flowrate. With these modifications, the tube-side pressure drop was estimated to be 0.64 kg/cm2, which was within the acceptable limit. 

To execute the modification with minimum site activities, it was decided to purchase a new channel barrel and floating head with modified partition plates for a four-pass arrangement. Extra grooves on the rear tubesheet were required to make it suitable for a four-pass arrangement. 

However, none of the vendors agreed to supply the front channel barrel and floating head due to the low value of these items. The plan was technically feasible but difficult to implement because of commercial reasons. Finally, new exchangers were purchased for this service, and the entire unit was replaced during shutdown (FIG. 4). 

FIG. 4. View of the existing and modified exchanger. 

A similar case involved the DW product stripper feed/vacuum column bottoms exchanger. The configuration of the existing exchanger was two AES shells connected in a series, with each shell containing four tube passes. It was decided to modify the tube pass configuration to only two passes. This was also not implemented due to commercial reasons, and the unit was replaced during shutdown. 

The use of existing exchangers for new services in the DAS unit. In addition to revamping the HCU LOBS unit, a DAS unit on the existing plot was installed as a part of the project—the project was part of the same EPCm contract. The installation of 11 new shell-and-tube heat exchangers in the DAS unit was required. Some of the services were similar to the LOBS unit, such as the feed/bottom heat exchanger and feed/overhead coolers.  

The existing exchangers that could not be modified/utilized due to commercial reasons were evaluated to be used for these services. The existing exchangers were found to be suitable in terms of TEMA type, metallurgy, design pressure and design temperature. The exchangers were further evaluated for thermal and mechanical adequacy and were found to be adequate for services in the DAS unit. Three shells of the existing feed/bottom exchanger were used for the feed/overhead cooler, the external splitter/feed exchanger and side-cut/feed exchanger, and all units of another exchanger were used for the DAS unit’s splitter feed/bottoms exchanger. The DAS unit installation was scheduled after the HCU LOBS TAR. The four existing exchangers from the HCU LOBS’s unit were removed during the TAR and moved to the DAS unit.  

Takeaways. Revamping existing operating plants is challenging due to constraints associated with brownfield work. The objective is to optimize the use of existing assets. There are methods to optimize revamp strategies, especially in the case of heat exchangers. These pieces of equipment can be reconfigured to meet the revamp conditions in several ways. Some options may not be practical, but some may fulfill the exact requirement of the revamp conditions and can be implemented within the allowable constraints. Modification options should be studied on a case-by-case basis and with an open mind. These options should be evaluated in close coordination among all the stakeholders involved, such as the thermal design team, licensor, vendors and the project execution team. This strategy can lead to the achievement of the overall objectives of the revamp project in terms of capacity augmentation, upgradation of product specifications, finding low-cost solutions and minimizing the TAR schedule. 

ACKNOWLEDGEMENT 

The authors would like to thank the management of both BPCL and Technip Energies for permitting the publication of this work.  

The Authors

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