PTQ Q2 2022 Issue

HC inlet

Finned rows

Observ. door

Steam lancing

Future rows provision

Observ. door

Steam lancing

Bare rows

HC outlet

Figure 2 Convection section future rows

Utilising future rows to increase the capacity of fired heaters Refineries worldwide are pushing their operating units to extract more from their existing hardware. The fired heater often jolts this ambition. The firebox temperature increases near the allowable limit, or the metal temperature, read by tube skin thermocouples, reaches close to its metallurgical limit. In such events, adding rows into the space provided can offer minor relief and an effective solution to increase the unit’s throughput appreciably. Refer to Table 1 for a comparative case study. As evident from the study, add- ing rows helps arrest the increase of radiant flux that is inevitable whenever higher throughput is pro- cessed through the heater. Even by a small margin, the increment in throughput can benefit the refiner, depending on the criticality and profitability of the product the unit generates. Utilising future rows for incremental efficiency improvement of fired heaters Furnace efficiency improvement has always been the lowest hanging fruit while auditing refinery fuel consumption patterns. Extracting more heat from flue gas has been an age-old proven technique. Utilising the future rows in the case of a natural draft furnace is a sim-

Figure 1 Fired heater convection section using provision for future rows as a stand-in solution for low-cost revamps

a fired heater or when the heater is required to be operated with more severity. Standards for the design of fired heaters (API 560) require space to be provided in the convection section for additional tubes, the so-called ‘future rows’. This article will focus on the pos- sibility of utilising the future rows provision included in the design of the fired heater convection section (see Figure 1 ) to keep these critical parameters within their intended operating limits and enhance the operating efficiency with minimal complexity. The American Petroleum Institute’s Standard 560 1 rigor- ously defines best design and sizing

practices for fired heaters. The API standard recommends including provision for two future rows in the convection section in the design of new fired heaters. The basic idea is to ensure space is available to install additional rows in the event of the need for further surface area aug- mentation without any major capi- tal or labour-intensive revamps. See Figure 2 for a visualisation of con- vection future rows. If these future rows are located at the breeching or top section of the convection section, utilising them will require changes to the heater terminals and the transfer line, which may not be an easy solution to implement

Increasing capacity of fired heaters by adding future rows of convection tubes

Base case Thru’put increase Thru’put increase without adding with installation future rows of two future rows

Absorbed process duty, MMKcal/hr

Thru’put, t/hr of feed

553

576

Average radiant flux, Btu/hr.ft 2 Bridgewall temperature, °C

12 000

12 500

12 008

880

892 477

881 413 405

Flue gas temperature ex-convection section, °C 470

Maximum tube metal temperature, °C

405 2332

407 2332

Convection surface area, m 2

3498

Table 1

64 PTQQ 2 2022