Replacing Radiant Tubes

 

INTRODUCTION

This document describes a recent project where our client was replacing the tubes in the radiant coil of a Crude Fired Heater. The heater was originally built in the 1970s, before the modern standards of API 560 and API 530.

The material of the original tubes was A312 TP 310 SS material, however, the client required a review of whether there was any benefit in changing the material of the new tubes to be installed. In order to make a valid assessment of changing the coil material, we had to review the current operational performance and duty requirements of the heater.

Therefore, we requested the full performance data for the heater over the last 5 years. This would allow us to understand peak duty requirements and the associated temperatures, pressures, flowrates, etc. In addition to the extreme operation conditions, our review of this data would also reveal the intensity or period of sustained operation at any given condition.

The table below outlines the maximum duty requirements of the heater over the previous 5 year period:

Feedstock Flowrate: 380,550 kg/hr

Inlet Temperature: 180 C

Outlet Temperature: 350 C

Vapour at Outlet:  40%

Radiant Coil Vibrations

Our client had also reported that they had also experienced significant vibration of the coils at certain times during operation. Given that the heater was a vaporizing service, it was suspected that slug flow regimes during certain operation case may be the cause of the coil vibration.

Flow Regime Map 

Important considerations for Replacing Radiant Tubes

- Safety: the criteria of the new tubes must be adequate to withstand the maximum operation temperature and pressures. In order to asses the suitability of the tube material and thickness, we used an appropriate industry standard such as API 530, ASME VIII or ASME I.

- Performance: The tube criteria must be able to suitable in order to achieve the performance requirements such as pressure drop and heat transfer surface, without exceeding the defined safety operational limits

- Lifetime: The lifetime of the tubes, tube supports and guides must be suitable in order to achieve a minimum acceptable lifetime at the specified performance criteria. For example, the typical lifespan considered when using API 530 Calculation & Design standard is 100,000 hrs. In most instances, API Fired Heaters are required to operate continuously 24/7 with typical maintenance shutdown cycle scheduled between 3 to 5 year intervals. The tubes, must therefore be able to operate at the specified conditions for the guaranteed lifetime.

API 560 Tube Material Temperature Limits

The API 560 Fired Heaters for General Refinery Service recommends the following temperature limits for tube materials listed below:

(Note: The design temperature should include a 15 C (28 F) margin above the calculated value, in accordance with API 530 Calculation of Heater-tube Thickness in Petroleum Refineries

API Fired Heater Simulation Review

We carried out a detailed fired heater simulation model of the key operational cases, assessing the impact upon the tube safety and performance

Normal Case 

Fired Heater Tube Profile Report (Normal Case)

Max Tube Temperature Case Results Summary

Feedstock Flowrate: 418,000

Tube Temp. Normal Case: 422.1 C

Tube Temp. Max Case: 499 C

Design Temperature: 580 C

The maximum tube temperature case involved an increased process flow rate of approximately 10%, therefore the pressure drop values differed significantly from the values reported on the original datasheet. However, the feedstock pumps and associated ancillary units at the refinery were to accommodate the different pressure profile.

In addition, the calculated tube temperature was different from the values reported on the original datasheet, however, they did not exceed the reported design temperature on the datasheet.

The tube material currently utilised were comfortably able to accommodate the higher temperatures current operation at the refinery. 

Flame Tube Impingement in Fired Heaters

Our analysis revealed that one operational cases was actually above the Design Rating case of the heater. The flames in this case were likely impinging directly upon certain tubes in the Radiant section. Flame-to-tube impingement within an API Fired Heater is a serious safety concern. 

Operation under these circumstances for extended periods will excessively strain and weaken the impacted tube and may result in tube rupture, leaking flammable feedstock fluid with potentially catastrophic consequences. These incidents threatens lives and significant financial losses.

Fired Heater Damage

Images from: Fired equipment safety in the oil & gas industry, by Jacques Dugué

Crude Fired Heater - corrosive crude feedstock

Another consideration when selecting the coil material is the actual nature of the process fluid itself. For example, Crude Heaters are often utilise P5 Grade tube material as a minimum although the tube temperature may not justify this selection. However, the reason for this selection is to withstand the corrosives species such as Sulphur, Sodium and Vanadium present within the fluid itself.

In addition, there is an increased pressure for refineries to have the technical flexibility to operate with a wider range of feedstock sources to mitigate political and logistical instability. Therefore, although being able to downgrade the coil material from 310 SS, the client decided to maintain the current material grade for the new coil, thereby maintaining flexibility to accommodate higher temperatures and corrosive crude feedstocks. 

Fired Heater HAZOP Review for Radiant Tube Replacement

In some instances, modification and upgrades to the Fired Heater may require a HAZOP review, in order to assess the safety integrity of the proposed modification. 

The HAZOP review involves segregating the various process aspects of the heater into distinct nodes. Usually, for Fired Heaters the distinct nodes are as follows:

- Feedstock Process Stream

- Fuel system (also separated for pilot fuels - if pilot is present)

- Flue Gas throughout the heater

- Combustion Air node (including FD Fan and Air Preheater if present)

- Flue Gas Ducting via Air Preheater (if Air Pre-heater is present)

The proposed modification to the tube had to satisfy the following scenarios typically considered during the HAZOP assessment, which are:

- High, Low and No Flow

- High, Low and No Pressure

- High and Low Flow Temperature

- Contaminated Flow

- Misdirected Flow

Further 3details of Fired Heater HAZOP and SIL reviews, are covered within another section 

Conclusion Radiant Tube Replacement

Based on our review and analysis of the refinery data, we were able to confidently conclude that replacement of the Radiant Coils in accordance with the original tube material A312 TP 310 SS.

This material grade provides a considerably margin above the maximum tube temperature operation cases and also provides sufficient capability to handle corrosive feedstocks sources that the refinery may utilise in future scenarios.