Establishing a Mechanical Integrity Program for Fired Heater Tubes

Introduction

Fired heaters are among the most critical equipment in refineries and chemical plants. Owing to the criticality of the equipment, tube failures in fired heaters present significant safety and financial impacts to the owner/operator. A mechanical integrity program is warranted to reduce the risk of failures. This article provides a general overview of the tasks that can be included in an effective mechanical integrity program for tubes in fired heaters. The article is intended to be a general outline, applicable to the majority of fired heaters.

It is noted that for the purposes of this article, “Fired Heaters” apply to equipment within the scope of coverage as defined in the API STD 560, “Fired Heaters for General Refinery Service,” with the tubes designed as required in API STD 530, “Calculation of Heater-tube Thickness in Petroleum Refineries.” Currently, this method is meant for refineries and some petrochemical services, but does not include alloys outside the coverage of API 530, such as used in steam methane reformer furnaces.

Overview of Heater Tube Design and Fitness-For-Service Considerations

The design criteria for calculating the required wall thickness of new tubes in fired heaters is governed by API 530, whereas Fitness-For-Service (FFS) calculations of service-exposed tubes are typically carried out per API 579-1/ASME FFS-1 procedures.

There are two different design considerations for heater tubes: elastic design and creep-rupture design. Elastic design is based on preventing bursting of the tubes due to short-term exposure (time-independent) to excessive stresses (greater than the yield strength of the material) at lower temperatures. In contrast, creep-rupture design is based on preventing failures under long-term exposure (time-dependent) to lower-than-yield stresses at higher temperatures. Creep being a time-dependent damage mechanism, creep-rupture design has to take into consideration the minimum intended life for heater tubes during the design stage. API 530 tubes are typically designed for 100,000 hours using the minimum expected creep properties for the tube material of construction.

The remaining life of a service-exposed heater tube can be controlled by (a) the creep-rupture life; (b) the elastic life; or (c) the tube retirement thickness set by the owner/operator. For heater tubes operating at temperatures high enough for the effects of creep to be significant, the remaining life is typically controlled by creep-rupture, unless there is significant corrosion or oxidation in the tubes. High corrosion rates, coupled with high relief valve set pressures, can cause the elastic design to govern even at higher temperatures. Corrosion is the controlling damage mechanism if the elastic design governs the tube life at a given temperature and pressure. Similarly, the tubes are expected to fail by creep if the tube life is governed by creep-rupture design. A third factor that may end up controlling the remaining life of a used heater tube is the retirement thickness set for the tube, which is again determined by dead loads and corrosion. API 530 does not provide any recommendations for tube retirement thickness and it is up to the owner to set a retirement thickness. Remaining life calculations based on API 579-1 do not take into account the retirement thickness set by the owner. In cases where tubes exhibit corrosion and the heater design/operating conditions are such that the tubes do not experience significant creep and has a low relief valve set pressure, the tubes may be paper thin by the time they fail by creep or by exceeding the elastic allowable stress (i.e., overpressure). In such cases, it makes practical sense to establish an overriding retirement thickness irrespective of the creep damage.