Apr-2017

Analysing FCC hot spots

Finite element analysis can be employed to improve the safety and quality of old or new designs and troubleshoot problems in the field

KENNETH FEWEL
Technip Stone & Webster Process Technology

Article Summary

Hot spots on FCC units are a common occurrence but relatively little is known about the implications for long term operation. Standard procedures in a FCC plant are to monitor on a scheduled basis the thermal scans of their equipment and use steam to cool if hot spots exceed a certain temperature threshold. Although this has served operators in a practical manner for years, there are larger safety issues that should not be ignored. ASME has developed standards for analysing thermal stresses that are suitable for design, maintenance and operation. Operators should be aware of these methods and take advantage of them for their hot spot maintenance and procedures.

Finite element analysis (FEA) is a method that enables accurate state-of-the-art analysis of stress and strain in all types of solid materials for all types of loadings, including thermal. Thermal stresses are usually calculated for a design that is working perfectly to insulate the steel shell from high temperatures inside the FCC vessel. FEA has been used to validate traditional calculation and more accurately define the state of temperature, stress and strain that will occur under normal operation in FCC units made from composite, refractory-lined steel, materials.

Recently, as a result of hot spots appearing on FCC equipment, there was a need to analyse and determine the severity of stresses and strains under hot spot operation. This has provided an excellent opportunity to use FEA to explore these situations using quantitative analysis and make recommendations to operators about the safety of their particular situation.

Before further explaining FEA, it is important to understand why hot spots occur and how stress and strain occurs.

Hot spot causes
Hot spots are caused by several mechanisms of operation that come together like a ‘perfect storm’. They do not appear randomly as there are reasons why hot spots form in FCC equipment.

1.    Erosion by catalyst
Catalyst is, by nature, irregular in shape and very hard compared to carbon or stainless steels. The refractory used to line steel vessels or piping is designed to provide insulation and strength for stresses and thermal strains; it is not designed purely for erosion resistance. Therefore, the material is susceptible to erosion by catalyst. Fortunately, this insulating refractory is thick, usually 4-5in in piping and vessels. However, catalyst can quickly erode through this thickness with the help of the next mechanism.

2. Thermal stress and strain
a. Wall stress: thermal wall stress can easily reach levels that will crack the refractory. In many designs some cracking cannot be avoided.

Longitudinal cracks in the refractory outer diameter of lining will form as soon as temperatures reach operational levels. This is due to thermal wall stresses in the refractory. These stresses have been determined from research experiments by Wygant and Crowley1 among others.2 The inner diameter of the lining is very hot, particularly in a regenerator, typically 1300°F (700°C). The inner refractory will go into compression when internal temperatures start to rise. The outer surface of the lining may not expand as much as the steel. Thus the lining will go into tension stress and the refractory may not bear upon the inner diameter of the steel piping. If so, thermal wall stress can cause the lining in the outer edges to crack.

b. Thermal bending stress: thermal expansion forces in the piping and vessel can crack the refractory in bending, often transverse to the longitudinal direction. In addition, compression bending stress can open gaps between the steel and lining, leading to large gaps through which vapour and catalyst can easily circulate. These are the most damaging stresses to the refractory that can occur. The resulting hot spots will exacerbate the problem, making the thermal stress and gap worse.

3. Worm holes
As a direct result of wall stress cracks in the refractory lining and gaps opened by thermal bending of the refractory on the outside of the lining, there is a strong possibility that the hot vapours and catalyst will find alternate paths through the piping. The wall stress cracks are probably not large enough for catalyst to circulate through, and they may fill with catalyst instead. A larger problem is the thermal bending strain in the refractory. Thermal bending stress and strain will open existing cracks or form new cracks large enough for catalyst to circulate through. Worm holes can then form, leading to catalyst circulating past the steel, overheating, and rapid erosion of the steel skin from the inside out.

The most apparent result of a combination of these mechanisms is a hot spot. The steel will often reach temperatures in excess of piping or vessel design values, typically around 650°F (340°C). When the skin reaches temperatures higher than design, the situation requires cooling steam to keep the temperatures under control. This is standard maintenance procedure, but what are the resulting safety and long term implications? When is a shutdown necessary? If a shutdown is planned, what needs to be done? Is the steel and/or lining material ruined or can it be reused after shutdown?

Usually, the answers are not so clear to the operator as a potentially dangerous situation arises. What if the steam pressure is lost temporarily? What if the temperature rises? Continuous monitoring of the steam cooling system is rarely done. The thermal scans are not continuously reviewed either, but done only periodically. How can an operator know that his plant is safe for the next day or week, or until the next scheduled shutdown?

These questions should not be answered by rough estimates or guesses. Hot spots, once formed, cause additional thermal stress. When a hot spot appears, the situation will worsen and thermal stress and strain can quickly become unstable.

How hot spots cause thermal stress
Take, for example, a thin plate held rigidly at the edges and heated with a torch at the centre of the plate. The material in the centre will begin to expand and thermal stress will occur. The edges, if truly rigid, will prevent expansion from occurring in the plane of the plate. Once the stresses exceed the proportional limit, the plate will begin to warp out of plane. Imagine that this happens on the surface of a large catalyst transfer pipe. The steel must bulge outwardly but will be resisted by the geometrical constraints of the adjacent cooler pipe and, to a lesser extent, refractory anchors. But the gap between the steel and refractory is larger now and the circulation of catalyst is naturally increased. The hot spot temperature is increased and the thermal stress becomes higher. This instability will not stop until one of the following mechanisms occurs:
1. The catalyst streaming in builds up to fill the void left by the steel strain and the gas bypass flow is reduced or eliminated.
2. The strain hardening in the steel, already in the plastic region, reaches a point of equilibrium before the rupture point is reached. This is usually assisted by a geometric constraint and/or steaming.
3. The steel reaches rupture point. Catalyst and vapours begin to spew from the opening.

Obviously every operator hopes for number 1 or 2 to occur. Despite denials, number 3 has been known to happen.

Based on the Goodier3 equation for thin circular plates unconstrained at the edges, the following relationship can roughly estimate the thermal stress rise due to additional temperature rise in the elastic region: