Apr-2015

Oxygen enrichment and tube sheet protection (TIA)

In sulphur recovery units (SRU), adequate protection of the waste heat boiler (WHB) tube sheet is needed. Many protection systems have failed either due to incorrect designs or operational problems.

Tobias Roelofs
Jacobs Comprimo Sulfur Solutions

Article Summary

As existing facilities seek to increase throughput, oxygen enrichment is being implemented more often. Because the Claus furnace with oxygen enrichment can reach continuous temperatures of 1500°C, a good and reliable WHB protection system is needed.

Jacobs Comprimo Sulphur Solutions has long experience with the design of WHB protection systems, and uses modern simulation tools to verify the designs. The following case study involves a facility with oxygen enrichment.

Jacobs Comprimo was asked to implement oxygen enrichment to increase the SRU capacity. The tube sheet protection system selected for this case consists of a two-piece square head ferrule which can be easily installed and replaced. Additionally, a two-piece ferrule provides resistance against thermal cycling. The ceramic ferrule is made from a high purity alumina. A ceramic felt gasket and ceramic paper wrap act as critical heat barriers in this design. The ceramic paper also ensures that the ferrule is positioned in the centre of the tube and that movement is limited. Additionally, bypass of hot gas is prevented as the ceramic paper located in between the square heads seals off possible flow paths.

The two-piece ferrule was analysed in a finite element (FE) model in order to calculate the highest tube sheet temperature. The model discretisation can be seen in Figure 1a. By inserting the actual geometry and physical properties, the model is able to predict which part of the tube sheet will have the highest temperature.

Figure 1 shows the results of the FE simulation. The highest metal temperature is expected near the tube-to-tube sheet weld. The model clearly indicates a large temperature gradient over the ceramic paper wrap at this position. The thickness of the ceramic paper wrap should be large enough to maintain a low metal skin temperature. A maximum metal temperature of 310°C is expected for this design as shown in Figure 2, which is well within the safe limit for corrosion and strength.

Other WHB problems may originate from the boiler design. As large heat fluxes have to be accommodated, good waterside boiler design is essential. Steam blanketing can occur which causes a rise in metal temperature with severe H2S corrosion as a consequence. The Jacobs Comprimo design uses a square pitch for the tubes. This ensures adequate space between the tubes and thereby facilitates steam removal. Although the typical flow pattern in this configuration reduces the risk of steam blanketing, the water circulation rate is also an important parameter to check. Jacobs’ WHB design ensures adequate water circulation which is required to refresh the hot tube surface.

Another source of WHB damage is poor quality control of the boiler feed water, leading to scaling and deposition of salts on the outside of the tubes. This results in an additional heat transfer resistance which causes high temperature sulphidic corrosion. Sufficient boiler blow down and regular checking of the boiler feed water quality can help to avoid scaling.

In conclusion, proper tube sheet protection and boiler systems should be selected for a reliable and robust heat recovery system downstream the Claus furnace, especially in the case of oxygen enrichment.

This short case study originally appeared in PTQ's Technology In Action feature - Q2 2015 issue.
For more infomation, please contact: Tobias.Roelofs@Jacobs.com