Feb-2017

Rapid waste heat boiler retubing at IPL Mount Isa sulphuric acid plant

Effective energy recovery in sulphuric acid plants can drive the economics of the plant every bit as much as the actual production of sulphuric acid.

Brian Lamb, MECS Inc
Michael Donaghue and Robert Gosling, RCR Energy
Luciano Leonforte, Incitec Pivot Limited

Article Summary

As such, many technologies have been used over the years to effectively recover energy. Examples include:
1. Waste heat boilers (WHB) for recovering heat from the oxidation of sulphur into sulphur dioxide in the furnace;
2. Economizers, superheaters, and gas to gas heat exchangers for recovering heat from the catalytic conversion of sulphur dioxide to sulphur trioxide in the converter;
3. Boiler feedwater preheaters to cool strong acid by heating treated water that is fed to the deaerator;
4. Recovery of the heat generated from the absorption of SO3 into sulphuric acid, such as MECS® HRS™
5. Latest generation, integrated technologies for optimizing energy conversion across an entire plant, such as MECS MAX3™

As energy recovery plays such a crucial role in overall plant economics, the reliable operation of energy recovery systems and the associated equipment is critical to the successful operation of a sulphuric acid plant. In the case of acid plant WHBs, ensuring reliable operation includes aspects such as proper design, operation, and maintenance.

Challenges for IPL Mount Isa
In the case of the IPL Mount Isa sulphuric acid plant, 2014 presented some very serious challenges associated with maintaining the site’s 16 year old No. 1 Waste Heat Boiler (WHB1).

WHB1 is a horizontal tubesheet boiler with 859 tubes, 70mm diameter x 9000mm long. The boiler shell is 3410mm ID and 45mm thick. Process gas from the sulphur burner directly upstream, enters a refractory lined plenum and enters the tubes at 951°C, producing 2400 kPa(g) steam in the boiler shell. The hot end tubesheet had circular ceramic ferrules consolidated with 100mm of high alumina refractory held on with “cowhorn” anchors.

IPL had recent experience with tube to tubesheet failures in their No.2 Waste Heat Boiler (WHB2), which was re-tubed under emergency conditions by RCR Energy, including of a redesigned tube to tubesheet joint. The original tube to tubesheet joint had a large root gap and a large “V” at the back of the tube hole, which seemed intended to wash the crevice and cool the tube welds. However, IPL preferred a conventional hole with face welding followed by expansion, although this configuration had the potential to increase the tube tip temperature. Whilst there was no refractory in WHB2, the methodology formed a basis for retubing WHB1.

Ultimately, it was the pitting corrosion found in the tubes of WHB2, attributed to water chemistry that led to the decision to re-tube WHB1.

With WHB1, there was concern that the tube tips may be experiencing high temperatures under the hot face refractory. Excessive tip temperatures can lead to Sulphidising and hence weld failure. IPL identified that a new hot face design, incorporating HexPro™ ferrules, provided time savings during re-tubing and afforded better inspection and repair options in the future. RCR Energy performed thermal calculations on the boiler and had Aurecon carry out thermal Finite Element Analysis of the redesigned tube to tubesheet joint, inclusive of the HexPro ferrules. Comparative analysis was carried out up to 1200°C and it was found that the HexPro ferrules provided excellent protection of the tube ends with slight improvement in temperatures at the tubesheet.

 In addition to the capital cost of a new boiler; the likelihood of removing and replacing a new boiler and its infrastructure during a 3 week shutdown is remote.  With tubing unlikely to last another 4 year campaign, the decision was made to re-tube WHB1, including new tubesheets and hot face refractory.

Retubing Considerations
A major time constraint identified was gaining access to the hot end, where the plenum has 3 layers of interlocking refractory brick. As personnel and tools could access the hot end through the Sulphur burner openings, it was decided that the 3 layer system would not be disturbed, and the hot end tubesheet would be brought through the boiler shell from the cold end. The tube supports would be sectioned to allow the tubesheet to pass through from the cold to the hot end. This necessitated pre-determining a tubesheet diameter that was accessible from the hot end and compatible with the refractory design. RCR Energy designed a rail system that could support the tubesheet in its optimal orientation during insertion.

Note: It was not possible to simply cut the tubesheet and “pull” the bundle as steam risers and down comers projected inside the shell. Further, impingement plates and tube support plates (baffles) were welded to the shell and support plates were not connected to either tubesheet with tie rods.

To commence repairs, access was gained from the cold end plenum by cold cutting the dished end from the plenum cylinder, as shown in Figure 1.

After removal of the hot face refractory, tubes were released from behind the hot end tubesheet and the hot end tubesheet removed in sections, as show in Figure 2. This allowed the tubes to be withdrawn from the cold end.

After the new tubesheet was installed in the hot end, openings in the tube support plates were restored. As there were no manways in the boiler shell, RCR Energy removed hand holes (in their entirety) to provide the access necessary to reinstate and inspect tube supports and guide tubes during insertion. The cold end tubesheet was installed and tubes were loaded, tacked, trimmed and welded, as shown in Figure 3.

Re-welding of the complete hand holes was carried out with temper bead welding to avoid stress relieving of the boiler shell, as shown in Figure 4. This technique was a success with weld hardness limits met throughout.

Reinstatement of the cold end dished end, combined with progressive non-destructive testing of welds and final hydrostatic testing of the shell allowed WHB1 to be returned to service in the allotted time.

Refractory Considerations
Another consideration for minimizing plant downtime was the refractory work to be done on the WHB tubesheet. Conventional boiler designs utilize cylindrical ceramic ferrules, as shown in Figure 5. Although inexpensive, the installation of conventional round ferrules can be quite time consuming, requiring the packing of monolithic refractory between the ferrules themselves, as well as a careful, lengthy curing and subsequent dry out of the refractory in order to ensure that, first, a ceramic bond is formed, and second, that all free and chemically combined water is removed from the lining prior to startup. Failure to do this can have catastrophic consequences for the refractory.

To make matters worse, this labour-intensive and time-consuming technique for protecting the WHB tubesheet often causes other adverse effects. The monolithic refractory structure that is created on the tubesheet is prone to cracking when the boiler is cycled from ambient temperature up to operating temperature, as shown in Figure 6.