How hex head ceramic ferrules cut the tubesheet work in an Australian sulfuric acid plant by 67%.
New tubes, new tubesheets and a new hot face, all inside one turnaround.
The ferrules were the part that could give time back
A sulfuric acid plant in Australia retubed its 16 year old main waste heat boiler, 859 tubes with new tubesheets and a new hot face, within a three week shutdown in 2014. Hex head ceramic ferrules, made by Blasch and supplied by DuPont MECS as HexPro, cut the ferrule installation time by 67% against conventional round ferrules set in castable, according to the case published in Sulphur in late 2016.
The boiler is a horizontal tubesheet unit with 859 tubes of 70 mm diameter and 9,000 mm length in a 3,410 mm shell. Gas from the sulfur burner enters the tubes at 951 °C, and the shell raises steam at 2,400 kPa(g).
The hot end tubesheet carried round ceramic ferrules buried in 100 mm of high alumina refractory, held by anchors. Pitting corrosion found in the plant's second boiler raised doubts about the first, and there was concern that the tube tips under the refractory ran hot enough for sulfidation and weld failure.
The tubes were unlikely to last another four year campaign, and a new boiler could not be swapped in during a three week shutdown. The decision was to retube in place.
On a tubesheet with 859 tubes, every hour spent packing and curing castable between the ferrules is an hour taken from welding and testing, so the ferrule design sets how much boiler work fits into the outage.
Hex head ferrules go into the tube ends without castable between them. Their six sided heads grow together at operating temperature into a closed face, sealed by fiber held in a groove in each head. Only a narrow band of refractory is needed at the edge of the tubesheet, so the long cure and dry out of a monolithic hot face falls away.
Before the installation, the boiler contractor had a thermal finite element analysis made of the new tube to tubesheet joint with the hex ferrules, up to 1,200 °C. It showed good protection of the tube ends and slightly lower tubesheet temperatures than before.
| What | Result |
|---|---|
| Ferrule installation time | 67% shorter than conventional round ferrules |
| Return to service | within the planned three week shutdown |
| Tubesheet temperature in the FEA | slightly improved |
| Pressure drop | lower, through the tapered ferrule inlet |
The plant paid more for the ferrules than for round ones. The authors state that the installation speed justified that cost.
Without mortar between the heads, each ferrule expands and contracts on its own, so the hot face does not crack the way a monolithic layer does when the boiler cycles. Single ferrules can be pulled for inspection at the next turnaround.
One piece ferrules for waste heat boiler tubesheets, from 20 to 200 mm tube diameter.
The first hex head ferrule installation, in a Claus sulfur recovery boiler.
Ceramic parts for sulfur, sulfuric acid and refinery units.
The plant had a three week shutdown. Removing and replacing a new boiler with its infrastructure in that time was judged unlikely, so the boiler was retubed in place.
HexPro is the name under which DuPont MECS supplies the hex head ceramic ferrules made by Blasch for sulfuric acid plants.
Only a narrow band around the edge of the ferrule field. Between the ferrules there is no castable, only the fiber seal in the groove of each head.
No. The thermal FEA of the new joint showed slightly lower tubesheet temperatures with the hex ferrules.
Send the tube outside diameter, the pitch and the gas inlet temperature. A Blasch engineer lays out the ferrules for your tubesheet.
