Where the hex head ferrule came from: a Claus boiler at a US Gulf Coast refinery, a reliability engineer and a tubesheet without castable.
Several boiler failures, then a question from the plant: does the tubesheet need castable at all?
The answer became the hex head ferrule
At a refinery on the US Gulf Coast, the first hex head ferrules from Blasch went into the waste heat boiler of a Claus sulfur recovery unit in September 1996. A full set took half a shift to install instead of the 2 days of the conventional ferrules in castable, the installer saved 2.5 to 3 days on that turnaround, and after 10 months the ferrules were in like new condition with no gas penetration to the tubesheet.
The conventional design in Claus waste heat boilers puts round high alumina ferrules into the tube ends, about 4 inches proud of the tubesheet, and buries them in castable or plastic refractory. On large boilers that takes the better part of two days, plus a couple more days of cure out.
Monolithic refractory cracks over time from the stresses of heat up and cool down. Anchors expand more than the refractory around them and corrode. Once a crack opens a path to the tubesheet, hot sulfur gas attacks tubes and welds. The refinery had replaced its whole tubesheet refractory after several boiler failures, and its reliability engineer asked Blasch for a ferrule system that needed no monolithic refractory at all.
Instead of fighting the expansion of the tubesheet face, the hex head ferrule uses it: each ferrule floats in its own tube, and the heads grow together into a closed face only when the boiler is hot.
Blasch and the refinery went through several keyed and stepped designs before settling on a simple hexagonal head. Two details made it work: the ferrules are cast accurately enough to close at the planned operating temperature, and a groove around each head holds a compressed high alumina fiber seal that does not block the closing. The design was later granted a US patent.
The first two sets went in in September 1996, installed by a maintenance contractor. The ferrules arrived prefired with all fiber applied. Only a thin band of castable around the ferrule field needed curing.
| What | Conventional ferrules in castable | Hex head ferrules |
|---|---|---|
| Installation of a full set | about 2 days, plus cure out | half a shift |
| Time saved on the turnaround | 2.5 to 3 days, reported by the installer | |
| Condition after 10 months | like new, no gas penetration to the tubesheet | |
| Tubesheet cleaning | hydroblasting | brushing only |
A later outage was caused by massive liquid carryover into the combustion chamber, which destroyed the bottom rows of ferrules and led to tube joint failures. The ferrules above the high water mark survived with some surface cracking and came out by hand. After inspection the whole tubesheet was refitted in less than half a shift, and the unit restarted without a long cure out.
Shorter turnarounds, a tubesheet that can be inspected ferrule by ferrule, and a hot face that recovers from an upset in hours. The ferrules were designed for a 3 to 5 year life, confirmed in later installations.
Today's one piece ferrules for Claus and sulfuric acid boiler tubesheets.
Hex head ferrules in a sulfuric acid plant boiler retubed in a three week shutdown.
Ceramic parts for sulfur recovery, FCC units and reformers.
If they were mortared together, the face would behave like a monolithic refractory layer and crack. Each ferrule is affected only by its six neighbors.
Yes. This installation showed that individual ferrules come out by hand, without breaking up the hot face.
Temperatures at Claus waste heat boiler inlets range up to nearly 3,000 °F, higher with oxygen enrichment.
Send the tube outside diameter, the pitch, the tubesheet thickness and the inlet temperature. A Blasch engineer sizes the ferrules for your boiler.
