If a steam generator’s shell is sound, replacing only the tube bundle costs about half as much as a new vessel and is quicker to deliver. In a quotation for an ammonia stripping plant, a complete replacement steam generator was offered with an option for the tube bundle and gaskets alone, both made in Thailand with a four-month lead time.
The challenge: a pressure vessel at the end of its life
The steam generator provides heat to the stripping process. When its tubes foul or corrode, it cannot supply enough heat, and the plant cannot run at capacity. It is a pressure vessel, so any replacement has to meet a code and be inspected.
The approach: two options
Option 1: a complete vessel
The full scope included all materials, fabrication, third-party inspection, U stamp (a mark showing the vessel was built to ASME pressure vessel rules), gaskets, packing and engineering. It served two stripping lines. Delivery was four months ex-works plus two weeks by sea.
Option 2: the tube bundle only
The second option supplied just the tube bundle with gaskets, to be fitted in the existing shell. The quotation priced it at under half the cost of the new vessel.
A related case: a leachate heater
A shell-and-tube leachate heater for another line was quoted with all materials, fabrication, testing, gaskets, packing and engineering documentation. Lead time was 14 to 16 weeks from Thailand plus seven to ten days by sea.
Why it works
The tubes are where wear is concentrated. A new bundle in a good shell restores performance and reuses the pressure boundary. A new vessel resets everything, including the code documentation, and is the better choice where the shell is corroded or the original design is no longer available. The choice depends on an inspection of the shell, which is why the quotation offered both.
In the UK, pressure equipment is covered by the Pressure Systems Safety Regulations; the HSE explains the duties in its pressure systems guidance.
What to consider for your own project
- Inspect the shell first. Thickness and corrosion decide the option.
- Specify the code and stamp you need.
- Ask for third-party inspection.
- Allow four months or more for fabrication.
- Plan the changeover. Fitting a bundle needs access and lifting.
For the smaller modules in the heat recovery section, see our post on economiser cassettes.
FAQ
What is a U stamp?
A mark showing that a vessel was built and inspected to the ASME pressure vessel code.
Can only the tube bundle be replaced?
Yes, if the shell is in good condition.
How long does a new steam generator take?
About four months to make, plus two weeks of shipping.
Is third-party inspection needed?
It was included in this scope.