Comments on my previous post “How Do You Keep Your Boiler Clean” reminded me of another problematic issue regarding the design and construction of boilers.
The number of rows of tubes in a bundle is a very important factor for efficient online cleaning or maintenance and inspection during shutdowns.
Unfortunately, we have seen situations where the tube bundles of the superheater, evaporator, economizer or air preheater consist of about 20 rows, with no gap between them.
Why do we recommend a gap of at least 600 mm between every 10 rows of tubes in a bundle?
The answer is obvious:
1. The gap makes it possible to install online cleaning devices for effective cleaning during operation. There is simply no such device in the world that can effectively clean online through a bundle of about 20 rows.
2. A 600 mm (two feet) wide gap between the bundles of about 10 rows with a manhole  (by the way, this manhole can be used to install an online soot cleaning device) allows you to accurately and in detail inspect the condition of the boiler during shutdowns. 
3. Due to easy access, this gap makes it easier to locate and replace damaged leaky tubes. There will be no need to cut a large number of serviceable tubes with the following complicated “mirror” welding only in order to gain access and replace the damaged ones.