🔍 Key Concepts

• heat blisters on boiler tubes and what physically causes metal to overheat and bulge • difference between localized flame effects and internal tube surface conditions • how water circulation and deposits affect metal temperature on the waterside

💭 Think About

• Ask yourself: Where do heat blisters actually form—on the fireside surface, the waterside surface, or within the tube wall—and what condition makes the metal hottest at that spot? • Which of these conditions would most likely create a small, very localized hot area on the tube metal itself, rather than evenly overheating a large section? • Think about which condition gradually acts from inside the tube over time, instead of being mainly about how the burner flame strikes the tube from the outside.

✅ Before You Answer

• Compare which choice most directly insulates the metal from cooling water, causing local overheating of the tube wall. • Consider which condition is described in textbooks as a common, long-term maintenance problem in boilers rather than an unusual firing condition. • Eliminate any options that mainly change combustion pattern but do not significantly reduce internal cooling at one spot on the tube.