Problem 13.7: A boiler produces 20000 lbhr of steam

From Chapter 13 Problem Set. Level 2: Applied Thinking.

Question

A boiler produces 20,000 lb/hr of steam. Feedwater conductivity = 50 µS. (a) At 40 cycles, what is the boiler water conductivity? (b) If cycles drop to 10 due to lost condensate, and feedwater conductivity rises to 200 µS, what is the new boiler water conductivity? (c) Using the ASME table, is this still within guidelines for a 150 psig boiler? Explain why conductivity alone does not tell the full story.

Answer

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(a) 50 µS × 40 = 2,000 µS. (b) 200 µS × 10 = 2,000 µS. (c) The conductivity is the same numerically, but the underlying chemistry is radically different. At 40 cycles on clean feedwater, TDS is composed of benign sodium salts. At 10 cycles on dirty feedwater, TDS includes calcium, magnesium, and elevated alkalinity - the species the softener should have removed. The 2,000 µS reading is within the 3,500 mg/L TDS limit for <300 psig, but the composition - not the conductivity - determines the risk. Hardness breakthrough means scaling risk is severe regardless of the conductivity reading.