Problem 13.8: Your blowdown conductivity controller reads

From Chapter 13 Problem Set. Level 3: System Integration.

Question

Your blowdown conductivity controller reads 2,000 µS. Your handheld meter reads 3,800 µS on a cooled grab sample. The blowdown valve has been closed for hours. (a) What has happened? (b) Estimate the actual cycles if feedwater conductivity is 80 µS. (c) Using the ASME table, is this boiler (300 psig) at risk? For what?

Answer

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(a) The conductivity sensor is fouled or out of calibration, reading artificially low. The controller believes the water is clean and has closed the blowdown valve, allowing cycles to climb unchecked. (b) Actual cycles = 3,800 / 80 = 47.5. (c) At 300 psig, the simplified reference table in this chapter gives a boiler-water TDS ceiling of 3,500 mg/L. Using the approximate conversion TDS ≈ 0.7 × conductivity, 3,800 µS/cm corresponds to approximately 2,660 mg/L TDS, which is below that simplified ceiling. However, the actual cycles have climbed to 47.5, the controller is operating from a false reading, and TDS alone does not establish safety. Silica, alkalinity, foaming tendency, steam-purity requirements, and the boiler manufacturer’s limits must also be evaluated. Immediate action remains appropriate: validate the manual result, restore controlled blowdown, clean and recalibrate the sensor, and confirm all boiler-water parameters before returning to normal control.