Carryover and High Conductivity After a Condensate Return Change
A 400 hp, 150 psig steam boiler has a maximum boiler water TDS of 3,500 mg/L. Make-up water has 350 mg/L total dissolved solids; condensate return has fallen from 70% to 30% because a process heat exchanger is leaking. The log says blowdown is 10% of feedwater, but the boiler water conductivity is 4,200 µS/cm, alkalinity is 600 mg/L as CaCO3, pH is 11.2, and feedwater hardness is 0.5 mg/L. The steam is carrying over: the sight glass is cloudy and the steam header temperature is fluctuating. The plant chemist wants to increase blowdown to 15%, while the energy manager wants to cut it to 5% to save fuel. The boiler has a continuous blowdown valve and a bottom blowdown valve; the conductivity probe is in the continuous blowdown line.
1. Calculate the allowable cycles of concentration and the blowdown required from the TDS numbers. What are possible reasons the measured conductivity is high despite a logged 10% blowdown?
Read the worked answer
With 70% makeup at 350 mg/L and 30% condensate at about 5 mg/L, feedwater TDS is approximately 0.70 × 350 + 0.30 × 5 = 246.5 mg/L. At a 3,500 mg/L boiler-water limit, allowable cycles are 3,500/246.5 = 14.2. The blowdown required to hold that is 100/(14.2 − 1) = 7.6% of feedwater.
At 10% blowdown, expected boiler TDS is about 2,465 mg/L. The 4,200 µS/cm conductivity is not directly comparable to mg/L TDS, but it is high enough to verify. Possible reasons: the actual blowdown is less than logged because a valve is stuck or a line is plugged; the conductivity probe or sample is fouled or not representative; feedwater TDS is higher than assumed because the leaking heat exchanger is contaminating condensate; or the lab/analyzer is in error. Check actual blowdown flow and probe calibration before changing the rate.
2. What are the likely causes of carryover, and what immediate steps reduce it without causing thermal shock?
Read the worked answer
Carryover is consistent with high dissolved solids and alkalinity, but the cloudy glass and fluctuating steam temperature could also come from the process leak. Immediate steps: verify the boiler water level is not high; reduce firing or load if possible; confirm the continuous blowdown is open and the sample probe is representative; test condensate for hardness, conductivity, and oil or organic contamination; and isolate the leaking heat exchanger.
Increase blowdown gradually, not by slamming the bottom blowdown valve, because a sudden dump can cause level and thermal shock. If steam is still wet after conductivity is back under limit, look for mechanical carryover-damaged steam separator, high water level, or rapid load swings. Do not add antifoam as a substitute for finding the contaminant source.
3. The plant wants to cut blowdown to 5% to save energy. Explain why that is risky and what control parameter should set blowdown.
Read the worked answer
Blowdown should be set by the boiler-water TDS or conductivity limit, with a margin for load swings-not by an energy target alone. At 5% blowdown, cycles would roughly double and boiler-water TDS/conductivity would rise, worsening carryover and possibly scaling. The 15% proposal may be more than needed and wastes heat, but it may be temporarily justified until the leak is isolated and the probe is verified.
The correct approach is to measure actual blowdown flow, calibrate the conductivity meter, calculate the heat loss, and use automatic conductivity-controlled blowdown if available. Restoring condensate return will reduce makeup and blowdown demand. The final rate should hold boiler water below the TDS limit under worst-case load, not just at average conditions.