Feedwater Heater Bypass and Boiler Carryover

Bypassing the feedwater heater means the feedwater enters the boiler at a lower temperature (perhaps 60–80°F instead of 220–250°F). Lower feedwater temperature reduces the heat available for evaporation per pound of fuel, so the boiler must fire harder to maintain the same steam flow. More importantly, the deaerating heater is also bypassed or ineffective, so dissolved oxygen remains high (45 ppb vs. 7 ppb limit). Oxygen pitting will accelerate on the boiler tubes and pre-boiler piping. At the same time, the softener is passing hardness (0.8 ppm vs. 0.3 ppm limit), which will form scale on the hot tube surfaces. Scale acts as an insulating layer, raising tube metal temperatures and promoting overheating and eventual failure. The high boiler water conductivity (4,200 µS/cm vs. 3,500 µS/cm limit) indicates high dissolved solids, which increases the risk of foaming and carryover. Carryover will contaminate the superheater and steam system with dissolved salts, leading to deposits and possible stress-corrosion cracking. The combination of scale, oxygen pitting, and high dissolved solids is synergistic: scale creates hot spots that accelerate oxygen attack, while high solids increase carryover that deposits on superheater tubes. The erratic drum level is likely caused by foaming from high conductivity, which can lead to low-water trips or water hammer. The shift engineer must recognize that the current operation is outside all three chemistry limits and that the risk of tube failure and steam contamination is high.

Immediate actions should focus on reducing the consequences of the chemistry excursions. First, increase blowdown from 5% to as high as the boiler can tolerate without causing low drum level or thermal stress. Increasing blowdown will reduce conductivity toward the 3,500 µS/cm limit, but it also increases feedwater demand and chemical consumption, and may worsen the low feedwater temperature problem because more cold make-up is needed. Second, if the deaerator is functional but the heater is bypassed, route feedwater through the deaerator and ensure the deaerating steam supply is adequate to heat the water to at least 220°F and strip oxygen. If the deaerator is also out of service, consider temporary chemical oxygen scavenging (e.g., sodium sulfite or DEHA) with frequent testing to maintain a residual. Third, increase the frequency of boiler water testing from once per shift to every two hours, and test for hardness, oxygen, conductivity, and phosphate/polymer residual. Adjust chemical feed rates accordingly. Fourth, reduce firing rate if possible by curtailing steam demand; lower firing rate reduces the evaporation rate and the carryover tendency. Fifth, monitor drum level closely and consider lowering the normal water level slightly to increase steam space, but never below the low-water trip point. Trade-offs: increased blowdown wastes energy and water; chemical overfeed can cause foaming or scaling; reducing firing rate may not be possible if the plant must maintain production. The engineer should document all actions and inform management of the risk.

The decision to continue operating or to shut down must weigh safety, equipment damage risk, and production needs. Key criteria: (1) Is the boiler water conductivity controllable below the limit with maximum blowdown? If conductivity continues to rise despite maximum blowdown, the risk of severe carryover and water hammer is unacceptable. (2) Is the drum level stable? If level control is erratic and low-water trips are occurring or likely, the boiler should be taken off line because low water can cause tube overheating and catastrophic failure. (3) Are there signs of tube overheating or leakage? If tube metal temperatures are not measured, look for discoloration, steam leaks, or increased make-up water. Any evidence of tube failure requires immediate shutdown. (4) What is the consequence of steam contamination? If the steam is used in a turbine, carryover can cause deposits and vibration; if the steam contacts food or pharmaceuticals, contamination is unacceptable. (5) Can temporary measures (deaeration, chemical scavenging, increased blowdown) bring all parameters within limits within a few hours? If not, shutdown is safer. The engineer should use a risk matrix: probability of failure (high due to scale and oxygen pitting) versus consequence (injury, equipment loss, production loss). Given that all three chemistry limits are exceeded and drum level is erratic, the safer decision is to reduce load to minimum and prepare for an orderly shutdown, or to shut down immediately if drum level cannot be stabilized. Document the decision and communicate with management.