Diagnosing a 210 MW Unit: Rising Back Pressure versus Air-Preheater Leakage, and the Cost of Waiting
A 210 MW coal-fired unit (design turbine heat rate 1950 kcal/kWh, boiler efficiency 87.5%, auxiliary consumption 9.5%, coal GCV 3800 kcal/kg) has drifted over one month. At a steady 200 MW the condenser back pressure has risen from 0.09 to 0.12 bar absolute while the circulating-water inlet temperature stayed at 32 degC; the CW outlet rose from 38 to 41 degC at unchanged flow. The air-preheater outlet oxygen reads 5.8% against 3.5% at the economiser outlet, and FD fan current is up 8%. Superheater spray has increased from 8 to 14 t/h. A vacuum-decay test performed before the shift showed the pressure rising 2.5 mbar/min after isolating the condenser. Coal landed cost is Rs 4200/t and the unit runs at 200 MW for 20 h a day. OEM guidance is about 1% heat-rate penalty per 0.01 bar of back-pressure rise above design, and about 1% boiler-efficiency loss per 3 percentage points of air-preheater leakage-related oxygen rise.
1. Separate the condenser problem from the air-preheater problem, and calculate the daily coal cost of each. Which would you attack first?
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Baseline. Gross turbine heat rate 1950 kcal/kWh; gross plant heat rate = 1950/0.875 = 2229 kcal/kWh. Net heat rate = 2229/(1 - 0.095) = 2463 kcal/kWh. Coal rate = 2463/3800 = 0.648 kg/kWh. Daily generation = 200 MW x 20 h = 4000 MWh, so daily coal = 4000 x 1000 x 0.648/1000 = 2593 t, costing about Rs 1.09 crore.
Condenser. Back pressure is up 0.03 bar, i.e. a 3% heat-rate penalty = about 78 t of coal a day = roughly Rs 3.27 lakh a day. But note the vacuum-decay result: 2.5 mbar/min is a severe air-inleakage signature, not tube fouling. Air raises the partial pressure of air in the condenser, degrades heat transfer, and also raises the dissolved oxygen in the condensate - which is a corrosion risk to the entire feed train.
Air preheater. Oxygen rise across the APH is 2.3 points, i.e. about 0.77% boiler efficiency, roughly 20 t of coal a day (about Rs 84,000), plus the 8% FD fan power increase.
Judgement. Attack the air in-leakage first. It is the cheapest, quickest fix (an online helium or ultrasonic leak search on the vacuum system, gland seals, and rupture discs), it carries the corrosion risk, and part of the apparent back-pressure penalty may disappear once it is sealed. Re-run the decay test afterwards before deciding on tube cleaning.
2. What is your operating risk assessment while the unit continues at 200 MW, and what limits would you set on continued operation?
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Risk 1 - loss of vacuum trip. With a decay of 2.5 mbar/min, any additional disturbance (CW pump trip, sudden load reduction) will take the condenser to the trip setting faster than the operator can respond. Test the actual behaviour: with the unit at stable load, isolate and measure the time to reach the alarm and trip values. If that margin is under about 10 minutes, the unit should be brought down deliberately rather than tripped.
Risk 2 - condensate chemistry. Air ingress raises dissolved oxygen; check the deaerator and condensate pump discharge DO immediately, and increase hydrazine dosing. If DO cannot be held, the feed train is being corroded and the case for an outage becomes urgent regardless of heat rate.
Risk 3 - exhaust hood and last-stage blades. Higher back pressure raises exhaust-hood temperature and, at constant load, increases the exhaust volumetric flow and windage; it also shifts the operating point on the last-stage blade. Monitor hood temperature and vibration.
Limits. Cap the unit at a load at which hood temperature and vacuum remain in range, keep both CW pumps running, keep the air-removal ejectors/vacuum pumps at full capacity, and set a written limit of a few days for sealing the leak. Simply raising the vacuum trip setting is not acceptable - the trip exists to protect the last-stage blades.
3. The increased superheater spray is offered as evidence that the boiler is also at fault. How would you test that claim before spending money on a boiler overhaul?
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Do not accept it at face value. Spray flow can rise for several reasons that have nothing to do with furnace fouling: air-preheater leakage changes the gas mass flow and the convective-to-radiant heat split, mill combinations change the flame position, and a change in feedwater temperature or unit load profile shifts the spray requirement.
Test 1 - normalise. Compare spray at the same load, same mill combination, same feedwater temperature and same excess air. A raw 8 to 14 t/h increase at an unstated load is not evidence of anything.
Test 2 - close the mass balance. Calculate the air-preheater leakage from the oxygen rise and the gas-side mass balance, and correct the boiler efficiency to a common reference. Then repeat the boiler efficiency test with the corrected excess air.
Test 3 - look for direct evidence. Check the furnace exit gas temperature trend, slagging on the water-wall and platen superheater, sootblower performance, and the attemperator valve position versus design. Fouling shows up as a rise in furnace exit gas temperature at constant load; a leakage problem shows up as increased gas mass flow with unchanged heat absorption pattern.
Judgement. Fix the leakage and the condenser first, re-test, and only then decide whether a boiler overhaul is justified - the air-preheater and condenser work can be done in a short outage and will change the spray figure anyway.