Cold Start After a Tube Replacement: Warm-Up Rate Versus a Four-Hour Deadline

Arithmetic: 406 F saturation minus 65 F starting water = 341 F of rise. At the 100 F/hr limit that is 3.41 hours of firing - and that number is a ceiling, not a plan.

The 3.41 hours excludes:

• filling and venting the boiler and confirming level;
• purge and light-off, and the low-fire stabilization period;
• rolling the burner and the slower heat input actually available at minimum fire;
• the fact that drum metal temperature lags saturation temperature - the metal is what the limit protects, so the metal rise trails the water rise and the pressure hold at the end takes longer than the water-side math suggests.

Answer: four hours is not achievable with any margin. A realistic plan is five to six hours, or a negotiated lower first pressure - getting on line at 100 psig cuts the required rise to about 273 F, roughly 2.7 hours at the limit, leaving room for the rest of the sequence.

There is also a technical reason to be more conservative than usual, not less: this is a boiler that was just reassembled. A fast warm-up on freshly rolled joints is an efficient way to create leaks.

Before light-off: confirm hydro blinds and plugs are removed, safety valves reinstalled, the gauge glass blown down and valves lined up, and the low-water cutoff and burner low-fire proved.

Steps and monitoring:

1. Fill to normal level and vent air from the drum and superheater - open the drum vents until steam appears, then close. Trapped air causes false level and uneven heating.
2. Open the superheater drains and leave them open until steam flow is established. With no steam flowing, the superheater tubes have no cooling and can overheat even at low fire.
3. Light off at minimum fire and hold there, or use short firing periods. The burner's minimum position is the only control you have over the rate of temperature rise early on.
4. Watch expansion: drum growth, the sliding feet and supports, and any restraint that should be free. A boiler that cannot grow will crack something.
5. Watch drum level behavior - swelling as steaming begins is normal; wild swinging is not.
6. Take a light bottom blowdown during warm-up to promote circulation and remove loose material.
7. Log saturation temperature versus time every hour and hold the rise to the 100 F/hr limit rather than using it as a target.

Shut down, controlled.

• Do not carry it up in pressure. A weeping rolled joint is already a leak path; raising pressure and temperature works the joint harder and can turn a weeper into a rupture at exactly the moment the boiler is at its hottest.
• The 'it will seal itself' argument is not reliable on a heat-exchanger tube. Thermal expansion can close a joint temporarily and then open it again under load, which is worse because it happens unattended.
• Cool down slowly. Stop firing, keep the boiler full, and do not dump cold feedwater into a hot drum or open drains before pressure is down. Fast cooling damages tubes and tube sheets the same way fast heating does, and it makes the repair bigger.
• Document the pressure at which the leak appeared and the firing history, because that record tells the repair crew whether the joint was under-rolled or the warm-up was too fast.
• After repair: re-roll or replace the tube, re-hydro, and restart on the same warm-up limit. If the schedule cannot absorb it, that is a production decision, not a boiler-operating one - and it should be escalated in writing.