Firing-Rate Control: Holding Steam Pressure
Steam pressure is the boiler’s main controlled variable. When pressure falls the boiler must fire harder; when it rises, less. Doing that safely means moving fuel and air together, in the right order, and with several boilers sharing the load sensibly.
You'll learn
- Describe pressure-based firing-rate control
- Explain fuel–air ratio control and cross-limiting
- Know how solid-fuel firing is controlled
- Sequence multiple boilers
Before this: On-off, modulating and PID, Air–fuel ratio and excess air
Pressure tells the boiler how hard to fire
If more steam leaves than the boiler makes, pressure falls; if less, it rises. A pressure controller compares steam pressure with its set point and adjusts the firing rate: the master demand. Fuel and air both follow that demand.
Small boilers use on-off or high-low firing between pressure switches; larger ones modulate firing smoothly. Modulation holds pressure steadier and avoids the losses of constant cycling.
Fuel and air together
- Mechanical linkage: one motor drives fuel and air valves through a cam, set at commissioning.
- Electronic ratio control: separate actuators on fuel and air, linked by a programmed fuel–air curve; repeatable and easy to retune.
- Oxygen trim: corrects the air around the curve using a flue gas oxygen measurement.
On solid-fuel boilers the master demand moves the fuel feed (screw feeder speed, grate speed, or feeders on a fluidized bed) and the FD and ID fans together, with the ID fan holding furnace draft.
Cross-limiting and multiple boilers
Fuel can change faster than air, especially when air is moved by a large fan and damper. If fuel rises first on a load increase, or air falls first on a decrease, the furnace briefly runs short of air: smoke, CO and, at worst, unburnt fuel. Cross-limiting prevents it: on a rising demand, air leads and fuel follows; on a falling demand, fuel leads and air follows.
- Lead–lag sequencing: one boiler leads and takes load swings; others come on line only when the lead reaches its limit.
- Load sharing: running boilers share load so each sits in its efficient range, rather than several running at very low load.
- Hot standby: a boiler kept warm and ready to pick up a trip of the lead boiler.
Key takeaways
- Steam pressure sets the firing demand; fuel and air follow it together.
- Mechanical linkage, electronic ratio control and O₂ trim keep fuel and air in proportion.
- Cross-limiting: air leads on rising load, fuel leads on falling load.
- Sequence and share load across boilers so each runs efficiently.