How boilers make steam

By June, the Riverbend repair is a page in Marcel Beaudry's notebook and Kiera Lafontaine is back on the Ironbridge Industrial list at Nisku. Rosa Delgado, the general foreman, has a plan for her summer and fall. Kiera will retube a packaged boiler at Larkspur Greenhouses, help inspect the heat recovery steam generator at the Northgate cogeneration plant, spend time in the fabrication shop while V-412, the knockout drum she helped fit last winter, is finished and tested, and go back to Riverbend for the Unit 2 fall inspection. In October, Northgate's turnaround brings a heat exchanger bundle into Ironbridge's exchanger shop to be retubed, and that job is the capstone of this course. Rosa's reason for the plan is simple. "You've cut tubes and fitted shells," she tells Kiera. "Now you need to know what the equipment does when it's running, because that's what tells you why it fails and how it has to be put back together."

This course is about the equipment boilermakers build and repair: boilers of every size, the parts inside them, pressure vessels, and the heat exchangers that fill refineries and plants. It starts with the physics that every one of those pieces depends on. A boiler is a device for putting heat into water under pressure until it becomes steam, and the way water behaves when it is heated under pressure explains most of what a boiler looks like.

Heat, temperature and pressure

Heat is energy moving from something hot to something cold. Temperature is how hot something is. When heat goes into water, two different things can happen. Below the boiling point, the water gets hotter; this is called sensible heat, because a thermometer senses it. At the boiling point, the temperature stops rising and the water turns into steam; the heat that does this is called latent heat, or the heat of evaporation, and it is much larger than the sensible heat. Heating 1 kg (1 lb) of water from the temperature of cold tap water to the boiling point takes about a sixth of the heat needed to boil the same water away.

The boiling point depends on pressure. At sea level, water boils at 100 °C (212 °F). Inside a boiler, the pressure is far higher and so is the boiling point. Water and steam that are in contact at the boiling point are called saturated; the temperature is the saturation temperature, and for each pressure there is exactly one. Riverbend's steam drum runs at about 10,800 kPa (absolute) (1,565 psia), where water boils at about 316 °C (601 °F). That is why a boiler can make steam hot enough to drive a turbine: the pressure lets the water get far hotter before it boils.

Engineers measure the heat content of water and steam as enthalpy, the energy carried per unit mass, in kilojoules per kilogram (kJ/kg) (British thermal units per pound (Btu/lb)). The values are listed in steam tables, which every boiler operator and engineer uses. A saturated steam table lists, for each pressure, the saturation temperature and three enthalpies: hf for saturated water, hfg for the latent heat, and hg for saturated steam, where hg equals hf plus hfg.

Steam that has been heated beyond its saturation temperature is called superheated. It is dry, it carries more energy per unit mass, and it can lose some heat without condensing, which is why turbines need it. Riverbend's steam leaves the superheater at about 510 °C (950 °F), nearly 200 °C (350 °F) above its saturation temperature. Those superheater tubes run hotter than any other tubes in the boiler, and they are made of chromium-molybdenum alloys for that reason, as BLR-210 showed when the crew welded them.

The water and steam circuit

Every boiler, from Larkspur's packaged unit to Riverbend, follows the same path for its water. Feedwater is pumped into the boiler at a pressure above the drum pressure. In a large boiler it passes first through an economizer, a bank of tubes in the cooler flue gas near the back end of the boiler, which heats it close to the boiling point. It then enters the steam drum, mixes with the water there and flows down large pipes called downcomers to the bottom of the furnace. From there it rises up through the tubes that form the furnace walls and the boiler bank, absorbing heat and partly turning to steam. The steam and water mixture returns to the drum, where separators split the steam from the water. The water goes round again; the steam leaves through the superheater to the turbine or the plant.

In a power plant, the steam that leaves the turbine is condensed back to water in a condenser, a large heat exchanger cooled by river or cooling tower water. The condensate is pumped through feedwater heaters and a deaerator, which removes dissolved oxygen, and back to the boiler. In a plant that uses steam for heating or for process, like Larkspur or Northgate, some steam is lost or contaminated and must be replaced with treated make-up water. Either way, the same water goes round the circuit over and over, and anything dissolved in it stays behind in the boiler when the water turns to steam.

The fire side

On the other side of the tube walls is the fire. Burners mix fuel with air and burn it in the furnace, where the flame radiates heat to the waterwall tubes. Riverbend burns natural gas. The hot products of combustion, the flue gas, leave the furnace at the top, turn and pass over the superheater, then down through the convection pass, where the boiler bank, the economizer and the air heater take more heat out of them. The air heater warms the combustion air with flue gas that would otherwise go up the stack. Fans push the air in and pull the gas out. By the time the gas reaches the stack at about 150 °C (300 °F), most of its heat has gone into the water and steam.

Boilermakers work on both sides of the tube wall. On the water side, they deal with drums, headers, tubes and internals. On the fire side, they deal with tube outside surfaces, burners and wind boxes, casing, refractory, ductwork and the dampers that control the gas. Most tube failures start from one side or the other: scale and corrosion from the water side, erosion, overheating and corrosion from the fire side.

Efficiency and where the heat goes

Not all of the fuel's heat ends up in the steam. Boiler efficiency is the heat put into the water and steam divided by the heat released by the fuel. The simplest way to measure it is the input-output method: measure the steam flow, the steam and feedwater conditions and the fuel flow, then divide. The losses are what the other method, the heat-loss method, adds up: heat carried out of the stack in the flue gas, heat lost through the casing by radiation and convection, heat lost in water drained off as blowdown, unburned fuel and moisture in the fuel and air. The stack loss is by far the biggest, which is why every boiler of any size has an economizer, an air heater, or both.

Lisa Chen, Riverbend's performance engineer, gives Kiera the numbers from a recent test at full load so she can work them out for herself. The code below interpolates the drum conditions from an abridged steam table, then works out the heat put into the steam, the fuel input, the efficiency and the equivalent evaporation.

# Riverbend Unit 2: steam conditions, heat added and boiler efficiency (metric)
# Abridged saturated steam table: pressure (kPa abs): (Tsat °C, hf, hfg, hg in kJ/kg)
SAT = {
    100: (99.6, 417.5, 2257.5, 2675.0),
    1_000: (179.9, 762.5, 2014.6, 2777.1),
    2_000: (212.4, 908.5, 1889.8, 2798.3),
    5_000: (263.9, 1154.5, 1639.7, 2794.2),
    8_000: (295.0, 1316.6, 1441.4, 2758.0),
    10_000: (311.0, 1407.9, 1317.6, 2725.5),
    12_000: (324.7, 1491.3, 1193.6, 2684.9),
}

def sat(p):
    """Linear interpolation in the abridged saturated table."""
    ps = sorted(SAT)
    for lo, hi in zip(ps, ps[1:]):
        if lo <= p <= hi:
            f = (p - lo) / (hi - lo)
            return tuple(a + f * (b - a) for a, b in zip(SAT[lo], SAT[hi]))
    raise ValueError("pressure outside the table")

drum_p = 10_800          # drum pressure, kPa abs
tsat, hf, hfg, hg = sat(drum_p)
print(f"Drum at {drum_p:,} kPa (absolute): Tsat {tsat:.1f} °C, hf {hf:.1f}, hfg {hfg:.1f}, hg {hg:.1f} kJ/kg")

steam = 180_000          # main steam flow, kg/h
h_out = 3_390           # superheated steam at the outlet, kJ/kg (from steam tables)
h_fw = 990             # feedwater entering the economizer, kJ/kg
q_steam = steam * (h_out - h_fw)          # heat put into the steam, kJ/h
print(f"Heat added per {'kg'} of steam: {h_out - h_fw:,.0f} kJ/kg")
print(f"Heat output: {q_steam / 1e6:,.1f} GJ/h")

fuel = 13_700            # natural gas, m3/h
hhv = 37_700              # higher heating value, kJ per m3
q_fuel = fuel * hhv
eff = q_steam / q_fuel
print(f"Fuel input:  {q_fuel / 1e6:,.1f} GJ/h")
print(f"Boiler efficiency (input-output method): {eff:.1%}")

# Equivalent evaporation "from and at" 100 °C: water at boiling point to dry saturated steam
ee = q_steam / 2_257
print(f"Equivalent evaporation from and at 100 °C: {ee:,.0f} kg/h")
print(f"Factor of evaporation: {(h_out - h_fw) / 2_257:.3f}")

Drum at 10,800 kPa (absolute): Tsat 316.5 °C, hf 1441.3, hfg 1268.0, hg 2709.3 kJ/kg
Heat added per kg of steam: 2,400 kJ/kg
Heat output: 432.0 GJ/h
Fuel input:  516.5 GJ/h
Boiler efficiency (input-output method): 83.6%
Equivalent evaporation from and at 100 °C: 191,405 kg/h
Factor of evaporation: 1.063

The boiler turns about 84 percent of the fuel's heat into steam, which is a normal figure for a gas-fired utility boiler of its age. Equivalent evaporation is an old way to compare boilers that run at different pressures and feedwater temperatures. It converts the actual output to the steam that would be made from water at the boiling point at atmospheric pressure, turning into steam at that same temperature, a condition called "from and at 100 °C". The ratio of the two is the factor of evaporation. Boilermakers meet both terms in the trade's exams and on the nameplates of older boilers.

Water treatment and why tubes fail

Because anything dissolved in the water stays in the boiler when the water evaporates, boiler water must be treated. Hardness minerals such as calcium and magnesium form scale on the inside of tubes. Scale is a poor conductor of heat, so a tube coated with it runs hotter than a clean one; in a furnace wall, a thin layer of scale can raise the tube metal temperature enough to cause an overheating failure. Dissolved oxygen causes pitting. Caustic and acid conditions under deposits cause gouging and hydrogen damage. Operators control water quality with make-up water treatment, deaeration, chemical feed and blowdown, a controlled drain of concentrated water from the drum. When a boilermaker cuts out a failed tube, the inside surface tells the story, and the plant's chemists and inspectors want to see it.

Why boilers are dangerous

A boiler holds a large volume of water far above its atmospheric boiling point. If the drum or a header ruptures, the pressure drops instantly and a large part of that water flashes to steam, expanding more than a thousand times in volume. The energy released is enormous, and historically boiler explosions killed thousands of people before codes, inspection and trained operators made them rare. The low-water condition is the other classic danger: if the water level falls below the tubes or the furnace crown sheet, the metal overheats, weakens and can fail, and adding cold water to an overheated boiler can cause a violent failure. Every code rule a boilermaker follows, from the welder's qualification to the hydrostatic test, exists because of this stored energy.