Running a Boiler Whose Gauges Still Read kg/cm²
Plenty of package boilers, economisers and steam headers commissioned before the metric-unit tidy-up are still in daily service, and their dials, name plates and log books all speak in kilogram-force per square centimetre. The steam tables on the wall, the modern transmitter that replaced a failed gauge and the burner controller's display are in bar. A shift operator crosses between the two every time a reading is written down.
Where the Two Units Meet in the Boiler House
Old iron, new paperwork
Pressure is really a temperature setting
The little "g" on the dial
Cross-Checking a Reading Against the Steam Table
The routine is short, and it is the same whether you are filling in a log, sizing a valve or arguing with a trend on the control screen.
Type the dial reading
Enter the kg/cm² figure the boiler gauge shows into the left field — 7, 10.5, 12.4, whatever the needle sits on. The bar value follows as you type, and a comma decimal is read the same as a full stop.
Add the atmosphere before the table
Steam tables are indexed on absolute pressure, so put 1.013 bar on top of the converted gauge value. A dial at 7 kg/cm²g becomes 6.865 bar g, which is 7.878 bar absolute — a saturation temperature near 170 °C.
Move the figure into the record
Each field carries its own copy control that lifts the number by itself, without the unit and without spacing, ready for a shift log spreadsheet or a maintenance form. Ctrl + C inside a field produces the same clipboard content.
Turn it round for a modern setpoint
When a controller is configured in bar and the plate is in kg/cm², the swap button (↔) runs bar → kg/cm² instead. The reverse multiplier is 1.019716213, so a 9 bar setpoint corresponds to 9.177 kg/cm² on the old scale.
Boiler Pressure and the Steam Temperature That Comes With It
Common package-boiler operating pressures, on the old dial and on a modern instrument, with the saturation temperature that belongs to each. Gauge values are converted to absolute before the temperature is read, so the table works straight off a kg/cm²g needle.
| Gauge (kg/cm²g) | Gauge (bar g) | Absolute (bar a) | Saturated steam | Typical duty |
|---|---|---|---|---|
| 0 | 0 bar g | 1.013 bar a | 100.0 °C | Open vent, boiler off line |
| 3 | 2.942 bar g | 3.955 bar a | 143.2 °C | Space heating, low-pressure calorifiers |
| 5 | 4.903 bar g | 5.917 bar a | 158.3 °C | Laundry, general process headers |
| 7 | 6.865 bar g | 7.878 bar a | 169.8 °C | Food and beverage cooking duties |
| 10 | 9.807 bar g | 10.820 bar a | 183.3 °C | Textile dyeing, rubber vulcanising |
| 12 | 11.768 bar g | 12.781 bar a | 190.8 °C | Chemical reactors, distribution headers |
| 16 | 15.691 bar g | 16.704 bar a | 203.5 °C | Long-distance mains before reduction |
| 21 | 20.594 bar g | 21.607 bar a | 216.3 °C | Upper end of ordinary package-boiler design |
The temperature column explains why the gap between the two units is worth respecting. Reading 10 kg/cm² as though it were 10 bar puts the header 0.19 bar too high and the steam temperature about 0.8 °C too hot — small on paper, but enough to blur a process specified to a tight band.
Details That Suit the Boiler House
Work down a log column without resetting
Both boxes take input, so a full shift of hourly readings can be converted one after another, and a bar figure from the control screen can be dropped in mid-way.
Face the manual, not the machine
A single press of the swap control reverses the pair when the document in your hand is written in bar and the plant in front of you is not.
MPa, psi and kg/m² are one search away
The dropdowns on both sides list all 26 pressure units in eight groups, which covers an imported valve rated in MPa or an inspection report written in psi.
Enough digits for a valve calculation
Results run to eight decimals with thousands separated by a space, so a set-pressure or accumulation figure keeps its precision on the way into a sizing sheet.
Boiler House Questions
The header gauge sits at 7 kg/cm² — how hot is the steam reaching the plant?
7 kg/cm²g converts to 6.865 bar g. Add the atmosphere and you are at 7.878 bar absolute, where saturated steam sits at about 170 °C. That single number is what the process actually feels: the latent heat arrives at that temperature no matter how much steam is flowing, which is why saturated systems are controlled on pressure in the first place.
Why is the boiler gauge marked kg/cm²g, and does the "g" change my steam-table lookup?
It does. The dial is zeroed against the air in the boiler house, so it reports how much the shell exceeds its surroundings. Steam property tables are published against absolute pressure, which means adding one atmosphere — 1.033 kg/cm², or 1.013 bar — before reading across. Skip that step at low pressures and the temperature error is large: 0 on the dial is 100 °C, not the freezing-cold nonsense a zero-absolute reading would imply.
How does the design pressure on the name plate relate to where relief valves are set?
Codes based on ASME Section I require at least one valve set at or below the stamped maximum allowable working pressure, with any additional valve no more than 3% above it, and total accumulation during discharge held within 6%. On a shell plated at 10.5 kg/cm² — 10.297 bar — that puts the highest permitted additional setting at about 10.815 kg/cm², or 10.606 bar. The conversion only tells you which number on the plate you are looking at; the settings themselves are the certifying authority's business.
Management wants the header dropped from 10 to 7 kg/cm² to save fuel — what changes downstream?
The steam gets cooler and bulkier. Going from 9.807 bar g to 6.865 bar g takes saturation temperature from about 183 °C to about 170 °C, so any heat exchanger sized on the higher temperature difference will transfer less. Specific volume also grows, so the same mass flow moves faster through mains sized for the old pressure. Standing losses do fall, which is the point — but the coils and the pipe sizing have to be checked first.
Should the gauge dip during a bottom blowdown, and how much is normal?
A short blowdown on a firing boiler usually shows a small, brief sag — a fraction of a kg/cm², so a few hundredths of a bar — that the burner recovers within a minute or two. A large or slow-recovering drop points at a blowdown valve that is passing, or at a duty far beyond what the boiler can make up. Log the reading in one unit consistently: mixing kg/cm² and bar entries in the same column is how a 2% shift gets mistaken for a real trend.
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