Steam Saturation Temperature Table

Pressure and Temperature Are One Setting

On the saturation line there is only one independent variable. Set the pressure and the temperature follows, which is what makes the steam saturation temperature table a working tool rather than a reference curiosity: a pressure reducing valve on a steam main is, in practice, a temperature controller. Reading down the columns, 6 at gives 158.1 °C and 20 at gives 211.4 °C, so a process that must sit at about 160 °C is held there by holding the supply at 6 at, not by any temperature device in the line.

The Unit: 1 at Is Not 1 bar

The pressure column is in technical atmospheres. One at equals 1 kgf/cm², which is 0.980665 bar, so 10 at is 9.81 bar and 80 at is 78.5 bar. German and Turkish plant documentation from the period this chart comes from is written in at, while every modern gauge and datasheet reads in bar. Treating the steam saturation temperature table as though the column were bar carries a systematic error of about 2 %, which is small at the bottom of the range and several degrees at the top.

The Pressures Are Absolute

The first row settles it: 1.0 at gives 99.1 °C, which is atmospheric boiling. Had the column been gauge pressure, that same row would read about 120 °C. So a plant gauge showing 6 bar means 7.01 bar absolute, which is 7.15 at, and the steam saturation temperature table is entered at that figure rather than at 6. Adding the atmosphere before reading is the single correction that turns this chart from approximately right into exactly right.

What the Temperature Decides on the Valve

The temperature column is where valve selection actually happens. Pressure classes derate as temperature rises, so a body rated PN 40 cold does not carry PN 40 at 250 °C, and the flange standard has to be read at the temperature the steam saturation temperature table gives, not at ambient.

Seat and seal materials set a harder limit. PTFE ball valve seats are generally good to about 200 °C, so at 20 at — 211.4 °C in the steam saturation temperature table — a soft-seated ball valve is already outside its material limit and a metal-seated or PEEK-seated design is required. The same applies to gaskets and to gland packing: PTFE packing runs out at roughly 230 °C, and graphite takes over above it. At the top of the table, 80 at is close to 294 °C, and every soft material in the valve has to be reconsidered.

Above the Line There Is Superheat

The steam saturation temperature table describes the boundary between water and vapour. Once steam is superheated, temperature and pressure become independent again and the chart no longer predicts one from the other: superheated steam at 20 at can be at 300 °C or at 400 °C. Superheat is good for turbines and for long distribution runs because it keeps the steam dry, but for heat transfer it is a nuisance, since the useful energy is released at the saturation temperature the table gives.

Flash Steam on the Condensate Side

The same figures explain what happens downstream of a trap. Condensate leaving at 8 at is at 169.6 °C, and a return line at 1 at can only hold liquid at 99.1 °C, so the surplus heat re-evaporates part of the condensate. Roughly 13 % of the mass flashes off in that example, which is why return lines and flash vessels are sized far larger than the liquid volume alone suggests. Reading both pressures from the steam saturation temperature table is how that percentage is worked out.

Read It at the Point of Use, Not at the Boiler

Pressure falls along a steam main, and the temperature falls with it. A boiler at 12 at feeding through an undersized line may present only 8 at at the far branch, and the steam saturation temperature table turns that into a real loss: 187.1 °C at the boiler against 169.6 °C at the user. A process specified for 180 °C will simply never reach temperature, and no amount of trap or control valve adjustment recovers it.

Where a plant has long runs, take the pressure reading at the branch itself, enter the steam saturation temperature table with that value, and compare the result against what the process needs before blaming the equipment. The same check is worth repeating after any pipe modification: extra length, additional elbows or an undersized isolating valve all move the delivered pressure down, and the steam saturation temperature table converts that drop directly into degrees lost at the heat exchanger.

Related Steam Tables

The steam saturation temperature table gives the temperature for a pressure; the two charts beside it complete the picture. The saturated steam table adds the enthalpy and specific volume at each point, and the steam pipe capacity table turns the resulting kg/h into a pipe diameter. Taken together they cover the three questions asked before any steam valve is ordered: how hot, how much energy, and how big.