Steam Distribution

Air venting

When steam is first admitted to a pipe after a period of shutdown, the pipe is full of air. Further amounts of air and other non-condensable gases will enter with the steam, although the proportions of these gases are normally very small compared with the steam. When the steam condenses, these gases will accumulate in pipes and heat exchangers. Precautions should be taken to discharge them. The consequence of not removing air is a lengthy warming up period, and a reduction in plant efficiency and process performance.

Air in a steam system will also affect the system temperature. Air will exert its own pressure within the system, and will be added to the pressure of the steam to give a total pressure. Therefore, the actual steam pressure and temperature of the steam/air mixture will be lower than that suggested by a pressure gauge.

Of more importance is the effect air has upon heat transfer. A layer of air only 1 mm thick can offer the same resistance to heat as a layer of water 25 μm thick, a layer of iron 2 mm thick or a layer of copper 15 mm thick. It is very important therefore to remove air from any steam system.

Automatic air vents for steam systems (which operate on the same principle as thermostatic steam traps) should be fitted above the condensate level so that only air or steam/air mixtures can reach them. The best location for them is at the end of the steam mains as shown in Figure 10.5.1.

The discharge from an air vent must be piped to a safe place. In practice, a condensate line falling towards a vented receiver can accept the discharge from an air vent. 

The discharge from an air vent must be piped to a safe place. In practice, a condensate line falling towards a vented receiver can accept the discharge from an air vent.

In addition to air venting at the end of a main, air vents should also be fitted:

• In parallel with an inverted bucket trap or, in some instances, a thermodynamic trap. These traps are sometimes slow to vent air on start-up.
• In awkward steam spaces (such as at the opposite side to where steam enters a jacketed pan).
• Where there is a large steam space (such as an autoclave), and a steam/air mixture could affect the process quality.

Other factors can be included in the equation, for instance, if a pipe is lagged with insulation providing a reduction in heat losses to 10% of the uninsulated pipe, then it is multiplied by a factor of 0.1.

Note: The constant 3.6 gives the answer in kg/h

Equivalent lengths:

• Pair of mating flanges 0.5 m
• Line size valve 1.0 m

Example 10.5.1

50 m of 100 mm pipe has 8 pairs of flanges and two valves, and carries saturated steam at 7 bar g. Ambient temperature is 10°C, and the insulation efficiency is given as 0.1

With reference to Table 10.5.1 and the application of Equation 10.5.1: determine the quantity of steam that will be condensed per hour:

Part 1 - Without insulation.

Part 2 - With the pipe insulated, but the valves and flanges are left without insulation.

Part 3 - Completely insulated.

Equivalent length of fittings:

• (8 pairs of flanges @ 0.5 m) + (2 valves @ 1.0 m) = 6.0 m of pipe
• Saturated steam at 7 bar g:

Part 1 - Without insulation:

Part 2 - Pipe insulated, but without insulation on the valves and flanges:

Consider the two elements separately:

Part 3 - Pipe and fittings insulated:

Relevant UK and International Standards

Symbols have been used to indicate, technically equivalent standards (=), and related standards (≠) respectively.

Summary

To summarise the ‘Steam Distribution’ Block of The Steam and Condensate Loop, the following checklist may be used to ensure that a steam distribution system will operate efficiently and effectively: