In any steam plant or any process plant effectiveness of steam distribution system is dependent upon the project specific conditions like location and layout of the process plant and its steam consuming equipment like heat exchangers, decorators etc.
Steam distribution circuit is one of the major link between the steam production point and the point of end use i.e. process plant. Primary steam generating source are co-generation plant and Steam generators. However it not the source of steam generation but the effective and efficient steam distribution system that decides right quality (pressure and temperature) and quantity of steam to reach to the process through it.
Thus designing of steam distribution is to be given due importance along with installation and subsequently maintenance during operation.
In the above shown Steam distribution network steam carry pipes are the major contributors factors in deciding the cost and the quality of steam reaching the process.
The success of any steam distribution system is depend upon meeting the end process steam pressure requirement of the process plant. Thus distribution system is influenced by the factors like:
- Steam Generating working pressure.
- Minimum pressure requirement at process end.
From the generating point to the user end, steam has to travel through the distribution network which comprises of steam pipe and other fittings and thus this steam have to undergone through the unavoidable pressure lose due to:
- Pipe network frictional-resistance.
- Condensation in the steam piping due to transfer of steam heat to the outside environment.
- Therefore necessary compensation should be kept for the above pressure loss while deciding the “Maximum working pressure of the boiler”.
Advantage of Steam Generation at High Pressure
Following are the advantages of steam generation at high pressure:
- Steam generation at higher pressure occupies relatively less volume than a lower pressure.
- Steam generated at higher pressure requires smaller size steam distribution lines.
- Higher steam parameters improves the quality of steam
- Smaller diameter steam lines require lower capital investment on pipes, flanges, supports and insulation.
Disadvantages of Steam Generation at High Pressure
Because of the high steam pressure following losses are increased:
- Flue gas losses
- Running losses
But the fuel consumption of the boiler is depend upon the connected load and not on the boiler output whether the boiler steam pressure at 4 bar(g), 10 bar(g) or 100 bar(g).
Generating and distributing steam at higher pressure, also requires necessary steam pressure reduction to suit the process steam pressure requirement at the point of use.
Pressure Reducing Valve (PRV)
In the above pressure reducing station separator is installed upstream of the reducing valve to prevent entrained and ensuring high quality steam to pass through. Safety valve on the down stream side protects the system, if PRV doesn’t work.
Sizing of Steam Distribution Piping
Steam piping plays a important role in meeting the end requirement of the process. Following are the critical aspects of this:
Proper steam pipe sizing to avoid any over or under-sizing of the steam mains.
Consequence of Over-Size Steam Pipe
- Results in larger diameter pipes, valves and other fittings and shall be more expensive.
- Requirement of increased pipe support, insulation etc.
- Cost of erection and installation shall be more.
- Cost of 80 mm pipe installation accounts for nearly 45% higher than the cost of installing 50 mm.
- Bigger size steam pipe generates more condensate and results in requirement of more steam trapping or else wet steam shall be delivered at the point of use .50 mm.
- Similarly heat loss from the insulated pipe (80 mm) pipe work shall be approximately 20 % higher than that of 50 mm pipe.
Consequence of Undersized Steam Pipe
- When the line size is undersized then we get the lower pressure at the point of use. So it the due this the performance of the process and equipment may suffer.
- Possibility of steam starvation can’t be ignored.
- Chances of greater risk of water hammer, erosion and noise due to under size steam pipe size.