At present time, my company intent to distribute/utilized the steam which generate from Co-Gen Plant to several facilities.. The Steam pipelines app.length is 20 km.
Need your advise regarding the piping code should be applied/refered..
B31.4 ??? or
B31.8 ??? or
B31.1 ??? or
else ???

due to steam's high corrosive effect on piping material and the temperatures reached, you should refer to 31.1 (power piping). However, i am not sure if the length you refer to could be within the scope of 31.1. One way is to separate the whole pipeline into smaller sections between the valve(Forged Steel Valves) stations - if any - and thus have smaller power piping parts.

I think you should apply B31.1 to your design.  B31.4 and B31.8 are primarily buried piping Codes and I think you will want your steam and condensate systems to be above ground (regardless, you will not want to bury the steam piping directly in the ground).  The B31.1 scope include "District Heating" piping and this is similar to your steam distribution application.  When steam distribution piping is placed underground it is common practice to place the piping in "steam tunnels" so that it can be accessed for maintenance.  Another option is to use a proprietary pre-insulated "conduit encased" modular piping system:

However, these systems require some experience to design and some special skill in laying out the expansion loops.  It is important to look closely at the internal pipe supports in these systems and to be sure that there is room to expand and contract near in in the expansion loops.  Also the placement of intermediate anchors is critical and the internal pipe expansion at bends must be considered.  The outer conduit may also have some amount of expansion/contraction and it might be necessary to place non-resilient foam blocks around the conduit at bends so that the soil will not interfere with the expansion/contraction of the outer conduit.

It is not a very common practice for our power plants to "sell" steam to other industries but I know of (and have worked on) several cases where this is done. For example, in Iowa one of the power plants sells steam to several large scale food processors. Some of these steam distribution piping systems are 5 miles long and longer.  The ASME B31.1 Power Piping Code was applied to the design, fabrication, erection and testing of these systems. In one case the condensate is returned to the steam plant and in other cases the condensate is not returned.  In the steam-water circuit, there are gas-to-vapor-to-liquid (condensate) phase changes and this causes the condensate to (over a period of time) become highly oxygenated and very corrosive.  In condensate piping it is important to treat the liquid such that its corrosive nature is reduced.  In very lengthy steam distribution systems there will be some amount of condensation and it is important to install steam traps to remove the liquid from the steam distribution piping.  If condensate is allowed to form "puddles" in the steam line there is a danger of "slug flow" incidents and those events may be very damaging to the steam piping system.  If the condensate is to be returned to the steam plant in a parallel piping system it may be necessary to install "heat tracing" under the condensate piping insulation to prevent the condensate from freezing on its return to the steam plant.

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