Following is a typical arrangement of blowdown pipe arrangement consisting blowdown valve (BDV), restriction orifice (RO), and downstream piping. It is recommended to maintain a minimum length of spool pipe b/w the BDV and RO.
This arrangement typically used to depressurized a system inventory to a safe level within a limited time to minimized catastrophic scenario. Between the Blowdown valve and Restriction orifice, there 600 mm spool piece between them. What the main purpose of this 600mm spool piece ?
Reason: There will be Joule Thompson (JT) effect at orifice due to hydrocarbon fluid expansion typically but to a high pressure let down. Generally the JT effect will produce a sub zero temperature (below 0 C) at RO and during the blowdown process low temperature will propagate to the BDV causing moisture from atmosphere will freeze on outside of the Blowdown valve.(Latent heat in the piping will transfer to cold fluid and slowly approaching this subzero temperature. The "coldness" will travel back to upstream of Restriction orifice and probably reaches Blowdown valve. It potentially cause the upstream Blowdown valve body temperature drops below subzero as well.) Operator will not be able to close the BDV after the blowdown and it could also cause backflow creating potential hazardous situation. Those "general good engineering practice is to locate Restriction orifice, 600mm downstream of Blowdown valve.
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