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Why Restriction Orifice is some distance from Blowdown

valve ?

Sunday, October 14, 2007

Very basic does not imply obvious...i really like this statement. Very often the answer is very

fundamental and basic to us, somehow it always does not obvious to us.

Lets look at the above sketch. It is a typical arrangement for a Blowdown Valve (BDV) with

Restriction Orifice (RO) , located downstream of the Blowdown valve. This arrangement

typically used to depressurized a 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 ?

Is this spool for straightening and smooth flow ?

Is this spool to avoid secondary choke ?

Guess what ?

Reasoning...Major pressure drop will take place at Restriction Orifice, downstream of

Blowdown valve during blowdown. Joule-Thompson (JT) effect results fluid temperature

downstream of Restriction Orifice drops below subzero (Less than zero degree Celsius).

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. Moisture from atmosphere will freeze at the

Blowdown valve body and potentially cause the stem stuck at position. Operator may not

possible to close the Blowdown valve after blowdown activities and may potentially lead to

back flow. Those "general good engineering practice is to locate Restriction orifice, 600mm

downstream of Blowdown valve.

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Useful Documents Related to Pressure Relief Valve

(PRV) - Part 1

Pressure Relief Valve (PRV) has been widely use in

overpressure protection of pressure containment

equipment. Regardless of vessel/column/drum/tank

designed in accordance to any pressure vessel code

our environment ? I m sure many of us.......

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Chemical & Process Technology: Why Restriction Orifice is some distance... http://webwormcpt.blogspot.com/2007/10/why-restriction-orifice-is-some...

1 of 3 8/1/2010 10:53 PM

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posted by Webworm, 7:21 AM

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