Sunday, November 23, 2008
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A reader MT (abbre.) has read "Why Restriction Orifice is some distance from Blowdown valve ?" and drop me a note. The question are more-or-less as follow :
i) For cases where huge pressure drop is not foreseen or low temperature is not expected downstream of restriction orifice (RO), is it still necessary to provide the minimum distance ?
ii) If the Flare header is Stainless steel (SS), and the process piping is Carbon steel (CS), the Spec break between CS and SS will be shown at the blowdown valve (BDV), i.e. BDV is designed for the most conservative material - SS. In this case, i do not seen the reason of the minimum distance.
The intention of the proposed arrangement in "Why Restriction Orifice is some distance from Blowdown valve ?" is to avoid the BDV stem stuck at position in case the depressured fluid temperature is dropped to sub-zero (below zero degree Celcious).
i) In case low pressure drop and the depressured temperature is still higher than sub-zero, this requirement is NOT necessary. As long as there is NO risk of fluid temperature drop below zero degree Celcious, then this arrangement is not required.
ii) The process piping upto BDV (excluded BDV) is CS and from BDV onwards is SS. This is a good arrangement with good spec break. The 600mm requirement is mainly related to frozen of moisture content (below zero degree Celcious), it has no/less relationship with the material. Thus, as long as the depressured temperature can drop below zero degree Celcious, regardless what the material of construction (MOC) is, the 600 mm arrangement still applicable.
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