Steam/Water Vapor Relief Distinction? 1

[Jefka]

API 520 (7th edition, Jan '00) provides a special equation specific for critical flow steam relief (section 3.7.1, eq. 3.8). This is obviously different than the normal critical flow equation for gases (section 3.6.2.1.1, eq. 3.2).

My question is as follows: if I have an ASME section VIII pressure vessel containing mostly water and I'm sizing a PSV for the "External Fire: Vapor Generation" case, do I use the steam relief equation 3.8 or would I use the normal sizing equation? Again, flow is critical.

Perhaps a dumb question, but it's caused some debate around the office here.

 

[skearse]

My understanding (from discussions with API committee members) is that you are "allowed" to use either. The steam equation is a little more conservative-I tend to use it when I have a water storage tank or similar that would be vaporizing liquid. The API training class slide I have has the statement "If desired, regular API vapor flow equation can be used with steam physical properties which can provide higher flow capacty."

 

[sheiko]

Jefka,

As far as i know, chemical engineers do not endorse the responsability of sizing the PRV nozzle. It is tipically the manufacturer...So, in my opinion, you can follow skearse advice and use the most conservative equation.

"We don't believe things because they are true, things are true because we believe them."