FV versus Half FV for BPVD?

[salehy]

Hi Guys,

I wanted to know if it is indicated any guidelines in ASME boiler and PV codes regarding the use of full vacuum or half vacuum for the design of a PV or a HX, as a general thumb of rule normally, we specify external full vacuum for the steam out condition, so when would considering half vacuum be more realistic and save costs? please guide

 

[doct9960]

salehy,

For ASME VIII div. 1 or 2 vessels, the Code just says that you should design your vessel to withstand external pressure or any coincident pressure and temperature during normal operation. The Code does not provide any guidelines on when to use full or half vacuum. The decision on whether to use FV or HV is up to the User or his designated agent. Process engineers are usually the ones who makes the call.

In most of the projects I worked in, process criteria for vacuum or partial vacuum is...
FV - for vessels having steam as a signifacant component during normal operation (e.g. amine regenerators)
- for vessels operating under full or partial vacuum during normal operation
- for vessels that require steamout as part of routine operation
HV - for vessels that require steamout during maintenance only and does not have steam during normal operation

 

[salehy]

Doct9960,

thanks for your elaboration, we are using the FV condition, in fact, for both cases, even when steamout si only during maintenance. I was wondering if in such cases when steam out is not used on the regular and continues operation, altering the design condition to have HV, would save some bucks.

 

[jte]

I was wondering if in such cases when steam out is not used on the regular and continues operation, altering the design condition to have HV, would save some bucks.

Yes, why worry about full vacuum when it only collapses the vessel during maintenance and not continuously...

Now, if the steamout is controlled with adequate venting etc - and with no possibility that one man turning the wrong valve can kill the venting - then a half vac design makes sense. But that is what the process engineers should be evaluating in conjunction with the owner/user representatives.

jt