NFPA 30 Week Roof-to-Shell Seam 1

[Leinster]

Table 22.4.1 (a) refers to a week roof-to-shell seam. To have a week roof to shell seam do you have to comply to the frangible roof joint as per API 650 or can you just have a week roof to shell joint by grinding portions out?

My second question.... Are you allowed to fabricate an emergency vent with a manhole with long untigtened bolts? If so how would you calculate the venting capacity of the actual manhole to suit the requirements set out in API2000?

Thanks

 

[stookeyfpe]

To comply with NFPA 30, the weak roof-to-shell seam design must comply with API 650. I recall that API changed the rules several years ago to prohibit this type of construction on tanks < 40 feet in diameter (and I'm unsure if this is the correct value) because the uplift forces under emergency venting conditions could cause separation between the tank shell and tank floor. You need to review API 650 before proceeding with such a design.

Regarding question 2, NFPA 30 requirements for emergency vents require it to be evaluated by an independant third party. If you choose to use a long-bolt emergency vent (which is allowed), it will require evaluation to be compliant with NFPA 30. As to the calculation method, the equations are clearly defined in API 2000.

 

[psafety]

I've been involved in small process tanks that definitely were NOT compliant with API-650 (which I thought was limited to 0.5-psig. NFPA-30 mentions several scenarios of different construction methods that should comply, but also up to 2.5-psig which (I'm not in refinery business) but I thought exceeded 650 guidelines.

I think I remember using the guide of the bottom should be 1.6 or 1.8 times the design pressure of the top. Which I think that parameter was from 650 frangible design.

Use an example of a 12-ft diameter vessel, no way to meet 650, but many of the principles could be applied.

 

[Leinster]

Could somebody please advise how one would go about doing the calculations to prove that an emergency vent designed using a manhole cover with long bolts will be able to vent as per the requirements.

In the attached calculation we have proved the pressure at which the manhole cover will begin to lift but cannot prove that it will be able to vent the total amount and at what pressure it will vent the total calculated amount.

Any assistance would be appreciated.

 

[IFRs]

If you know the pressure differential across the PV vent then with some basic assumptions (basically choose a factor for entrance and exit losses) and the open area (circumference x cover lift) you can figure the flow. Also, API 650 now has frangibility criteria for self anchored tanks smaller than 50 feet in diameter.

 

[chicopee]

Refer to the JPEG attachment to answer your second question. Reference is included.

 

[Leinster]

That's excellent... Thank you very much chicopee.

 

[Leinster]

Chicopee...

I just have a couple of queries regarding your calculations.

1) Are the units constant throughout ie. is the manhole area to be in in2 or ft2?

2) Absolute pressure inside tank... Is this the maximum design pressure taken from API 650 Appendix F or the pressure at which the manhole will begin to lift?

Look forward to your response.

 

[Leinster]

I know this is quite frowned upon however I would really like someone to be able to assist me in my query for emergency venting capacity based on the calculation below.

CFH = 1,667*Cf*A*?(Pt-Pa)

Where;
Cf = Flow coefficient of valve
A = Area of manhole or area of manhole lift
Pt = Absolute pressure inside the tank
Pa = Absolute pressure outside the tank

I would like to know:
1) Are the units constant throughout ie. is the manhole area (A) to be in in2 or ft2?

2) Absolute pressure inside tank... Is this the maximum design pressure taken from API 650 Appendix F or the pressure at which the manhole will begin to lift?

Any advice would be greatly appreciated.