2 replies to this topic

[Leinster]

I was recently given the calculation for emergency venting as per NFPA 30 using a "manhole with long bolts" as per the below.

CFH = 1,667*Cf*A*√(Pt-Pa)

Where;
Cf = Flow coefficient of valve (set at 0.5)
A = Area of open manhole or area of manhole lift
Pt = Absolute pressure inside the tank (inH2O)
Pa = Absolute pressure outside of the tank (inH2O)

My questions which nobody has been able to answer (hopefully until now) are as per below.

1) Is the absolute pressure the pressure at which the manhole cover begins to lift or is it the design pressure of the tank or the maximum design pressure as set by Appendix F of API 650?

2) The orifice are of the manhole. Are these units meant to be in "inches" or "feet"?

Thanks in advance.

[Zauberberg]

Orifice area is expressed in square inches, as per NFPA 30.

As per my understanding of the paragraph (a), Pt should be the opening pressure i.e. the pressure at which the flow area is fully open.

[Leinster]

Many thanks for the response Zauberberg.

How would you calculate the pressure at which the manhole is fully open? The opening pressure is simple to calculate however I am not really sure where to go once the manhole has begun to lift.