Emergency Vent with Long Holes 1

[Clamont]

I really apologise but I posted the below post over a year ago and did not get a response... I am now in need of the answer again and I am hoping that someone will be able to assist. I apologise if this double posting is frowned upon but it was such a long time ago that I figured it would not be such a problem.

Here goes;

I was recently given the calculation for emergency venting as per NFPA 30 using a "manhole with long bolts" 22.7.3.2.1 (2) 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 is as per below;

1) Is this calculation correct and where can I verify it?

2) How do you determine the pressure when the manhole cover is in the fully open position as required by NFPA 30 22.7.3.10 (2). I can easily calculate the opening pressure however do not know how to calculate the pressure when fully open assuming I had a predetermined length of bolt.

I look forward to replies.

 

[stookeyfpe]

Clamont:

What you are proposing is to utilize a tank manway as an emergency vent under NFPA 30. These are commonly employed on large shop-fabricated tanks and on moderate sized API 12F series ASTs.

I'm confused about your question 1. The equation (it's not a calculation) is found in the section you cited in NFPA 30. No calculation was presented, so I'm unclear of what you're attempting to ask.

Concerning question 2, the height that the manway lifts will establish the vent area. UL 142 Section 8.1 requires that when long-bolt manway emergency vents are used, the bolts be installed such that it has 2-inch of free space to allow for the lifting of the manway. UL 142 also required that every other bolt opening be left unfilled by a bolt.

I am not a big fan of long bolt emergency vents. They can be easily defeated and depending on the thickness of the manway plate, can deflect and warp due to freeze/thaw, which can allow water inside the tank. If possible, I prefer a pallet type emergency vent such as those manufactured by Clay & Bailey, Morrison Brothers or OPW.

 

[Clamont]

Stookyfpe,

Thanks for your reply. I was given the equation by someone else on querying how to calculate the size of the manway. If this equation is not correct what is the correct equation if trying to calculate the maximum flow of the manway when fully open? Calculating the pressure at which the manway covers begins to open is simple however the maximum flow when fully open is what I am after.

Thanks

 

[IFRs]

Max flow can be approximated by knowing the pressure across the vent ( which you know and is the lifting pressure ) and the area of the vent when fully open ( which you also know and is the max lift times the inside circumference of the nozzle ). Assume a conservative flow coefficient ( perhaps 0.60 ) and use classical flow equations ( Bernoulli ) to calculate the velocity across the hole and then with the area calculate the flow ). If you are not near the required flow, don't bother to refine your calculations, get a bigger vent or install more.

 

[chicopee]

That formula was probably taken out the Flammable and Combustible Liquid Code Handbook published by the NFPA. The attachment shows two pages from that NFPA handbook relevant to your formula.

 

[IFRs]

It is in my opinion necessary to understand the basis of even the simplest of equations that you use so you can exercise good engineering judgment on the results and understand the units used. This equation is easily derived from Bernoulli's. The constants are density and flow coefficient, both of which come with some units that were used to derive the numbers. This is a basic flow formula easily dissected and understood. Good luck and hopefully you will glean a proper understanding of the theory and application behind the formulae.