Atmospheric Storage Tank Relieving & Overpressurization

[tankerator]

Hello, I’ve got a question on over pressuring of a tank. I have a small (app. J, API 650) tank that requires a N2 blanket. According to the company standards, our regulators must fail open in order to keep the blanket. The design condition that I’m getting off of the old valve datasheets is 200 psig. That poses an issue for this tank, which is only designed for 1.5”WC. The flow is not horribly high at 1200 scfh. The tank has a small, 4” pressure/vacuum vent that is set for about 1”WC.

At first I thought I could treat the N2 as an ideal gas (neglecting temperature changes, auto-refrigeration) and set P1*V1 = P2*V2. Taking the length component of volume to be equal and reducing the equation to

P1*A1 = P2*A2

Using
P1 = 200 psi,
A1 = 0.864 in2 (area of 1” pipe coming in to tank),
A2 = 12.73 in2 (area of 4” outlet/PV vent)

I still get the pressure equaling about 13.6 psi which is obviously too much for the tank.

What this analysis does not include is the flow rate. Say, for example, there is a pinhole orifice that is leaking a small amount into the tank, but the pressure upstream is 200psi. The vent would obviously be able to handle that because the flow is so small. How do I incorporate the flow into the analysis? Is this a Bernoulli’s equation based problem?

Also, does anyone know where I can find N2 tables?

Thanks for the help.

Matt

 

[StoneCold]

Tankerator
There are a couple of things you could do. You could add an orfice plate down stream of the regulator to reduce the flow if the regulator fails. Assuming the orfice will allow enough flow for normal service. You could also two stage the situation. Say make the outlet of the first regulator 10 psig. Install a relief valve in this piping at maybe 20 psig. Then the max differential accross the second regulator is only 20 psi instead of 200 psi and you will have reduced the fail open flow considerably.

Regards
StoneCold

 

[JohnGP]

You must consider flow rate. If the max flow is 1200scfh, then the vent must have at least the capacity to pass 1200scfh with a driving head of 1.5"WC, otherwise pressure will increase above the tank design pressure.

If vent capacity is deficient, the internal pressure will increase intil it reaches a value that is sufficient to drive 1200scfh through the vent.

You should also check the N2 supply. Is the 1200scfh a constant - i.e. at that flow does the upstream pressure remain at 200psig, or does it diminish and thereby reduce flow of N2 over time?

Just a couple of thoughts.
Cheers,
John

 

[tankerator]

Thanks for the help, guys. Gases aren't as intuitive as incompressible fluids (m_dot_in = m_dot_out). I am still going though API 2000 but it seems like the vent calcs and tables are more for fire cases.

We are exploring the possibility of an orifice plate and relief vent on the inlet line.

The flow I listed is actually the max, design flow. It will probably be about half that (which isn't much) but I needed to size for the max case.

Thanks again.