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# Calculating Vacuum on a blocked in tank

## Calculating Vacuum on a blocked in tank

(OP)
I am trying to figure out how much vacuum would be drawn on a tank of hydrocarbon that is blocked in. The scenario is the tank is ~90% full and has been isolated. The tank normally has a vent but in this case the vent is also obstructed. I am just interested to see what vacuum the tank would experience due to cooling from say 90F to 70F. I did a quick calculation using the ideal gas law (assuming the liquid stays a constant volume and only the vapor changes) and varying the pressure with the change in temperature and got a reasonable answer of ~14"wc. Does this approach make sense? Is there a better way to look at it?

### RE: Calculating Vacuum on a blocked in tank

So you assume worst case that the temperature of the gas phase drops faster than the liquid phase. But condensation might still take place. So you will have to check the vapor pressure at the given temperature and see if this is lower than your calculated pressure drop from the gas law.

--- Best regards, Morten Andersen

### RE: Calculating Vacuum on a blocked in tank

I agree with Morten - It's more of an issue to see if you have the same mass of gas in there.

But even 14" WC will collapse a normal tank.

You could try using Hysys or similar to simulate this.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: Calculating Vacuum on a blocked in tank

hi,
To me you need to consider to install a Nozzle with a rupture disk or PVRV to prevent your tank to collapse . Check for hot tapping procedures and contractors to support .
Good luck
Pierre

### RE: Calculating Vacuum on a blocked in tank

(OP)
Thanks for the replies... the tank has a normal vent. We are doing a risk assessment and this would be a special case when someone makes a mistake and the tank is full but has no vent.

If i were to use the vapor pressure is it just as simple as assuming the tank is at atmospheric pressure when it is isolated (it is). Then, look at the vapor pressure difference between T1 and T2 and subtract this number from the atmospheric pressure to determine the expected vacuum on the tank?

### RE: Calculating Vacuum on a blocked in tank

Week I think for your case you're t trying to be too precise.

Basic issue is what sort of vacuum can the tank withstand?

It is often very very low so it matters not what your answer is, if it can happen, what can you do to stop it collapsing your tank.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: Calculating Vacuum on a blocked in tank

Attaching a snip of your P&ID showing the tank would go a long way to understanding what you are doing. For example, "the tank has a normal vent" is not very specific.

Good Luck,
Latexman

### RE: Calculating Vacuum on a blocked in tank

Jeff, for the hypothetical that you're analyzing, the tank will almost certainly fail. Furthermore, vacuum failure is also the outcome for almost any hypothetical case you can think of, if the vac relief is blocked in. As has already been stated, low-pressure tanks have a shockingly low tolerance for vacuum. So rather than doing this type of analysis for hypothetical vacuum scenarios, you're baseline assumption should be that the tank will experience vacuum failure if the vacuum relief is blocked in.

About 6" WC vacuum is all it takes to suck in the majority of low pressure tanks.

### RE: Calculating Vacuum on a blocked in tank

@OP

I guess its difficult to give so precise answers when you are looking at a specific case that we have data on. But i agree with your general approach

1) Initially air and some hv vapours at T1
2) closed vent and temperature drops to T2
3) wichever is lower
a) gas law
b) condensing a fraction (calculate difference in vapor pressure of HC) difference must be the drop in pressure since the noncendensables will just remain

Alternatively as Don says, assume damage to tank if the vent gets closed. So maybe remove the block valve from the vent line or install a vacuum breaker?

Best regards, Morten

--- Best regards, Morten Andersen

### RE: Calculating Vacuum on a blocked in tank

Dear Jeff,

although your tank as stated by you is liquid full which means it has a buffer inventory heat sink which stiffens the pressure inside the tank, however, vacuum development is all about the quenching of the vapor space due to : 1) tank is subject to significantly cold ambient conditions (initially hot tank exposed to intense cold rain conditions as stated by API 2000 7th edition) 2) pump out from the tank with no pump in which is mainly under shutdown conditions of the upstream facilities, again as explained by API 2000 section 3.4.

However, no credit shall be taken to the liquid presence in the tank as the tank can be isolated during maintenance and left out empty filled with condensable/or non condensable gases. As per API 2000, an open vent can be considered as a vacuum relief however for hydrocarbon containing tanks where the risk of flammable mixture being present and the fluid's flash point is less than 60 C, consideration to providing flame arrestor to protect the tank against the consequences of internal explosion can also be considered.

I don't support the simplified approach of estimating the vacuum development inside the tank based on only the temperature difference of the vapor space in the tank as other multiple factors can be involved such as ambient weather conditions and thermodynamic considerations which were analyzed by BS14015 and ISO 283000 and further considered by API 2000 in their latest 2014 edition. Hence, the methodology presented by API is recommended and can be optimized with consideration to the scenarios involved. Additionally, relying on open vent for vacuum relief is associated with significant product loss and the use of pressure vacuum safety valve is warranted to minimize the same

Regards,

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