Leak at LPG storage inner tank

Annulus pressure?

I always thought the annulus was kept at atmospheric pressure and any rise in pressure indicated a leak.

Can you safely vent the space to see if it pressrises back?

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

Thank you for your suggetions.
it's a new LPG tank and it's the first filling with LPG liquid.

The annular space temp of the tank is equal to inner tank temp.

The contractor proposed to perform one of the two techniques to confirm that there is or no leak on inner tank explained below:

1. to check the variation of liquid level inside inner tank during a period.
2. to empty the tank, for internal inspection.

which technique is better?
For time being I have no other infromation.

Thank's

Doing troubleshooting with such limited information is very poor practice, and it will almost certainly bring no results. I sympathize with you, but there must be a way to collect key information/documents, and do some field measurements.

You need to find out where the temperature sensor is physically located (perhaps installed too close to the inner wall) and verify readings of other transmitters in the annular space.
Next, look at the drawings and see if there is any purge (usually N2) through the insulation between the two walls, and where this purge is routed (flare or vent), and get some samples for gas composition.

Dejan IVANOVIC
Process Engineer, MSChE

Until you find out or give us more information about the tank, specifically what is in the annulus and what sort of process measurements you have available there is not much more I can advise.

The whole point of a double skin vessel is that an leak in the inner tank is contained, but also that you are aware of it by means of pressurisation or other sensors within the annulus.

Both "techniques" you mention seem quite poor. There should be no guesswork about whether you have a leak or not.

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

I figure something like this might offer some insight.
claimed 50 ppm

seems to claim 1 ppm

Keep in mind that the tank contractor is motivated to not find a leak.

1 seems insufficiently accurate to rule out a leak, and offers no information about the location if it does suggest that there is a leak.

thank you for your reply and advices

you can find herein attached two documents: one related to sensors position around the tank and second document is related to temperature readings from 20 may 2015 3:00 PM to 27 may 2015 12:54 PM.

temperatures readings file attached

Tommy,

Your sketch shows "LNG". Is it LNG or LPG in question? It makes a whole lot difference with regards to evaporation temperatures.

Secondly, what is the storage temperature? What is the temperature of the liquid phase inside the tank?
Moreover, is there any Nitrogen purge through the insulation between the walls?

Your Excel file shows that all 8 temperature sensors are showing the same value, and according to the sketch they are distributed inside the perimeter of the outer (concrete) tank wall, and closer to the outer wall. I don't think this indicates a leak.

Please reply with the requested information.

Dejan IVANOVIC
Process Engineer, MSChE

Refrigerated LPG would be at approx -35degC, while LNG would be at approx -160degC for what looks like an atmospheric storage tank. In either case, if the annulus temp is -3 to -4degC, that is still miles away from -35degC, so is there a problem?