5 replies to this topic

[danyelson]

Hi everybody,

 

I want to ask you the following question:

 

We have in our plant a tank containing such kind of residual oil fractions (IBP > 150 oC and S.G. > 0.94, water content  max. 1%, atmospheric tank, two protection devices, one sample hole, steam coil present, MOT 75 oC, DT 100 oC).

At each 2 days we are pumping the product back to the unit, in order to recover the losses.

 

On 1st of September the pumping was started at about 70 oC and 6 meters for level height, and two days later, the pumping was finished, at 110 oC on the tank and at a level about 2 metres.

On next days, the temperature is continuing to rise up to 140 oC (probably the valve on the steam coil does not work properly), when, in a few seconds the tank roof was damaged completely, after one very strong and quickly explosion (no fire).

 

After, we verified the steam coil, that was not damaged. No possibility to introduce additional water to the tank (all the lines were verified).

 

Could temperature to determine such a high pressure, able to destroy the roof in a few seconds, at the lowest tank level, for this heavy product and for a water content at about max. 1%?

 

Thanks in advance.

[fallah]

danyelson,

 

Please provide complete info about the safety device(s) by which the tank is protected against the overpressure and undervacuum along with the operating info...

[danyelson]

Hi,

 

The tank is equipped with 2 breathing valves, one for inbreathing and one for out breathing, in fact, very old protection devices, never verified, with missing documentation. In the meantime, before the incident the sample hole in 8" was opened. The protection devices total orifice area is lower than 8", so that, I suppose that the safety devices weren't sized for this scenario.

 

Thanks.

[aanita]

Is it due to vacuum create as you mentioned level reduced to 2m. Provide a sketch to better understanding.A PFD will be more clear with design data. Recently I have seen an ammonia tank exploded and displace from the base location. Still under investigation, prime suspect was PSV not functioning. Please upload some pictures if possible.

[prakash23]

Temperature rise to 140 deg C indicates there is a possibility of either steam valve or steam trap passing even in closed position. If this is not the cause then following reasoning can be thought out.

 

Does the material stored in the tank contain unsaturated hydrocarbon.If yes,they may have polymerized  spontaneously releasing heat which caused temperature and pressure to rise and not released due to possibility of safety devices on the tank not got operated.

 

Implosion due to vacuum creation may be ruled out as

- Drop of 4 mt level was in two days ( sufficiently slow)

-  Incidence happened with rise in temperature not fall in temperature,i.e. with rise in pressure not drop in pressure.

[danyelson]

Yes, in fact the manual steam valve doesn't work properly, there was continuous steaming through the steam coil, and the temperature increase continuously.

There wasn't an implosion, the roof was pushed/bent to the outside.

For such a quickly incident, I think there are few possibilities:

- some accidentally cold water entrance into the tank (this was exclude - all the valves closed after pumping);

- the protection devices work improperly, leading to vapor accumulation inside the tank (there was a sampling hole in 8" open, and for a tank at 600 m3 I think 8" is higher than both protection devices area;

- vapor pressure of the mixture oil-water was increased dramatically at 140 C, attaining a pressure higher than DP of the tank (that is close to ATM), and pushed the roof to the outside (so strange, taking into consideration IBP for the product 150 C);

- water layer (1%) it was displaced, passed the oil layer and quickly vaporized, giving this effect (in fact, both phases are vaporized together);

 

Thanks