Desiccant breathers for outdoor storage tank

[Orga78]

Hi,

We have an outdoor storage tank with chemical, equipped with N2 padding system and Desiccant Breathers at the tank's vent line.

Since the N2 padding works as barrier to prevent outside contaminants and moisture ingress into the system, appreciate if you can share your thoughts on how important it is for the desiccant breather to operate at its highest efficiency in filtering and drying the air?

Will it be enough to have only the N2 to do the padding/blanketing the tank's volume to have the system "sealed" from contaminant when the tank is breathing (assume the desiccant bag has saturated)?

Thanks!

 

[mk3223]

IMO, it may be an environmental concern and requirement to have a venting with a desiccant breathers. And, it should have a maintenance procedure for changing the depleted desiccant .

 

[Orga78]

Thanks mk3223.

What is the recommended interval or best practice to replace the desiccant? I've heard different opinions from different vendors - replacement after 1 year of use, +30% of saturated weight, +40% of saturated weight.

Another thing is, with N2 padding active, and given my desiccant is already saturated, what is the likelihood of wet air ingress into the vent line when tank is breathing (inhaling)? Will the N2 barrier be able to prevent it?

 

[mk3223]

Sorry, I don't know the replacement timing of desiccant. The vendor may provide a better guideline based on your process condition and the desiccant material.
IMO, if the N2 padding is active all the time, the likelihood of the wet air ingress into the vent is the same no matter if the desiccant is saturated or not. The question is how much in a given time.

 

[LittleInch]

Can you tell me what N2 "padding" is please. Blanketing I know which requires a small positive pressure at all times but if you have a desiccant breather this and the comments implies a normal vent.

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

 

[Orga78]

Thanks mk3223. We are using Des Case desiccant breathers. Local vendor recommended 1 year of service life or 30% of + saturated desiccant bag's weight whichever comes sooner. Yes, our N2 padding is active, all the time, fully-automated.

LittleInch, my understanding is N2 padding is synonymous to "blanketing", pls correct me if I'm wrong. The function is the same, to fill the empty column in the tank wrt materials' volume in the tank. As you mentioned, it gives + pressure to the vent line, and it does blanketing job as well to prevent contaminant ingress into the tank.

 

[LittleInch]

Well that's why I asked. Blanketing normally works on the basis that there is no normal inbreathing of outside air as there is a positive pressure of your blanketing gas at all times. This positive pressure is usually controlled by a pressure control valve otherwise you spend a fortune just pumping Nitrogen in to have it vent out into the atmosphere.

Hence I just don't understand why you have dessicant breathers unless this "padding" is just someones bright idea to dump a bit of Nitrogen in there and have it magically sit there and not just vent out or gradually mix with air as the tank level changes.



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

 

[Orga78]

Thanks LittleInch. Yes, you're right. There is a pressure control valve to control the amount of N2 injected into the tank when level changes.

See below for the vent line config of the tank:

N2 ---- Vent Line ---- Carbon Filter ---- Breather Valve ---- Desiccant Breather ----> atmosphere
|
|
Tank

N2 supply is connected to the vent line, and goes to inject N2 into the tank for blanketing/padding. Tank's breathing system is equipped with carbon filter + breather valve (pressure/vacuum relief valve) + desiccant breather.

From the above system, is the N2 blanketing effective to function as barrier from outside air ingress during tank in-breathing via?

 

[LittleInch]

That looks quite odd.

Most blanketing systems consist of an inert gas feed with a pressure regulator set at a certain pressure, say 20 mbar, with sufficient capacity to exceed the volumetric outflow from the tank.

They then have a pressure relief system set at say 30 mbar to relieve the pressure caused by inflow into the tank and are sized to restrict the pressure to a certain level based on maximum inflow into the tank.

Your system seems to be some sort of strange retrofit and I really don't understand what the desiccant breather or carbon filter are doing other than acting as inflow / outflow in the absence of the Nitrogen.

Most systems incorporate some sort of emergency / high pressure relief vents to prevent tank collapse or overpressure, but these are not normally in use.

You seem to be neither in one camp or the other here...

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

 

[mk3223]

In a LNG facility, I saw the setup of the tank N2 blanketing with a vent line and back pressure CV, but without carbon filter or desiccant. The vent line is 3m high in vertical to atm. I believed it's part of the PHA requirement to have additional vent line instead of discharging through the PVRV.

 

[Orga78]

There's a discrepancy in my previous post. I put [spaces] so the tank line will be below Vent Line. Instead it goes to the 1st line of the 2nd paragraph, makes tank connected direct to N2 (after I submitted my previous post).

It's supposed to look like this:

Atmosphere
|
Desiccant Breather
|
Breather Valve (PVRV)
|
Carbon Filter/Flame Arrestor
|
Vent Line <----< N2 blanketing supply
|
Tank

N2 blanketing operates as per normal, to fill in the empty column of the tank wrt tank's volume during pump-in & pump-out.

Breather valve (PVRV) controls the tank's pressure during in-breathing and out-breathing.

Desiccant Breather to reduce odor, filter the vapour from tank release to atmosphere.

Is the N2 blanketing effective in keeping the tank's content from contact with incoming air during in-breathing via PVRV (Breather Valve)?

 

[mk3223]

Is the N2 blanketing effective in keeping the tank's content from contact with incoming air during in-breathing via PVRV (Breather Valve)?

IMO, it may depend on N2 blanketing flow rate and the PVRV vacuum pressure set point. The PVRV will be open if the N2 blanketing flow is inadequate during the liquid pump-out.
Where is the desiccant located in the PVRV, i.e. the pressure outlet or the vacuum inlet?

 

[Orga78]

Where is the desiccant located in the PVRV, i.e. the pressure outlet or the vacuum inlet?

Desiccant breather at + pressure outlet of PVRV.