Doubt: Expansion tank, venting atm with control valve, (vent outlet underneath process) whyyy!!!??

Question

Doubt: Expansion tank, venting atm with control valve, (vent outlet underneath process) whyyy!!!??

SF6-146 (Chemical)

(OP)

20 Jan 22 10:54

Hello to all,

I have a small question. I have a proces as show in the pic:

We pressurise an expansion tank with Nitrogen to 300 mbarg. The vent outlet of the tank has a control valve at 400 mbarg because it is 1.5 m below the thermal oil process piping (1.5 m oil column is 0.100 mbarg).

I don't understand why we have to pressurise the tank to at least 1.1 bara and cannot have the outlet to the atmosphere as normal if the vent outlet is above the entire process.

Can you help me please?

MJCronin (Mechanical) · Answer 1 · 2022-01-20 15:04:03

I assume that this tank is upstream of your thermal oil pump .... Could we see more of your system ?

Could this have something to do with suction conditions at your pump ?

Does the pump have a history of poor NPSHa conditions ?

MJCronin
Sr. Process Engineer

LittleInch (Petroleum) · Answer 2 · 2022-01-20 15:57:04

We really need to see the whole system and it's heights etc.

Usually an expansion tank is pressurised sufficiently so that at the highest point in what I assume is a closed system is higher than 1 bara.

Hence why, if the expansion tank is below the level of the highest point, it needs to be pressurised to accommodate that.

I assume that the level in the tank is somewhere below the top?

Hence the height of the vent is irrelevant as all it contains is nitrogen or air.

As the fluid expands and contracts this design maintains the pressure between 300 and 400 mbarg. So your critical height is the bottom of the tank / lowest liquid level and the highest level of the system to maintain a positive pressure compared to atmospheric pressure.

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

MJCronin (Mechanical) · Answer 3 · 2022-01-24 22:29:56

Again, the original poster seems to have lost interest in his topic ...

I will request that the site administrator delete this thread in a couple of days ...

MJCronin
Sr. Process Engineer

MortenA (Petroleum) · Answer 4 · 2022-01-26 12:03:08

The PSV has a set-point of 0.5 barg - seems like its not a pressure vessel then? Maybe the design is just to prevent frequent lifting of the PSV?

--- Best regards, Morten Andersen

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Doubt: Expansion tank, venting atm with control valve, (vent outlet underneath process) whyyy!!!??