tank blanketing calcs

[koshyeng]

I am calculating amount of nitrogen required for one of our tank blanketing during gravity transfer to other tank. Flow rate of toluene is 180 US gpm. How should I calculate equivalent amount of nitrogen required for tank blanketing to maintain 15 psigin the tank. Nitrogen feed pressure is 30 psig.

Thanks for your kind assistance

 

[JimCasey]

Size the flow of Nitrogen for the maximum drain rate of the tank. You mention a flowrate of 180 gpm at 15 psi.
180 GPM is 24 ACFM, and converting that to standard would be about 48 scfm.

15 psi blanketing pressure seems extreme unless you are using the pressure to force the toluene forward in the system. I think that is called "Padding" instead of "blanketing". Typically "Blanketing" is just inserting an inert atmospher to prevent combustion, condensation, or ocntamination by atmospheric constituents. As long as the pressure is greater than atmospheric, this is accomplished.

When you pump toluene into the tank, where does the Nitrogen go? Typical blanketing pressure is 2-3" WC. That lets the blanketing gas be pushed through a conservation vent when filling, then the blanketing regulator senses when the vessel is draining and replaces the volume of toluene flowing out with Nitrogen.
At 15 psi your conservation vent can't be used and you will need an ASME Section VIII safety-relief valve, which "pops" instead of modulating. Probably will need a full control loop with a control valve, positioner, controller and a pressure transducer just to prevent overpressure when you fill the tank. You might have other concerns that force you to need this.

 

[786392]

I wonder what type of tank we are discussing? An AST made in accordance with API 650,then Normally the typical value of pressure is few inches of water and the scheme
must cater for sufficent vent(out) of blanketted vapour space
and sufficient Nitrogen inflow rate to avide negative pressures inside(vaccume) while pumping out.
hope this helps

Best Regards
Qalander(Chem)

 

[koshyeng]

My bad. I should I have furnished more details here.

This a ASME pressure vessel with a relief valve setting at 60psig.

When pumping toluene into tank, vent valve opens to a scrubber. Then we chare rubber crumbs in to the tank and dissolve it at 15psig prssure with chilled water circulating in the jackets. After dissolution, it is transferred to (gravity and pressure) a ractor. Initial dump rate is 180 gpm. During transfer pressure falls to 4 psigresulting in an increase in transfer time.

I would like to know how can I maintain padding pressure at 10 -15 psig during transfer.

Nitrogen line is a 3/4 inch maintained at 30 psig. upstream of regulator is 100 psig.

 

[JimCasey]

First: Apologies for my poor typing in the earlier post.

I ran your numbers thru a valve sizing program, and the N2 Regulator needs a Cv of around 3, based on the valve's geometric parameters such as Fl, Xt, etc.
A 1/2" self-relieving filter-regulator might just be perfect for this. You could pipe the self-relieving port to the scrubber as long as it stays at atmospheric pressure. Most such regulators have Buna-N seals, But my o-ring chart says you'd need Viton for Toluene.

If you have a couple extra inputs available on your DCS, just input the tank pressure and have the output drive a small control valve. A 3/4 Worcester CPT with a 60-degree V-seat looks to be sized appropriately, and all the elastomers are PTFE-or PEEK. I mention this valve becasue of zero leakage at shutoff, so the idle tank pressure won't creep up due to leakage. THen you could use a similar valve to vent to the scrubber.

 

[koshyeng]

Thank You Jim.

In the first post, you mentioned 48 SCFM of flow rate. Is it sufficient if I maintain Nitrogen flow rate at 48 SCFM to maintain 15 psig pressure in the tank? Please help me out. Tank vent valve is controlled with tank pressure set at 15 psig via DCS.

Thanks

 

[JimCasey]

>>48 SCFM of flow rate. Is it sufficient if I maintain Nitrogen flow rate at 48 SCFM to maintain 15 psig pressure in the tank? <<

48 scfm is about the same volume as 180 gpm at 15 psi.
So if you are draining the tank at 180 gpm, and introducing 48 scfm N2, the lost volume of toluene is replaced the pressure of 15 psi will be maintained.

If you control with a regulator, it will sense the pressure drop when you start to drain the tank, and will open to allow N2 in to maintain pressure. It should be a soft-seat regulator so that the N2 does not continue to leak in and overpressure the tank. Either locate it several feet back in the N2 line where there will be no toluene fumes, or order it with Toluene-resistant elastomers.