7 replies to this topic

[mtlmoody]

Dear All,

 

I am working on a NGL storage proj which is a part of frac train debottlenecking proj where it was suggested that the 140 m^3/hr NGL feed (Dethanized liq) should bypass frac train and be stored in the existing spherical storage (Dia =16.69m, 690 kPag design pressure) during shutdown/commissioning time of frac train. 

 

The vapor pressure of NGL that enters the storage is 517 kPaA @ 14 deg C, whereas the design pressure and PSV set P =690 kPag (780 kPaA). Tank is uninsulated and during the time of bypass the ambient air temp may go up to 31.7 deg C on a certain day. 

 

 I simulated the stored NGL temperature as 26 deg C when its vapor pressure will reach 90% of set pressure and PSV may start to lift at that point. After reaching here, I took the unsteady state heat transfer calculation route to solve this.

 

M*Cp*dT/dt = U*A*(Tair -T) ..(neglecting flow out of the sphere for being conservative, i.e. 200 m^3/hr)

M= stored mass at 90% full vessel, density =540 kg/m^3.

Cp = average mass cp, based on 14 deg C and 26 deg C.

T = storage temperature at time "t" , deg C

t = time, hr=?

U = 26.7 W/m^2-Deg C (4.7 BTU/hr-ft^2-deg F), based on the Aerstin and Street article at wind velocity of 10 mph. for uninsulated tank. I think I can rigorously resolve U from Kothari article as well to double check value.

Volume of liquid at 90% vessel height is 2364 m^3.

 

The equation above is conservative as I didn't include NGL entering and leaving the storage. In my opinion, this vessel should have an insulation cuz if the mass M is small, then it will reach PCV set pressure before it lifts PSV in less than an hour. The liquid is pumped out at 200 m^3/hr.

 

Question1: MCp dT/dt = U*A*(Tair-T) - M(in)*Cp*(T-Tngl)-M(out)*H, where H is enthalpy of liquid pumped out at different T, but it doesn't change a lot, can I take it as constant?

 

if the above equation in question 1 is set up right to include all heat sources and could you guide me how to solve them or a simpler alternate solution.

 

All criticism, help and advise are welcome. Thanks in advance.

 

[PingPong]

it was suggested that the 140 m^3/hr NGL feed (Dethanized liq) should bypass frac train and be stored in the existing spherical storage (Dia =16.69m, 690 kPag design pressure) during shutdown/commissioning time of frac train.

Does this mean that you start with a (nearly) empty sphere, fill it up at a rate of 140 m³/h until it is nearly full while the frac train is down, and then pump it out to the frac train when it is taken back into operation? If so, then there is no simultaneous fill and withdrawl.

 

But what does this then mean:

(neglecting flow out of the sphere for being conservative, i.e. 200 m^3/hr)

Is that simultaneously with a fill rate of 140 m³/h? And why is it bigger than that 140 m³/h?

 

 

Question1: MCp dT/dt = U*A*(Tair-T) - M(in)*Cp*(T-Tngl)-M(out)*H, where H is enthalpy of liquid pumped out at different T, but it doesn't change a lot, can I take it as constant?

I don't think that formula is correct, even when ignoring solar radiation and difference in U value for vapor and liquid contacted parts of the sphere wall.

 

In any case: it matters also where the feed enters the sphere, and where the product is withdrawn.

Do you have a drawing of the sphere showing nozzles and internals?

[mtlmoody]

At first sphere is nearly empty when frac train is bypassed but inflow 140m^3/hr is continuous and since outflow is 200m^3/hr (rated for pump at the outlet) the level in sphere will drop. Since the sphere doesn't have heat coil source inside it and vapor and liquid are always in equilibrium that's why I assumed no temperature difference in two phases, but you are right that U could be different for dry and wet part of vessel.

 

I'll attach a hand-sketch of vessel GA in the next post, as right now my desktop is acting up. Thanks so much ping.

[PingPong]

At first sphere is nearly empty when frac train is bypassed but inflow 140m^3/hr is continuous and since outflow is 200m^3/hr (rated for pump at the outlet) the level in sphere will drop.

It is still not clear to me how this system operates before, during and after the frac train is down.

 

vapor and liquid are always in equilibrium

Not quite so.

At best, the vapor just above the liquid level is in equilibrium with the upper liquid layer.

 

That's why I am curious how the (cold) feed is introduced into the sphere. If that is via a sparger in the top of the sphere, above the liquid level, then the upper liquid layer is fairly cold, as long as the cold feed is entering. If the feed enters near the bottom of the sphere it is a whole different situation. Perhaps the feed inlet and product outlet even use the same nozzle? In those cases the upper liquid layer is old and nobody knows what its composition and vapor pressure is.

Edited by PingPong, 13 March 2014 - 11:23 AM.

[mtlmoody]

Currently, this sphere stores the propane product continuously at a rate of 55 m^3/hr but during shutdown/commissioning of frac train instead of storing propane product it will store bypassed NGL.

 

I checked the sphere drawing the 4" inlet is shown to enter from the top head of sphere and 10" outlet is leaving from the bottom.

[PingPong]

I checked the sphere drawing the 4" inlet is shown to enter from the top head of sphere and 10" outlet is leaving from the bottom.

And what about internals in the sphere?

 

Currently, this sphere stores the propane product

NGL is propane mixed with with butanes and heavier, so it has a lower vapor pressure than pure propane. If there presently is no problem with propane vapor pressure in that sphere, why do you then expect a problem with NGL?

[mtlmoody]

not too sure about internals..could you be please a little specific about internals..I suspect the vapor pressure close to PCV set pressure cuz the NGL feed temp (14 deg C) to storage is higher than propance vapor pressure at -9 deg C.

[PingPong]

With internals I mean: internal pipes, baffles, et cetera. Should be indicated on the sphere drawing cross section.