7 replies to this topic

[Afshin445]

Dear Sirs,

I try to design a Surge Drum for LPG Vaporizer in Fuel Gas System for a crude Refinery.
The LPG routed to surge drum via a pump located in LPG Storage Tank area.Pressure in Battery Limit of Fuel Gas System is about 26 Barg and there is a LCV before Surge Drum.

The surge drum is only in level control and there is not any pressure control in Surge Drum.
My question is how i can estimate operating pressure of Surge Drum.


Regards,
Afshin

[joerd]

If there is no vapor connection, except for safety relief or depressuring, the pressure in the drum will be the saturated vapor pressure at the drum's operating temperature - which should be close to the pressure in the LPG storage tank. Dependent on the configuration of the system, the answer may be somewhat different, so you should take a close look to see what makes sense.

[Zauberberg]

The design/concept is quite strange. The make-up rate of LPG into the fuel gas system has to be controlled somehow - otherwise how will you know what quantity to vaporize?

One scheme you can try to visualize is to have PC valve on the surge drum discharging into fuel gas system, connected with the vaporizer heat supply (usually steam or electrical heater). A drop in pressure will push more heat into the system and evaporate more LPG. Level control is a separate control loop that controls the inflow of LPG into the surge drum.

[shan]

Dear Sirs,

I try to design a Surge Drum for LPG Vaporizer in Fuel Gas System for a crude Refinery.
The LPG routed to surge drum via a pump located in LPG Storage Tank area.Pressure in Battery Limit of Fuel Gas System is about 26 Barg and there is a LCV before Surge Drum.

The surge drum is only in level control and there is not any pressure control in Surge Drum.
My question is how i can estimate operating pressure of Surge Drum.


Regards,
Afshin

The surge drum pressure is 26 Barg if you ignore the friction and elevation change. Otherwise, how could you explain to Mr. Bernoulli?

[Afshin445]

Dear Zauberberg

Please find attached schematic of LPG vaporizer system. as u can see there is PCV in outlet line of vaporizer.
My question is how i can estimate operating pressure of surge drum?

Attached Files

•  LPG Vaporizer.pdf   26.76KB   135 downloads

Edited by Afshin, 11 October 2009 - 12:19 AM.

[joerd]

You're not showing the vapor space connections on the surge drum. Like I said before, if the pressure is not controlled, the LPG in the surge drum is most likely at its bubble point. Once you know the temperature (and the composition), you know the pressure. In Hysys, set the vapor fraction to 0, and leave the pressure blank.

[Afshin445]

Jored,

It's depending me to use gas blanketing on Surge Drum, in this scheme
there is no pressure control and gas blanketing but as it appear there
is enough space above liquid for gas separation in Drum.
Is my explain is enuogh

[joerd]

In that case, the pressure will be the bubble point pressure, at the operating temperature.
When the level drops, the pressure will decrease (expansion of the vapor space), but then a little bit of LPG will vaporize, because there is no longer an equilibrium between vapor and liquid, until the pressure is back to where it was (actually the vaporization will take energy, so the temperature will drop, and the pressure will be a little lower than before). If the level rises, the reverse happens.
So, the pressure will fluctuate with the operating temperature along the vapor-liquid equilibrium line of the LPG. This is the same as in your propane cylinder at home. See also Wikipedia.