4 replies to this topic

[icasensio]

Dear All,

I have found a spherical tank with two lines to load LPG into it ( in a P&ID ), and I don´t know the reason. This tank is a pressurized vessel, non refrigerated, and pressure inside is about 145 psi.

LPG composition is C3: 60% w/w and C4: 40 % w/w.

In the upper loading connection, the spherical tank has a device to disperse LPG, and I have read somewhere that It could be to lower temperature.

Anybody knows the reason of upper or lower loading LPG into a Spherical Tank?

Many Thanks in advance.

[Art Montemayor]

Icasencio:

A few words of advice:

• Never specify a pressure without stating if it is absolute or gauge. It is confusing and can cause errors - such as in your case, where it is vital to know the vapor pressure of your stored LPG mixture.
• Always find out what the MAWP and MAWV of your storage vessel are. Also, make sure you fix the feasible storage temperature range your LPG will have to tolerate. Since the LPG is stored at SATURATED conditions, it is very important to know if your vessel is within the range of vapor pressure exerted by the LPG at the varying storage temperature - expecially at mid-day.
• The very first thing you should do is to find out what is the mixture range of the LPG and calculate the resulting vapor pressures for the upper and lower compositions of the low boiler(s) - butane, in this case. Make sure you obtain a definite assurance that the low boiler compositions will not vary any higher than that promised. In my opinion, you are working very close (perhaps too close) to the 145 psig (if that is what you mean).

[ankur2061]

icasensio,

Refer section 4.3 in the link below:

http://sunrise-energ...Application.pdf

Regards,
Ankur.

[kkala]

I happened to find an old (1968) practice of a known engineering company, "Pressure storage facilities for LPGs", noting among others.
- Storage can be horizontal cylindrical vessels (up to 200 m3), Vertical cylindrical vessels (up to dia=10m h=25 m, 1750 m3), Spheres (from 400 m3 on).
- Only one product line shall be connected to the bottom of the vessel and be used for filling, discharge and draining. However operational considerations may dictate use of a separate top connected filling line.
- Top connected filling and vapor lines may be flange connected to the vessel.
Practice does not advise what these operational considerations can be, but it confirms that top filling line in addition to bottom line is possible. No mention of "dispersion device".

However web reference of post No 3 (6x1728 m3 burried LPG vessels) reports spray loading (para 4.3.2) and splash loading (4.3.3), with subsequent auto cooling. Cooling is difficult for me to understand, assuming saturated LPG vapor in the vessel. Some explanation would be welcomed.

Edited by kkala, 16 April 2012 - 08:27 AM.

[ankur2061]

The mechanism of spray loading as described in Shell Design Engineering Practice for "LPG Bulk Transfer and Transportation" is as follows:

LPG is transferred to the receiving vessel through a pipe in the vapour space provided with spray nozzles, orifices or slots. The use of a spray bar improves contact between the incoming liquid and the vapour in the vessel and thus reduces pressure build-up through condensation of the vapour. In some cases spray loading may eliminate the need for a vapour return line.

The idea of a spray system is not for cooling or reducing the temperature, it is to reduce a pressure build-up in the vessel. The small degree of cooling that might occur would be purely be coincidental due to the rapid condensation of the vapors by the intimate mixing between the liquid and vapor when liquid is sprayed in the vapor space compared to a continuous slug of liquid entering the vessel.

Ankur