3 replies to this topic

[Butterfly]

I now there are many related topics, but I cannot find the exact information.

I have a hot oil close circuit. Oil comes out the expansion vessel and is pumped through a WHRU. It then enters some reboilers, and goes back to the expansion drum.

This drum has blanketing using natural gas. Its volume is 725 ft3, and the blanketing consumption estimated by the engineering company is 270000 ft3/day. Since hot oil is continually flowing through the reboilers where it is used, I cannot see how they calculated such a big amount of fuel gas for blanketing. I just would like to have a way of doing a quick estimation.

Design/operating pressure of the vessel are 125 psig/5 psig

Thanks in advance

[fallah]

Butterfly,

Even though the submitted information isn't adequate to be able to response on the query, it seems such a big amount of fuel gas is using for sweeping applicacation instead banketing.

Anyway if you are using blanketing system, it sounds due to continuous operation of the mentioned system the fuel gas is mostly consuming for thermal breathing rather than out-flow compensation.

Fallah

[ankur2061]

Butterfly,

When dealing with hot oil expansion vessels the following needs to be considered:

1. The vent from the expansion vessel should normally be connected to a flare system

2. When hot oil system is operating at or above the normal boiling point of the hot oil the pressure in the vessel should be maintained at least 1 to 2 bar above the vapor pressure of the hot oil at the operating temperature. This is done by providing a blanket gas flow to the expansion vessel at a pressure of at least 1-2 bar above the vapor pressure.

3. The blanket gas supply should be equipped with a split-range controller and a non-return valve, which will regulate the blanketing gas supply and the quantity being vented (lost to the flare).

4. There should be a dead pressure zone between the blanketing gas supply pressure and the vent set pressure to the flare. In this dead zone, the pressure of the expansion vessel is not controlled and is allowed to float freely while both the blanketing gas supply and the vent-to-flare valves are closed. This dead zone will reduce the blanketing gas consumption and will also ensure non-accumulation of low boilers due to the hot oil degradation by lowering the vent pressure.

5. Low boiling compounds of the hot oil will be vented periodically to the flare by pressure control through the split-range control system. In case the condensation and accumulation of the low boilers from the hot oil in the flare knockout drum is unacceptable due to operational or HSE reasons, the vent line should be routed via a condenser to a collection vessel and the vent connection from the collection vessel should be connected to the flare for removal of non-condensibles.

6. The basic criteria for determining the maximum instantaneous blanketing gas flow of the expansion vessel would be the reduction in hot oil level of the expansion vessel from the maximum operating level to the minimum operating level as per the guiidelines given in API STD 2000 for inbreathing due to liquid movement out. However, as mentioned in 4 the blanketing requirement is not a continuous one by using the split-range control system and you need to tell the engineering company to consider the 'dead pressure" zone while estimating the blanketing gas flow rate.

Hope this helps.

Regards,
Ankur.

[Butterfly]

Yes, you are right, there is a split-range pressure control system, connected to flare (and to supply source). There is also the "dead zone" (between 3-8 psig).

I'll have a look at API 200, to see if I can make some calculation for the thermal breathig.

Thank you