6 replies to this topic

[Oliz]

Dear members,

 

I recently got the task to improve existing drain systems on our two butane spheres and design new drainage systems for our two propane spheres (we have no water draw off on the propane today). Butane and Propane is stored under ambient temperature (-20 to 30 degrees C).

 

I have read both API 2510 and 2510A as well as had a look in the Shell DEP to gather some insights into this problem.

 

The existing system (see attached sketch) on the butane spheres consist of a manually operated hydraulic valve followed by a ball valve. These valves are opened and water/butane mixture is drained into the drain pot. The upstream valves are then closed. The operator then opens the first down stream valve fully and throttles the flow of water/butane out of the drain pot with the second valve.

The two ball valves downstream of the drain pot are separated more than 600 mm but close enough for one operator to operate both valves just according to both API and DEP. The last valve is a quick acting spring return valve also according to API and DEP.

The drainage is a "batch" process and the operator has a level gauge to his help to see when there is no more water to drain out of the sphere.

 

The problem with this system is that the water and butane drained to the drain pot have to be drained to atmosphere.

 

The intent with the new installation is to route the butane to the flare system. I have come up with a first proposal that I would like to get some opinions on.

 

The TSV will be removed and the drain pot will be permanently lined to the flare header operated slightly above atmospheric (0.1-0.2 barg). By doing this, butane entering the drain pot will vaporize and be routed to flare. The water will be drained to the oily  water sewer system. The problem that I see is that during winter temperatures are sometimes below 0 degrees C and the boiling point of butane is -0.5 degrees C at atmospheric pressure. This means that on cold days, butane might be in the liquid phase when drained out of the sphere which could lead to routing liquid butane to the oily sewer system. Does anyone have experience of a similar installation? One alternative that I was thinking about is to have steam coil in the vessel so that you can easily heat it up to evaporate butane to the flare system or if it is enough just to trace the pipe in to the drain pot. Another alternative that I have not looked into the details on yet is to install pumps and route the liquids to our slop oil tank but I would prefer the first option.  

 

On the propane spheres, we don't have any regular draining today and I was thinking of installing a similar installation on this system as on the butane system. The only difference is that propane will not be in the liquid phase, hence no heating is required. 

 

Any help is appreciated.

 

Many thanks,

 

Ola

 

 

 

 

 

 

 

 

Attached Files

•  sketch.pdf   57.31KB   152 downloads

[Zauberberg]

I wouldn't keep open connection with the flare system. It may introduce additional hazards while it does not bring any visible benefit.

 

As for the proposed routing of oily water, you need to look at possibilities/hazards of light liquid hydrocarbons escaping to the oily water sewer (dissolved in water or via blow-by). Operator error or level gauge malfunctioning could lead to continuous gas flow from Flare system to the oily water sewer - another reason why Flare connection should not be of an open type.

 

Another issue: is there enough head to drive oily water to the new destination point? I believe the drain drum operates at near atmospheric pressure.

[Oliz]

I have tried to look for references on this topic but it is really hard to find any good ones.

 

Shell DEP at least states that the water draw off line shall be connected to a knock-out vessel from where the gas is routed to flare. I does not mention anything about a continuous connection but I assume that is what they mean. They don't talk about how the water/heavier hydrocarbons are taken care off.

 

The intention was that the water would flow with gravity to a sump from where it is pumped to the water treatment facilities so there I don't see a problem regarding head requirements. However, the malfunctioning and risk of getting LPG in to the oily water sewer system and gasing off at other open drains might be a risk that makes this design a bad idea.

 

I used to work in the upstream industry and I compare this vessel with a typical closed drain drum. They are, at least to my experience, continuously connected to the flare system with pumps (on/off) starting when the level in the vessel gets too high, pumping the water/HC back to main process.

 

At least the current system is not a good design. The risk today is that the operators fill up the whole drain pot with HC and then draining that 0.1 m3 out on the ground some 50 meters away from the tank. 0.1 m3 of liquid butane is about 23 Nm3 of gas which is quite a big cloud.

 

Does anyone have any suggestions of a well proven design on these kind of systems?

[Zauberberg]

The main problem lies with disposal of light Hydrocarbons. Water or aqueous materials that are withdrawn continuously or intermittently from vessels where they directly contact hydrocarbons must be disposed of in such a way that entrainment or inadvertent withdrawal of hydrocarbon will not create a hazard. Routing of oily water to an open sump is equally - if note even more - potentially hazardous as the current setup. Is there a way you can connect the oily water discharge to a closed drain system?

 

The connection to the flare you can leave as it is, I see no reason for any design change.

[Oliz]

I will definetely look into the possibilities of routing the oily water discharge to a closed drain system.

 

I do not see the problem with an open connection to the flare though. Not leaving it open will leave the system with no where to release any gaseous hydrocarbons drained to the drain pot except if the pressure increases by thermal expansion until the temperature safety valve pops open. 

[Zauberberg]

If you find a way to connect the discharge to a closed drain system, no worries for backflow from flare or potential relief scenario.

 

The hazard of routing LPG and heavier components to an open sump (assumed below ground level) could lead to a very dangerous scenario: accumulation of explosive materials inside the sump.

 

If the spheres are located offsite and far from any equipment or potential ignition sources, you may leave the drain system as it is.

[badelita marius]

Hy Oliz,

 

I read your post and i thought about other problem. When the draining sequence is over, and you close the hydraulic valve downstream the butane tank, you will have also some liquid butane on the pipe. 

Asuming the KO drum is operating at atmosphere pressure.

My question is: When liquid butane from pipe will meet atmospheric pressure it will depressurise, and it is possible to frezze the watter inside the pipe?