30" Floating suction

[jimmoore]

I have a settling tank here in Nigeria that we have recently revamped. It originally had a 30" fixed pipe set at 40 feet for evacuating the product. We decided to go with a floating suction instead and done away with the fixed pipe. Now we seem to be getting a vapor lock in the line and the floating suction just pops right up out of the product. We installed a bypass line from the shell to the outlet nozzle for the initial fill of the tank and as long as the bypass is left open there is no issues. Unfortunately we can not leave it open as it is installed in the water table of the tank and hence can not pass lab tests once in operation.

Has anyone ever seen this before?

Floating suction is aluminum
Swivel Joint is Stay-tite

Thanks

 

[JStephen]

The floating suction needs an air vent out at the outer end. A fairly small hole will vent the pipe while the tank is filled.

 

[jimmoore]

JStephen ,

Thanks for your response. There is presently an air vent in that spot but we feel it may not be big enough due to the amount of gasses we have in our product here. Gas breakout is a big problem here. We are starting to think that gas is building up in the elbow (while in service) were the small vent is located and that the vent hole is not big enough to accomidate the relief needed. We have been in touch with the manufacturer and they are thinking on the same lines.

Do you know of any solution that would keep us from going into the tank. Due to the cost and time it would take. To clean a tank in Nigeria takes up to 6 months to a year thats not including setting up the contract to the contractor(no joke).

Note: this problem is occuring after the tank has been in service for some time (1-5 days of service). Not during initial fill.

Thanks

 

[IFRs]

The buoyancy calculation may deserve to be reviewed. Otherwise I agree that the vent hole size and location need to be checked, although even a small hole should relieve the gas over time - dies the swingline get lower over time?.

 

[IFRs]

Something is missing. If the floating suction works during the initial fill, how can air get into it after that? Presumably is it in the "up" position, under the liquid level and only getting water into it. How can it get air in it in that position? Someone must be pumping liquid/air into the suction line?

 

[jimmoore]

now you understand .... what is missing??? .... thats the key. We know we have alot of gas coming in from offshore here in Nigeria.. this seems to be only happening when we get a increase in production ... say 490,000 bls a day to 590,ooo bls a day .. we have 5 settling tanks to handle the bls per day....so i am thinking that it is gas pockets building up in the elbow portion of the floating suction and that the small vent is not able to release the build up. Also I just found out 2 days ago that this system is using a centrifical pump ...which does not help matters much.

Note that the old system was a fixed open ended pipe that allowed the oil to just overspill into the pipe .... hence no air build up..

I am lost... the trick is to try and fix this without going into the tank to save money. I am thinking I might not have a choice.

 

[IFRs]

If the gas comes in via a different pipe, how can it get into the elbow? Are you SURE that the floating suction operates properly on initial fill? Is it possible that the 30" floating suction is undersized for the pump? - there is little NPSHa when the liquid level is low. Does the liquid level vary much? Again, if it works on initial fill when the entire pipe is full of air then the vent is not the problem. Floating suctions don't usually pop right out of the water - it's just not possible given the geometry. How are you determining the elevation of the elbow? Is there a hold-down chain that limits the swing of the pipe? Do you have a history of liquid levels vs floating suction operation? If you have drawings and calculations we can review?