5 replies to this topic

[tomwhite]

Dear All,

I need some help from you all regarding the vaccum issues with Prodcued water disposal to sea

The hydrocyclone outlet treated Produced water is being dispsoed into sea onboard on FPSO.The flowrate is so high and due to very high velocity in the pipe line we are unable to get sample downstream of hydrocyclone downstream of pressure control valve and it is sucking air basically the moment we open the smaple point.The disposal tube is not submerged in the sea.It is suspended in the air and produced water is realsed that falls into sea.

To my udnerstanding putting a manual value clsoe to the discharge point and close it to soem some % and leave it to increase the back pressure in the line there by velocity head will get convert to pressure head( flowrate reduction) and we can get sample from the smaple point.But by this way opertaor got to adjust the valve often depending on the production profile.

Normally produced water disposal caissions or disposal tube will be provided with a vent line at the elbow which is basically used for venting incase if the produced water has some air pockets and has gas bubbles.Froude number to be less than 0.3 to avoid air pockets.Is this vent normally sized for what flowrate .is it for the same flowrate as discharge water? If this vent is sized for same flowrate as water dsihcarge flowrate,can we avoid vaccum issue at the outlet of hydrocyclone pressure control valve?. Even though if this vent is sized for the same water discharge flowrate,would this avoid vaccum genertaion?


I like to have people thoughts and recomemdnations on this issue and what are all the solution we have to this vacuum problem .Dwonstream of the Pressuer control valve we are having some instruments and it is not reading corretcly due to this high velcocity and vaccum issue. If the disposal tube is submerged in the sea will that helps in anyway.

I would apprecaite it if any one could help me in this regard

[Dazzler]

G'Day Tom

One possible scenario is that if the hydrocyclone outlet line just goes down a drop of many meters, and there is no vent on top section, and line is not too big (restrictive), then a syphon could easily develop and when this occurs the flow increases greatly sucking higher and higher flow, and developing some level of vacuum at the top of the pipe. I've no experience with your type of setup, but have observed this in other situations. When I've seen it I feel that the scenario can be unstable and occassionally the flow suddenly slows or reverses due to perhaps a cavitation effect if the vacuum gets too much. Flow disturbances and vibration are observed.

Some ways around this is by having a suitable vent, or actually lowering the discharge pipe pressure drop by increasing the line size enough so the line enters gravity drainage regime, in which case air can go back up the pipe in reverse direction to the falling liquid and prevent creation of vacuum. Perhaps a combination of the two is best.

I achieved this one by removing a valve in the outlet line as it was significant deltaP, with no need to increase line size. There was already a vent that helped partly.

You should be able to find formulae or calculators online for determining line size for gravity drainage regime.

Good luck.

Dazzler

[tomwhite]

Dear Dazzler,

Thanks for the reply and apprecaite your valuable suggestions.The line size cannot be changed now as the FPSO vessel is under opertaion.The present line size is not rightly deisgned for the discharge flowrate and hence we are getting very high velocity there by vaccum generation.

The hydrocyclone outlet pipeline goes ups and downs with many bends and finally travels horizontally and then takes 90degree elbow and goes downwards close to the sea level but it is suspendend and not submerged in the sea water.

If I put bigger size vent that is sized for the same discharge flowrate,then please advise would this solve the vaccum problem if I putt his vent exactly on the 90 degree elbow pointing upwards. I beleive this vent pipe should have enough elevation higher than hydrocyclone.correct me if i am worng. do we need to consider to submerge the water discharge pipe in the sea water as well.

i look for your advises.thanks

[Majid-Process]

Hi there,
There is specific control system on produced water hydrocyclones that is applicable to all similar cases. To get best separation efficiency, there should be specific proportional differential pressure between hydrocyclone overflow and underflow line.

Two DP transmitters plus two control valves linked to a PLC, always fluctuate in a way that a pre-set figure is met in all operating conditions. Therefore puting any new item on the hydrocylone outlet will disturb the separation performance. As a result, if even you succeed to get a sample, it will not be a real sample of you operating condition.

To propose a solution, more info is required. If you like, you may share any drawing from your system to see that what can be done.

m_zolfkhani@yahoo.com

I Hope above can help.

[riven]

The attached file made by member Art Montemayor I believe can also give some ideas even though it is for gravity flow.

Attached Files

•  Gravity Flow.xls   1.12MB   67 downloads

[chemsac2]

Tomwhite

Your system is not clear from what you have described. A sketch would be very helpful. Nonetheless, here is what I think of your system:

Since your stream is from hydrocyclone, I am note sure if Froude number criteria would be applicable.

Most probably it is result of column separation in a vertical loop. Rising leg of loop is against gravity whereas discharge leg is in direction of gravity. Thus, upflow may not match downflow and partial vacuum would occur at highest point. Since pipe is not submerged in liquid, pressure at pipe outlet must be atmospheric even if one calculates it from conditions in pipe. Thus, pressure at topmost point in vertical loop would have to be below atmospheric to ensure that after considering static head pressure at pipe outlet is atmospheric. Flow would be irratic if height of discharge pipe is more than 10 m.

A block valve in outlet line near point of discharge would help as it adds resistance to discharge leg. Large driving force available for downflow due to gravity is now partially resisted by block valve. With proper operation, it would be possible to keep discharge pipe flooded upto block valve.

Such a trick is also used in systems where process flow drops to such a low value that it can not occupy full pipe cross sectional area. Boiler feed water to steam drums is one such application.

Hope it helps.

Regards

Sachin