4 replies to this topic

[mykid]

Hi all,

 

I have attached the produced water flow scheme installed on one of our vessel.

 

Normally the produced water from degasser can be discharged overboard if it meets the oil in water specification. However if it's offspec they will be routed to slop tank.

 

The produced water flow under gravity from topsides degasser to the slop tank via a 8" drain header. The water is allowed to settle in the slop tank for oil and water separation. The clean water is pumped and discharge overboard when its on spec or recirculated back to the slop tank via the same drain header.

 

My question is:

When the slop pump is running to transfer the produced water to topsides hydrocyclone, is it possible to cause the water backup to the Degasser ?

 

Since the pw degasser is flowing under gravity to slop tank, the back pressure in the system is negligible. 

The flow from slop pump is controlled via a restriction orifice. Strictly speaking when the two system joined at the drain header, it should see the same back pressure, and shouldn't cause backflooding in the degasser right? 

Attached Files

•  slop transfer.pdf   85.94KB   31 downloads

[samayaraj]

Hi mykid,

 

Pressure at the drain junction = opt. pr. of slop tank + pressure drop taking place from junction to slop tank.

 

To calculate the pressure drop from junction to slop tank, you should consider the max flow from degasser and flow from slop pump via RO together in 8 inch line.

 

If the pressure at the junction is more than the operating pressure & head of water in degasser, it will flood. Else it wont.

[mykid]

Samayaraj, thanks for the reply.

We calculated the pressure drop for max flow, it is minimal as the line also gaining static head before entering slop tank.

 

But in actual operation, the PW degasser was flooded when they started the slop pump.

Given that during normal operation the water is drained by gravity flow, it is not possible to include a check valve on the line to prevent backflow from slop pump. Is there any way to prevent the PW degasser being flooded?

[mykid]

In addition, i noticed that the existing drain header is not sized as self venting line. The line probably liquid full when the slop pump is running or even when produced water is flowing under gravity.

 

If the slop pump is at close to atm pressure, and the feed is having a bottom entry, i.e. 26m head gain, does that mean the the drain header on the topsides is seeing pressure below atm?

[samayaraj]

Hi Mykid,

 

It seems the line is not sized properly for maximum flow condition. If the degasser is getting flooded means, available head is not sufficient which causes the inlet flow to degasser is greater than the outflow when the slop pump is operating. You have to change the line size from junction point to slop tank to reduce back pressure to avoid degasser being flooded during slop pump operation.

 

Another problem may with the RO. If it is not sized properly it will not deliver the intended flow and this will increase the back pressure.