2" SCH40 line of oily water

[GasOilEng]

Hi everyboy

I am process engineer in LPG plant. The oily water generated from regeneration phase of MolSiev bed of natural gas is sent from regeneration drum working @ 90bar & 50 to 60°C to API oily water treatement unit (Working pressure 1bar, ambient temp) passing by control valve through 2" SCH40 line.
To be noted that the control valve working from 90bar to 1bar is not 100% seal when it is shut @ 100% (There is always leak, formation of ice on outer surface of this line).
What happened is that the deformation of the latter line, see attached photo, has been occured.
Is any one has an explaination of this phenomenon.

If yes, Is it correct if we change the diameter from 2" to 3 or 4".

Thanks for your helps.

Best Regards

 

[LittleInch]

Difficult to tell but maybe it grew a lot of ice and in it's rather flimsy shape and lack of supports it sagged down.

Changing size won't do a lot for you, but without seeing an iso and a bit more idea what and where the supports are it's not easy to say.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

This appears to be a poorly designed disposal system, process wise. The icing on the line is obviously due to gas blowby / JT chilling through the leaking valve. From a process safety perspective, it is unlikely the low pressure API separator will be able to withstand a level failure / gas blowby event at this drain valve on the regen gas condensate KOD and dispose of the blowby gas safely.

The lower design temp of this line should be the gas blowby stream temp at the pressure downstream of the drain control valve (probably ASTM A333 Gr.6 or SS316L or similar). Control valve trim should be good for erosive letdown conditions, preferably some kind of choke valve. The life of the choke valve can be further extended by installing a thick plate RO downstream to enable a 2 stage letdown. An oversized choke /control valve can lead to very high on/off dump rates, leading to intermittent gas blowby also.

All high pressure oily water process drains in this plant should first be routed to a central oily water collection drum (process design pressure of 350-700kpag) which floats on the LP or LLP flare header. The drum liquid exit stream may then be routed to this API sep on level control. Each HP drain feeder line (and its pipe supports) to this HP process oily water drain drum should be able to withstand the high velocity / high pressure of its upstream gas blowby failure incident and materials of construction also good for the gas blowby low temps generated. Blowby gas from one feeder line should also not reverse flow into other feeder drain lines. Looks like this drain line has suffered more than one too many gas blowby events, leading to high vibration / knocking the pipe off its supports. Get a reputable engineering contractor to sort all this out for you.

 

[EdStainless]

Just hanging out in the air like that any expansion/contraction will cause deformation.
At the same time you don't want to over constrain it, that could break something.
You need to do some analysis of the stiffness of this line.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[bimr]

The weight of ice on the outside of the pipe combined with lack of pipe supports probably caused the pipe deformation.

The vertical section of the pipe needs a pipe support.

 

[GasOilEng]

Hi everybody

Thanks everybody for your technical approaches, but still give you more details for the damaged 2" line oily water in order to give more explanation of that phenomenon.

LV's and SDV valves are for level control of these drums.


Best regards.

 

[chicopee]

the blowby gas described by georgeverghese is being throttled from 90 bar to 1 barpsig. Get a Mollier diagram of LPG and knowing the gas temperature on the high pressure side you can determine the temperature on the low pressure side under constant enthalpy.

 

[georgeverghese]

In this new diagram, an intermediate oily water flash drum is included, so that should in theory be able to handle gas blowby from these HP level dumps - check if this is the case for all dump lines. As suggested, check that these dump control valve trims are as small as they can be and that CV trims are erosion resistant tungsten carbide or silicon carbide or similar. Also dump lines and supports should be able to withstand high momentum forces resulting from full gas blowby stream loads going to flash drum. Level transmitter (dp cell ) impulse lines should be remote sealed. To further improve safety, add an independant low level trip SDV in series with LCV on each dump line. Level low low on each source vessel should be at least 100-200mm above vessel bottom seam line and a vortex breaker should be included on liquid exit nozzle.

 

[GasOilEng]

Hi

Thanks for this thoughts which are valuable for me. Especially Mr georgeverghese , your thoughts make things clearer.
What I think is that there is gas blowby from the reg KO drum when there is LT failure, this may cause knocking and hammering , hence line deformation. Because we notice over 03 years that there is almost no liquid accumulation in the coalescer filter drum (SDV On-Off control level & LCV1007 almost always closed) but gas leaking happening (outer Ice formation).

Please could this statement above be the reason for this deformation.

Regards;

 

[georgeverghese]

There may some solid particles stuck in the lcv/sdv trim that is causing the gas leak?