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[shekhar dhuri]

Hi,

 

I am a Process Engineer & presently working designing the On-Spec & Off Spec Condensate Storage System.  

For On Spec Condensate (True V.P = 70 KPa), I am going ahead with External Floating Roof Tank.  However, I am unable to decide the type of Tank  for Off-Spec Condensate. I checked with my past Projects refernces which have used ‘Fixed Cone Roof Tank’ with Nitrogen/Fuel Gas Blanketing. According to me it correct since Off-Spec Condensate is unstabilised & hence flashing. In our case, V.P of unstabilised off-Spec Condensate is ~200 KPa. Though it has low flash point.   

 

However, my problem here is We do not have ATM Flare. So where to route blanketing gas outlet line ??  Quantity of Flash gas as Off Spec is as high as 1 MMscfd.  Can i vent this much quantity to safe location based on dispersion ?? Or  can I connect vent line to Low Pressure Flare (3.5 barg). 

 

We do not have N2 generation plant & N2 is going to be purchased in Cylinders.  Hence I am thinking to use Fuel Gas Blanketing. Is it OK ??

 

Kindly suggest.  

[fallah]

Hi,

 

1- You have to connect the vent line to low pressure flare because it cannot be dispersed enough to be vented to safe location...

 

2- I think if you want to use fuel gas blanketing you have to consider flame arrester in relevant PVRV connection line which already installed on the tank as conservation vent...

 

3- Due to relative high desigh pressure, let say around 15-17 kpag, the tank is normally designed as dome roof...

[ankur2061]

Shekhar,

 

You need to do a flare system analysis before providing any new connections to any existing flare. This analysis would include the effect of the new connection on the back-pressure at the existing vent and relief devices connected to the flare as well as the velocity in the flare header.

 

If your fuel gas is treated (moisture and solids free) and is inert in nature, then fuel gas as a blanketing option instead of nitrogen is perfectly feasible.

 

Atmospheric dispersion is increasingly becoming obsolete due to stricter environmental regulations being enforced worldwide. Check the local regulations regarding permissible pollutant levels. You may perform a dispersion analysis to know the ground level concentrations of the pollutants from your proposed cold vent. Many countries have now categorically defined the emission limits of typical pollutants from stacks, chimneys and cold vents. Exceeding the emission limits is now a criminal liability in many countries leading to heavy penalties and in extreme cases forced closure of the defaulting industry.

 

Regards,

Ankur.

[paulhorth]

Shekhar,

 

The need for an off-spec tank for crude or condensate, and the design of this tank and its venting arrangements, is a process design problem which always arises with oil and gas production facilities. The solution usually involves engineering compromises which can be different from one project to another and which can change during the course of the design as various reviews are carried out. There is no easy answer.

 

Here are a few comments:

You are right to select floating roof tanks for the on-spec condensate storage. However if your condensate can go off-spec on vapour pressure you cannot risk discharging it into a floating roof tank. So, you are right to opt for a fixed-roof tank with fuel gas or nitrogen blanketing for off-spec storage. This tank should normally be empty and only be used in an upset condition. On this basis itis possible to argue for a remote atmospheric vent from this tank, becuse normally there will be no emissions. However I recognise and accept the cautions advised by Ankur on the emissions limitations. To overcome this problem you might need to install a dedicated LLP flare system for this tank (and possibly there will be other LLP discharges on your plant).

You cannot connect the vent from this fixed roof tank to the LP Flare with a design pressure of 3.5 barg as any significant back pressure (not necessarily as high as 3.5 barg) will damage the tank. However, on some projects the offspec tank has at the inlet a degassing vessel which is connected to the LP Flare. Normally the LP flare and this vessel will be at atmospheric pressure, but if some live condensate comes through, the flash gas will be caught here before it enters the tank. On a PID from a recent project that I have, the degassing vessel is at high level and connected to the tank through a 14 m seal, which can help to protect the tank from some back pressure in the LP flare.

 

But first you should consider what kind of upset will give rise to offspec condensate, and what can be done to eliminate it. 1 MMSCFD of flash gas sounds like a serious upset condition to me. On one project, we on the process design team managed to convince the client that the offspec tank could be deleted, because there was sufficient redundancy in the heating system for the stabiliser reboiler that any offspec duration would be brief, and would be quickly corrected, and the produced liquid would be diluted away in the main tank.

 

Paul

[shekhar dhuri]

Hi,

 

Thank you Gold Members for your valuable comments. I am going ahead with Fixed Roof Tank with Fuel Gas Blanketing. Fuel gas is treated (moisture and solids free) and is inert in nature. However, could not conclude on the routing of Vent line.

 

Paul, to estimate the off-spec load, I assumed Stabliser Column as a simple vessel (& not column),  I removed vapour from stabliser column inlet steam and then flashed the vessel bottom stream to atm pressure. The load I got is ~1 MMSCFD. May be it is crude way of doing & option of redenducy will be explored separately.  

Secondly, We are providing degassing boot inlet of the Off Spec tank with Vent combining with Tank PVRV outlet. Liquid from the boot will flow to the tank with inverted 'U' loop to avoid vapour breakthrough.  

 

I am thinking of avoiding routing the vent line to safe location even though off-spec is an intermittent operation. Ofcourse dispersion analysis will be certainly done if we wish to do so. I am thinking of routing vent line to LP Flare. We do not have HP & LP Flares & no LLP flare is envisaged.  

 

In our Project, Inlet Separation Sys, Cond. Stabilizers, Sweetening, GDU, NGL Recovery Sys, Sour & Sweet Gas Comp, LPG Bullets are operating > 10 barg. I will route them to HP Flare. Only systems relieving to Low Pressure are :

 

a) Amine Regeneration Column – CV Failure Case, 

Produced Water Degasser – Gas Blow by 

c) Off-Spec Cond.  – As per API 2000

 

Hence they will not relieving simultaneously with the load which can exert backpressure on Off-Spec Tank PVRV.  For Fire Case though, I may have check in detail based on layout.

 

Do you still feel, need for separate LLP flare sys ??

 

Please suggest.

 

Regards,

Shekhar Dhuri

[fallah]

Hi,

 

Separate LLP flare system is strongly suggested as a subsystem of the LP flare network without dedicated stack. Obviously, vapor coming from LP and LLP KODs are separately introduced to the same flare stack (LP stack).

Edited by fallah, 08 September 2013 - 06:40 AM.

[paulhorth]

Shekhar,

 

I support Fallah's suggestion. If you follow this advice you will protect your tank from the back pressure generated by the LP header during the other relief cases, while providing a safe discharge for the vent and blanket gases.

You need to avoid the LP flare back pressure, from say the amine regenerator, being imposed on the tank, even when the tank itself is not in an upset condition.

 

The upset condition which gives rise to your offspec condensate flash gas appears to assume the total loss of reboiler heating, while production continues. You should perhaps consider if this is a credible scenario. If the entire heating system to the plant is lost, then production would have to stop, since you would lose gas sweetening and dehydration etc as well as stabilisation. For loss of the stabiliser reboiler, I could see perhaps a spurious fail closed of the ESDV on the heating supply as a cause. If this happens, and you want to continue production, then I guess this meets your scenario if it can't be fixed quickly.  The TCV on the heating medium supply could fail closed also, but that malfunction could be put right with a bypass or a handwheel.

Other upsets would give rise to less than the maximum flash gas that you calculated. These might include:

• excess feed to column
• low feed temperature
• change in feed composition
• High column pressure
• malfunction of reboiler temperature controller

Paul

[SUPRIYAPRA]

We use this off spec storage tank flash gases to Fuel gas and Power generation system. 1 MMSCFD is not a small amount to flare. Environment regulatioins do not allow to flare such a huge gas amount.

[shekhar dhuri]

Thanks all of you for your valuable time & suggestions.