8 replies to this topic

[vnpetroleum]

Dear Everyone,

I am reviewing the flare system in our platform. Our current peak rate is about 580 mmscfd. We intend to increase the gas production. Therefore, flare system is require to be reviewed.

Please help to give some tips to re-calculate the peak rate by using Depressurise tool from HYSYS. How to define some potential scenarios for flare system (typically)?

Thanks in advance for your help

Vnpetroleum.

[paulhorth]

vnpetroleum,

HYSYS Depressurise is used to calculate blowdown rates. The blowdown rate will not change if you increase production, but only if you add more equipment or raise the operating pressure.

However, there may be a relief rate which has to change due to increased production, for example if you are connecting more wells to the first separator you might have to increase the relief capacity on that separator. HYSYS is not used to define this.

It worries me that you have been asked to review a flare system without the right level of experience and understanding.

Paul

[vnpetroleum]

Hi Paul,

Your answer is very interesting. Let's me clear my question. Because we need two more wells to increase the gas production, the operating pressure of some vessels will be increased. Existing conditions is about 90 barg for Production Separator. Howerver, new condition is about 110 barg. A HYSYS model is updated.

That's why I have been asked to review the flare system. Please help me how to calculate the new peak rate by using HYSYS (tips or steps)

Your comment will be highly appreciated.

vnpetroleum

[demank]

fist, you must define everything that enter the flare system.
such as PSV's and BDV's.
second, you define the peak rate of each PSV's/BSV's.

what parameter that you need for defining the rate of these PSV and BDV?
I think thats are you need, rite?

[paulhorth]

vn,

OK, so you are raising the operating pressure of some equipment. Hopefully you are still within the design pressure??

The inital blowdown rate is approximately proportional to: p1/(p1-p2). ln (p1/p2) where p1 is inital pressure, p2 is final pressure. You might like to google Grote Equation.

Calculate the value of this factor for the 91 bara case and the 111 bara case. Take the ratio of these.

You can use this to estimate the increase in inital blowdown flow from the equipment with increased pressure (i.e. p1 increased, p2 stays same).. This assumes that the blowdown time is unchanged, and it does not take account of the liquid in the vessels. Take the blowdown flow for the existing conditions (which should be in data in the flare and blowdown report for your plant) and multiply by this ratio.
Then add this new flow to the flow from the rest of the equipment which won't have changed.

You need to check the flare radiation contours for the higher flow to see if flare radiation will exceed the limit for personnel exposure.

As for using HYSYS Depressurise, this is not suitable for explanation in a post like this. The input to the latest version is rather complicated. I have run many blowdowns over the years, and I find it difficult to get Hysys Depressurise to do what I want. I prefer the previous version which I found much easier to use. But for an initial check of the flare system capacity, the above hand method will serve.

Paul

[fallah]

Hi Paul,

Your answer is very interesting. Let's me clear my question. Because we need two more wells to increase the gas production, the operating pressure of some vessels will be increased. Existing conditions is about 90 barg for Production Separator. Howerver, new condition is about 110 barg. A HYSYS model is updated.

That's why I have been asked to review the flare system. Please help me how to calculate the new peak rate by using HYSYS (tips or steps)

Your comment will be highly appreciated.

vnpetroleum

vnpetroleum,

Seems,your issue has more wider dimentions than you limited it to difference between peak rate of depressurizing before and after some changes in the system.

IMO,at first you should perform a RE-HAZOP due to adding two wells to existing system.Then,using the action sheets as HAZOP study output,you should modify previous "applicable scenario report" and following that "relief load summary report" included relief load for each scenario.After that using possibility of simultaneous cases you can obtain new peak rate,may relates to a scenario other than previous one.

HYSYS just can give you peak flowrate of a BDV in depressurizing of an isolated part of the process and you can use new process conditions as HYSYS input to calculate new peak flowrate of each BDV.

Fallah

[vnpetroleum]

Many thanks for your response. It makes me better now.

Regards,

vnpetroleum

[daryon]

Hi Paul,

Your answer is very interesting. Let's me clear my question. Because we need two more wells to increase the gas production, the operating pressure of some vessels will be increased. Existing conditions is about 90 barg for Production Separator. Howerver, new condition is about 110 barg. A HYSYS model is updated.

That's why I have been asked to review the flare system. Please help me how to calculate the new peak rate by using HYSYS (tips or steps)

Your comment will be highly appreciated.

vnpetroleum

Lots of good points made already, I just wanted to add one....you should review what the original blowdown calculations assumed as a starting pressure. It's not unusual to perform blowdown calculations assuming the vessel is at PAHH (this is a conservative assumption - and some companies insist it is done). If the vessel PAHH was used to determine the peak blowdown rate and you have only raised the operating pressure whilst maintaining the PAHH for the vessel, then the peak rate may not change significantly.

Provided the system volume remains constant, the fluid compositions/properties are not significantly altered by the new well fluids coming online, then your peak blowdown rate will be similar if the original calculations assume starting pressure of PAHH and you haven't changed this high pressure trip setting. Obviously you must still perform the new calculations rigorously to check.

To help you with getting to grips with HYSYS dpressuring utility I've attached a guide - it's quite old (from the days when Hyprotech owned HYSYS) - but the fundamentals haven't changed. I found it very useful when learning about the utility and it's capabilities/limitations. Its so old i'm pretty sure i'm not infringing on any copy right laws / intellectual property rights by posting it.

Attached Files

•  Dynamic Depressuring - A Practical Guide.doc   373KB   109 downloads

[paulhorth]

Daryon,

I agree with your point about blowdown from the PAHH setting, I wish I had mentioned that in my post.
And thank you for the HYSYS article, it is very useful. It can be a struggle to work out how to use all the features in Depressurise just by trial and error.

Paul