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Psv Discharge Line Sizing

psv sizing diameter

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#1 Amanda Ferrari

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Posted 16 April 2014 - 01:50 PM

Hi Sirs

 

I´m discussing with some friends about PSV´s discharge line sizing during individual discharge (for instance,  closed outlet case). Some of them are saying that it´s necessary to design discharge lines considering as back pressure the minimum back pressure, that means something near to atmosferic pressure.

 

In my point of view, this approach is not realistic and can lead to oversized diameters. Let´s try to explain it considering the figure attached.

 

 

At the instant t=0, when back pressure is almost atmosferic (minimum back pressure), we have the highest velocity (green curve). Imagine that in this case we have "plugged" flow (Mach=1)  in which is not possible to reach the flow through PSV. What will happen? The pressure immediately downstream PSV starts to increase and consequently system pressure will increase too (yellow curve). As the pressure increases, the velocity decreases (considering the same flow rate). This intermediary condition takes place until the rated flow be reached. At this point the flowing through  the system reaches the equilibrium and stabilizes itself. So, but how much this pressure can increase? Till reach the maximum pressure of the piping (red curve). In this case I would have the maximum pressure in the base of the flare (already definied in the project)

 

So, trying to summarize, if I design PSV discharge lines considering as back pressure the maximum at base of flare, evaluating if I reach or no piping allowable pressure I would have a more realistic case leading to smaller diameters.

 

What do you think about that?

 

Thank you!!


Edited by Amanda Ferrari, 16 April 2014 - 01:51 PM.


#2 Amanda Ferrari

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Posted 16 April 2014 - 01:58 PM

Sorry, I forgot to attach the figure.

Attached Files



#3 Lai.CY

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Posted 16 April 2014 - 07:22 PM

Hello Amanda,

 

Normally the way I do it is, in the event of Emergency Shut Down (ESD), if there is a fire, the whole process plant blowdown valves (BDV) will open.

Due to that, pressure will build up in the Flare Knock Out Drum (FKOD) and the PSVs will experience maximum backpressure.

Therefore, the PSVs which are sized for Fire Case, I say it is credible to use a maximum backpressure value.

 

However, when there is an emergency relief from the PSV not due to fire, let's say, blocked discharge, gas blowby, block-in heatup, check valve failure, and so on... The FKOD is still operating at it's 'near atmospheric' pressure.

 

If, the relieving rate is massive, and you suspect a substantial amount of backpressure will build up which will result in a smaller tail pipe, you can always use Flare simulators to check, or you can perform a manual calculation and accumulate the pressure loss in the pipings (and equipments, such as FKOD, stacks...) and that will be your backpressure (for single relief).



#4 Bobby Strain

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Posted 16 April 2014 - 07:32 PM

Amanda,

     Your concerns are unnecessary. Just size the piping for steady state values.

 

Bobby



#5 aroon

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Posted 16 April 2014 - 07:54 PM

Amanda,

 

Reality and conservativeness are two different approaches for the design of any system (here it is PSV outlet piping). However, conservative approaches are followed worldwide since beginning due to additional safety margins and to account uncertainty in the operation/process specs itself. API also advises to go with conservative approaches. Nowadays, many peoples are using dynamic simulators to capture the reality in cases and optimize the sizes (either the PSV or inlet-outlet piping) as they have strong reasoning (dynamic consideration) to prove their approach.

 

Now in your case, difference between minimum backpressure and header backpressure may not be that much significant and this factor will be compensated by available standard size of the pipe. Many times our piping is oversized by 10-15% due to standard sizes. I would suggest you to go with minimum backpressure as maximum and minimum values are generally set capture the worst cases and those sometime happen in reality.



#6 fallah

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Posted 16 April 2014 - 11:17 PM

Amanda,

 

In general, the sum of all pressure losses due to full rated flow (here in a single case) starting back from the flare stack's base (where the pressure is normally considered to be fixed at around 1-2 psig and included pressure losses due to such flow through the used seal, flare tip and stack riser) up to the relevant PSV yields the total back pressure for mentioned PSV. This back pressure must be lower than the maximum allowed back pressure in the relevant header and, if there would be simultaneous relieving from several PSVs to mentioned header, corresponding to the lowest set pressure of the psv...

 

Now let's know your real concern...



#7 Amanda Ferrari

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Posted 17 April 2014 - 06:07 AM

Hello Sirs,

 

Thank you for your attention with my topic.

 

Bobby,

 

Actually, I´m sizing the piping for steady state values. But, while, for a single relief, I think I can use values higher than atmosferic pressure as backpressure, my friends believe they need to use atmosferic pressure.

 

In my point of view I can use the maximum backpressure at Knock out drum as backpressure, since I have checked that I won´t reach the pipe spec limit.

 

Fallah,

 

For Emergency Shutdown of the entire unit (full rated flow), I´m doing what you have mentioned in order to check if all PSVs can work properly.

 

 

Aroon,

 

My intention in keep smaller diameter comes from an arrangement´s point of view. On offshore plants we have reduced space, so we need to optimize that. But of course keeping security as a primordial requirement.

 

 

Lai,

 

Let me see if I understood what you said (probably is the same I´m thinking): you size flare network for a ESD case and than you get the maximum backpressure. If a have margin to increase this backpressure in the tail line of each PSV (till the limit of piping spec), I can reduce the diameter. So the maximum backpressure obtained can be used to design PSVs tail lines.

 

Thank you!

 

Regards,

 

Amanda



#8 Bobby Strain

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Posted 17 April 2014 - 10:58 AM

You have all the tools and skills to calculate the pressure profile from the relief valve to the flare tip (or backward from tip to psv). Use them and you can't go wrong.

 

Bobby






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