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Psv Sizing For Fire Case

psv sizing for fire case

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#1 krishnamurthy

krishnamurthy

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Posted 29 March 2023 - 08:42 AM

I have attached PSV sizing (simulation model as per Shell DEP)  for fire case for Production separator , which is a 3 phase separator. This is how we do in our earlier company, i could not understand why we flash 5% and 10% mass flow and why we subtract those mass flows to get the latent heat . Please explain this . 

 

Another question is that , the temperature is also adjusted by using SET to bring the temperature of stream downstream of the heater to the temperature of the stream near the first 2 phase separator . Can anyone please explain this. 

 

Please find the attached file your information. 

 

Thanks in advance. 

Attached Files



#2 PaoloPemi

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Posted 29 March 2023 - 11:22 AM

there are several threads in this forum discussing this topic and the details depend from the tools utilized...

 

since you export the results in Excel I briefly describe the procedure which I adopt,

1)  given the specific volume vs of the mixture (at operating conditions) you can calculate the (initial) relieving temperature (for PSV) solving a volume-pressure flash operation where vs is the initial specific volume which doesn't change depending from volume of container (separator) and the pressure is the PSV relieving pressure, 

you can solve the flash in different ways (see the threads at cheresources discussing this topic)
I have Prode Properties library which works with Excel  and I solve this flash with two macros in Excel, the first to calculate the specific volume vs and the second to solve the flash

=StrV(stream)

=VPF(stream, pset, vs)

2) the VPF() macro returns the  (initial) relieving temperature for PSV (this temperature will change as liquid will evaporate) with relieving temperature, pressure and mixture composition (found solving the flash operation) you can calculate all the required properties including latent heat (as difference of vapor and liquid enthalpy)

 

but, as said, there are different ways to get similar results
   



#3 Bobby Strain

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Posted 29 March 2023 - 12:54 PM

None of the simulators offer a pool boiling simulation. I would conduct 20-30 sequential flashes of 1% of the liquid from the previous flash. Calculate the orifice size required based on the properties of each flash. You should find a maximum size in this manner. Shell has the right idea, but doesn't go far enough. But maybe they know something I don't. If you can extract the information you need from HYSYS or others to Excel, It's not a difficult task.

 

Bobby



#4 Fletch

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Posted 01 April 2023 - 10:22 AM

Hopefully i can explain your questions

 

I have attached PSV sizing (simulation model as per Shell DEP)  for fire case for Production separator , which is a 3 phase separator. This is how we do in our earlier company, i could not understand why we flash 5% and 10% mass flow and why we subtract those mass flows to get the latent heat . Please explain this . 

For a multi-component mixture is very important to evaluate the relieving rate through out the duration of the fire.

 

At the start of the fire, the lighter ends will boil off first (Stream 46) - the first 5%

The remaining liquid (47) continues to boil and the next components boil off (48) - the next 10%

 

Often we continue the process, boiling the remaining liquid (49) until the majority is vapourised.

We can then determine the required reliving rate at each step.

There might be a particular component concentration that all boils together that governs this fire case

 

 

why we subtract those mass flows to get the latent heat . Please explain this .

 

Another question is that , the temperature is also adjusted by using SET to bring the temperature of stream downstream of the heater to the temperature of the stream near the first 2 phase separator . Can anyone please explain this. 

 

This appears to be a method your company uses to determine the latent heat of vapourisation.

There must be some acknowledgement that the latent heats of vapourisation that Hysys reports don't represent this particular flash step

 

They are looking to determine heat or duty required to vapourise just stream 48 (the NET energy required)

E-101-2 is the total stream flow, including all the work to heat up stream 55 and to vapourise stream 48

.

Steam 55 is returned to the initial temperature, as this is the residual liquid that didn't vapourise, so this is the duty didn't go into vapourising stream 48

E-102 is the duty of this residual liquid stream, while its cooled down, its representing part of the duty in E101-2.

 

By subtracting the two duties [kW or kJ/s], you are left with the energy to vapourise stream 48

By then dividing by the mass flow [kg/s], you are determining the heat of vapourisation of stream 48 [kJ/kg]






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