10 replies to this topic

[dgoyal]

Dear All
I am doing blowdown calculation for two- phase separator vessel on hysis.
I am using heat input model as FIRE API521.

My vessel design pressure is 15 barg and operating pressure is 12 barg and operating temperature is 100 deg F.

What the temperature and pressure I should consider for blow down calculation.

This blow down valve is design to operate on High-pressure switch and manually by operator

Please help me.

Thanks
Deepak

[JoeWong]

Deepak,
I would suggest you blowdown from High-High pressure trip setpoint with maximum liquid inventory (high level alarm).

I am not sure what type of facilities you have. The API521 Fire module is considering Pool fire. Please assess the potential of jet fire.

JoeWong

[dgoyal]

Thanks Joe
i wouul do that but in case of pool fire as iam doing what temprature at blowdown i should consider

[JoeWong]

I would guest you can base on the temperature generated by HYSYS. Personally, i would think the temperature have quite minimum impact to blowdown as compare to pressure.


JoeWong

[dgoyal]

Hi Joe

I have seen great impact by start temperature also and hysis is not calculating any temperature increase with time during depressurization. So I am not sure about what you are recommending.

Hope if you can give some other idea or shall I take normal operating temperature

Deepak

[JoeWong]

You may assess what the possible maximum normal operating from the system. This probably will give you maximum blowdown rate.

JoeWong

[janson_kaniaz]

Hi Joey,

I was wondering why we need to conduct blowdown theoretically from design pressure or psv set pressure and at normal operating temperature. Is that practically sound?

In what condition will we have/ reach design pressure and operating temperature?
Many literatures I have read so far only pay attention to state that the worst case should be at design pressure and none touches on what the temperature should be. While it is true that it gives less significance as compared to pressure, shouldn't we be at least clear on what it should be, at least practically?

Thanks Joe
i wouul do that but in case of pool fire as iam doing what temprature at blowdown i should consider

Use maximum operating temperature as the input for fire case calculation( It is asuumed that heat exchanges are stopped)

[aju_1807]

Dear All
I am doing blowdown calculation for two- phase separator vessel on hysis.
I am using heat input model as FIRE API521.

My vessel design pressure is 15 barg and operating pressure is 12 barg and operating temperature is 100 deg F.

What the temperature and pressure I should consider for blow down calculation.

This blow down valve is design to operate on High-pressure switch and manually by operator

Please help me.

Thanks
Deepak

The depressurzation should be started from design pressure to 6.9 bar g or 50% of design pressure whichever is lower in 15 minutes. As per API521 we can depressurize system upto 50% of DP within 15 minutes if the vessel thickness is more than 25 mm.

Further, depressurization should be always started with highest denisity case (for worst relief load). Lowest temperature and highest mol wt is gthe worst case for sizing.

I hope this clarifies your query.

Regards,

Aju

[JoeWong]

...depressurize system upto 50% of DP within 15 minutes if the vessel thickness is more than 25 mm.

Vessel strength subject to material type. The quote figure may be applicable to Carbon Steel. However, it may be different for "softer" material such as Stainless steel.

I used to believe "...depressurize system upto 50% of DP..." until recent findings tell me that this may require some concept adjustment...
My advice is maximize flare load capacity by depressure system at shortest time.

[aju_1807]

...depressurize system upto 50% of DP within 15 minutes if the vessel thickness is more than 25 mm.

Vessel strength subject to material type. The quote figure may be applicable to Carbon Steel. However, it may be different for "softer" material such as Stainless steel.

I used to believe "...depressurize system upto 50% of DP..." until recent findings tell me that this may require some concept adjustment...
My advice is maximize flare load capacity by depressure system at shortest time.

I agree but if we demonstrete that the vessel is not lossing it integrity then its applicable for other vessels also. Further, its not always required to depressurize the system to 6.9 bar g (as we are unnecessarily increasing the flare capacity). Of course, this philosophy always requires clinet approval as some of the clinets alwyas prefer to depressurize upto 6.p bar g irrespective of API clause..

This phenomena unnecessarily introduces the sequential depressurization (which is quite complicated to implement). If we are depressurizing all vessels with teh same criteria of 6.9 barg in 15 minutes then it makes your flare system and capacity very bigger (particularly in case of UPS failure on demand).

Regards,

Ajay Sharma