7 replies to this topic

[M_Menessy]

Dear all,

 

Thanks for your great efforts that encourage me to ask the following question;

 

Based on API RP 512, blowdown valve is required when the operating pressure is 1700 kPa or higher. Also, the blowdown system should depressurize the system down to 690 kPa or half of the design pressure within 15 minutes. Here comes the question, one of our engineers suggested to have a PSV (designed based on fire case) that sets at the half of the design pressure of a horizontal separator knowing that the operating pressure of that separator is lower than 690 kPa. She says that she is following the standard however, I am not comfortable with that solution.

 

What do you think about this solution as she claims that the PSV will replace the blowdown valve and as a result the blowdown valve will not be purchased.

 

I know that PSV does not protect the system from fire case and the solution during fire emergency is to depressurize the system (manually for example) using blowdown valve.  

 

Thanks in advance

[flarenuf]

Dear M,

 

it isnt really clear from your post as to what is being suggested here but.

 

you cannot replace a PSV with a DP valve and vice versa

 

they are two different functions. a psv is a protection device that needs no operator/control intervention.

for a vessel you should have one installed and sized for fire case and , if need be ,blocked outlet.

if you have a DP valve installed then this will be controlled by the F&R logic or be manulally activated.

the PSV set pressure cannot be higher than the design pressure.

 

you say " I know that PSV does not protect the system from fire case"

this is incorrect , if it sized properly then it will do

 

a vessel protected by a PSV does not necessarliy have to be depressurised in an emergency

 

flarenuf

[fallah]

Hi,

 

At first API 521 is correct...

 

Let's know the design pressure of mentioned vessel by which a proper response can be submitted...But in general, if a BDV is replaced by a PSV with set pressure of half of design pressure of the vessel; as long as the vessel pressure wouldn't reach to half of design pressure, the PSV wouldn't be activated and the operating pressure should be well lower than half of design pressure...

[lokeshmiddha]

Dear M,

 

I am not clear about your design philosophy. If you can set your PSV set pressure as half of the equipment design pressure which means there is still sufficient margin available between operating pressure and half of design pressure. then why to design at such high pressure.

 

blowdown valve is required when the operating pressure is 1700 kPa or higher but in your case operating pressure is lower than 690 kpa so you can avoid blow down valve and can use only PSV. 

 

Blow down valve is used for controlled flaring to reduce the system pressure down to 690 kPa or half of the design pressure to reduce the stress developed due to heat input. typical time is depend on the stress value for example, for vessel design pressure upto 20 barg typical time is 5 minutes not 15 minutes. and it does not mean that after reaching half of the design pressure or 690 kpa system depressurization  stops whereas PSV can only limit the pressure rise. i believe blow down system can not replaced by PSV.

[Bobby Strain]

    No one mentions that the requirement is the lesser of 690 kpa or 1/2 the design pressure.

 

    It is not uncommon to use a PSV also for blowdown. But the PSV musty be pilot operated with a reliable means to dump pressure from the main valve manually. Usually this means using a pneumatic operated system to block the pilot outlet and dump the pilot pressure input line to the PSV. Whatever is done, check with your insurer.

 

Bobby

[M_Menessy]

Dear gents,

 

Thanks again for your great answers. 

 

Our engineers aims to use PSV for both cases (pressure rise and emergency blowdown). For the first look, this may not be applicable. However, she thinks that is the PSV sets at the half of design pressure (which should be the value recommended by API 521 to reach after 15 minutes as practice), then there will be no need to have BDV. I concluded two things from your answers( correct if I am wrong);

1- As operating pressure is lower than 690 kPa, no need for BDV system.

2- PSV can not replace BDV as PSV closes after pressure decreases to the reseat pressure while BDV continues to discharge the inventory.

3- If the operating pressure is below 690 kPa, why to design a PSV to set at the half of the design pressure to replace the BDV knowing that BDV is not required anymore.

 

Thanks again for your great help 

[fallah]

Hi,

 

1- As per API 521, emergency depressuring for the fire case should be considered just for large equipment operating at a gauge pressure of 1700 kpa or higher.

 

2- If the operating pressure is lower than 1700 kPag, basically no need to consider BDV for depressuring and a PSV should be installed just for code compliance. If the operating pressure is higher than 1700 kPag, as per the reason you mentioned BDV cannot be replaced by a PSV and each of them should play its own role in relevant vessel protection.

 

3- Yes, if the operating pressure is below 690 kPag (or even 1700 kPag), as I mentioned, it's basically no need to BDV; then replacement by a PSV is automatically removed from the investigation and only a PSV would be installed for code compliance...

[M_Menessy]

Dear Fallah,

 

Thanks a million

 

Best wishes