6 replies to this topic

[SandeepanGoswami]

"PSV Design conditions:

Opinion-1: As a process engineer, design pressure = relieving pressure and design temperature = relieving temperature.

Opinion-2: Vendor understanding is that design pressure = relieving pressure and design temperature = process design temperature of the vessel on which is the PSV is mounted.

As a process engineer i think opinion-1 is correct as when a PSV pops it shall be emergency scenario and PSV is considered as a final safety device hence it should be designed to handle relieving pressure and relieving temperature."

Request all readers to share their views

Edited by Sandeepan, 10 March 2014 - 12:43 AM.

[aroon]

Hi Sandeepan,

 

Don't mess with the terminology. Always try to understand the meaning of the terminologies as given in API 521 Section-3 (5th edition). As far as answer of your question, anyone can interpret your both options wrong.

 

For the sizing of particular scenario;

 

Relieving pressure = Set pressure of PSV + allowable overpressure (10%, 16%, or 21% of the set pressure)

 

10% ..... is for design contingencies with single PSV

16% ..... is for design contingencies with multiple PSVs

21% ..... is for fire contingencies irrespective of number of PSV present.

 

All the above values are valid for design pressure above 30 psig with ASME VIII code stamp vessels. For vessels design with ASME VIII code stamp and design pressures falling in between 15 to 30 psig, overpressures are as follows:

 

3 psi ..... is for design contingencies with single PSV

4 psi ..... is for design contingencies with multiple PSVs

21% ..... is for fire contingencies irrespective of number of PSV present.

 

Note that ASME Section I code stamped vessels and low pressure tanks are having different criteria's for overpressure.

 

Hope this may be useful for you.

 

Rgds,

Aroon

[fallah]

"PSV Design conditions:

Opinion-1: As a process engineer, design pressure = relieving pressure and design temperature = relieving temperature.

Opinion-2: Vendor understanding is that design pressure = relieving pressure and design temperature = process design temperature of the vessel on which is the PSV is mounted.

 

 

Hi,

 

Option 2 (Vendor) is correct other than required corrections on relieving pressure as Aroon well described...

[SandeepanGoswami]

Dear Aaroo / Fallah,

 

Please note that I meant PSV design conditions for Fire Case PSV.

Hence Relieving Pressure = Set pressure x 1.21

However, the query is mainly to determine the PSV design temperature (inlet flange for PSV + spring for PSV) of a  conventional PSV, should it be considered same as relieving temperature?

 

As per current project:

Vessel Design Pressure = 144 kg/cm2g

Vessel Design Temperature = 50°C (DT)

Fire Case PSV Set Pressure = 144 kg/cm2g

Fire Case PSV Relieving Pressure = 174.24kg/cm2g (RP)

Fire Case PSV Relieving Temperature = 207 °C (RT)

Fluid is at super-critical condituons at RP/RT

 

So, what should I advise vendor to consider as design conditions.

If combination is RP + DT then as per API 526 inlet flange rating is 900#

If combination is RP + RT then as per API 526 inlet flange rating is 1500#

 

Also note the piping MOC for inlet to fire case PSV is DSS (Duplex Stainless Steel)

Edited by Sandeepan, 10 March 2014 - 12:15 PM.

[fallah]

Hi,

 

Because the vessel design temperature is already fixed equal to 50 C, i.e. relieving temperature of the fire case hasn't been considered in mechanical design of the vessel, then no need to consider relieving temperature in PSV design conditions. It means, if fire case would be happened, before the vessel can be subject to relieving temperature has been failed.

 

Therefore the fire case PSV should be designed based on combination of RP (or even SP)+DT...

[SandeepanGoswami]

Dear Fallah,

 

So, as per experience do you think my design is ok or should I intimate my mechanical counterpart of the same.

It shall be very helpful to know from your experience as to how you have designed your pressure vessels

[fallah]

Sandeepan,

 

The info you provided isn't adequate to be able to evaluate the vessel design, also here the main matter is PSV design conditions instead of the relevant vessel design itself.

 

Anyway, as i mentioned the fire case PSV should be designed based on RP (or even SP)+DT...

 

Please don't hesitate to submit any clarification you might need in the way you are going...