10 replies to this topic

[tamueng]

Hello all,

 

I have some questions regarding the relief scenario and sizing basis of a PSV protecting a heat exchanger.

 

The exchanger is a heater with the tube side being liquid product entering the heater at 140 degrees F and 180 psig. The shell side is steam at 400 degrees F and 240 psig. The PSV is located on the tube side outlet line and has a set pressure of 400 psig. 

 

Per the typical 2/3rds or 10/13ths standard, a tube rupture scenario would not be considered. The product vapor pressure at shell side temperature (244 psig) is less than set pressure (400 psig), and pump deadhead is less than set pressure, so blocked outlet scenario is not considered.

 

All other scenarios simply result in a higher temperature product, which downstream equipment can accommodate. Per API 521, fire scenario is considered due to the heater being at grade level. 

 

This is the first sizing I have done being a tube side fire scenario with shell side being steam. Is it realistic to consider the sizing basis to be a fire scenario resulting in product vapor being discharged through the PSV, or have I overlooked something that I should also look into?  If so, would it be safe to assume the wetted surface area is equal to the total surface area of the vessel in this scenario?

 

Please see generic diagram below.

Attached Files

•  Screenshot 2023-11-07 153114.png   461.01KB   1 downloads

Edited by tamueng, 08 November 2023 - 11:05 AM.

[breizh]

Hi,

To support your work, you can review the documents attached. In part 2 you have a revision of the calculation.

Breizh

Attached Files

•  PRV sizing for exchanger tube rupture ( 2 ).pdf   1.62MB   23 downloads
•  PRV Sizing for Exchanger Tube Rupture (1).pdf   2.34MB   20 downloads

[fallah]

Hello all,

 

I have some questions regarding the relief scenario and sizing basis of a PSV protecting a heat exchanger.

 

The exchanger is a heater with the tube side being liquid product entering the heater at 140 degrees F and 180 psig. The shell side is steam at 400 degrees F and 240 psig. The PSV is located on the tube side outlet line and has a set pressure of 400 psig. 

 

Per the typical 2/3rds or 10/13ths standard, a tube rupture scenario would not be considered. The product vapor pressure at shell side temperature (244 psig) is less than set pressure (400 psig), and pump deadhead is less than set pressure, so blocked outlet scenario is not considered.

 

All other scenarios simply result in a higher temperature product, which downstream equipment can accommodate. Per API 521, fire scenario is considered due to the heater being at grade level. 

 

This is the first sizing I have done being a tube side fire scenario with shell side being steam. Is it realistic to consider the WCDS to be a fire scenario resulting in product vapor being discharged through the PSV, or have I overlooked something that I should also look into?  If so, would it be safe to assume the wetted surface area is equal to the total surface area of the vessel in this scenario?

 

Please see generic diagram below.

 

Hi,

 

In order to provide a proper response, please let's know the design pressure of the shell/tube sides...

[latexman]

Hi,

 

In order to provide a proper response, please let's know the design pressure of the shell/tube sides...

 

tamueng,

 

In addition to fallah's request above, no relief device is shown for shell side, which is required by Code.  Please let us know the details on that too.

[tamueng]

 

Hello all,

 

I have some questions regarding the relief scenario and sizing basis of a PSV protecting a heat exchanger.

 

The exchanger is a heater with the tube side being liquid product entering the heater at 140 degrees F and 180 psig. The shell side is steam at 400 degrees F and 240 psig. The PSV is located on the tube side outlet line and has a set pressure of 400 psig. 

 

Per the typical 2/3rds or 10/13ths standard, a tube rupture scenario would not be considered. The product vapor pressure at shell side temperature (244 psig) is less than set pressure (400 psig), and pump deadhead is less than set pressure, so blocked outlet scenario is not considered.

 

All other scenarios simply result in a higher temperature product, which downstream equipment can accommodate. Per API 521, fire scenario is considered due to the heater being at grade level. 

 

This is the first sizing I have done being a tube side fire scenario with shell side being steam. Is it realistic to consider the WCDS to be a fire scenario resulting in product vapor being discharged through the PSV, or have I overlooked something that I should also look into?  If so, would it be safe to assume the wetted surface area is equal to the total surface area of the vessel in this scenario?

 

Please see generic diagram below.

 

Hi,

 

In order to provide a proper response, please let's know the design pressure of the shell/tube sides...

 

 

Fallah,

 

Thanks for the response. The design pressures are as follows: S/T: 300/400 psig.

[tamueng]

 

Hi,

 

In order to provide a proper response, please let's know the design pressure of the shell/tube sides...

 

tamueng,

 

In addition to fallah's request above, no relief device is shown for shell side, which is required by Code.  Please let us know the details on that too.

 

 

Hello,

 

There is currently no overpressure protection on the shell side of the vessel, but I will look further into this.

 

Thanks!

[latexman]

tamueng,

 

For the tube side, heat is transferred from the fire through the heads, and from the shell side to the tube side. The most conservative approach is to use the heat  exchanger’s external surface area (shell and heads) as the exposed surface area for the tube side. If this approach results in an unacceptable Pressure Relief Device size, a more exact approach is to account for the two different heat paths.  On the part of the heat transferred from the shell side to the tube side, the limiting factor may be either the heat transferred from the fire to the shell side, or the heat transferred through the shell side media to the tubes.  You have to dig into the details to find out.

 

Your profile says you are in Texas.  IIRC, Texas does not require pressure vessels be Code stamped, so technically, on the shellside pressure vessel with no PRD, no laws are being broken, but it is RAGAGEP for both sides of a S&T HX to have PRDs.  However, there are some rare, legitimate reasons one might not want a PRD on a pressure vessel that is addressed by UG 140 in ASME VIII.  I'd recommend seeking guidance from a senior "PRD Engineer" in your company, unless, of course, you are that guy!

[Bobby Strain]

Your starting premise is incorrect. With the PSV as shown, it must be sized for liquid flow equivalent to vapor generation. Or liquid expansion.

 

Bobby

[breizh]

Hi,

Probably good to read.

Breizh

Attached Files

•  Fires, Vessels, and the Pressure Relief Valve CHE May 2000.pdf   865.83KB   0 downloads
•  Smith-Burgess.-Relief-Device-Inlet-Piping.pdf   387.71KB   9 downloads

[fallah]

 

 

Hi,

 

In order to provide a proper response, please let's know the design pressure of the shell/tube sides...

 

 

Fallah,

 

Thanks for the response. The design pressures are as follows: S/T: 300/400 psig.

 

 

Hi,

 

Then you cannot ignore considering tube rupture case; because the relevant criteria in the last Ed. of API 521 wouldn't be met and the hydrostatic test pressure of the low pressure side (300*1.3=390 psig) is lower than the design pressure of the high pressure side (400 psig)...

[breizh]

Hi,

To add to my previous replies, a note from Aspen tech about PSV sizing with examples.

Breizh

Attached Files

•  Aspentech equations-and-example -benchmark-calculations-for-emergency-scenario-required-relief-loads_compress.pdf   10MB   14 downloads