7 replies to this topic

[SPC]

Dear all,

 

       I have done calculation for following :

 

Purpose : Thermal relief Valve / TSV Sizing located on Pump discharge header pipeline.

                                                                                              ( Pipe size: 3" & 1700ft long).

 

( Although 3/4" X 1" is sufficient as a  good engineering practice, however calculation is requirement of my Client.)

 

 

 

Problem : Input Data for Calculation not available :

 

Set Pressure / Design Pressure of Pipeline to be protected by TSV.

 

Resolution / Methodology :

 

I have calculated Pump Shut-off Pressure. PSV Set pressure kept 10% More than set pressure.

 

Design Pressure of Pipeline decided to have same i.e. 10% higher than PSV Set pressure.

 

 

Your advise / help is highly appreciated.

 

Regards,

 

SPC

 

 

 

 

 

[J_Leo]

Hello SPC,

 

I haven't done thermal relief sizing myself. If you have to do it, I think you can calculate the relief rate base on the liquid coefficient of expansion, on condition that no vaporization will occur due to temperature increase.

 

Relief rate = Liquid Coefficient of Expansion * Protected Pipe Volume * Temperature Change Rate

 

That is my thought.

 

 

Leo

[Saml]

Line design pressure or a "class" should be marked on P&ID.

If not, you can check the piping and accesories involved and take the lowest.

And your PSV set pressure can be as high as permitted by the design code (ie. ASME B31.3)

[SPC]

Hi Saml / Leo,

 

           Thanks a lot for your suggestion.

 

Saml: In this case,  I have checked following:

 

It shows for ASME B31.3 Pipe for pump discharge line 3" / Thickness-0.216mm ( Considered 40Schedule) as below:

Maximum Allowable Pressure (psi) = 2766psi  for Pump shut-off pressure 146 psig & Pump Max. operating Temp= 140 degree Celcuis.

 

Pipe class details is 150 Rating , Pipe steel Maximum Allowable non-shock Pressure (psig) is 260psig & Hydrostatic Test Pressure is 450 psig)

 

 

Query: Shall I consider Hydrostatic Test Pressure as per Pipe class 150 rating table OR Max. allowable Pressure for Pipe as per ASME B31.3?

 

OR  Pipe design pressure = 1.1 x Pump Shut-off Pressure

 

 

 

 

Your advise is highly appreciated.

 

Regards,

 

SPC

[Saml]

Here I would like the input from another more experienced on this topic forum members. This is what I understand by I know I may be missing some details.

 

The issue with design pressure or MAWP for vessels is that the design codes are a "one way" methodology. That is you define the design pressure and calculate the line or vessel thickness, and that is your "design pressure" or "MAWP". The codes require that you have to set the pressure relief devices at this originally assumed pressure regardless of the overdesign that comes from selecting the next commercially available pipe or plate. So let's say that your chosen pressure is 70 psig. You select a sch 40 pipe can be used up to 260 psig. Your design pressure is 70 psig. Had you initially selected 120 psig, the designed pipe would probably be exactly the same in mechanical terms, but your design pressure would be 120 psig.

 

Many companies have internal "Classes" or "Piping Standards" that define all the details from maximum design presure,  pipe to use, accesories, branches configurarion, welding procedures, hydrostatic test pressure, etc so you don't design every line on its own design pressure. You take one that fits your need and design to that class.

 

So you have the following options:

 

- You have the design calculations of the line and the pressure test documents. Your relief has to be set at the design pressure.

 

- If you are designing you can pick wathever fits your need. 1.1 of shutoff pressure may be an option. I would take a wider margin myself if it does not incur in extra costs (that is, thicker or higher class accesories). 10% operating ratio is OK for main process PSV, but for small TRV that receive less attention and is subject to  startup cycles and probably pressure spikes, I would take a higher value. There is no rule on this.

 

Just be aware that the case of the system blocked in with only the suction of the pump closed may be a credible one. Then your weakest part would be the pump casing.

 

-  The line is designed, build and tested to a specific company "class". Then your set pressure is whatever that class establish as desing pressure.

 

- You have nothing. Then you have to inspect the line and do a fitness for service calculation.

[SPC]

Dear Mr. Saml,

 

          Lot many thanks for clearing my confusion regarding :

 

1. Selection of Design Pressure  from various options for its data / sources.

 

2. System blocked-in - Yes, I do agree & understand that from suction (Isolation Valve) to discharge Line (Isolation Valve), Weakest part is Pump casing. Hence for this reason our client might have recommended one TSV just near Pump (suction side flange ).

 

Lot many thanks.

 

 

Regards,

 

SPC

[J_Leo]

Hello SPC,

 

Generally, I see practice to use the pump shutoff pressure as the design pressure of downstream piping and equipment. Thus, the thermal relief PSV set pressure will be the design pressure of the piping. The pump shutoff pressure will have to be confirmed when the vendor is selected. 

 

Not quite sure why you need a margin over the shutoff pressure.

 

 

Leo

[SPC]

Yes Leo,

 

       I have done one mistake in my  project!

 

You are correct. Pump shut-off pressure can be taken as design pressure for Piping & equipment located at the downstream side / discharge side of Pump .

 

Once again thanks to you for your valuable suggestion.

 

Regards,

 

SPC