6 replies to this topic

[gunjan]

Dear all,

 

I have a query regarding the sizing method for the rupture disc.

 

As per section 5.11 of API STD 520 Pt-1, when you have independent rupture disk, sizing can be by two different method:

 

1) Rupture Disk Sizing Using Coefficient of Discharge Method (Kd = 0.62)

2) Rupture Disk Sizing Using Flow Resistance Method

 

However for the discharge coefficient method can be used only if rupture disk discharges to atmosphere, is installed within 8 pipe diameters from the vessel nozzle entry, has a length of discharge not greater than 5 pipe diameters and has nominal diamters of the inlet and outlet discharge piping equal to or greater than nominal pipe size of the device.

 

In my current project, none of the above condition is satisfied, still vendor (REMBE) is using the Discharge Coefficient method. As per vendor, none of the supplier in the market use Flow Resistance Method to size the Rupture disk. I have requested to provide KR factor (for resistance method) and try to validate the size selected and found that the size of the orifice which supplier was offering with Discharge coefficient method is actually smaller than required based on Flow Resistance Method.

 

Can anybody share their experience regarding the above discrepancies and way forward.

 

Thanks and Regards,

Gunjan

[breizh]

Consider this resource.

 

Breizh

[gunjan]

Hi Briezh,

 

I have done calculation for Flow Resistance Method using this document only. I am curious why vendor are reluctant to use this method even when the discharge coefficient method is not conservative as per my comparison.

 

Regards,

Gunjan

[fallah]

gunjan,

 

Coefficient of Discharge Method may only be used when a RD discharges directly to the atmosphere, while Flow Resistance Method applies in all other cases. Then in your case in which the conditions for using first method aren't satified you have to use the second method for RD sizing. If you (or vendor) use the first method for your case, it's obvious that the size of the RD orifice to be smaller because it has wrongly been supposed that the inlet and outlet lines of the RD are limited in length and almost no superimposed back pressure is available to be considered in sizing calculation...

Edited by fallah, 03 June 2014 - 04:03 AM.

[gunjan]

Hi Fallah,

 

Thanks for your reply. Vendor argument is they have taken care of superimposed back pressure by subtracting that from the burst pressure for sizing of the Rupture Disk with Discharge Coefficient Method.

 

In your experience, rupture disk sizing is done by Process Engineer or by Vendor? If by vendor, have they made the calculation by Flow Resistance Method? I just want to be double sure that what vendor trying to convince me is not correct that none of the vendor does the Rupture Disk Calculation with Flow Resistance Method.

 

Regards,

Gunjan

[fallah]

gunjan,

 

RD preliminary sizing is to be done by instrument/process engineers, but final sizing has to be done by vendor mostly using coefficient of discharge method. Such sizing would be supported by performing the subsequent standard tests.

 

Vendor is right to say: none of the vendors does the RD sizing with Flow Resistance Method; because this is the responsibility of the system designer. In fact, in such method RD is treated as a component in relevant relief system and vendor hasn't the system characteristics and geometry to do so even if do want. System designer has to check the RD certified capacity by using Flow Resistance Method and system characteristics to ensure the system relieving capacity is greater than the required process capacty.

Edited by fallah, 03 June 2014 - 05:17 AM.

[Bobby Strain]

If you don't agree with the vendor sizing method, then tell all who are providing bids to use the method of your choice. After all, it's your money.

 

Bobby