3 replies to this topic

[Karthikram]

Rupture disk (RD) is provided at the compressor aftercooler on the cooling medium side (outlet line) for the protection against tube rupture scenario.

 

Required relief flowrate has been calculated considering upstream pressure PAHH and downstream as relieving pressure and relief flow is estimated based on assumption of two orifice during tube rupture as a conservative approach.

 

RD size follows the inlet pipe size as a common approach. Outlet piping has been designed based on required relief flowrate (verified Mach and Rho.v2) as per API 521, 7th edition, section  5.4.1, table 8. The same has been specified in the Client specification to follow required relief rate for sizing.

 

Vendor has designed the RD based on required relief rate. Nominal size follows the inlet pipe size (general design approach) and provided free flowing area after rupture & other dimensions. Also, vendor has provided maximum flow through the rupture disk (rated flow/peak flow) based on free flowing area in vendor calculation sheet.

 

Client has requested to verify integrity of downstream piping based on peak flow based on Vendor data. Client knows that it is out of process/ piping design scope as per contract, however they insist to verify even though peak flow is expected for only few seconds & discuss the commercial issues later. They extend this comment for all compressor aftercoolers.

 

We performed FIV/AIV check for downstream piping and both fails.

FIV LOF > 1, need a piping support arrangement with natural frequency of 14 Hz to qualify the FIV LOF limit., which is difficult to achieve as per piping. FEA  study is recommended by third party.

AIV PWL is higher and LOF >1 which requires additional mitigation for the piping support.

Reaction force (provided by RD vendor) considering peak flow is almost double, current piping geometry and support structure is not designed for this load.

 

 

Can you clarify the points below,

 

- Is it mandatory to design the RD downstream piping considering short term peak flow. Any technical justification to avoid such requirement.

(Evidence/confirmation from API 521, Client Process Design Criteria & Vibratec (FIV/AIV third party) is provided but client needs additional justification)

 

- Can someone clarify the following statement as per  API 521, 7th Edition Section 5.4.2 (highlighted in red).

 

"For rupture-disk or buckling-pin devices installed as a stand-alone device (i.e., not upstream of a pressure-relief valve), the required

relieving rate is typically used to size the piping and the relief device. The design of downstream equipment, particularly scrubbers and

thermal oxidizers, should consider the higher load that can be encountered based on the upstream pressure at which the relief device opens.

The piping mechanical design should also consider this higher initial capacity."

 

As per my understanding, higher initial capacity/higher load refers to required relief flowrate considering Upstream PAHH condition (same as relief flow estimation mentioned above) and this is not the peak flow corresponds to free flowing area. Correct me if am wrong

 

- API 520 Part 2, 7th Edition, Section 6.5, RD outlet piping shall be designed based on transient loads. Is transient loads (reaction force) to be calculated based on short time peak flow or only based on required relief flow as per considering upstream pressure conditions. (as per RD vendor reaction force will be higher considering short time peak flow)

 

6.5 Stresses in Discharge Piping During Release

The reaction forces and stresses that originate in the downstream piping as a result of the release of a PRD

are typically not significant once flow is established and has reached steady state cond'tions, due to small

cnanges in pressure and velocity within the closed system components. However, large ‘orces may result if

tl"ere are sudden pipe expansions within the system or as a result of unsteady flow conditions during the

iritial activation of the relief device. Additionally, large reaction forces can be created at e bows as a result of

two-phase fluid flow in the slug flow regime. Mechanical loads from the initial flow from a rupture disk (both

steady state and transient loads) shall be included in the mechanical installation design. 

 

- Any previous experience on the similar subject. Any recommendations are provided in ASME/other design codes (SHELL DEP etc)

 

Appreciate your valuable inputs on this subject.

 

Thank you

Edited by Karthikram, 20 August 2025 - 02:59 AM.

[Pilesar]

My interpretations of codes and guidelines are not always consistent with industry practice. I try to choose design based on common sense and the behavior of the real world as I understand it. Then I compare what the codes say to see if there is agreement. Sometimes I learn that the code is reasonable, sometimes I am convinced the code is a barrier to good engineering. So I am not an official resource for you but will offer my opinion:

 

All equipment and piping should be designed to contain the fluid. Designing equipment for an average flow and pressure is not sufficient. Piping mechanical design should consider the expected peak flows and pressures so that there will be no loss of containment. Transient load is short time peak flow. Reaction forces are greatest at the initial activation. The piping should be designed for the largest reaction forces.

[Karthikram]

 

My interpretations of codes and guidelines are not always consistent with industry practice. I try to choose design based on common sense and the behavior of the real world as I understand it. Then I compare what the codes say to see if there is agreement. Sometimes I learn that the code is reasonable, sometimes I am convinced the code is a barrier to good engineering. So I am not an official resource for you but will offer my opinion:

 

All equipment and piping should be designed to contain the fluid. Designing equipment for an average flow and pressure is not sufficient. Piping mechanical design should consider the expected peak flows and pressures so that there will be no loss of containment. Transient load is short time peak flow. Reaction forces are greatest at the initial activation. The piping should be designed for the largest reaction forces.

 

 

Mr. Pilesar

 

Thanks for your opinion.

 

Is that recommended to perform AIV and FIV study for the short term peak flow condition? Can you suggest any reference.

AIV could be exempted performing actual No. of cycles to failure calculation considering Tube failure rate for the duration of few seconds.

However FIV fails (LOF>1), which has higher impact to piping to increase natural frequency & needs further FEA studies (commercial impact). Is it mandatory to consider short term peak flow in FIT point of view?

 

Thank you

 

- Karthik

[Pilesar]

If there is no expert here able to help you, perhaps there may be similar forums for piping engineers and mechanical engineers. I have no further suggestion. If you find a definitive answer, please teach the rest of us.