4 replies to this topic

[Cue]

Dear all,

I would like to get some opinion regarding how to size the bursting disc.

I have high pressure gas (PSHH at 310 barg) at the tube side and cooling medium at the shell side. Shell side design pressure is 10 barg.

To size the bursting disc, i try to optimize the relief line and bursting disc size to be within the allowable pressure drop limit (shell side hydrotest pressure - flare backpressure). Refer to my query as per following:

1) To determine the relief load, the gas mass flow rate is calculated based on critical flow equation at PSHH and then volumetric flow is calculated based on the gas density at relieving condition (1.1 x shell side design pressure). The calculated volumetric flow is the liquid displacement rate for the bursting disc sizing. But since the upstream pressure is very high compare to the relieving pressure, very high volumetric is observed. i am abit doubt with the results. Is this method correct to determine the liquid displacement rate?

2) Pressure drop from the ruptured tube to the relief line connection is considered in the overall pressure drop to determine the relief line & bursting disc size. I have the data for pressure drop during the normal cooling medium flow through the exchanger. Can this data be used to determine the shell side pressure drop with relieving rate. Is the relation of dp proportional to flow^2 which is applicable to pipe flow can be used here or simple ratio between the flow and pressure drop will do?

Many thanks.

[Mark-TR]

Hi

I advise you to review API 521 chapter 5.19 , there you have a detail discussion on how to design for tube rupture scenario.

API does not state clearly if liquid displacement shall be considered when a tube rupture scenario is considered for the design. Nevertheless, it clearly states that in fire scenario you should consider the liquid displacement in equipment full of liquid due to vapor production in the vessel see API 521 5.15.3.3.

Therefore, it is a conservative assumption to consider liquid displacement. Furthermore, in your particular case, and due to high difference in pressure, it seems to me that the rupture disk is needed to avoid a surge in the equipment. Surge pressure happens because of the liquid sudden displacement and can be only protected by rupture disk or antisurge valve. PSV are not fast enough.

Consequently consider liquid displacement at the beginning of the event seems quite correct.

Volumetric displacement of liquid shall be equal to volumetric gas flow at relieving conditions.

Please note that API 521, allows to consider the operating pressure at the high pressure side instead of the design pressure. These criteria may significantly reduce your relieving flowrate.

Usually the pressure drop on the heat exchanger is neglected in the design and only pipe lines pressure drop is considered. As it is normally difficult to evaluate the pressure drop on the heat exchanger and it is a more conservative approach. Less pressure drop on the downstream system relieving system=> more diferential pressure on the tube rupture and then more flow.

As the objective of the rupture disk is to protect for surge peak pressure, I advise you to install it as close as possible to the heat exchanger. Don’t forget to also consider the gas relieving flow on your design as the steam may be the sizing scenario for the pipe line and/or your rupture disk.

i advise you to also check API 520 chapter 4.3 you have the discussion on how to size rupture disk, I advise you to carefully select the burst pressure considering the manufacturing range of the rupture disk. it is an industry common mistake to neglect this value, you can find the details on API 520 chapter 4.3.

regards

Edited by Mark-TR, 05 July 2012 - 05:08 AM.

[fallah]

cancer,

IMO, "PRV sizing for exchanger tube rupture" By: Wing Y. Wong in Hydrocarbon Processing, Feb. 92, presents a reasonable procedure for relief load determination in tube rupture case. Bursting disc (or any proper relief device) sizing can be easily performed after precise relief load calculation.

Fallah

[lokesh_09]

Hi,

Please refer IP guidelines "Guidelines for the Design and Safe Operation of Shell and Tube Heat Exchangers to Withstand the Impact of tube failure" document for rupture disk sizing procedure and design consideration.

Regards,
Lokesh.

[Kunal_DNV]

Hi Fallah.

Can you please email "PRV sizing for exchanger tube rupture" By: Wing Y. Wong in Hydrocarbon Processing, Feb. 92" on kunal14284@gmail.com

Thanks in advance.