12 replies to this topic

[ekar]

Dear All,

 

In an overpressure condition due to tube rupture, the tube, which is the high pressure side, is ruptured, resulting in its fluid pressurizing the shell (which is the low pressure side) and causing overpressure. According to my company guide for this scenario (based on "PRV Sizing for Exchanger Tube Rupture" article), the relieving rate is a function of pressure drop (among other factors). Pressure drop is defined as high side design pressure less the overpressure value of the low pressure side or the critical flow pressure, which ever is greater.

 

I wonder why the high side pressure should be the design pressure. It seems that tube rupture doesn't happen only when the tube side reaches its design pressure. Instead, the rupture can ostensibly happen at the tube flow's operating pressure, or others. (this is assuming the tube side operating pressure is higher than the shell side design pressure).

 

There may be a simple explanation for this. I would appreciate your input/explanation.

 

Thanks!

[fallah]

ekar,

 

According to API 521 for tube rupture case, using maximum possible system pressure instead of design pressure may be considered as the pressure of high-pressure side where there is a substantial difference in the design and operating pressures for the high pressure side of the exchanger...

[S.AHMAD]

1. My suggestion is you calculate the flowrate based on operating pressure and design pressure.

2. For safety reason, select the worst case flowrate as the basis for determining the PSV size hen you can figure out the reason why the company guideline is based on design pressure.

[fallah]

AHMAD,

 

No need to do the calculation. It is obvious, worst case flowrate would be achieved based on design pressure...

[S.AHMAD]

Fallah

Yes I know. I just want ekar to think about it so that he understand the reason for using design pressure.

[ekar]

Fallah & Ahmad,

 

Thank you for your replies.

 

Fallah - I'm aware of the API 521 clause. Do you have any figure / criteria on how big should the difference be for the clause to apply? If the difference is 20 psi, would it be considered "substantial"?

 

Ahmad - yes, the bigger PSV size will be for design pressure consideration. I can understand it from safety consideration (take no chances). I was just curious because in other cases (for example, blocked valve) it could be seen more easily how the pressure could go up to design pressure, while for tube rupture, the rupture might happen before design pressure was reached.

 

Thanks again for your input & guidance.

[fallah]

ekar,

 

[quote]...Fallah - I'm aware of the API 521 clause. Do you have any figure / criteria on how big should the difference be for the clause to apply? If the difference is 20 psi, would it be considered "substantial"?...[unquote]

 

In fact, there is no generalized criteria shows how big should the difference be and the matter should be investigated case by case, but if there wouldn't be any danger of operating pressure change due to process upset or operator error and the difference would be at least 20%, using operating pressure instead design pressure could be reasonable... 

[narendrasony]

ekar,

 

According to API 521 for tube rupture case, using maximum possible system pressure instead of design pressure may be considered as the pressure of high-pressure side where there is a substantial difference in the design and operating pressures for the high pressure side of the exchanger...

 

What could be the reason for this statement in API-521 when it is obvious that design pressure will be always higher than the maximum operating pressure (maximum possible system pressure) and hence worst case flow rate would be at design pressure only . This is confusing.

 

Regards

Narendra Kumar

[fallah]

Narendra Kumar,

 

In fact, by that recommendation API 521 has tried to mitigate the relief load value of the PSV in tube rupture case when there is a substantial difference between operating pressure and design pressure of the high pressure side. Then applying this recommendation will lead to lower relief load and consequently smaller PSV orifice area...

[narendrasony]

Thanks Fallah.

But is it appropriate to take credit for this substantial difference between operating pressure and design pressure of the high pressure side (tube side)? How can we ensure that operator will always maintain this difference?

 

 

Regards

Narendra Kumar

[fallah]

Narendra Kumar,

 

If you note to 7th post, i mentioned:

 

 ...but if there wouldn't be any danger of operating pressure change due to process upset or operator error...

 

Then, besides the substantial difference the above conditions should also be existed to enable you to mitigate the relief load by taking operating pressure instead design one...

 

To wrap up the matter it could be said that in most cases due to a limited difference, the design pressure of high pressure side would be taken and just in rare cases with substantial difference because the huge relief load will lead to selecting very large PSV(s), the operating pressure has been considered. Obviously, in those rare cases the above mentioned conditions (bold line) should already be established and relevant operating manual should be included the procedure to prevent any process upset leading to rising the normal operating pressure more than prespecified value... 

Edited by fallah, 06 March 2013 - 07:15 AM.

[kkala]

<http://www.cheresour...ture#entry63526> may be also interesting to the query of post no 1. Normal operating pressure is considered in the high pressure side..

[chemsac2]

ekar,

 

One scenario that can have significant difference in design and operating pressure would be centrifugal pump. Centrifugal pump discharge is designed for pump shut-off which considers design pressure of suction system, highest suction liquid level and shut-off head of pump.

This gives large difference in operating and design pressure. 

 

I am aware of at least one engineering company guideline which asks for maximum operating pressure for tube rupture in such cases.

 

Also, low pressure side can be blocked for maintenance or process reasons (e.g. SDV closure/control valve throttled for temperature control etc) for long durations. High pressure side can reach its design pressure sometime during this period. Tube may be already ruptured or leaking. If it can be proven that such blocking of exchanger is not possible, maximum operating pressure of high pressure side can be used.

 

Regards,

 

Sachin