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Preventing Vessel Rupture Due To Pressure Increase


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#1 SP500

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Posted 25 September 2016 - 07:50 AM

I am currently learning about process safety. I have encountered a question asking how could you prevent an enclosed process vessel rupturing due to the increase in pressure arising from an explosion inside the vessel.

 

So, my thoughts are that a pressure relief valve is inadequate for this purpose as the rate of pressure rise will be too high in comparison to the flow that valve will be able to accommodate.However what about a safety valve? It seems that the name should be self-explanatory that these are designed for such purposes. I believe a rupture/burst disk will do for this purpose but I am unaware of the technicalities, what I understand is that rupture disks allow higher venting areas for pressure relief and are thus more effective, but what happens to the released gas? Does it go to a flare? is it recirculated? Hopefully it isn't disposed in the atmosphere , probably it depends on the process. Could perhaps water spray and steam curtains do the trick? However these wont assist with the pressure rise but rather with any subsequent fire escalation control, so perhaps not.

 

I would appreciate some clarity on the matter, many thanks


Edited by Alex1995, 25 September 2016 - 07:53 AM.


#2 fallah

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Posted 25 September 2016 - 08:17 AM

Alex,

 

In most cases the pressure increasing rate is such that a proper selected PSV can accommodate the vessel pressure not to exceed higher than allowable limits (1.1 or 1.21 times of design pressure); but there might be a case, say a high rate chemical reaction, in which the pressure spike doesn't allow using a PSV at all and it will force using a RD to relieve the overpressure faster than that of a PSV to avoid passing the pressure limitation ...



#3 SP500

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Posted 25 September 2016 - 08:26 AM

So essentially, a PSV is used to accommodate pressure relief during normal operation but in emergency a RD does the job? Is there any difference between a PSV and PRV? Apart from a RD is there anything else than can give the necessary pressure relief in such situations?

 


#4 fallah

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Posted 25 September 2016 - 12:01 PM

So essentially, a PSV is used to accommodate pressure relief during normal operation but in emergency a RD does the job? Is there any difference between a PSV and PRV? Apart from a RD is there anything else than can give the necessary pressure relief in such situations?

 

Alex,

 

No, a PSV as last safeguard is used in the cases in which the pressure rises; more than normal operating pressure due to a process upset or an external heat source; at a rate such that a limited relieving area can accommodate relevant overpressure relief.

 

In the cases which there might be a pressure spike with very high pressure rising rate a PSV cannot accomodate the sudden overpressure relief hence a RD with having higher relieving area has to be used.
 



#5 sparsha

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Posted 25 September 2016 - 11:36 PM

Usually explosion is associated with huge increment in volume and relief valves and RD may not be able to accomodate it. You don't design your sytem to withstand explosions.



#6 Art Montemayor

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Posted 26 September 2016 - 05:36 PM   Best Answer

Alex:

 

After 53 years as a practicing engineer, I want to take this opportunity to address your query for your sake as a student (as well as all other students reading this) because I consider this topic as a very important one for chemical engineering students.

 

I strongly endorse and back up what Fallah has categorically answered - and in precisely the same tone:  there is no such thing as calling an operation a "normal" one while a safety relief device is being activated and venting excess pressure.  THAT IS NEVER to be considered as a normal operation.  That is either a major process upset and/or an emergency situation where remedial action must be taken to relieve and stop the cause of the upset.  A Pressure Safety Valve relieves excess pressure; unfortunately it cannot stop the cause of the excess pressure.  This is an ABnormal operation - not a normal operation.

 

For your sake as a student studying process safety, I highly stress the need for your acceptance of the above fact: a PSV is used to protect the human and machinery environment in the event of an abnormal process pressure increase.  The specific PSV is designed to have a relief capacity for a specific worst case scenario and that scenario is studied and identified as to credibility and flow capacity by process design engineers (probably you or other chemical engineers) encharged with that duty.

 

As Fallah infers, in the event that there might occur an extreme and sudden pressure spike (such as a runaway reaction, a deflagration and/or detonation, a process blow-through, etc.) a rupture disk or buckling pin might have to be incorporated in the system as a means to handle that possibility.  It is also the Chemical Engineers designing the process to identify the possibility of that occurring and size the appropriate relief device.

 

A normal operation is one where there are no relief devices in operation and all steady state flows, temperatures, pressures, and compositions are well within the design range for the unit.  When a process pressure upset occurs, the immediate response is to relieve the source of the pressure to atmosphere.  In the special case of toxic fluids, this relief must be done in a controlled manner - i.e., to scrubbers or containment systems where the gases or vapors can be handled and managed safely without entering the atmosphere.  Flaring a gas relief is used to avoid the gas proper entering the atmosphere as such, but rather as an oxide of the components - which generally are acceptable as emergency contaminants to the atmosphere - such as CO2.  How you control and manage such emissions is another subject and one that is also assigned to Chemical Engineers.  This requires a thorough knowledge of all available Unit Operations and Unit Processes in order to identify one that can be applied to the relieved gases.

 

I do not agree with Sparsha's comment about not designing your system to withstand explosions.  I have designed acetylene generation plant piping systems to withstand deflagration AND detonation.  This is a normal piping design requirement for acetylene generation plants and existing standards and codes exist.   I don't know where Sparsha got his/her belief, but it is not true.  If an internal detonation is possible, then you MUST design for it.  Otherwise, to design such a system without that capability would be a crime.



#7 SP500

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Posted 08 October 2016 - 11:26 AM

Thank you very much for your answers.


Edited by Alex1995, 30 October 2016 - 05:14 PM.





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