Heat Exchanger Over Pressure Protection

[AKBennett]

Hi everyone,
I am new to the forum and am a recent graduate of mechanical engineering from the University of British Columbia. I have just started my first job with a natural gas company and one of my first assignments is sizing and selecting over pressure protection for our heat exchangers. The tube side can run up to 800 psig of NG and the shell side is running at extremely low pressure (water)and rated for 150 psig. The problem is the heater we use is only rated for 30 psig so my question is if a tube were to rupture at 800 psig is it even possible to relieve the shell side before it pressures up to just 30 psig. It seems to me the best route to go is a rupture disk. Dose anyone have any experience with a situation like this or can they point to towards some manufactures / vendors who would be helpful for sizing? Thanks.

 

[TD2K]

Hi there (class of 1981, Chemical Engineering).

Yes, for ovepressure protection like you have described a rupture disk is usually the way to go. First, a PSV will not react fast enough and secondly, at those pressures the volume of gas to be relieved would likely rule out a conventional safety valve.

this question just came up on the site not long ago but I can't seem to find the link. the other company I would talk to is Rupture Pin Technology.

 

[25362]

Visit:

ftp://ftp.fike.com/fmpdocs/TB8100.pdf
ftp://ftp.fike.com/fmpdocs/TB8102.pdf

as tutorials.

One may also consider adding PRVs on the water piping to protect the water system from temporary peak pressures that may develop in the exchanger.

 

[25362]

BTW, I once read that, depending on the shell length, three rupture discs would be advisable. One in the middle and the two others at 1/4 way from each end.

This would probably save the shell. However, the shock may bring about water hammering to the water system, which should, thus, be protected as well.

 

[Montemayor]

AK:

As a recent grad, you would be rewarded if you visit:

http://www.cheresources.com/asiseeit.shtml

Where you will find a series of 6 excellent and detailed articles on the design and application of rupture discs. I recommend you digest the expert information given there.

TD2K is precisely correct in pointing out that this is an obvious application for a rupture disc or a buckling pin. A conventional PSV cannot compete with the reaction time of these devices in this type of application. I no longer recommend rupture discs in light of the great advantages and characteristics of the buckling pin. The buckling pin reacts even faster than the rupture disc and gives you:

*Accuracy and repeatable results;
*immunity from corrosion; more economical;
*simplicity;
*can be used in positive or negative pressure applications;
*and many more advantages.

With an appropriately designed buckling pin you should have no problem venting the mixture of water and natural gas resulting from a gas tube rupture. For details on buckling pins go to:

http://www.rupturepin.com/

I hope this helps you out.


Art Montemayor
Spring, TX

 

[AKBennett]

Thanks for all the info guys, I'm working my way through the links and associated articles. I really appreciate the feedback and will post the outcome of the project and reasons for the preticular design selection.

 

[EGT01]

AKBennett,

To add to your reference material, if you don't have a copy of API RP-521 then you should get one, API RP-520 as well. There is a section in API RP-521 that covers heat exchanger overpressure protection with which you should become familiar and will assist you in how to determine your required relief rate. There is also some discussion about the issue to which 25362 refers that is related to transient pressure surge.

Keep in mind that "sizing" of your relief device is only one step in the overall "design" of your relief system. A relief device manufacturer or vendor will be able to aid you in sizing your relief device but only you can design the system. From your first post, it appears that you have given conflicting information about your low pressure system design pressure. You will need to resolve that before you can determine the proper relief set pressure or start your sizing calculations.

 

[BenThayer]

no mention was made of the size of the heater or at least i missed it.

you might consider replacement of the heater if it gets too absurd to relieve it. you might only need a design for ~ 600 psig.

the references to other sites (montemayor) and the use of RP-520, etc. will point you in the right direction. when your operating pressure gets too close to the set-point of the rupture disc, you can expect some unexpected "bursts" due to transient conditions such as valve shut-offs, etc.

if the heater is quite small, replace it with a suitable pressure and take the failed tube scenario out of the list of credible scenarios.

 

[EGT01]

Increasing the pressure rating of the low pressure side of the exchanger itself will help to ensure that damage to the exchanger due to a tube failure may be unlikely but to properly assess the consequences of the tube failure, you should also consider the effects to the entire low pressure system into which the high pressure side can flow.

I would suggest reviewing API RP-521 on the subject. As additional info, take a look at the technical interpretations for API RP-521

http://committees.api.org/standards/tech/reti.html

there are a number of interpretations related to heat exchanger overpressure protection and tube failure.

http://committees.api.org/standards/tech/docs/521ti.xls