Wetted Surface Area 2

[npf]

All:

I am in the process of calculating surface area for 3 stacked shell & tube heat exchangers and hence calculate the relief rate. Shell length is 24ft and ID is 37".
Should I take my total length to be 72 ft(24*3) and ID -37" and determine the surface area for horizontal cylinder.

Also I am trying to determine the liquid flow rate for my tube rupture case, can anyone provide a method to determine this.........

Help would be appreciated.

Regards,
sp77

 

[MortenA]

With regards to the fire case:

Yes assuming tha the HX is liquid on the shell side then the area=area of all HX's.

Is it also liquid in the tubes?

Anyway: Decide how large your rupture is (a hole in the pipe or total sewerance óf the pipe and then do an "orifice calculation" to determine the flow rate with the downstream pressure equal to the PSV set point plus margin (10%) and the upstream equal the max presuure on the tube side and the tube ID (or hole size) as the orifice size. I would disregard any other dP since they most likely will be quite small compared to the loss in the rupture.

Best regards

Morten

 

[npf]

Thanks Morton:

Using the orifice formula W = 1891*Y*d1^2*C*(dP*rho)^0.5, I calculated the flow rate to be 133,700 lbs/hr.
Also I read that as aconservative approach, the flow should be based thru 2 orifices. Could you please explain that.
If so then it would be aound 267,450 lbs/hr.

Again thanks for the help
Regards,
sp77

 

[MortenA]

I will refer you to API 521 sec 3.18.3:

In the first set of bullitpoints the "c" bullit reads:

"The high pressure fluid is assumed to flow both through the tube stub reamining in the tube sheet and through the other longer section of tube"

Disregarding the fact the flow from the opposite side oif the rupture will be less due to pressuredrop in the long pipesection is a conservative measure. If you are cutting it close then maybe you can get a somewhat ower flow from the "long" side - but also a more complicated calculation.

Read the section before (3.18.2) carefully with regards to set pressures etc.

Best regards

Morten

Best regards

 

[pleckner]

For your fire case, you only need to consider the first 25 feet above the source of the flame (API RP521 3.15.1.1). IF your exhanger sits 1 foot off the ground above the fire, then you only need to consider 24 feet in you determination of wetted surface area.

 

[MortenA]

forgot about that - one star for pleckner!

Best regards

Morten

 

[EGT01]

SP77,

Regarding your question about whether you should use 3*24=72 ft, the heat absorption equations were developed recognizing the fact that larger vessels were less likely to be completely engulfed by flames compared to a smaller vessel. Hence, the wetted area is raised to a fraction of a power, for example A^0.82 is used to determine total heat input, btu/hr.

What this means is you will get a lower heat input rate by considering your 3 exchangers as being one long exchanger because (3*24*37/12)^0.82 < 3*(24*37/12)^0.82. I believe the proper way to determine your heat input would be to consider the heat input to each exchanger separately using the latter approach.

 

[npf]

Thanks all,

EGT01 -you are right I need to evaluate both the scenarios. Also I have one question , how do we determine whether the fluid is supercritical? And if it is how do I determine the vapor properties for my fire case scenaroio?

Regards,

sp77.

 

[EGT01]

A fluid becomes supercritical when the system temperature and pressure exceeds the fluid's thermodynamic critical temperature and critical pressure. Beyond that point, there is no distinction between liquid phase and gas phase. Here's a link to a little more information and a video clip of CO2 undergoing a phase transition to supercritical fluid.

http://www.chem.leeds.ac.uk/People/CMR/whatarescf.html

For information about how to handle relief of supercritical fluids, see the discussion in this thread...
thread1203-137692

Unfortunately, I see that the links are no longer working. The details of the article from Chemical Engineering Progress are...
"Rigorously Size Relief Valves for Supercritical Fluids", by Ryan Ouderkirk, Chemical Engineering Progress, August 2002

Keep in mind, that you may have a supercritical fluid upstream of your relief valve but you need to consider what happens to the fluid as it "depressures" through the relief valve into the relief valve discharge system.

 

[npf]

Thanks EGT01!!