8 replies to this topic

[FancyWhale092]

I am verifying the sizing calculations on a PSV in my facility. The PSV is on the cooling water line that feeds into the tube side of a shell and tube heat exchanger. The water operates normally at 100 psig, 80F, and 2.3 million lb/hr. The PSV has a set pressure of 150 psig, and it is a 6x8 PSV with an orifice area of 17.78 sq. in. 

 

On the shell side, is 900,000 lb/hr of CO2 operating at 140F and 1400 psig. There is a PSV protecting the shell side of the exchanger that relieves at 2450 psig.

 

Again, the PSV in question is the PSV on the water line (tube side). My question is, do I need to do calculations based on a tube rupture scenario, and if so, how do I go about doing that, given that the water is in the liquid state, and the CO2 is supercritical? 

 

I am a process engineer almost 2 years out of school, and this is really my first time doing PSV calculations. Thank you for your help in advance. 

Edited by FancyWhale092, 11 March 2022 - 04:48 PM.

[Bobby Strain]

Well, you have a real problem. You best consult with someone in your organization. A PSV does not offer protection against tube rupture as normally defined.

 

Bobby

[Pilesar]

Do you know the conditions at the relief valve when it opens? Can you know the pressure of the fluid at the PSV? Can you determine the temperature? Can you determine the fluid phase at that pressure and temperature? Will the fluid be vapor, liquid, solid, or some mix? Go through the calculation steps to find the answers. Then determine what the flow rate would be through the PSV. If the flow rate is less than required, then the PSV would not provide adequate protection for that scenario.

[Bobby Strain]

We should not encourage an inexperienced engineer to analyze this system. It's the dynamics that create great overpressure. It's unlikely any of that system would survive a complete tube rupture. The original installation was based on faulty analysis. Nobody with the proper experience ever looked at the system. But, this type of hazard is more prevalent than you might expect. You are likely to see it on projects by major engineering companies. The only remedy is not to use a shell & tube exchanger. But use an air cooled exchanger. And, what would one do if the exchanger used heating media? Mostly ignore it and pretend to provide adequate overpressure protection is the usual solution.

 

My guess is this hazard will never be remedied. The owners staff will just hope that a complete tube rupture never occurs.

 

Bobby

Edited by Bobby Strain, 11 March 2022 - 11:35 PM.

[breizh]

Hi,

Bobby is right , you should get support from senior peers within your organization or third party consultants.

Attached a reference to support your work .

Good luck

Breizh 

[Art Montemayor]

Fancywhale:

 

For my own education, could you please cite the type of (TEMA?) heat exchanger being used and its size with a copy of the calculations you are verifying?  Is this an existing unit, or a new, proposed one?  I assume this is a real-life industrial application.

 

A nominal size of 1,000,000 lb/hr of supercritical CO₂ placed on what is normally the weakest structural side of a heat exchanger is enough to raise the eyebrows of a lot of people – especially if the shell is designed for approximately 2,450 psig as MAWP.  That’s why I’m curious of the type and construction of the heat exchanger in question.  This raises a lot of questions regarding the scope of work and basic data… stuff that can’t be handled in a forum posting.  Bobby, Pilesar, and Breizh are right on top of the importance and logical way to handle this type of process problem: get local, expert and experienced help and advice on this assignment.  I’m not afraid of supercritical CO₂ – and I’m sure neither are Bobby, Pilesar, or Breizh.  I’ve worked with this fluid for many years in my young engineering years.  But we are all – I am sure – aware of the many strange things that are exhibited by this “4^th”, not fully understood, phase that oft times defies the predictions of current Equations of State.  The stuff seems to be very compressible.  I even handled it in reciprocating compressors.  It exhibits extraordinary properties in dissolving and absorbing a lot of stuff.  It has been used to selectively decaffeinate coffee beans for over 60 years.  It continues to be studied.  See the attached.

 

My main point here is that the advice heeded by Bobby, Pilesar, and Brieizh should be followed if you are indeed encharged with designing a relief system for a heat exchanger handling supercritical CO₂ – not because you are unqualified, but mainly because you lack sufficient years of experience in the field of handling this type of problem.  You deserve all that help and the project needs it.  It’s the smart, engineering thing to do.

 

 Transportation of CO2 in Dense Phase January 2012.docx   226.11KB   12 downloads

[breizh]

Hi,

To add to my previous reply , consider this handbook about CO2 properties .

Good luck

Breizh 

[FancyWhale092]

Thank you all for the responses. I have reached out to a more senior level engineer at my company to assist me with this. 

 

To answer your questions: 

 

Art - it is a TEMA Class R exchanger, and it is an existing application. Unfortunately, we are not able to find the calculations that were done to size this PSV, which is why I am having to do it. Thank you for providing your perspective on supercritical CO2. The whole point of my plant is CO2 removal from natural gas - we process almost 1 BCF of CO2 every day, and all of it makes it way out of the facility using reciprocating compressors, so interesting to hear!

 

I think overall the thing that jumped out to me about this was having high pressure fluid on the shell side of the exchanger. 

Edited by Art Montemayor, 15 March 2022 - 09:56 AM.
attaching prior posts & attachments unneeded

[FancyWhale092]

Breizh:

 

Thank you for providing these. I dove into Aspentech file and gave it a try.