6 replies to this topic

[vinay]

Dear All,

I have come across a problem in my project which i am sure is not uncommon. We have a exchanger circuit for Alkylate product ( please see the attached sketch), The exchanger in question is a makeup heater where tube side is operating at 24.8 Kg/cm2,g (design pressure 60 Kg/cm2,g) & shell side is operating at 7.6 Kg/cm2,g.

In order of following 10/13 rule , our detailed engineering contractor has fixed the shell side design pressure as 10/13*60 = 46.15 ~ 47 Kg/cm2,g.

The discharge & suction side (with respect to makeup heater ) equipments & their design pressure are also displayed in the sketch.

My line of thinking is that even though the shell side of makeup heater is protected in case of tube rutpure & doesn't require any PSV ,the suction & discharge side equipment & piping are not protected as they are designed for considerably lower pressure . In this case do we need to provide a PSV on the makeup heater shell side (or suction/discharge piping) to protect upstream/downstream equipments in case of tube rupture of makeup heater ?

I have asked this question to my seniors & other colleagues but so far they have not given any satisfactory answer to convince me hence i thought that many of you might have come across some scenaios & in better position for answering above question ?

Regards,
Vinay

Attached Files

•  Sketch_for_Circuit.xls   33.5KB   157 downloads

[djack77494]

vinay,
You pose an interesting question. By setting the exchanger's shellside design pressure to 47 kg/cm2g, you have satisfactorily addressed that particular exchanger. Commonly, I would dismiss concerns further up or downstream in cases like this BASED ON there being an open flowpath for relief of any abnormal fluid flows tending to overpressure the equipment. In your case, however, I suspect that reverse flow of your alkalate is prevented by a check (non-return) valve at your pump's discharge. Forward flow may be in question because of the presence of a control valve. The end result is that you cannot be certain that there are no overpressure scenarios. In this case, I would just set my exchanger's shellside design pressure to where it would normally be set (i.e. much lower than 47 kg/cm2), and then engineer a PSV that CAN handle the tube rupture case.
Good luck,
Doug

[JoeWong]

Yes. In case of Alkaline water exchanger & Make-up iC4 heater tube rupture, it potentially overpressure Alkylate pump, Alkylate cooler and associate piping.

In addition, tube leakage is one the problem occur without knowing. In the event, the Alkylate pumping system down and liquid trapped-in, the pressure
in the alkylate line will slowly built-up due to leakage.

What's the purpose of PSV set Pr -41 ?

[vinay]

Thank you dear Joe & djack for your quick replies. I have also thought of providing a PSV on the shell side of Makeup heater ( & lowering the shell side design pressure) , however after reading some articles on tube rupture case I am thinking on line of providing a rupture disk with connection to flare for quick reliving of pressure (PSV may take more time to open)? Does any guidelines in this regard exists which specify when to choose rupture disk (specially for tube rupture scenario) ?

To answer Joe's question the PSV of Alkaline water exchanger is set at 41 Kg/cm2,g as this is the shut in pressure of the pump supplying tube side fluid (alkaline water).

Regards,
Vinay

[JoeWong]

It is good to hear that your are aware of instanteneous surge pressure when tube rupture occur.


With my limited experience, i don't have any rule-of-thumb...

Rupture disc is "understood" to act "much" faster than PSV and in many events burst prior to peak pressure exceeded equipment's MAWP. Nevertheless, rupture disc do not 100% guarantee as there are studies have shown the failure. Can not be denied that rupture disc reduce the risk of catasthrophe.

Another aspect is the compressibility of fluid in low pressure side (LPS) and expandability of high pressure side (HPS). Liquid in shell (LPS) with gas in tube (HPS) would have a high risk of surge pressure exceeded the MAWP. More disccusion in "Tube Rupture : Pressure Relief Valve (PSV) or Rupture Disk (RD) ? ". For other combinations could have lower risk.

[djack77494]

I have a preference for using rupture disks when attempting to safeguard a S&T heat exchanger shell during a tube rupture case. Granted, even a quick acting device such as a rupture disk will NOT guarantee protection during an extremely fast acting surge condition, as stated by JoeWong, it is still an incremental improvement over a relatively slow acting PSV in this service. Just because I may not be able to quantify the advantage does not negate it.

If using a single device, I prefer to locate it in the middle of the exchanger where it will be most effective for a quick acting hazard. There are some unsteady state analysis tools available that can possibly put some numbers to these risks, but outside of being pretty sure they are CFD programs, I am not familiar with them.

Good luck in reaching a conclusion,
Doug

[vinay]

Dear Doug & Joewong,

Thanks again for your inputs.

Regards,
Vinay