9 replies to this topic

[domiBod]

Dear experts,

 

I have some concerns about the extreme low temperatures we can get in relief lines of steam exchanger in case of coil rupture.

I have the practical case where the effluent pressure and temperature at the entrance of the coil are 2100 psi and -16°C, respectively.

 

The relieving pressure of the PSV mounted on the steam vessel is 250 psi. To determine the PSV relieving temperature, I assume isenthalpic expansion from the coil pressure (~2100 psi) down to the PSV relieving pressure (250 psi). Note that I consider that the steam will be evacuated rapidly and finally it plays no role.

 

Applying isenthalpic expansion process, the calculated PSV relieving temperature is around -90°C (for coil temperature -16°C). The temperature becomes obviously lower downstream the PSV.

 

Our available piping for relief line is not rated for so low temperature ...

 

I'm wondering whether my reasoning is correct ? If yes, there is apparently no possible mitigation ?

 

Thanks for your support.

Regards

Dominique

[Mahdi1980]

Hi,

What do you mean by PSV here? Do you mean Pressure regulator or a throttle valve?

What do you mean by a steam -16 C and pressure 2100 psi? How can we have a 140 bar-steam at such a low temperature condition?

what is the function of your system?

Please let us know more about your operation by providing a detailed sketch.

[Bobby Strain]

Please provide more information about the exchanger. Sounds like you have a real problem. Years ago I designed a similar application that could reach low temperature with tube rupture. I ignored it, but the relief discharge was to atmosphere. As far as I know there were no problems over these 30 years. Fortunate I guess.

 

Bobby

[domiBod]

Thanks for the answer, I'm going to try to provide more pertinent information ...

 

the purpose is to heat up a gas (mainly methane) flowing at high rate. When the gas enters the heater, its temperature is around -16°C and the pressure is ~2100 psi in the piping line. It flows through coils and exchanges heat (by conduction through the coil wall) with steam contained in a vessel. The steam is at the saturation temperature. The steam vessel is protected by a pressure safety valve (PSV) with a set pressure of ~250 psi (~PSV relieving pressure). The scenario we want to simulate is a coil rupture where the gas flows accidentally in the vessel and the PSV opens to evacuate it in the relief line.

 

When the coil fails, the gas undergoes a depression from 2100 psi to ~250 psi. I assume that this gas expansion is an isenthalpic process and I calculate the temperature accordingly. This calculated temperature is the PSV relieving temperature. Note that I assume that the steam contained in the vessel at the time of the coil rupture is instantaneously evacuated through the PSV and plays no role.

 

For the study case, i.e. isenthalpic gas expansion from (2100 psi, -16°C) to 250 psi, I obtain a temperature which is around -90°C. This temperature is the PSV relieving temperature and therefore the temperature downstream the PSV is even lower since we have again a pressure drop through the PSV.

 

The line downstream the PSV must be rated for this extremely low gas temperature (~ -100°C), and here is the problem, our piping can not manage such low temperature.

 

I would like to get confirmation about my reasoning, it there something I miss ?

 

Thanks again for your support

 

Best Regards

Dominique

[Bobby Strain]

We understand the basics. What is a high rate? Steam supply pressure? What is the coil design? You have omitted many essential details. Maybe you can include a drawing.

[domiBod]

Bobby,

 

For the studied case, the gas rate is around 50 MMSCF/d, but it should be higher for other cases (~ 100 MMSCF/d). The steam pressure in the vessel is close to atmospheric, but there is a check valve on the steam supply line so that there is no steam feed when the PSV opens.

The coil are thick pipes (ID=6.4 cm and thickness=1.9 cm). Internally in the flow direction, we have a high pressure coil, then an adjustable choke orifice and downstream the choke a low pressure coil (low pressure if the choke is not fully opened).

 

I apologize but I'm afraid I'm not authorized to provide drawings of equipment.

 

My concern was about the assumption of isenthalpic expansion when the coil fails. For a short period of time, the pressure in the coil should be maintained at a high pressure level, and for sure the gas will experience a significant pressure drop from coil pressure to PSV set pressure. I guess it requires transient calculations on the entire setup to have an idea of the coil pressure versus time in case of failure. However I did a quick check and the "propagation" of extreme low temperature through pipe wall (by conduction) is more and less instantaneous. Therefore I think that the piping of the relief line downstream the PSV will "see" extreme low temperature for a short time. How acceptable is it? What is the maximum duration a pipe can undergo such low temperature?

 

thanks again

Dominique

[Mahdi1980]

Hi Dominique,

 

Oh! your case is now clarified. It is a good sample of process engineering design.

 

Tastes are different but I believe that it is conservative to replace the material of pipeline at the discharge of PSV with a Low temperature steel.

 

In tube burst/ rupture cases the abrupt pressure drop may lead to a dramatic reduction of temperature.

However in your case, the fluid come into the vessel from coil will contact with steam and its temperature will be rose up.  You should consider the temperature obtained from pressure drop of 250 psi to atmospheric. 

You should also check the size of PSV to handle the fluid come into vessel during the tube rupture.

[Bobby Strain]

With your brief description I presume that the coil is fabricated from pipe and not tubing. This was likely done to preclude any tube rupture. ASME code does not require overpressure protection for rupture for such an exchanger. Any evaluation that you conduct is essentially useless.

 

Bobby

[domiBod]

Gentlemen, thanks for your help.

 

Effectively the coil are usually 4" XXS Schedule pipe. Our concern is more about erosion in the U-turns of the coils inside the vessel. Since the effluent (gas or multiphasic) flowing through the coil can convey a significant amount of solid, we may have erosion in these coil U-turns. I guess it will start with a small hole, but the fluid leaving the coil through this small hole will experience expansion (and therefore low temperature) independently of the rate through the hole. 

Asking around me to experienced people, nobody has never seen such coil rupture in our business ...

 

Best Regards

Dominique

[Bobby Strain]

Then you best eliminate the solids before the fluid enters the exchanger. Your guess about the leak starting as a small hole may not be correct. If you get a major failure, you are in great trouble.

 

Bobby