LTCS use for piping 1

[gasoperations]

I need the expert's advise on following design situation:

Hysys simulation indicates that while initial startup up temperature will drop to -30degF in the Gas piping system (due to JT effect). But after the process lines are pressurized and during normal operations, the temp will be above ambient. The pressurization will take about 30 minutes or so and the low temperature is expected to be limited to a small length of piping downstream of pressurization point. Source of pressurization is at 60 bar and normal operating pressure is about 20 bar.

Do we still need to design the piping for low temp and use LTCS like A333 or normal CS piping can be used.

Thanks

 

[zdas04]

The energy transferred to the pipe in a J-T cooling scenario is a function of both the dP and the mass flow rate. Your dP is a given, but your mass flow rate is not. You could take 90 minutes to pressurize the line instead of 30 minutes to lower the mass flow rate and eliminate the problem. The model predicted a condition that creates a risk. You can implement a (very expensive) materials solution to mitigate the risk, or you can do it for almost free with a procedural solution.

I know how I would do it.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[LittleInch]

minus 30f is not so low and it depends on what your piping material is good for. Often you just need to test it to a lower temperature to make sure the Charpy values are still acceptable.

As Dave says, what you need to do is actually work out what the metal temperature could be based on mass flow, size of pipe, heat transfer form the ambient ( you need to use the lowest ambient). Another issue is that sometime the safety guys get involved and then make you insulate it against cold contact, which just makes it worse!

In reality, the metal temperature of the pipe is often considerably higher than the calculated fluid temperatures and especially for gas, the specific heat capacity is often quite low. Especially for short duration events, you won't get icing to any degree, but long term at lower temperature than ambient air, it will attract a lot of condensation.

Depends on what it will cost to investigate it as opposed to just specifying the first 10m downstream to be A333 or Stl Stl. You could easily burn more costs from analysis than just specifying something that will cope with the min fluid temp.

I assume you've looked at potential hydrates, water drop out, HC liquid drop out?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[EmmanuelTop]

If pressurization is the only scenario during which low temperature excursions might occur, then limiting the pressurization rate or provision of warmer gas for pressurization purposes will solve the problem. I haven't encountered a case (plant) so far in which material selection would be driven by startup pressurization - it doesn't make a lot of sense to do things that way.

However, it seems like the designer did not evaluate other possible scenarios that could result in temperatures below the design figure: depressurization from process conditions, evaporation of liquid inside the system (auto-refrigeration) - if any, depressurization from operating pressure and ambient temperature (cold blowdown for maintenance), etc. etc. If there are multiple scenarios in which low temperatures might occur, then it could be possible that more conservative material selection is needed.

Look at the Propane refrigeration units and Propane storage (bullets). Propane boiling point is around -40 degC at atmospheric pressure, and yet all Propane units I have seen so far are built of Carbon Steel.

Dejan IVANOVIC
Process Engineer, MSChE

 

[SNORGY]

Also, don't dismiss the provisions in the Codes like B31.3 323.2.2(b) / UCS-66. Impact test qualified (low temperature materials) are not necessarily mandated, depending on the stress levels present at the time of the temperature excursion. Below -45 C or -50 F (soft conversion) you might have to take a look at the effects of nil ductility or DBTT (Ductile-Brittle Transition Temperature), but otherwise, most of these J-T issues that I have seen similar to what you describe are, as zdas04 suggests, effectively mitigated by operating procedures.

 

[georgeverghese]

If you dont have the computational tools and the time to simulate the metal temp drop, the price differential between A106 and A333 is minimal, so I'd switch out to LTCS if I couldnt tell how the metal temp changes with time and distance for this short pipe spool for this repressure ops - I've been in similar situations before in my past design life.

 

[BigInch]

That indicates your start up procedures should most likely be revised to preclude excursions to low temperature ranges.

 

[gasoperations]

Thanks all for valuable advise.

The impact test is not envisaged as the worst case condition returns low temp. of -30 and the test may be required only below -45 deg or so.

It appears the modifying SOP ( Standard operating procedure ) to control the rate of cooling is the best solution as the phenomenon is only limited to start-up and that too for a relatively short duration.

Thanks