3 replies to this topic

[rogered]

This query relates to problems related to boiling in a crude oil heater. This heater operates intermittently downstream of a crude oil pipeline. The heater uses LP steam, and the crude which feeds a downstream tank (at 1.1 bara) has a bubble point of 85°C at 1 bara. The crude skin temperature is > 85°C at low flows but then above 20% flow it falls below this.
If some limited boiling occurs in the heater is this a problem and why?

[Zauberberg]

Roger,

Your query seems to be a bit confusing. When you say "crude skin temperature" it is a contradictory term - it could be either tube skin temperature, or crude film temperature (laminar sublayer near the tube wall), but not the crude skin temperature. I believe you are referring to the tube metal skin temperature since this is the only thing that can be measured.

Normally, as the flow increases, the cooling effect of flowing crude oil increases as well, and that is why you are seeing skin temperature decrease at higher flows. It's hard to say whether there is any localized boiling close to the tube wall but it is very unlikely since the crude oil flows at a higher pressure than the final tank pressure, and the crude oil film temperature is always lower than the metal skin temperature. As long as you are getting the crude oil bulk temperature below initial boiling point at storage tank conditions, you should be fine.

I'm not sure if this was your original query, but in case you had something else in your mind please re-post the question and attach an appropriate sketch as well.

Best regards,

[rogered]

Zauberberg

Thanks for your reply/clarification,

I think that in this case the (crude) film temperature and (crude) skin temperature terms are used interchangeably.

I believe that the concern relates to the possibility of localised boiling inside the exchanger (no concerns about the downstream tank), and the fact that when this occurs the exchanger performance may not be as per design, ie. unpredictable. I do not think that the crude itself suffers any degradation, or or that the exchanger suffers mechanical damage due to bubbles collapsing.

So for these reasons I think that the whole issue is a red herring but I wanted to confirm with other engineers in case I was missing an important point.

I only have a PFD-type sketch which I don't think adds much to the picture.

Best regards,

Edited by Art Montemayor, 20 December 2010 - 11:07 AM.

[Zauberberg]

You have my vote on that as well. If there are no problems in the operation of crude heater and crude storage facilities, why to create an issue where there are no concerns?

To clear your conscious and sleep tight, you can create a crude assay in Hysys (having corresponding initial boiling point) and see what would be the IBP at the exchanger operating pressure.