6 replies to this topic

[Beergson]

Dear All,
I need Your help with sizing thermal relief from steam traced pipeline

Pipe is 2 kilometers long, used periodically, insulated and steam traced (1 steam tracing). There is no possibility to empty this pipeline beetween using period so it has to be protected by PSV

What heat flux should be took into consideration to calculate thermal relief for PSV? Normal working temperature is 40 deg C, and heating medium is low pressure steam (pressure ca 6 barG, ), nominal diameter of pipe is 100 mm, steam trace - 20 mm. PSV will be set at 5 barG. Flowing medium in pipe is off-spec p-xylene or benzene.
Any your help will be appreciated

[bam88]

Dear All,
I need Your help with sizing thermal relief from steam traced pipeline

Pipe is 2 kilometers long, used periodically, insulated and steam traced (1 steam tracing). There is no possibility to empty this pipeline beetween using period so it has to be protected by PSV

What heat flux should be took into consideration to calculate thermal relief for PSV? Normal working temperature is 40 deg C, and heating medium is low pressure steam (pressure ca 6 barG, ), nominal diameter of pipe is 100 mm, steam trace - 20 mm. PSV will be set at 5 barG. Flowing medium in pipe is off-spec p-xylene or benzene.
Any your help will be appreciated

Refer to below

Edited by bam88, 23 September 2009 - 07:58 PM.

[bam88]

I think I have misunderstood your information.

Is your stagnant line blocked in by two isolation valves? If so, then a PRV is necessary.

If only one isolation valve is used, then thermal expansion can dissipate along the pipeline, and PRV is not necessary.

[Beergson]

I think I have misunderstood your information.

Is your stagnant line blocked in by two isolation valves? If so, then a PRV is necessary.

If only one isolation valve is used, then thermal expansion can dissipate along the pipeline, and PRV is not necessary.

Yes, the line is blocked by gate valves and PRV is necessary

Edited by Beergson, 23 September 2009 - 11:36 PM.

[herrani]

Hi Beergson

I believe you need to use the heat flux from the steam line. Since the steam tracing line will not be insulated, you can assume that the temperature in the steam line will be constant at 165 C (approx. 6 bar g of pressure) and that the initial temperature in the line will be 40 C.

With that, and the heat transfer coefficient for the pipe material(s), you should be able to calculate the heat flux from the steam tracing into the isolated portion of the pipeline.

I guess that your question is whether you should use the maximum heat flux (from a temperature difference between 165 anf 40) or an average one (since the temperature difference will decrease with time). In API 521, section 5.14 sizing of RVs is covered in the case of thermal expansion of liquids. As an example, they recommend sizing RVs for the maximum heat exchager duty possible.

In your case, I would use the maximum heat transfer rate, unless you end up with a huge RV, in which case you would need to look again closely into the problem.

I hope this helps!

[shan]

Dear All,
I need Your help with sizing thermal relief from steam traced pipeline

Pipe is 2 kilometers long, used periodically, insulated and steam traced (1 steam tracing). There is no possibility to empty this pipeline beetween using period so it has to be protected by PSV

What heat flux should be took into consideration to calculate thermal relief for PSV? Normal working temperature is 40 deg C, and heating medium is low pressure steam (pressure ca 6 barG, ), nominal diameter of pipe is 100 mm, steam trace - 20 mm. PSV will be set at 5 barG. Flowing medium in pipe is off-spec p-xylene or benzene.
Any your help will be appreciated

1. Figure out the bubble point temperature of pipe line contents (off-spec p-xylene or benzene) at the relief condition 5 barG.
2. Calculate how much heat duty, including both latent heat and sensitive heat, available for 1 kg 6 barG steam to the content bubble point.
3. Get total heat duty available by Answer 2 x Trace Steam Flow Rate.
4. Obtain vapor rate by Answer 3 / Latent Heat of Content.
5. Size your PSV based on Answer 4 vapor rate.
6. Ask the credit from your boss. This is all.

[ARAZA]

Dear All,

I don't think there is any complicated calculations required to size a thermal relief. I've seen lots of TSV's on various pipelines in potential blocked condition and on cooling water outlet lines. All the thermal relief are the smallest you can get, typically they are 3/4" inlet x 1" outlet. The sizing of the TSV's is related to thermal expansion due to radiant heat in blocked condition. The co-efficient of thermal expansion of liquids isn't much hence most of the time the smallest size is sufficient.

Araza