thermal relief flowrate of steam traced pipe blocked in 1

[gelsi]

Hi
I'm going to define an overpressure protection device for a steam traced & insulated pipe-line.
How can calculate or estimate the heat-trasfer coefficient U or the max thermal flux to the fluid?
There is a short-cut method?
Tx very much

Main data:
fluid heavy oil
normal temperature of fluid 70°C
steam satur. temperature 150°C
case: pipeline blocked in

 

[EmmanuelTop]

I think you should use the same W/m heat loss figure that was used for sizing the steam tracing system. As a reference, see page 5 of the Spirax Sarco guideline:

http://www2.spiraxsarco.com/pdfs/sb/gcm_12.pdf

This is the maximum steam flow so heat gain cannot be higher than that. It can only diminish as the blocked-in fluid temperature increases by time.

Pressure rise due to heat gain in liquid-full systems occurs much faster than in gas-filled or gas-liquid systems. So the TRV will open much before the maximum fluid temperature is reached (150 degC in this case). There is no reason to consider the maximum steam temperature as relieving temperature of the fluid.



Dejan IVANOVIC
Process Engineer, MSChE

 

[TD2K]

Getting an accurate estimate of the maximum heat input is not easy since you don't really know how well the tracer is in contact with the piping unless you are using a heat transfer cement. The calculation is all about the U. The good news is that unless you have an odd system (a very long piping system protected by this thermal relief valve or multiple tracers with heat transfer cement), it's likely a moot point.

You can calculate how much a D orifice (or whatever orifice size you are using) PSV flows for your fluid and set pressure. Then go back and compare this to the calculated flow you need for the heat input following the method suggested by Dejan. My experience is that typically the capacity of the PSV is orders of magnitude greater than the calculated flow rate. Granted, there are exceptions but you need a lot of heat input to exceed the nominal capacity of a D orifice PSV set at even 100 psig. For most cases, I don't even bother calculating a flow rate, I just use a nominal capacity of 0.5 gpm and fill in the data sheet. Now, for those systems where there is a significant heat input and a significant flow rate of fluid has to be relieved, that calls for a deeper evaluation to ensure my PSV is large enough.

 

[georgeverghese]

Net U value is significantly higher for cemented tracers - typically 10x that for uncemented tracers.
TRV exit backpressure should be taken into account when selecting the type of TRV. TRV inlet and exit lines should also be insulated where required.

 

[gelsi]

Tx
It is a very easy method even if not rigorous.

Below my calc
300 mm pipe od
100 mm insulation tk
min ambient T -7°C
DT (product - ambient) 70+7 = 77 ~100°C, max heat loss from pag 5 table 1: 75 W/m
DT (product - ambient) 100+7 = 107 ~125°C, max heat loss from pag 5 table 1: 94 W/m (to take in account a middle temperature
heat transfered to fluid =~ max heat loss

 

[EmmanuelTop]

For calculating steam tracing, you can probably refer to the related project specification. Spirax Sarco guidelines should give you the result in the same ball park, though.
If you like to play with steam tracing calculations, attached is a free spreadsheet from ChE Resources (make sure you have the Solver add-in installed in Excel). Below is the link to Thermon software (free).

http://thermon.com/software/computracest.aspx

Dejan IVANOVIC
Process Engineer, MSChE