High Temperature thermal relief

[Bill3752]

Have a couple of questions related to thermal relief of cooling water.
1. When process (hot side) temperature is less than the saturation temp for the cooling water side, I have a liquid thermal release. Should I use Cp based on relief temp, or the average temp from inlet CW to relief?

2. When process temp > saturation temp of the CW, I assume I should design based on steam? To calculate the amount of steam generated should I base on the Hvap at relief temperature? Or based on inlet CW temp? If the former, should I divide the heat duty by the Hvap or by (Hvap+CpdT)where dT is temperature change from inlet CW to relief temp?

In all cases I know the heat duty of the equipment.

Thanks

 

[CMA010]

1: What do you want to use Cp for?

2: Hvap at relief conditions. The relief load is based on the amount of steam generated once the CW has been heated up to relief conditions.

 

[ggordil]

Bill3752,

1. I am not sure you need the heat capacity for this situation. Look up thermal expansion in API 521. It gives a formula.

2. For vaporization, the relief temperature is the saturation temperature at your relief pressure plus allowable accumulation. You know the duty of the exchanger therefore it is a simple division.

relief rate= duty/ heat of vaporization

 

[Bill3752]

Thanks for the feedback. A couple of followup items:

1. Use Cp to calculate the thermal expansion for liquid; the mass rate is proportional to 1/Cp - wondered if one should use the heat capacity at flowing (relief) conditions, or an average across the temp range.

2. My question regarding release when the temp is > Ts is a little more complex. In thinking about this a little more, should we assume that the we have "total steam" at the RV, or is a more likely case that we have saturated liquid that flashes across the RV? After getting your responses I checked a couple numbers using one of the scenarios I am evaluating - the orifice size based on the steam assumption (where mass rate = duty/Hv) is over 2 sq in; assuming liquid flashing across the RV, the orifice required is ~ .2 sq in. I am struggling with the correct "model" to use. This started off as a simple thermal protection case that has gotten much bigger.

Thanks for your help.

 

[CMA010]

1: Never heard of that approach, but i've never calculated the relief rate due to thermal expansion. The API STD 521 formula (1 & 2) use Cp, I would use Cp at the initial conditions.

2: The second "stage" of your relief scenario is not thermal expansion in my opinion. You have identified an additional scenario. After blocking in the cold side of your exchanger you'll have three "stages". Thermal expansion, two phase (vapour and flashing liquid) and vapour. I've personally also once found a hex with a PSV on the inlet piping (basically the PSV was located below the hex), so the generated vapour would push the liquid out through the PSV.

 

[ggordil]

I have run into a few similar case where blocking in the cold side while leaving the hot side causes vaporization. In reality the process is dynamic. Meaning that as the liquid starts to vaporize the pressure goes up in the exchanger but then so does the boiling temperature of the fluid. I have found that the best way to "see" what is going on is to use dynamic software.

Based on the above, my opinion is that the worst case is total vapor relief.