PSV sizing for glycol cooler

[processengineer1998]

the concern is regarding sizing a PSV for fire case for Glycol Cooler in Glycol Regeneration Package.
Data is as below:
1-PSV set pressure=10 barg
2-glycol cooler inlet conditions:temp=73 c,pressure=6.3 bar
3-glycol cooler outlet conditions:temp=37 c,pressure 5.8 bar
4-wetted surface area considering 20% margin for associated piping=70 ft^2
5-actual glycol flowrate=13.5 m^3/h
As far as I know the condition before relif in fire case is p=relief pressure(21% higher than set pressure) and temp=liquid boiling pressure.please correct me if I am wrong.
For glycol with purity about 99.2% mol the boiling point in 13.1 bara(21% above set pressure) is around 440c which is higher than design temperature for carbon steel(material of glycol cooler).
please explain me in depth the procedure for sizing PSV for such case.

 

[processengineer1998]

4 days,no idea.
is it difficult to answer or my explanation is not clear?

 

[CMA010]

It's not unusual for the relief temperature in the case of fire to be above the design temperature of the vessel. This is generally considered to be quite acceptable. In your case however the relief temperature is rather high and you might want to consider additional protective measures. See ISO 23251 / API STD 521, § 5.15.

 

[StoneCold]

Ok
What design code is being used for the unit?
What exactly is the "carbon steel piping"?
440C is up there. What is the design pressure of your system? Is it 10 bar at ???C.

Regards
StoneCold

 

[processengineer1998]

Dear StoneCold;
glycol is in shell and seawater in tube.
the design pressure for glycol cooler shell is 11.36 bara and 15.01 bara for tube.
the material of shell is A516 Gr. 70.
piping material for glycol input and output lines and psv line is carbon steel NACE MR 0175.
material for seawater line is copper nickel.

please advise.

also I am confused for case of sizing of psv.shall it be fire case or I need to consider thermal expansion case?

 

[Chance17]

If there is significant vapor being generated by the Fire, the calculations are two-phase flow.
Otherwise the calculation is thermal expansion.
The problem is not hard - but it is also not easy.
This is not a good forum for the in-depth procedure you requested.
I would recommend one-on-one coaching by a professional.

 

[StoneCold]

ProcessEngineer1998
Ok.
You should evaluate all the scenarios that could require relief for your system. Thermal expansion, pump shutin, fire case if there is a local source of flammable material, or other cases your client might ask for.
Because your relief temperature is so high, you might want to check the yield strenth of the materials at that temperature and see if you can actually contain the material up to that 440C temperature. If not then you may need to bring the relief pressure closer to the operating pressure, or have some type of dump valve for overpressure scenarios.
Are you above the critical point at those relief conditions? You might want to investigate that, because if you are you need to treat it a little differently.

Chance17 may be correct that you need an expert to walk you though this or double check your work.

Regards
StoneCold

 

[CJKruger]

processengineer1998, in general I agree with the comments made by CMA010.

Two other points:
- Do you need a relief valve just for the exhanger ? If you only have a fire case here, you may be able to rely on a relief valve on a connected vessel (see AMSE 8 Appendix M).

- For hydrocarbons we usually assume they start to crack and limit the relief temperature to 800F. Not sure about glycol.