5 replies to this topic

[Jiten_process]

hi all,

I am stuck up with confusion. Firstly please see attached drawing. There is vacuum system with condensors. Now I am to size the PRV-1 (clouded on drawing) on the first condensor's cooling water line. And my problem is

1) Should I consider only thermal expansion scenario in order to arrive the size of PSV? And further API 521 says for thermal expansion reliefe it is reasonable to take PSV of size 1/2", 3/4" or 1". But how should i arrive the actual venting required flow rate. Because here my heat exchager area is considerably large, in order of 754m2

2) what are all other scenarios i should consider for such systems. Fire case is anyway not to be considered because the area is non fire area.

Plz suggest...and let me know if further data is required to comment.

Attached Files

•  H.T._PFD.pdf   191.15KB   83 downloads

[ARAZA]

Dear Jiten,

From the attached drawing it seems that these Thermal safety valves are located on the cooling water return lines on the three H-E's. These are provided to protect the piping from damage from thermal expansion of the CW if the cooling water gets blocked inadvertantly while the hot process fluid still running on the other side.

I've used a lot of these and in my expereince there is no particular sizing required, you can just pick up a standard size which is available in 3/4" x 1" OR 1/2" x 3/4".

As a side note, do not confuse these Thermal safety valves with Pressure safety valves. If these exchangers require a PSV sized for a Fire case, than PSV would be located on the process side NOT on the cooling water side.

Hope this helps.

Best Regards,

ARaza

[Qalander (Chem)]

Dear Jiten Hello/Good Night,

In general, I feel the reponse from ARAZA is appropriate; however, occassionally it has been observed during my stay at my previous refinery that either one or two of the water coolers for final R/Down stream were observed to even produce steam momentarilily during Cooling Water upset i.e. lower than design C.W. flow rate/pressure few equipments for end of the loop process units may be suject to starvation.

Here is the role for this small sized PRV (orTRV) to help save us from Water (steam) Hammer possible damages.

Hope this helps
Qalander

[ankur2061]

Jiten,

If my memory goes right Art Montemayor had once said that it is a waste of time and effort in performing calculations for relieving rate of thermal relief valves. Even if you do a relief rate calculation you will find that the relief rate is so small for a thermal expansion case due to blocked outlet the time spent on doing this exercise is not justified.

Since you are not sure, I would suggest a 3/4" X 1" relief valve with orifice designation as "D". A even safer scenario would be to consider a 1" X 1" relief valve.

Hope this helps.

Regards,
Ankur.

[Jiten_process]

Thanks to you all, that really helped me a lot to remove confusion.

[djack77494]

Jiten,
I'm not sure I can concur with the advice given. Perhaps there is some confusion regarding the nomenclature. In my understanding, the so called thermal expansion design case is a "catch all". You can use it to satisfy code requirements for a PSV when no particular relief scenario is evident. In this situation, it can protect against blocking in some piping, equipment, etc. and then having some possibly unidentified heat source slowly leak into your system. A minimum size PSV will suffice to protect your system against this.

You, on the other hand, have potentially a large and readily identified source of external heat entering your isolated cooling water circuit. On your shell side you may have a hot gas stream consisting of hot process gases plus motive steam added to the steam ejectors. Calculate the heat input entering your isolated CW side of the exchanger. Knowing the heat input, look at both (subcooled) liquid expansion and also vapor generation. Use the controlling case for sizing a suitable PSV. That's how I think it should be done.
Doug