8 replies to this topic

[Faisal Ghafoor]

We are in the process of laying a line from ammonia storage to Urea plant. I want to know what is the criteria of installing thermal relief valve between two isolation valves w.r.t to minimum trapped volume. I have seen such PSVs installed in trapped volume of 500 litre (0.5 m3). What are the general practices of defining PSV requirement for trapped volume of ammonia? Is there any standard mentioning min volume requirement for thermal PSV installation on ammonia service?

[Padmakar Katre]

Hi,
There must be a lo point drain/high point vent provided to empty out the dead portion between two isolation valves. What scenario you envisage the requirement of PSV on piping. Waiting for your comments.

Edited by Padmakar S Katre, 11 July 2012 - 04:29 AM.

[fallah]

Faisal Ghafoor,

As far as i know, there is no fixed value for minimum volume of trapped liquid after which it would be needed to TSV installation to protect relevant piping from overpressure due to thermal expansion.

In fact, TSV requirement is a funtion of expanded volume that, in turn, would depend on trapped volume and temperature change. Then theoretically, if piping MOE (modulus of elasticity) can afford the effects of expanded volume with an acceptable margin no need to TSV installation, otherwise it should be considered.

Fallah

[Robert Montoya]

My friend, there is a criteria which depend of the kind of liquid, tightness category . In the attachment file is the chapter 06 of the relief system handbook you can find more information.
Robert

Attached Files

•  thermal relief.pdf   129.88KB   149 downloads

[kkala]

I have not observed thermal liquid expansion valves on anhydrous ammonia lines in the fertilizers (1975-81), yet this is mentioned in http://www.wagner-me...aSafety2006.pdf , page 3, "thermal expansion" (hydrostatic relief valves). The output should be directed to safe location.
A general rule (not specific for NH3) is: TRVs shall be typically installed between two valves, in case that pipe length is longer than 30 m, or liquid volume is more than 1.0 m3, http://www.cheresour...n-relief-valve - post No 23.
Nevertheless http://www.eng-tips.....cfm?qid=322203 '> http://www.eng-tips.....cfm?qid=322203 , post by BenThayer, recommends thermal expansion valve between each pair of valves on NH3 lines (more conservative). I would try to follow it, due to potential hazards of NH3 handling.
However my opinion is not based on actual experience on NH3 piping design.

Edited by kkala, 11 July 2012 - 04:18 PM.

[Faisal Ghafoor]

We have two plants. In the new plant we have generally kept 0.5 m3 trapped volume as criteria for PSV installation. For the old plant, nothing is mentioned in specification. However valve is present even in 0.3 m3 trapped volume locations. The figure of 30 m length is from some engineering standard or is based on engineering practices common in ammonia plants.

Edited by Faisal Ghafoor, 11 July 2012 - 10:30 PM.

[breizh]

http://www.scribd.co.../Thermal-Relief

Consider this resource (page 19)

Breizh

[kkala]

Concerning post No 6, it is clarified that the rule of 1.0 m3 or 30 m pipe length is not specific for ammonia. This according to post No 5 (look at web reference cited there, post No 23). Post No 5 reports a more conservative rule for NH3.
Attachment by breizh is more complete, as well as more conservative.

Edited by kkala, 12 July 2012 - 02:15 AM.

[Faisal Ghafoor]

• Thanks all. Every thing is very useful in defining criteria in our plant.