Hello,
I will divide this in 2 parts, part 1 : materials and part 2: construction
1. First of all, all material selection approuval should be verified by someone with technical knowledge of metallurgy / mechanical enginneering. That being said, in most cases the standard metallurgy proposed by most manufacturer can be used safely for most process, and these manufacturer give guidelines for this selection. (the 3 main brand/company in Canada and the US are Consolidated, Anderson Greenwood Crosby and Farris)
In your case, water at 70°C will generally be relieved to the ground or to a drain / sewer system.
So you would normally give the option to each manufacturer to make a quotation for their standard thermal relief PSV:
Consolidated Series 19000
Farris Series 2700
Anderson Greenwood Crosby Series 900
A closer look a these 3 manufacturer will show that PSV bonnet is generally in Carbon steel , ASTM Grade WCC CS or ASTM Grade WCB CS.
The internals : nozzle, disk, spindle are generally in Stainless steel, 316 SS or variations and 400 Series SS
The spring can be in Stainless Steel, Chrome Steel and other alloys
In all cases, you should ask assistance from a technically competent person in metallurgy / mechanical engineering to select the materials of construction.
If you dont have access to this kind of assistance, In your case of water at 70°C I wouldnt worry too much and select the normal material of construction.
You should notice that for all 3 manufacturer, the nozzle is full and also act as the base for the PSV. The body / bonnet is screwed to this base.
So for material, use 1 of the 3 manufacturer as a basis for selection, ask for quotation from each (with "or equivalent" mention) , and put in the material of the selected manufacturer on final datasheet. Consult with metallurgist / mechanical engineer if possible.
2. First of all , you have to make clear if the 70°C is a maximum during normal operation or if it is only during a relief event.
If this temperature can be reached in a scenario that doesnt lead to overpressure and relief ( Ref ASME SEC VIII DIV 1), you will need a lifting lever as per ASME.
The type of lifting lever is normally selected depending on your process conditions / fluid involved ... and relief path.
The cap is almost always screwed to the PSV, only specific PSV application require the cap to be bolted.
The bonnet is normally closed, only specific reason may warrant the use of a open bonnet, a good exemple being boilers that require Open Bonnet Per ASME regulation.
For thermal relief in your case, use closed bonnet, screwed cap, plain lifting lever (if relieving to atmosphere) or packed (if relieving to pressurised system or flare).
If you dont have an isolation valve on the entrance of PSV and need to perform hydrotest while the PSV is in place, you need to ask for a testing rod top allow proper hydrotest.
regards,
Marc-Andre
Edit: typos
Edited by Marc-Andre Leblanc, 31 October 2014 - 12:29 PM.