Registering Steam Heated Molds
PVE-5242, Last Updated: June 17/11, By: LRB
In some provinces, heated molds need to have CRN registration. For example, steam heated platens and steam heated cavity molds need CRNs in Ontario. Tire molds with steam cavities are registered in Quebec. Each province has its own requirements – contact your local jurisdiction.
A designer of a mold or platen would not normally consider the pressure vessel design rules unless specifically required to do so. The primary stresses in the mold normally come from the use of the mold, not from the steam in its heating passages.
VIII-1 U-l(c)(2) Based on the Committee’s consideration, the following classes of vessels are not included in the scope of this Division… pressure containers which are integral parts or components of rotating or reciprocating mechanical devices… where the primary design considerations and/or stresses are derived from the functional requirements of the device.
I interpret this passage to mean that you can use the code rules to design these items if you like, but don’t be surprised if the most important loads and stresses in your device are not pressure related. Design to code rules only could prove inadequate.
The registration process:
- The mold is usually registered as a fitting based on the inside volume of the passages being less than 1.5 cuft. As a fitting, a statutory declaration must be filled out and declared by someone with an appropriate quality control program for manufacturing a pressurized fitting – which can be an adventure for a mold manufacturer. This is even more difficult for a mold that is in service with uncertain lineage.
- The mold is not built to ASME code rules, so the material will normally not be ASME listed. As a fitting, the material can be adopted if the tensile and yield properties are known or can be determined by testing. See Unlisted Materials for more information on using unlisted materials.
- Applying code rules can be interesting. Typically a platen or mold will have a number of gun drilled heating passages. These passages will have intersecting passages to connect and direct the flow of steam and condensate. The passages are surrounded by material that could be considered to be the wall of a pipe. The intersecting passages could also be considered to be a header to branch pipe intersection. The mold ends up being calculated as a piping system!
Tire molds have more complex internal cavities that require more difficult calculations. The calculated code stresses from internal pressure usually end up being a small fraction of the stresses that the mold experiences during operation.