May we know why you only describe class and or material as the limiting factors? I think you're still missing some important points.
Kindly note that valve is connected to its counter piping. And piping is designed as per Schedule or wall thickness and not 'Class'
Just general, the risk of thread:
- during thread rolling or machining, there is a risk of collapse or damage the wall thickness. As we all know, wall thickness is critical factor for with subject to PED or CE marking. Depending on the liquid or gas classification, e.g. valve sizes bigger than DN 25 should conform prescribed wall thickness
- Thread is more prone for crevice corrosion due to water ingress, etc. Once badly corroded, may subject for leakage path. Nobody wants a leakage especially when the fluid is hazardous or flammable
- Thread connection often relies on PTFE tape or similar, which limit the operating temperature of the line.
- The higher the torque moment applied, is not necessarily make the female thread more tightened against male thread. Torque moment prescribed mainly for two things: to achieve correct seating stress shall gasket applied e.g. airfin plug, body to bonnet valve, etc. And as guidance in case of uniformity (same brand, same design) e.g. tighten flywheel on old cars (moment value was given) due to limited space and visual constraint.
Operator or maintenance sometimes think, ahhh bigger class, so i have to put my best strength (with tools) to tighten this thread. This may damage the thread self
So, in conclusion there are many references that says, unless it is DN 50 or smaller, unless non hazardous application (that may allow the use of PTFE, or small leakage between threads is generally acceptable), unless the schedule is minimum 160 (to compensate wall thickness loss) for class higher than 300, then threaded join is not allowed or not advisable.
Appreciate your response, Valves type forged steel Gate, Globe, Check & Ball, sizes <DN50 used for boiler applications so IBR approval required,
But in IBR Max. Pressure for screwed pipes 103.5 bar, for nominal pipe size up to & including 20mm, So should we advise our customer to go for Welded ends.
Again, it is possible to use thread but higher schedule is deemed required.
Some common counter argument from end user is, "we use threaded equipment in this line also e.g. flow-meter, etc.". Then you have to explain more from engineering or standard point of view.
