How should pressure rating be specified for industrial heating elements?

Specify the pressure rating of an industrial heating element as a design pressure-temperature pair, not by wattage alone. First define normal and maximum pressure with the corresponding sheath temperature, then verify material allowable stress, tube diameter and wall thickness, flange class, welds and sealing method, also considering how mounting affects loads on the tube. Excessive surface load raises wall temperature and reduces pressure margin, so temperature control should include over-temperature protection and pressure interlock. State the fluid, corrosion, temperature combination, flow and test pressure, and require the supplier's calculation basis plus hydraulic or pneumatic test records; at acceptance, check material certificates and weld inspection reports.