PN vs ASME Class for valves: compare pressure ratings
“What ASME Class equals PN 40?” sounds like a simple conversion question, but there is no universal one-line answer. The designations belong to different standards systems, while allowable pressure depends on material and temperature. A safe replacement requires comparison of the full rating basis, connection geometry and acceptance requirements.
TAZ Engineering & Quality Team · · 10 min
1. PN and Class are designations from different systems
PN is a nominal designation used across European and international piping-component systems. ASME Class is also a pressure-class designation, not an allowable pressure expressed directly in psi. The numbers cannot be placed in a universal formula to create an automatically interchangeable valve.
In both systems, the engineer consults pressure-temperature rating tables in the governing standard. The result depends on material group, temperature and component type. The useful question is which design and material provide the required design pressure at the design temperature under the project standard.
2. Temperature and material change the allowable pressure
Metal strength changes with temperature. The same ASME Class can correspond to different allowable pressures for WCB carbon steel, low-temperature carbon steel or stainless steel. Likewise, one PN designation does not remove the need to verify material, product standard and temperature limits.
State design pressure and design temperature as a pair instead of providing only normal operating pressure. Include start-up, shutdown, static head, surge, vacuum, external loads and system test cases. Select the body material together with bolting, gaskets, trim and any impact-toughness requirements.
3. Verify flange compatibility as geometry
Even when the rating is adequate, two valves can have incompatible flanges. Compare outside diameter, bolt circle, number and size of holes, facing, thickness, gasket type and bolting requirements. DN and NPS serve related purposes, but they do not guarantee an identical bore or installation dimensions.
For a brownfield replacement, collect the piping class, flange standard, nameplate photographs, existing drawing and field dimensions. An adapter or replacement mating flange requires separate engineering approval. A matching bolt count is not evidence that two connections are compatible.
4. Face-to-face dimensions and actuation affect replacement
Face-to-face standards can specify different series and dimensions. A valve with apparently suitable flanges may not fit between the existing mating faces. For gate and globe valves, verify total height, stem travel and bonnet-removal clearance. For quarter-turn valves, check gearbox envelope and free lever movement.
A higher pressure class can increase wall thickness, mass and required operating torque. Recheck supports, piping loads, lifting arrangements and actuator sizing together. Confirm an electric actuator against maximum differential pressure, supply, enclosure rating, hazardous-area classification and required operating time.
5. Test pressure is not the operating rating
Shell and seat tests demonstrate pressure-boundary integrity and closure performance under stated test conditions. A short-duration test pressure is not the allowable continuous operating pressure. The test record must identify the valve, document and edition, medium, pressure, duration and acceptance criterion.
When moving between PN and Class, agree the product standard and testing standard in advance. API 598, ISO 5208 and EN 12266-1 address related subjects, but differences in required operations and leakage criteria call for a compliance matrix. The phrase “hydrostatically tested” without parameters gives the purchaser weak acceptance evidence.
6. Use a traceable comparison workflow
First, establish the design cases and project code basis. Select the material group and locate the required rating in the applicable table. Then compare flange geometry, face-to-face dimension, bore, trim and soft-part materials, operating torque and testing. Record every deviation before purchase.
For EU supply, separately determine PED applicability, equipment category and conformity-assessment route. A PN or Class marking alone does not demonstrate conformity with Directive 2014/68/EU. The purchaser needs an agreed datasheet, dimension drawing, compliance matrix, inspection and test plan and final manufacturing records.
Checklist
- design pressure and design temperature
- governing design and product standard
- material and pressure-temperature table
- DN/NPS and actual bore
- flange standard, series and facing
- face-to-face and installation envelope
- trim, seals and bolting
- torque and actuator
- test standard and parameters
- PED and contractual conformity records