When selecting or replacing butterfly valves, many engineers and purchasers assume that wafer butterfly valves with the same nominal diameter (DN) are interchangeable. In reality, this assumption can lead to serious installation problems.
One of the most frequently overlooked differences is how the face-to-face dimension is defined. Although an EPDM/NBR rubber seated wafer butterfly valve and a PTFE lined wafer butterfly valve may share the same external wafer-style appearance, their published face-to-face dimensions are measured from different reference planes.
Understanding this structural difference is essential to prevent installation errors, PTFE lining damage, and leakage after commissioning.
The face-to-face dimension is the axial distance between the two sealing reference planes of a valve. It determines the installation space required between pipeline flanges and is one of the most important dimensions for valve replacement.
However, the measurement reference differs depending on the valve design.
For conventional EPDM or NBR rubber seated wafer butterfly valves, the face-to-face dimension is measured:
Metal Body Face → Metal Body Face
This measurement method complies with commonly accepted butterfly valve standards such as:
Because the metal body determines the installation width, published dimensions are always referenced from the metal faces.
Although a PTFE lined wafer butterfly valve has a similar wafer-style body, its face-to-face dimension is measured differently.
The published dimension is:
PTFE Sealing Face → PTFE Sealing Face
rather than
Metal Body Face → Metal Body Face
To achieve complete corrosion protection, the PTFE lining covers every wetted surface inside the valve.
Unlike rubber-seated valves, the PTFE liner extends slightly beyond both metal body faces, creating raised sealing lips.
During installation:
Therefore, manufacturers define the face-to-face dimension using the outer PTFE sealing surfaces instead of the metal body.
Important:
Although both valves are wafer-type butterfly valves with nearly identical external profiles, they use completely different measurement reference planes.
Ignoring this dimensional difference can create costly installation problems.
If the pipeline spacing was originally designed for a standard rubber-seated valve:
The opposite situation also causes problems.
Since the PTFE sealing lips are absent:
Even when two valves have the same DN size, they should never be considered directly interchangeable without verifying the dimensional drawings.
The distinction extends beyond face-to-face dimensions.
| EPDM/NBR Rubber Seated Valve | PTFE Lined Butterfly Valve |
|---|---|
| Elastic molded rubber seat | Rigid PTFE liner |
| Seat integrated with valve body | PTFE liner supported by silicone or elastomer energizer |
| Suitable for water and general industrial service | Designed for highly corrosive chemicals |
| Metal body determines installation width | PTFE sealing lips determine installation width |
Because PTFE is considerably less elastic than rubber, the valve body must be specially designed to support the lining while maintaining reliable sealing performance.
To avoid installation failures, always follow these practices:
Always verify the manufacturer's 2D dimensional drawing before replacing any butterfly valve.
Ask the valve manufacturer whether the published dimension is measured from:
When upgrading from a conventional butterfly valve to a PTFE lined butterfly valve for corrosive media:
Although EPDM/NBR wafer butterfly valves and PTFE lined wafer butterfly valves often appear nearly identical from the outside, their face-to-face dimensions are based on different reference planes.
Understanding this seemingly small distinction is essential for successful valve replacement and reliable pipeline sealing. Before ordering or installing a butterfly valve, always verify the manufacturer's dimensional drawings and confirm the face-to-face measurement reference. Doing so can help prevent installation difficulties, protect the PTFE lining from damage, and eliminate the risk of costly leakage or rework.
