Thoughts inspired by issues found at the pressure transmitter interface
Thread Content
I have found that many design institutes use 1/2'' NPT internal threads when designing the process interfaces for pressure transmitters, but in practice, they convert this to a Φ14 pressure tapping pipe when installing the instruments. This will require a 1/2'' NPT (F)-M20*1.5 adapter. Why not choose the M20*1.5 external thread interface directly during selection? What are the considerations when choosing between 1/2'' NPT internal threads and M20*1.5 threads for connections?
M20*1.5 is a regular thread, generally used to connect equipment components, with an emphasis on the connection itself. NPT threads are 60° sealing pipe threads, with an emphasis on sealing. The process interface of the transmitter must be sealed, so NPT threads are naturally more suitable for such applications.
It’s very simple: a 1/2\" tube is used for the pressure conduit, so no conversion is needed. Think twice before thinking again~
Based on the selection options, various interfaces are available; it seems that the design institute did not consider ease of installation either, perhaps out of habit. Now, for the selections here, I change them to M20x1.5 unless there are any special requirements.
What was said on the first floor makes sense, but the design department also didn’t base its design on the actual needs of operation; if the process connectors were replaced with external ones, it would save a lot in terms of maintenance costs.
For 1/2\" NPT, when converting to a 14mm ferrule, there is no need to use M20x1.5; the conversion can be done directly.
This is purely a design flaw; simply using a 14 to 1/2 NPT terminal fitting will do； NPT uses thread sealing, while M uses face sealing.
M20*1.5 is a regular thread; its threads are approximately perpendicular to the shaft, allowing it to withstand higher pre-tension forces, but its sealing performance is poor. NPT threads are 60° tapered threads, with the thread teeth forming a 60° angle with the shaft, which provides excellent sealing performance; they are mainly used for sealed connections in pressure-bearing equipment or pipelines
For the classification and uses of common threads, refer to this table
Can it be done in one step? The threaded pressure transfer tube must be fitted with a connector; this is not a design flaw