what is the standard for female npt depth?

[tk369]

say i need a 1/2" npt in a valve body. this will be used to feed hydraulic fluid into the valve.

the material i'm tapping is 1" thk. from machinery's handbook, 1/2" npt has effective ext thread length of .5337, yet an engineer says to drill/tap thru.

isn't that rather illogical? how deep is too deep? hand-tight engagement is only .32. running the nipple in will gouge up and destroy the first threads, won't it?

i appreciate any replies.

peace..out. tk369

 

[KENAT]

I asked a similar question a few weeks ago.

thread1103-216997

KENAT, probably the least qualified checker you'll ever meet...

 

[CheckerRon]

The thread that KENAT sited is good. There you will see that depth of an NPT thread is not specified. A 1/2" internal NPT depth is L1 + L3 or .5343 from the Machinery's Table 3. beyond that you would only need your access bore thru.
For those familiar with Eugene Megyesy's "Pressure Vessel Handbook", there is a nice table in there on "Length of pipe engagement to make a tight joint". For a 1/2" nipple it gives 1/2" engagement to make a tight joint. Megyesy's table is just a little less than L1+L3 for all pipe sizes, and I have found it to be accurate so I use it to calculate pipe lengths needed.

 

[mechengdude]

From "The Pipe Fitters Blue Book" by W.V. Graves, the length of engagement for a "tight joint" for a 1/2 NPT is 9/16"

 

[fcsuper]

A general rule is that full engagement is 3 to 4 threads.

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

 

[CheckerRon]

The "General rule" is wrap it with teflon tape, tighten it until it won't go no more and check for leaks.

3 or 4 threads at 1/2" NPT - 14 pitch is only .21-.29" and that is probably not going to seal.

 

[eddiewdi]

Keep in mind gentlemen these are tapered threads. The thread engagement is not the same as straight threads.

 

[fcsuper]

Exactly. 3 to 4 threads is a seal. (There is no seal on a straight thread, but 10 threads is considered a flame barrier.) Anymore than 5 threads on an NPT engagement may cause issues with binding. Teflon thread would help that, but is not 100% as it will just be cut by the threads at a certain point. If one tigthens to 3 to 4 threads, one generally won't need teflon tape.

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

 

[fcsuper]

My above posting is in reference to virgin mating.

Additional point, over tightening can cause deformation of that threads that reduces the NPT's ability to seal on subsequent engagements.

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group