B16.11 Fitting Design

[loilfan]

Hello, we are trying to register the design of a threaded fitting similar to a B16.11 bushing, except that the inner NPT 1/4 threading faces the opposite direction.

We have three sizes 1/2", 3/4", and 1". I can prove that the two largest sizes have enough thickness between the threads that the backwards threading won't make a difference. Since these are NPT threads and the tapers aren't parallel, I can't show that a 1/4 NPT thread in a 1/2" bushing meets all the dimensional requirements of B16.11.

B16.11 only gives dimensions for the heads and the external threads which I can easily match.

The material used is A105 - a B16.11 accepted material. We would like to certify them as Class 6000 fittings.

When using B16.11, normally we would look at the schedule of pipe and look at Table 7 to determine which Class to use. How does the reverse work? These fittings will go into a variety of pressures and temperatures, so how I can establish them as being Class 3000 or 6000? There's no calculations or proof tests for me to verify.

We want to hydrotest these fittings before using them since they aren't exactly standard - how do you determine what pressure to use if it's not required by Sec 2.2 of B16.11?

 

[metengr]

Look here in B16.11
1.1.2 Special Fittings. Fittings with special dimensions,
threads, or counterbores may be made by
agreement between the manufacturer and purchaser.
When such fittings meet all other stipulations of this
Standard, they shall be considered in compliance therewith,
provided they are appropriately marked (see
section 4).

2.2 Pressure Test Capability
Pressure testing is not required by this Standard, but
the fittings shall be capable of withstanding a hydrostatic
test pressure required by the applicable piping
code for seamless pipe of material equivalent to the
fitting forging and of the schedule or wall thickness
correlated with the fitting Class and end connection of
Table 7.

 

[loilfan]

Thanks for the response metengr.

Since the only two dimensions that B16.11 specify for a bushing are thread length and head length, it's not difficult to achieve those. The manufacturer will also be the purchaser here. It reads to me that B16.11 will accept alternative threads, but relies on Engineering Best Practice to justify. In that case, I took the internal pressure and compared it against the tensile area between the thinnest area between the two threads and divided it by 2.1 from Peterson's Stress Concentration Factors for tapered threads. Is that how you would approach it?

For the hydrotest, if my fitting is A105 Class 6000 and 3/4", then would I be looking at doing a hydrotest at the following pressure for B31.3?
D=1.050"
t=0.308"*0.875=0.270"
E=1
W=1
Y=0.4
S=23,300 pi (Table A-1 at ambient temps)
Using t=P*D/(2*(S*E*W+P*Y)), I get P=15,051 psig.

Does that mean that my fitting should be hydrotested to 1.5*15,051=22,577 psig to ensure that it is safe enough for all uses in XXS B31.3 piping?