Threaded joint for steel pipe
View Patent ↗A threaded joint for steel pipe is provided that ensures good sealing performance and reduces crevice corrosion. A threaded joint includes a pin ( 10 ) and a box ( 20 ). The pin ( 10 ) includes a first shoulder surface ( 11 ), a first male thread ( 14 ), a second shoulder surface ( 18 ), and a second male thread ( 17 ). The box ( 20 ) includes a first shoulder surface ( 21 ), a first female thread ( 24 ), a second shoulder surface ( 28 ), and a second female thread ( 27 ). Supposing that the distance between the shoulder surfaces of the pin ( 10 ) is represented as L pin and the distance between the shoulder surfaces of the box ( 20 ) is represented as L box , and the interference δ shld is defined by Equation (1) given below, then, the threaded joint is constructed such that Equations (2) and (3) given below are satisfied. In Equations (2) and (3), P is the thread pitch of the first male thread ( 14 ), Δ min and Δ max are the lower and upper limits, respectively, for the difference between the numbers of tightening rotations, and λ is the elongation of the portion of the pin ( 10 ) located closer to the tip than the second shoulder surface ( 18 ) is. [Formula 1] δ shld =L box −L pin (1) P×Δ min +λ≤δ shld ≤P×Δ max +λ (2) Δ min =− 9/100, Δ max = 3/100 (3)
1. A threaded joint for steel pipe including a tubular pin and a tubular box, wherein the pin and the box are fastened together as the pin is screwed into the box,
wherein the pin includes, in order from its tip, a first shoulder surface, a first sealing surface, a first male thread constituted by a taper thread, a second shoulder surface, a second sealing surface and a second male thread constituted by a taper thread,
the box includes a first shoulder surface, a first sealing surface, a first female thread constituted by a taper thread, a second shoulder surface, a second sealing surface and a second female thread constituted by a taper thread corresponding to the first shoulder surface, first sealing surface, first male thread, second shoulder surface, second sealing surface and second male thread of the pin,
the pin includes a nose located between the first shoulder surface and the first sealing surface and contiguous to the first sealing surface, and further includes an unthreaded extension located between the second shoulder surface and the second sealing surface and contiguous to the second sealing surface,
the box includes a recess corresponding to the nose of the pin, and further includes an unthreaded extension corresponding to the unthreaded extension of the pin,
upon fastening, the first shoulder surfaces come into contact, the first sealing surfaces come into contact, the second sealing surfaces come into contact, a gap is formed between the nose of the pin and the recess of the box, a gap is formed between the unthreaded extension of the pin and the unthreaded extension of the box, the first male thread and the first female thread engage each other, and the second male thread and the second female thread engage each other,
wherein the distance, as measured in a pipe-axis direction, between the first and second shoulder surfaces of the pin before fastening is represented as L pin , the distance, as measured in the pipe-axis direction, between the first and second shoulder surfaces of the box before fastening is represented as L box , and the interference δ shld between the second shoulder surface of the pin and the second shoulder surface of the box is defined by Equation (1) given below, then, Equations (2) and (3) given below are satisfied:
[Formula 1]
δ shld =L box −L pin (1)
P×Δ min +λ≤δ shld ≤P×Δ max +λ (2)
Δ min =− 9/100, Δ max = 3/100 (3)
where P is the thread pitch of the first male thread, Δ min is the lower limit of the number of tightening rotations after the first shoulder surfaces come into contact or the second shoulder surfaces come into contact during fastening, Δ max is the upper limit of the number of tightening rotations, and λ is the elongation of the portion of the pin located closer to the tip than the second shoulder surface is during fastening, and,
wherein the inner diameter of the pin is represented as ID, the innermost diameter of the second shoulder surface of the pin is represented as D ms , the outermost diameter of the first shoulder surface of the pin is represented as D is , and the interference between the first male and female threads is represented as δ th , then, λ is represented by Equations (4) and (5) given below:
[
Formula
2
]
D
int
=
(
D
ms
+
D
is
)
/
2
(
4
)
λ
=
{
(
D
int
+
ID
)
(
D
int
+
ID
-
2
δ
th
)
-
1
}
L
pin
(
5
)