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Instability And Anti - Strain Design Of Ball Valve Seat Valve
- Jul 06, 2017 -

In a large pipeline ball valve made in a test, there is abnormal leakage. After inspection, the seat inner diameter of the residual deformation, the valve seat under the action of external pressure instability. In this paper, the stress and strain analysis of the external valve seat are carried out, and the design and calculation method of the seat strength and strain are put forward.

Seat instability occurs

Pipeline ball valve seat can be regarded as a thin-walled cylinder, the valve seat as a valve inside the pieces, usually do not consider the valve seat instability. However, for a large-diameter high-pressure ball valve seat, according to API-6D DIB-I test, the valve seat is a thin-walled cylinder by external pressure, there may be potential risk of instability, to be stress, strain analysis And calculation.

Class900, NPS56 pipeline ball valve, in a DIB-I test, the two sides of the seal seat a lot of leakage, disintegration inspection, seat diameter from 1360mm reduced to 1358mm, the valve seat residual deformation, confirmed seat instability.

Stress Analysis

The typical seat structure is shown below. By the metal seat, triangular ring, O-ring, support ring and other parts.

Typical seat structure

Metal seat 1, in the DBB functional test, the valve seat in the role of internal pressure qa, as shown in Figure 2a. In the DIB-I function test, the valve seat in the role of external pressure qb, as shown in Figure 2b. In the hydraulic strength test, the valve seat in the qa, qb and the two ends of the common role of qc. As shown in Figure 2c.

In this way, we can treat the seat as a thin-walled cylinder, by the internal pressure qa, external pressure qb, both ends of the role of pressure qc. Assuming an infinite length of the cylinder, qc = 0 makes the radius of the cylinder a, the outer radius is b, it is clear that the stress component at any point on the cylinder is a function of radius r. The cylindrical seat is an axisymmetric part. The stress component at one point is expressed by polar coordinates: the circumferential stress σθ, the radial stress σr, and the axial stress σz. Because we assume that the infinite length of the cylinder, the two ends of the pressure qc = 0, so σz = 0. For two - way stress state considerations.

Engineering Calculation of External Pressure Shell

The strength of the shell by the external pressure, that is, the wall thickness can be calculated according to ASME - (1) UG28 "outer shell and pipe wall thickness". The purpose of the calculation is for a known wall thickness of the cylinder material and its physical parameters to obtain a critical external pressure [P] value.

The critical external pressure [P] is related to the structural parameters L / D, D / t and the yield limit σs of the material. D is the inner diameter of the cylinder, t is the wall thickness, L is the cylinder length.

The first step, according to L / D, D / t ASME-D in the first three points in the parameters A.

The second step is based on the obtained parameter A, as well as the material and the operating temperature in ASME D.

The third step, according to the following formula to calculate the critical working pressure [P]

[P] = 4/3 * B × t / D (10)

If some material lines are not available on the chart, the Shanghai Branch of the valve recommended B value of the material to allow the pressure of 1.5 times. When taking the safety factor n = 2, then B = 0.75σs; when n = 1.6, then B = 0.9σs.

in conclusion

Pipeline Ball Valve For valves requiring DIB-I function, the strength, deformation and instability of the valve seat under external pressure are required. Especially high-pressure, large-caliber metal seat. In the sealed design, the radial deformation of the seat increases the sealing gap, may lead to sealing failure, should be noted.