What are the selection and application of metal bellows for valves? - 2

Release time:

2024-03-18

Answer: introduce the application, design and selection of metal bellows in stop valve, gate valve, regulating valve, safety valve and ball valve. The solution to the problems in the use of metal bellows for valves is given.

9) bellows rigidity: the force required to make bellows produce unit displacement. The rigidity calculated by design is called nominal rigidity or rated rigidity. In general, the nominal stiffness does not directly use the design value, but the corrected value after the product prototype is tested.

10) one cycle: the movement process of bellows from fully open to fully closed and then to fully open along with the valve rod.

11) cycle life: the maximum number of cycles that the bellows will move back and forth without damage under the specified pressure, temperature and axial stroke. The service life of the same product is related to the working state, cycle times, frequency, pressure and temperature of bellows.

12) effective area: the magnitude of the converted concentrated force when the displacement of the elastic element is zero under the action of unit pressure. However, the effective area changes with the increase of pressure, so it is not constant. In the force balance instrument, the effective area should be the same as possible.

(3) model selection

1) typical structure of bellows valve: the application of bellows assembly is to provide a metal spacer that can be axially deformed between the valve rod and the valve body process fluid to form a dynamic seal to eliminate the leakage at the valve rod.

2) selection requirements: A. select the inner diameter of the bellows according to the diameter of the valve rod. The inner diameter of the bellows shall be 1.5mm larger than the diameter of the valve rod. The outer diameter of the bellows is selected according to the inner diameter of the bellows. Outer diameter and inner diameter are connected by the ratio of outer diameter to inner diameter. Generally, the ratio of outer diameter to inner diameter is 1.3 ~ 1.5.

B. the wall thickness, number of layers, number and length of bellows shall meet the requirements of pressure resistance, stroke and cycle life of bellows. In general, the larger the pressure is, the thicker the wall thickness of the bellows is. In order to increase the displacement or reduce the rigidity of the bellows, and improve the service life, we can reduce the wall thickness of single layer and increase the number of layers of bellows.

C. guide structures shall be designed to prevent distortion of bellows. At the same time, local contact between bellows and valve rod or valve body shall be avoided to prevent wear and premature failure. Anti twist structure shall be designed to prevent the bellows from twisting and deformation.

D. when designing the valve, the compression and tensile stroke of the bellows shall be limited to prevent the bellows from bearing the compression displacement and tensile displacement beyond the limit of the cycle life test. In general, the stroke of the bellows shall not exceed 25% of the free length, otherwise the cycle life of the bellows will be affected. Especially the bellows valve in high temperature and high pressure environment.

E. bellows shall be of the same pressure and temperature class as the valve. The bellows shall be able to withstand the pressure test of the nominal pressure of the main valve at 38 ℃ and 1.5 times of the nominal pressure at 38 ℃. During the pressure test, the weld shall not crack or leak, and the bellows shall not be twisted.

F. the materials for bellows components shall be selected within the allowable service temperature range, and the materials for bellows shall have good forming performance, welding performance and compatibility with the medium.