Long term maintenance and assembly requirements for valves

Long term maintenance and assembly requirements for valves

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The valve resistance coefficient ζ depends on the size, structure and cavity shape of the valve product. Each component in the body cavity of the valve can be thought of as a system of components that generate resistance (fluid turns, expands, contracts, turns again, etc.). Therefore, the pressure loss in the valve is approximately equal to the sum of the pressure loss of each valve component. It should be pointed out that the change of resistance of one component in the system will cause the change or redistribution of resistance in the whole system, that is to say, the medium flow is mutually affecting each pipe section.
When the fluid passes through the valve, its fluid resistance loss is represented by the fluid pressure drop △P before and after the valve.
For turbulent liquids:
Where △P — pressure loss of the valve under test (MPa)
ζ — flow resistance coefficient of valve;
P — Fluid density (kg/mm)
U — Average flow velocity of fluid in pipe (mm/s)
Fluid resistance of valve components
The valve resistance coefficient ζ depends on the size, structure and cavity shape of the valve product. Each component in the body cavity of the valve can be thought of as a system of components that generate resistance (fluid turns, expands, contracts, turns again, etc.). So the pressure loss in the valve is approximately equal to the sum of the pressure loss of each component of the valve, that is:
In the formula, the resistance coefficient of valve components with the same medium flow rate in the pipeline.
It should be pointed out that a change in the resistance of one element in the system causes a change or redistribution of the resistance in the whole system, that is to say, the medium flow affects each pipe segment mutually. In order to assess the influence of various components on valve resistance, resistance data of some common valve components are used. These data reflect the relationship between the shape and size of valve components and fluid resistance.
(1) Sudden expansion
As shown in Figure 1-12, sudden expansion causes a large pressure loss. At this point, part of the fluid velocity is consumed in eddy formation, fluid agitation and heating. The approximate relationship between the local resistance coefficient and the ratio of the cross-sectional area A1 before expansion and A2 after expansion can be expressed by equations (1-9) and (1-10). The drag coefficient is shown in the table
Figure 1-12 Sudden expansion
(1-9)
(1-10)
Type in the %
ζ — resistance coefficient at medium velocity in expanded pipeline;
ζ — Drag coefficient at medium velocity in the tube before expansion.
Table 1-32 ζ values of local drag coefficient during sudden expansion
(2) Gradually expanding As shown in Figure 1-13, when θ 40°, the resistance coefficient of the gradually expanding tube is smaller than that of the suddenly expanding tube, but when θ=50° -90 °, the resistance coefficient increases 15%-20%. Gradually expanded better expansion Angle θ: circular tube θ=5° ~6°30′; Square tube θ =7°~8°; The local resistance coefficient of rectangular tube θ= 10° -12 ° can be calculated as follows:
(1-11)
ζ — coefficient, as shown in Table 1-33;
λm — average resistance coefficient along the path,
λ1λ2 — are the drag coefficients corresponding to small and large tubes respectively.
Figure 1-13 expands gradually
Table 1-33 ζ values
(3) Sudden shrinkage is shown in Figure 1-14. The local resistance coefficient of sudden shrinkage is shown in Table 1-34. ζ can also be calculated by the following empirical formula:
(1-12)
Figure 1-14 zoom out
(4) Gradually shrinking As shown in Figure 1-15, the pressure loss generated by gradually shrinking is small, and the local resistance coefficient is calculated as follows:
(1-13)
ξ C — coefficient, as shown in Table 11-35;
ε — coefficient, see Table 1-36
ζ values can also be obtained directly from Figure 1-16.
Figure 1-15 gradually diminishes
Table 11-34 ζ values of suddenly reduced local drag coefficients
Long term maintenance and assembly requirements for valves
Valve is the control part of the fluid conveying system, with cut-off, adjustment, diversion, prevent countercurrent, pressure regulator, shunt or overflow pressure relief and other functions. Valves for fluid control systems, from the most simple globe valves to extremely complex automatic control systems used in a wide range of valves and specifications. Valves can be used to control the flow of air, water, steam, various corrosive media, mud, oil, liquid metal and radioactive media and other types of fluids. So, in the long-term use of the valve process should be how to maintain?
1. Do not use long levers or wrench wheels when closing or opening the valve.
2. Stem threads often friction with stem nut, should be in the screw to maintain a certain amount of oil, lubrication, to ensure the free movement of the stem, flexible and good. Valve mechanical transmission, to timely on the gearbox additives, to prevent biting.
3. Open the valve for a long time, the sealing surface may be sticky with dirt; When closing, the valve can be gently closed first, and then open a little, so that the dirt can be washed away by high-speed flow of medium, and then shut again.
4. The valve installed outdoors should be protected with a protective sleeve on the valve stem to prevent rain, snow, dust and rust
5. The valve should be cleaned and checked frequently to keep the valve parts clean and complete. Do not place heavy objects on the valve and do not stand on the valve.
6. Before opening the steam valve, remove the congealing water in the system, and then slowly open the valve to avoid the impact of soda water; When the valve is fully open, turn the handwheel back a little.
7. The spare valve should be placed in a dry place indoors, and the interface should be sealed with wax paper board or plug to avoid the entry of dirt.
Valve assembly requirements
Cleaned parts must be sealed for installation. Requirements for the installation process are as follows:
1. The installation workshop must be clean, or set up temporary clean areas, such as newly purchased color strip cloth or plastic film, to prevent dust from entering during the installation process.
2, assembly workers must wear clean cotton work clothes, wear a pure cotton cap, hair can not leak, feet wear clean shoes, hands wear plastic gloves, degreasing,.
3. Assembly tools must be degreased and cleaned before assembly to ensure cleanliness.


Post time: Jun-30-2022

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