What are the characteristic advantages of globe valves?

Advantages and disadvantages of shut-off valves

1、Advantage

a, compared with the gate valve, the structure of the globe valve is simpler, usually only one sealing surface on the valve body and valve flap, small sealing area, saving valuable materials, low cost, and therefore better manufacturing process, easy maintenance.

b, the sealing surface wear and abrasion is light, good sealing. No relative sliding between the valve flap and the valve body sealing surface when opening and closing (except for the conical sealing surface), so wear and abrasion are not serious, and good sealing performance, and long service life.

c, opening height is small, generally, only 1/4 of the diameter of the valve seat channel, compact structure, save installation space.

2、Disadvantages

a、Large structure length.

b, opening and closing torque is large, opening and closing effort. When closing, because the direction of movement of the valve flap is opposite to the direction of media pressure, must overcome the force of the medium, so the opening and closing torque is large.

c, large flow resistance. The media channel inside the valve is more tortuous, flow resistance, power consumption, especially in the hydraulic device, this pressure loss is particularly obvious. In all types of cut-off valves the valve flow resistance is the largest.

d, the direction of media flow is restricted.

Media flow through the shut-off valve, at the valve seat channel can only be from the bottom up or down, so the media can only flow in one direction, and can not change the direction of flow.

e, when the valve is closed, the sealing surface may be trapped between the solid particles in the fluid medium.

f. Installation is inconvenient.

 

Operating characteristics of the shut-off valve

The sealing form of the globe valve is forced sealing, and sufficient force must be applied to the valve flap when the valve is closed to achieve sealing must be above the specific pressure to achieve sealing. Since the sealing force and medium pressure are on the same axis, when the medium enters from the lower part of the valve flap (low to high), the valve closing torque is much larger than the opening torque due to the opposite direction of the medium pressure and sealing force, the maximum stress on the valve stem is compressive stress, so it must have sufficient strength and rigidity, otherwise, the valve stem will produce flexural deformation. Because of this, when the shut-off valve bore size greater than DN150 (NPS16), in order to reduce the valve operating torque and thus reduce the stem diameter, usually change the media flow direction from the upper part of the valve flap into (high into the low), at this time the media pressure and sealing force direction is the same, under the action of media pressure, the valve is easier to achieve sealing, closing torque becomes smaller, opening torque increases, the maximum stress on the stem is tensile Stress, at this time the stem diameter can be reduced accordingly.

When the shut-off valve is opened, the opening height of the valve flap reaches 25%-30% of the nominal size of the valve, the flow rate has reached the maximum, which indicates that the valve has reached the fully open position, so the fully open position of the shut-off valve should be determined by the stroke of the valve flap.

When the globe valve is closed and opened again, it is similar to the forced seal gate valve, that is, after the valve is closed, sufficient operating force should be applied to the sealing surface to achieve sealing. Therefore, the closing torque of the valve should be determined by increasing the operating torque to the specified value. When the valve is opened again, the valve opening torque is usually larger than the closing torque in order to open the valve reliably due to the need to overcome the effects of static friction and thermal expansion, and therefore should be considered in the design.

In order to reduce the high shock pressure formed in the opening and closing the valve, the globe valve should be operated slowly, in the sense of producing a rate of change consistent with the flow rate.

The operating torque of the shut-off valve shows the following characteristics.

① "low into high", the valve from the fully open position to start the closing stage, the operating force to overcome the resistance is the stem and packing friction resistance and medium pressure in the valve flap cross-section (axial) caused by the thrust, with the valve flap down, the fluid in the valve flap before and after the formation of pressure differential, prevent the valve flap down, and this resistance will be with the valve flap descending and rapidly increasing, with the valve completely closed, the stem exerts a forced sealing force, the pressure difference between the front and rear of the valve flap reaches the maximum value of the working pressure of the medium, when the stem is subjected to the maximum closing torque. In the valve opening process, the medium pressure or the pressure difference between the front and rear of the valve flap from the same thrust and valve opening direction, but it should be noted that in the opening moment because to overcome the larger static friction torque between the sealing surface, the opening operating torque may also exceed the closing torque.

② "high in low out", the valve from the fully open position to close the stage, the operating force required to overcome the resistance is still the above two forces, as the valve flap down, the fluid in the valve flap before and after the pressure difference until the valve is closed is conducive to valve closure. The situation is exactly the opposite when the valve is opened because the medium pressure and the pressure difference before and after the valve flap caused by the thrust are opposite to the valve opening direction, so the valve opening process requires a much larger operating torque than the closing torque.

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