Globe valves also are called stop valve, belongs to forced sealing valve and is a kind of block valve. There are three connection modes: flange joint, screwed connection and welded connection. The flow direction of the globe valves is from top to bottom, so it has directionality during installation. This type of globe valves is very suitable for cut-off or regulation and shut-off. Because the valve stem opening or closing stroke of this kind of valve is relatively short, and has a very reliable cutting function, and the change of the valve seat opening of the globe valves is directly proportional to the stroke of the valve disc, it is very suitable for the regulation of flow.
The globe valves are designed with inlet low and outlet high to reduce the flow resistance and save effort when opening the valve. At the same time, when the valve is closed, the padding between the valve casing and bonnet and the stuffing around the valve stem are not stressed, which will not be affected by the medium pressure and temperature for a long time and can prolong the service life and reduce the probability of leakage. In addition, the stuffing can be replaced or added when the valve is closed, which is convenient for maintenance.
Many people think that the globe valves are inlet low and outlet high, in fact that was not the case. Generally, the globe valves are inlet low and outlet high, but there are some special cases where the globe valves are inlet high and outlet low.
(1) High pressure globe valves with diameter greater than 100mm
Due to the poor sealing performance of the large-diameter valve, the medium pressure acts on the valve clack when the globe valves are closed, so as to increase the sealing performance of the valve.
(2) Two globe valves are connected in series on the bypass pipeline, and the second globe valve requires “inlet high and outlet low”
In order to ensure the tightness of the globe valve in a maintenance cycle, two globe valves in series are required for valves that are often opened and closed. For the bypass system, the installation functions of this bypass are:
① Balance the front and rear pressure of the main pipeline valve, make it convenient and labor-saving to open, and reduce the wear of the main pipeline valve.
② During startup, warm up the pipe with small flow
③ On the main water supply pipeline, the water supply flow is controlled to control the pressure rise speed of the boiler, and the boiler hydrostatic test is carried out. According to the medium flow direction, the bypass globe valves are primary valve and secondary valve respectively. During normal operation of the unit, the primary valve and secondary valve are closed, and both of them are in direct contact with the medium. In order to prevent the padding between the secondary valve casing and bonnet and the stuffing around the valve stem from being affected by medium and temperature for a long time, and the stuffing of the valve can be replaced during operation, the installation direction of the secondary valve is required to be “inlet high and outlet low”.
(3) Exhaust the boiler and vent the globe valve
Exhaust the boiler and vent the globe valves for the boiler are only used in the process of boiler startup and water supply, and the opening and closing frequency is small, but the working medium loss is often caused by lax sealing. Therefore, some electric power plants install these globe valves in the direction of “inlet high and outlet low” in order to improve the tightness.
(4) Electromagnetism quick shut-off valve
The function of the electromagnetism quick shut-off valve is to close quickly and cut off the fuel supply quickly. The structure of the electromagnetism quick shut-off valve is similar to that of the globe valve. If the electromagnetism quick shut-off valve is also a working medium entering from the lower part and flowing out from the upper part, the fuel has a great force on the lower part of the electromagnetism quick shut-off valve disc, and the weight of the electromagnetism quick shut-off valve is much smaller than it. Therefore, if the working medium enters the quick shut-off valve from the lower part, the torque generated by the heavy hammer is less than the torque generated by the fuel pressure. When the quick shut-off valve is working, the fuel cannot be cut off, so the expected purpose cannot be achieved.
Generally, it is difficult to close the valve if inlet low and outlet high are used under large diameter and high pressure. If inlet low and outlet high are used under high pressure and large diameter, the valve stem is easy to deform and bend under water pressure for a long time, affecting the safety and sealing of the valve; If inlet high and outlet low is selected, the valve stem diameter can be smaller, which will also save some cost for valve suppliers and users.