Why are globe valves designed to be low in and high out?
Globe valves are designed to be low in and high out in order to reduce flow resistance and save effort in opening valves.At the same time, when the valve is closed, the padding between the valve housing and bonnet and the packing around the valve stem are not stressed. Without being subjected to medium pressure and temperature for a long time, the service life can be prolonged and the probability of leakage can be reduced.In addition, packing can be replaced or added when the valve is closed for easy maintenance.
Many people believe that globe valves are low in and high out, but they are not.Globe valves are generally low-in and high-out, but there are special cases where globe valves are high-in and low-out:
1. High pressure stop valve with diameter greater than 100mm
Due to poor sealing performance of large diameter valves, media pressure is applied above the disc to increase the sealing of the valves when the globe valves are closed in this way.
2. Two shutoff valves in series on bypass piping, and the second one requires high inlet and low outlet.
To ensure valve tightness during an inspection cycle, two shutoff valves in series are required for frequent opening and closing operations.For bypass systems, this bypass is installed to:
(1) Balance the pressure in front of and behind the main pipe valve to make opening convenient and labor-saving, and reduce wear of the main pipe valve;
(2) Heating pipes with small flow during start-up;
(3) In the main water supply pipe, the water pressure test of the boiler is carried out by controlling the water flow to control the pressure rising speed of the boiler.According to the direction of media flow, bypass stop valves are primary and secondary valves respectively. Primary and secondary valves are closed when the unit is in normal operation, and both of them are in direct contact with media.To prevent the padding between the secondary valve housing and bonnet and the packing around the valve stem from being subjected to medium and temperature for a long time and to replace the packing of the valve during operation, the installation direction required for the secondary valve is "high in and low out".
3. Boiler exhaust and vent stop valves
Boiler exhaust and vent globe valves are only used in the process of boiler startup and water feeding, and their opening and closing frequency is low. However, they often cause loss of working substance due to poor sealing. For this reason, in order to improve the tightness, some power plants will install such globe valves in the direction of "high in and low out".
4. Solenoid quick stop valve
The function of the solenoid quick-shut valve is to shut off the fuel supply quickly.The structure of the solenoid quick-disconnect valve is similar to that of the globe valve. If the solenoid quick-disconnect valve also enters from the lower part and flows out from the upper part, the force of fuel acting on the lower part of the solenoid quick-disconnect valve disc is great and the weight of the solenoid quick-disconnect valve is much less than it.Therefore, if the working fluid enters the quick-stop valve from below, the torque generated by the weight hammer is less than that generated by the fuel pressure, and the fuel can not be cut off when the quick-stop valve operates, thus the expected purpose can not be achieved.If the working fluid enters from the upper part of the quick-stop valve, the force acting on the lower part of the valve disc will soon decrease to zero due to the rapid decrease of pressure after the quick-stop valve is actuated, while the force acting on the disc by the fuel and the weight of the hammer and lever will form the force.
Generally, it is difficult to close the valve when using low-in and high-out in large-bore and high-pressure conditions. If using low-in and high-out in large-bore and high-pressure conditions, the valve stem will be easily deformed and bent by water pressure for a long time, which will affect the safety and sealing of the valve. If using high-in and low-pressure conditions, the diameter of the valve stem can be smaller and the cost will be saved for manufacturers and users.