Reduce cavitation and flashing in the globe valves
Knowing the principles by which cavitation and flashing occur in a globe valve... how we can prevent these.
23 February 2021
Globe valves are fundamental components within water systems and allow you to adjust one of the characteristic parameters (pressure or temperature), changing the flow of the fluid that passes through them. The most characteristic aspect of this type of valve is the possibility of having a linear regulation of the flow through the conformation of the component itself.
The components of the globe valve
The globe valve consists of: - Body: it is the supporting structure of the valve; - Shutter: it is the element that blocks the flow of fluid inside the duct; - Rod: the movement of this component allows the movement of the shutter; - Sealing seat: represents the area where the shutter must be located in order to have a total closure of the valve; - Stuffing box: used to ensure isolation from the external environment, allowing you to avoid problems that may arise; - Flanges: necessary for connection with the pipe by means of threads.
When the shutter is in the sealing seat, the valve is completely closed. Conversely, when the stroke of the rod is at the minimum it is in the maximum opening position.
The problems of cavitation and flashing
The passage inside the globe valve is characterized by a section reduction with a constant flow rate.
where Q is the flow rate, v is the velocity of the fluid and A is the section of the duct in correspondence with the valve.
Therefore, based on this consideration, there is an increase in v which is a parameter of the Bernoulli equation:
where ρ is the density of the fluid, g is the acceleration of gravity, h is the average potential share and p is the pressure.
The last term is constant because there is no difference in height, so the speed variation involves a pressure reduction. The problem arises if the pressure drops below the vapor pressure with the formation of vapor bubbles. There are two different cases based on the pressure downstream of the valve p2: - p2 > pv: in this case there is the problem of cavitation with bubbles that implode due to the pressure greater than that of steam. Over time, continuing to explode, they cause both resistance and noise problems in the system, damaging it. - p2 < pv: there is the problem of flashing with steam bubbles that do not explode and lead to an excessive increase in noise.
To solve these problems it is necessary to act both in the design phase and in the regulation phase of the valve, avoiding such speeds as to induce an excessive reduction of pressure and the appearance of these phenomena. Through the study and analysis of the different variables (pressure, density, speed, section, etc.) it is possible to understand the most effective methodologies to guarantee a longer duration of the system and reduce the unfavorable auditory effects that cavitation and flashing can cause in globe valves.
The name "globe valves" derives from the shape of the spherical outer casing that characterizes them. These components allow the regulation of a parameter between pressure and flow rate, favoring the efficiency of the system and reducing losses to a minimum.