Valve Components and Cavitation Phenomena

Valve Components

Valves are essential mechanical devices used to control the flow of fluids. Their various components work in concert to achieve precise regulation. The upper structure of the valve often serves as a guide for the stem's movement and houses the packing and gland.

Actuator

Responsible for performing the movement of the valve, which can be either manual or automatic.

Body

The main part through which the fluid flow takes place and within which the shutter (or plug) moves.

Rod (Stem)

The element responsible for transmitting the movement from the actuator to the shutter.

Plugger (Plug/Disc)

The moving part within the fluid stream that determines a section of the flow path according to a given characteristic (e.g., fast, linear, isopercent).

Seat

Provides support to the shutter and performs the work of closing the valve.

Guide

An item that aligns with the valve seat, ensuring proper movement and sealing.

Packing

An element whose primary function is to prevent fluid leakage to the outside of the valve.

Gland

Forces the packing to adjust against the walls of the packing box and presses it around the stem, ensuring a tight seal.

Understanding Cavitation in Valves

What is Cavitation?

Cavitation is the transformation of part of the circulating liquid into vapor during the rapid acceleration of the fluid as it passes through the valve's internal passages. Subsequently, this vapor returns to the liquid phase downstream of the valve, once it has passed through the regulation orifices.

How Cavitation Occurs

This transformation into vapor occurs because the fluid near the valve's internal passages experiences an acceleration, which significantly lowers the pressure acting on the fluid. If this pressure decreases to a point below the liquid's vapor pressure, bubbles will form in the liquid (evaporation). This phenomenon persists as long as the fluid is accelerated. Once the fluid slows down after passing through the valve, it undergoes pressure recovery.

Consequences of Cavitation

Pressure recovery has very negative consequences for valves. The return of the vapor to the liquid phase results in a local vacuum. This vacuum causes material erosion, leading to the detachment of material and the formation of holes and channels within the valve. Such damage severely impairs the valve's closing capacity and regulation, leading to serious and often irreparable harm.

Preventing Cavitation

Cavitation depends significantly on the pressure recovery characteristics of the valve, which in turn vary depending on the valve's internal construction. In 63, the critical flow coefficient (Cf) is introduced, which defines the pressure recovery for each type of valve. We can avoid cavitation by reducing the pressure drop below the critical pressure drop for a given valve and operating condition.