Introduction
A control valve is an important device that regulates the flow rate of fluid that passes through a given pipe or aperture. Depending on the nature of fluid and function, users have to consider numerous attributes of control valves. The association between stem travel and control valves determines flow characteristics and pressure drop. According to Monsen, the pressure drop in control valves is dependent on inherent and installed features. Moreover, the pressure drop in a fluid that goes through a control valve is subject to numerous factors, relating to the design of aperture, type of fluid, piping system of fluid, temperature, and pressure. Therefore, it is evident that a combination of characteristics of the control valve, installation system, and attributes of fluid are vital factors that influence pressure drop in control valves.
Inherent Characteristics of Valves
Valves have inherent characteristics that are dependent on the engineering design. Manufacturers incorporate engineering features on valves to ensure that the low rate of fluid has constant pressure and delivers a continuous volume of fluid. Control valves with equal stem travel and uniform flow rate tend to generate constant pressure. However, valves with unequal stem travel and variable flow rate cause a high degree of pressure drop. A control valve with an ideal characteristic assumes constant pressure and uniform flow rate without the influence of systemic effects. Nevertheless, the existence of pressure drop owing to variation in aperture size and flow rate of fluid distorts the ideal characteristic of a pressure valve.
A control valve can also cause a non-linear relationship between flow rate and stem travel, owing to different levels of pressure drop. In a non-linear relationship, small changes in stem travel at lower levels cause small rises in the flow rates, while substantial changes in stem travel at higher readings lead to significant upsurges in the flow rate. In this case, the stem travel of a control valve determines the pressure drop and flow rate increases.
The purpose of engineering design is to ensure that stem travel and changes in the flow rate have a linear relationship with a constant pressure drop. For instance, globe valves exist in designs that allow linear and non-linear relationships in stem travel and the flow rate of fluids (Table 1). In contrast, rotary valves are available in fixed forms that do not provide for versatility in the regulation of flow rates. Control valves that employ the design of full or segmented balls depict relationships of non-linear, which show an equal percentage increase in the flow rate over stem travel. Additionally, butterfly valves with high performance adopt a moderate relationship that occurs between linear and equal percentage curves. Hence, the design determines the inherent characteristic and influences the pressure drop of control valves.
Table 1: Types of Valves and Their Characteristics.
Installed Characteristic of Valve
Installed characteristic is also another factor that influences the pressure drop in the control valve. During manufacturing, the design features of control valves assume the existence of constant pressure and uniform flow rate of fluid in a system for a linear relationship to occur. However, the functioning of control valves is dependent on their installation in different systems with variable flow rates and pressure drops. Monsen defines installed characteristics as a product of the flow rate, inlet pressure, valve position, centrifugal force, and piping pressure losses of the specific system. A significant pressure drop occurs when the flow rate is low, inlet pressure is small, the valve position is distant, centrifugal force is minimal, and the piping system is extended. In installing control valves, engineers have to consider numerous factors that influence pressure drop. The overall objective of the effective installation of control valves is to optimize pressure by ensuring there are minimal losses for optimal flow rates of fluid within a system.
Stem travel and pressure exhibit negative relationships in a valve with installed characteristics. Pressure in a pipe system has a negative association with the magnitude of the size of the aperture in the control valve. When the control valve is slightly open, the pressure is high, and the flow rate is low. In contrast, when the control valve is wide open, the pressure is low, and the flow is high. Variation in the degree of opening the control valve does influence not only the flow rate but also the pressure drop in a system with a lot of piping. Consequently, the interaction between the piping system and the control valve creates a characteristic of that approaches the linear trend. A comparison of the control valves reveals that those with a lot of piping systems exhibit a pattern of equal percentage, while those with little piping system depict the trend of a linear relationship (Table 2).
Table 2: Installed Valve and Flow Characteristics.
Attributes of Fluid
The type of fluid regulated by the control valve affects the pressure drop in various systems. Gas and liquid are two types of fluids that dictate the degree of pressure drop across different types of control valves. Pressure drop is high in liquids than gases owing to their differences in the degree of viscosity. In large volumes, liquids exhibit a trend of equal percentage, while gases depict a linear trend in the flow rate owing to differences in pressure drop (Table 3). Temperature affects the pressure drop by varying the viscosity and the density of the fluid in the control valve. High temperatures reduce the viscosity and the density of the fluid, and consequently decrease the pressure drop in a flow system. On the other hand, low temperatures increase the thickness and the density of the fluid, leading to an increased pressure drop across the control valve.
Table 3: Type of Liquid and Flow Characteristics.
Conclusion
A critical examination of the control valve shows that it varies in design, installation, and the nature of fluid it controls. These variable factors affect the pressure drop across a given control valve. Manufacturers design the control valves to ensure that the pressure drop does not influence the flow rate of fluid they regulate. Installation of the control valve in a piping system requires consideration of the size and length of pipes to avert pressure losses and significant pressure drop. The use of liquids and gases as fluids to regulate by the control valve influence the pressure drop. While liquids have a lot of pressure drop due to viscosity and high density, gases tend to have a low level of pressure drop. Therefore, the design features of the control valve, the size of the installation system, and types of fluid are significant factors that affect the pressure drop in control valves.
Reference List
- F. Hamad, F. Faraji, C. G. S. Santim, N. Basha, and Z Ali, “Investigation of pressure drop in horizontal pipes with different diameters,” International Journal of Multiphase Flow. 91, pp. 120-129, 2017.
- Engineering ToolBox, “Control valves and flow characteristics,”. Web.
- J. Monsen, “Control valve flow characteristics” 2015. Web.
- J. Praveen, “Flow coefficient analysis for a globe valve by using CFD,” International Journal of Science and Research, vol. 6, pp. 757-763, 2015.
- D. Hellmann and D. Agar, David, “Modeling of slug velocity and pressure drop in gas‐liquid‐liquid slug flow,” Chemical Engineering and Technology, vol. 42, no. 10, pp. 2138-2145, 2019. Web.
- D. Harrold, “Valve characteristics selection guidelines,”. Web.