What are the characteristics of a pressure regulator?
Release Date:
2023-02-15
A so-called small-flow regulating valve, as its name suggests, is a regulating valve with very limited flow capacity. The flow capacity of a valve is a performance indicator that quantifies the valve’s throughput under standardized conditions. In China, this parameter is denoted by the C value, which is defined as follows: when the valve is fully open, with a pressure differential of 1 kg/cm² across the valve and a fluid density of 1 g/cm³, the volume of fluid that flows through the valve per hour (in m³/h). For incompressible fluids, under fully turbulent flow conditions, the valve’s flow capacity depends solely on the valve’s intrinsic design. When calculating the required flow capacity of a valve, it is important to recognize that differences in the fluid properties or in the flow conditions can lead to significant variations in the flow regime within the valve.
As the name suggests, a small-flow control valve is a control valve with very limited flow capacity.
The flow capacity of a valve is a performance indicator that characterizes the valve’s capacity under standardized conditions. In China, it is denoted by the symbol C. By definition, C represents the volumetric flow rate (in cubic meters per hour) through the valve when the valve is fully open, the pressure differential across the valve is 1 kg/cm², and the fluid density is 1 g/cm³. For incompressible fluids, under fully turbulent flow conditions, the valve’s flow capacity depends solely on the valve’s intrinsic design. When calculating the required flow capacity of a valve, it is important to recognize that differences in the fluid properties or in the flow conditions can lead to significant variations in the flow regime within the valve.
Under low-flow conditions, particularly when handling viscous fluids at low pressures, the flow regime is typically laminar or a transitional state between laminar and turbulent flow. In the laminar regime, the flow rate through the valve exhibits a linear relationship with the pressure differential across the valve. As the Reynolds number increases in the transitional regime, even if the pressure differential remains constant, the mass flow rate through the valve will rise. Only under fully turbulent flow does the flow rate become independent of changes in the Reynolds number. Nevertheless, when selecting small-flow control valves, conventional methods and empirical formulas are still commonly employed. However, the calculated values often deviate significantly from the actual performance; according to available data, for Cv values below 0.01, the Cv rating serves merely as a sizing indicator with limited practical significance. The actual flow capacity should be determined based on engineering experience.
As the flow capacity decreases, the valve’s turndown ratio will decline. However, it can still be maintained within the range of 10:1 to 15:1; if the turndown ratio becomes even smaller, flow regulation will become difficult.
When valves are used in series, the pressure differential across each valve varies with the degree of opening, causing the valve’s characteristic curve to deviate from the ideal performance. If the pipeline resistance is high, the linear characteristic will shift toward a quick-opening behavior, resulting in a loss of control capability; conversely, an equal-percentage characteristic will tend to become linear. At low flow rates, however, where pipeline resistance is minimal, these deviations from the ideal characteristics are small, and the need for an equal-percentage characteristic virtually disappears. From a manufacturing standpoint, when the flow coefficient Cv falls below 0.05, it is no longer feasible to achieve an equal-percentage side profile. Consequently, the primary challenge for low-flow valves is to maintain the flow within the desired range.
From an economic standpoint, users often desire a single valve that can both shut off flow and provide regulation—and such functionality is indeed achievable. However, for control valves, the primary objective is to regulate flow, with shut-off being a secondary function. It is a misconception to assume that small-flow valves, because their inherent flow capacity is low, can easily achieve shut-off when closed. Internationally, leakage limits are typically specified for small-flow control valves as well. For example, when the Cv value is 10, the valve’s leakage is limited to no more than 1% of the full-flow rate under a supply pressure of 3.5 kg/cm².
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