Only Testing Reveals the True Performance of Hydraulic Valves

The Limitations of Theoretical Calculations

In the field of hydraulics, it is impossible to accurately determine the true performance of hydraulic components, including hydraulic check valves, through theoretical calculations alone. While theory provides a valuable foundation, real-world conditions introduce variables that cannot be fully accounted for in mathematical models.

Testing, which encompasses both measurement and experimentation with a strong emphasis on precise quantification, is essential. Hydraulic testing involves measuring parameters of hydraulic systems, components, and their parts under actual operating conditions or in specially designed test environments. This process allows us to determine their true characteristics.

Even though complex operating conditions may require sophisticated analysis and processing of test data, only through rigorous testing can we uncover the real performance of hydraulic check valves and other critical components. The nuances of fluid dynamics, material behavior, and manufacturing tolerances all contribute to performance characteristics that theoretical models simply cannot predict with sufficient accuracy.

Testing as the Foundation of Hydraulic Technology

Hydraulic technology, including the design and application of hydraulic check valves, has been built and developed on a foundation of rigorous testing. Theoretical advancements certainly play a role, but practical experimentation has been the driving force behind our understanding of hydraulic systems. The following examples illustrate this fundamental principle.

1. Flow Regime and Reynolds Number

As previously noted, the pressure difference across an orifice determines the flow rate through it. However, the flow regime—whether laminar or turbulent—exerts a significant influence on the actual flow rate. With the same pressure difference, calculations based on turbulent flow versus laminar flow formulas can yield results that differ by several times. The Reynolds number, which marks the boundary between these flow regimes, was determined by Osborne Reynolds in 1883 after thousands of experiments.

For hydraulic check valves and other components, understanding these flow regimes is crucial. The upper critical Reynolds number, where flow transitions from laminar to turbulent, was approximately 12,000 according to Reynolds' own experiments, though later researchers achieved values up to 40,000 in specially controlled, low-disturbance environments.

Flow Regime Visualization

The transition between laminar and turbulent flow significantly affects hydraulic performance. This visualization demonstrates the distinct characteristics of each flow regime, which must be considered when testing hydraulic check valves.

The lower critical Reynolds number, where flow reverts from turbulent to laminar, is generally accepted as 2000 for circular pipes (Reynolds originally suggested 2300). For concentric annular gaps, this value is around 1100, and for spool valve ports, it is approximately 260. Importantly, these values were not derived from theoretical formulas but were determined through extensive testing (see reference [18]).

The Reynolds number-friction factor charts commonly found in university textbooks (see Figure 5-3a) are actually derived from extensive experiments by researchers like J. Nikuradse (1932) and others, as shown in Figure 5-3b. These empirical relationships are essential for understanding fluid behavior in hydraulic systems, including the performance of hydraulic check valves under various operating conditions.

Reynolds Number-Friction Factor Relationship

The relationship between Reynolds number and friction factor, derived from experimental data, is critical for understanding fluid flow in hydraulic systems including hydraulic check valves.

2. Thin-Walled Orifice Flow Formula

The pressure difference-flow rate formula for thin-walled orifices, widely used in hydraulic technology (see equation 3-3), operates under specific assumptions. Many details can significantly affect the actual flow coefficient. For example, slight variations in orifice geometry, such as those shown in Figures 3-9 and 3-10, can cause the flow coefficient to vary from 0.7 to 2 or more. These variations, which directly impact the performance of hydraulic check valves and other flow control components, were discovered through systematic testing.

Standard Orifice Geometry

This standard orifice configuration typically yields a flow coefficient around 0.7, commonly referenced in hydraulic design calculations for components including hydraulic check valves.

Modified Orifice Geometry

Small geometric modifications can significantly alter flow characteristics, with coefficients reaching 2.0 or higher, demonstrating why testing is critical for hydraulic check valves.

These findings have profound implications for the design and application of hydraulic check valves. A valve that performs optimally under one set of conditions may exhibit unexpected behavior when operating parameters change, underscoring the necessity of comprehensive testing across the full range of intended operating conditions.

Testing Instruments in Hydraulics

Flow rate in pipes, while somewhat observable and estimable at outlet points, cannot be accurately determined by visual inspection alone. This challenge becomes even more pronounced in closed steel piping systems where direct observation is impossible.

Pressure, a critical parameter in hydraulic systems, is even more difficult to estimate without proper instrumentation. Before the invention of the pressure gauge in 1849, numerous serious accidents occurred with steam engine boilers due to the inability to accurately determine internal pressure. This historical context highlights the importance of reliable measurement in fluid power systems, including those utilizing hydraulic check valves.

As hydraulic technology has advanced, numerous specialized testing instruments have been developed. It is no exaggeration to state that modern hydraulics, including the precise operation of hydraulic check valves, would not exist without these instruments. While all measuring devices have some degree of error, this error can be quantified and accounted for through calibration—comparison with higher-accuracy instruments. Modern hydraulic testing instruments typically offer accuracies ranging from 2% down to 0.1%, making test results far more reliable than theoretical calculations alone.

Hydraulic Data Acquisition Systems

Modern hydraulic testing often employs sophisticated data acquisition systems where measured parameters are converted into electrical signals by sensors. This conversion allows for the recording, processing, display, and transmission of dynamic changes in system behavior—critical for understanding the performance of hydraulic check valves under varying conditions.

Advanced Testing Systems

Modern data acquisition systems provide precise measurements of pressure, flow, and temperature, enabling detailed analysis of hydraulic check valves and other components under dynamic conditions.

These systems can capture transient events and subtle performance variations that would be missed by traditional instruments, providing a more complete understanding of hydraulic component behavior.

Advantages of Recording Test Instruments

1

Recorded information can be objectively and completely transmitted, shared, analyzed, and discussed, even across regions with different languages. This facilitates global collaboration in the development and improvement of hydraulic check valves and other components.

2

Modern pressure sensors offer dynamic performance exceeding 5000Hz, sufficient to capture pressure transients in typical hydraulic components and systems, including rapid pressure changes across hydraulic check valves. This high-frequency capability allows for detailed post-analysis of transient events.

3

As previously noted, the performance of hydraulic check valves and other hydraulic components cannot be adequately described by just one or two numerical values. Performance curves provide a more accurate and comprehensive representation. Recording test instruments facilitate automatic data collection and curve generation, making it easier to visualize and analyze complex performance characteristics.

Practical Application

When testing hydraulic check valves, data acquisition systems can record pressure differentials across the valve at various flow rates, temperatures, and fluid viscosities. This comprehensive data set enables engineers to create performance maps that accurately predict valve behavior in real-world applications, ensuring optimal system design and operation.