Pressure Drop-Flow Characteristics of Hydraulic Valves
Understanding the relationship between pressure drop and flow rate is fundamental to analyzing hydraulic systems. This comprehensive guide explores the essential characteristics that define hydraulic valve performance, including the role of hydraulic valve symbols in system design and analysis.
Hydraulic valve system with pressure and flow measurement instrumentation
1. Pressure Drop-Flow Characteristics
Regardless of how hydraulic valves are classified, their fundamental nature is simply their ability to change flow passages and openings. Therefore, every hydraulic valve contains at least one flow passage. Whenever a flow passage is open and there is a pressure difference between its two ends, there will be flow. When either the opening or the pressure difference changes, the flow rate changes accordingly.
The valve's opening and its variation are determined once the valve is manufactured. However, the actual opening area during operation cannot be directly measured. In principle, the spool displacement can be measured, but this generally requires modifying the valve – installing a rod connected to the spool, linked to a displacement sensor. For this reason, the opening and variation of hydraulic valves are always represented by pressure drop-flow characteristic curves, which are essential components in hydraulic valve symbols used in system diagrams.
Figure 3-46: Factors Affecting Opening Variation Determine Valve Pressure Drop-Flow Characteristics
- 1 - Constant opening
- 2 - Opening varies with flow rate
- 3 - Opening varies with pressure drop
- A - Maximum opening area
All hydraulic valves have at least one pressure drop-flow characteristic curve, though they may appear in different forms and sometimes go by different names in technical literature and hydraulic valve symbols.
The openings of directional valves and shut-off valves generally do not change with pressure drop or flow rate. Therefore, their pressure drop-flow characteristics are roughly like those of a fixed hydraulic resistance, as shown by curve 1 in Figure 3-46. This behavior is clearly represented in standard hydraulic valve symbols used in system design.
The openings of pressure valves generally change with flow rate: the greater the flow rate, the larger the opening. Consequently, the pressure drop can remain approximately constant across different flow rates, resulting in a characteristic curve (curve 2 in Figure 3-46) that is roughly parallel to the flow axis. Understanding this behavior is crucial when interpreting hydraulic valve symbols in system schematics.
The openings of flow control valves generally change with pressure drop: the smaller the pressure drop, the larger the opening. Therefore, the flow rate can remain approximately constant across different pressure drops, as shown by curve 3 in Figure 3-46. This important characteristic is standardized in hydraulic valve symbols to ensure consistent interpretation across designs.
However, both pressure valves and flow valves have limited openings that cannot increase beyond a maximum point, after which they become fixed hydraulic resistances. At this point, their pressure drop-flow characteristics correspond to area A in the figure. This limitation is an important consideration when working with hydraulic valve symbols in system design.
Key Insight
The pressure drop-flow characteristic curve is the most accurate way to represent a valve's performance, as it accounts for all operational variables. This is why hydraulic valve symbols often include references to these curves in technical documentation.
2. About Flow Rate
(1) Determining Flow Rate
ISO 5598 "Fluid Power Systems and Components - Vocabulary" defines rated flow as: "A confirmed flow through testing, at which a component or piping is designed to operate." The corresponding national standard GB/T 17446 translates this as "rated flow." It would be incorrect to interpret this as a "specified flow," as no hydraulic valve must operate at a single specific flow rate. Therefore, "determined flow" is a better understanding.
This defined flow rate is an important parameter included in technical specifications and is often referenced alongside hydraulic valve symbols to provide complete performance information. Engineers rely on this data, combined with hydraulic valve symbols, to properly size and select components for hydraulic systems.
ISO 5598 Standard
Established international guidelines for fluid power terminology, ensuring consistent communication across global industries. This standard helps in standardizing hydraulic valve symbols and their associated performance parameters.
GB/T 17446 National Standard
Chinese national standard corresponding to ISO 5598, adapting international terminology for local application while maintaining compatibility with global hydraulic valve symbols and specifications.
(2) Nominal Flow
Many manufacturers use the term "nominal flow" in their product specifications, but there is no unified definition for this term. This lack of standardization can create confusion when comparing different products, even when their hydraulic valve symbols appear similar.
For directional valves and shut-off valves, nominal flow usually refers to the flow rate that can pass through at a certain pressure drop. While this definition seems simple, it is actually ambiguous, a fact that should be considered when interpreting hydraulic valve symbols from different manufacturers.
Sources of Ambiguity in Nominal Flow Specifications
- Varied Pressure Drop References: Major manufacturers in Europe and America often select different pressure drops, with some using 0.5MPa and others 0.7MPa (derived from the imperial 100psi). Flow rate values measured at different pressure drops are not comparable, even when hydraulic valve symbols suggest similar performance.
- Multiple Flow Path Considerations: Some valves, such as directional valves, often have multiple flow paths, which may result in multiple different pressure drop-flow characteristic curves. The question then arises: which curve should be used to determine the nominal flow rate referenced in hydraulic valve symbols?
For pressure valves and flow valves, nominal flow actually often refers to "determined flow," roughly representing the maximum achievable flow rate. This distinction is important when working with hydraulic valve symbols in system design, as it affects component selection and performance expectations.
Therefore, the nominal flow rates provided in product catalogs should be used only as references, as their value is limited. It is better to compare the pressure drop-flow characteristic curves, which provide more comprehensive performance data. Some manufacturers, such as Hydraforce, sometimes do not provide nominal flow rates at all, instead directing users to consult the characteristic curves alongside their hydraulic valve symbols.
It is perhaps due to these considerations that "nominal flow" was not included in either the 1985 or 2008 editions of ISO 5598 "Fluid Power Systems and Components - Vocabulary." This omission highlights the challenges in standardizing this parameter across different types of hydraulic valves and their corresponding hydraulic valve symbols.
(3) Operating Flow
If nominal flow refers to the flow rate that can pass through a valve at a certain pressure drop, such as 0.5MPa or 0.7MPa, it is important to note that nearly all hydraulic valves can operate under much higher pressure drops. Naturally, the operating flow rate that can pass through them will be much higher under these conditions.
The flow rate that can pass through a flow control valve is basically manually adjusted. In theory, there is no limit to the flow rate that can pass through a throttle valve or relief valve: the greater the pressure drop, the greater the flow rate, until the pressure exceeds the burst pressure and the valve is destroyed. However, solenoid directional valves have a certain operating range due to the limited thrust of the solenoid (see section 4.5) and can only switch normally within a certain flow range. This operational limitation is an important consideration when interpreting hydraulic valve symbols in system designs.
Advanced hydraulic valve testing facility measuring flow rates across various pressure differentials
Understanding the relationship between operating flow and pressure drop is crucial for proper valve selection. Engineers must consider not just the nominal values but the entire operating range when working with hydraulic valve symbols in system design. This comprehensive approach ensures that the hydraulic system will perform reliably under all expected conditions.
(4) Port Size
However, if a valve is to handle large flow rates while maintaining low pressure losses, the actual opening of the flow passage must be large enough. This has led to the use of port size, DN, and DG to characterize the size of valves. These size designations are important elements in hydraulic valve symbols, providing quick visual information about a valve's capacity.
Initially, the diameter of the valve's port in millimeters was generally used as a reference, following SAE (Society of Automotive Engineers) standards, such as 06, 08, 10, etc. However, as technology has advanced, port diameters have been modified to fully exploit valve flow capacity and achieve better structural characteristics.
| Nominal Size | Approximate Port Diameter (mm) | Typical Flow Range (L/min) |
|---|---|---|
| 06 | 6-8 | 30-60 |
| 08 | 8-10 | 60-100 |
| 10 | 10-12 | 100-160 |
| 16 | 16-18 | 160-250 |
| 25 | 25-30 | 250-400 |
Therefore, nominal size, port size, DN, DG, etc., are now only loosely related to the actual manufacturing dimensions of the port, serving as rounded values for reference: a size 08 valve is definitely larger than a size 06 valve. These size designations are standardized in hydraulic valve symbols to facilitate quick system design and component selection.
Since moving the spool is the only means of regulating flow, and flow rate determines the movement speed of hydraulic cylinders, the speed at which the spool moves affects the rate of change of flow, which in turn affects the acceleration of the hydraulic cylinder. The acceleration of the hydraulic cylinder movement, in turn, determines the smoothness of the load movement.
Therefore, how to move the spool and control the speed of spool movement is crucial to the smoothness of load movement. This principle is fundamental to understanding hydraulic system dynamics and is reflected in the design and application of hydraulic valve symbols in system schematics. The precise control of spool movement remains a key area of development in hydraulic valve technology, with ongoing advancements improving both performance and energy efficiency.
When designing hydraulic systems, engineers must consider not just the static pressure drop-flow characteristics but also the dynamic response of valves to spool movements. This dynamic behavior is often indicated in technical specifications alongside hydraulic valve symbols, providing a complete picture of valve performance under various operating conditions.
The interaction between port size, flow rate, pressure drop, and spool dynamics creates a complex relationship that defines overall system performance. By understanding these interrelationships and properly interpreting hydraulic valve symbols, engineers can design more efficient, reliable, and responsive hydraulic systems.
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