When the valve is cycling, the quality of control can be observed by measuring and plotting the flow leaving the valve and operating the valve input, such as current (for valves controlled by a coil) or step length (for valves controlled by a stepper motor). This article will discuss the characteristics of the valve performance curve—in particular, where the flow starts, and how the trend of the curve indicates certain valve characteristics and their consequences.
When selecting a valve, you need to look at many parameters, but when sensitive equipment is involved and accuracy is critical to the application, such as in fluid analysis, the lifting characteristics of the flow should be considered, because the initial flow may surprisingly affect the life and life of the entire system. run. For example, if the valve allows a sudden surge in flow, it may cause a hydraulic shock (also known as water hammer or fluid hammer). This can be seen in Figure 1, where the circled area shows the sharp flow peak that occurs when the valve is lifted or opened.
The sudden lift is usually due to how the internal sealing surface of the inlet (usually the nozzle) engages with the actuating sealing surface of the valve, most commonly a poppet valve with rubber adhered to its surface. Unfortunately, the nature of rubber (fluororubber, EPDM, etc.) makes it easy to stick, deform and degrade, which in some cases can cause the poppet to stick to the nozzle before it suddenly detaches. Mechanical irregularities may also cause or contribute to a sudden increase in flow, but these are very related to the design of the valve, such as springs and even friction.
Certain types of valves have characteristics that make them more suitable for reducing flow lift or surge and hammering, such as gate valves, ball valves, or needle valves. In order to limit the inrush current in your application, the operating orifice of the valve needs to be variable in its free area. A typical control valve, such as a poppet valve or a spool valve, has a fixed orifice. Therefore, the lift on the sealing surface acts on the entire circumference of the orifice. This leads to an initial peak in flow. For example, a 0.006 orifice plate with a control lift of 0.001 will represent 6.5% of the full flow, which is equivalent to a sudden lift-off or jump.
Hydraulic shock can cause vibration, noise, and even rupture/failure of pipes or seals. Especially for sensitive systems, ensuring a smooth and consistent increase in flow is the key, not just for the life of the machine or maintenance schedule. Sudden changes in flow, no matter how short-lived, can be particularly harmful and costly in controlling critical applications (such as medication or ventilation control), and this is also true when using instruments to measure various aspects of the system, such as miniature blood pressure cuffs. The sudden peak of the flow may be too high and too fast for the meter to measure and transmit in time, which will result in inaccurate or incomplete readings.
A sharp flow peak may also mean that premature or excessive media enters the pipeline. For critical applications (such as sample extraction), this may waste expensive reagents/solvents and even lead to inaccurate analysis. It is also necessary to consider the state of the medium itself, such as blood or other sensitive fluids that are prone to hemolysis or homolysis, where the cells or compounds in the fluid are broken down. Turbulence will increase the loosening of chemical bonds, so minimizing interruptions or turbulence in the flow will prevent damage or destruction of delicate samples. Therefore, valves used in precision applications must provide a flow profile that minimizes turbulence—protecting components, internal instruments, and fluid components from damage. For this reason, it is recommended to use “soft start” (see Figure 2). Unlike the boost shown previously, the “soft start” provides a gentle flow introduction before smoothly easing the proportional flow (each current or step provided).
Ensuring that the flow curve follows the same valve-to-valve trend, especially the lifting characteristics, is important for consistency in production and building confidence in the product; after all, excellent low-end control that allows microliter flow is only excellent when it is repeatable . However, one must also admit that no two valves have exactly the same performance, because no two valves can be exactly the same. Therefore, there should be a range in which all valves will operate. When selecting valves, the range should be understood and understood so that all valves in any given batch are sufficiently uniform and can operate as expected in a given application. This especially involves lift-off.
Therefore, whenever you choose a valve, it is important to pay attention to the lifting characteristics of the valve and consider whether it will support or hinder the application.
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Post time: Dec-01-2021
