How exactly does a flow-limiting orifice plate achieve flow limitation?
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At critical flow, the flow rate increases as the pressure difference increases; whereas at subcritical flow, the flow rate no longer increases with the pressure difference, and the flow velocity reaches the speed of sound? For example, at the rated operating point of the pump, ① assuming the total flow rate at the pump outlet is 100 liters, the head is 100 meters, the main pipeline capacity is 80 liters, and the minimum backflow amount is 20 liters – assuming that this minimum backflow returns to the tank, which is at atmospheric pressure, and without considering height differences – then a flow-limiting orifice plate is needed to dissipate this 100-meter head loss. This orifice plate is designed to create a pressure drop of 100 meters, with a maximum flow rate of 20 liters. How does this orifice plate manage to limit the flow to 20 liters? Has subsonic flow at the speed of sound been achieved? ② If the minimum return line is not taken into account, how does the orifice plate installed on the main pipeline achieve flow restriction? ③ Compared to control valves, which use feedback to adjust the actuator in order to increase or decrease the flow resistance in the flow path, the orifice plate of a flow-limiting valve has a fixed diameter, resulting in a constant pressure drop. I’ve always been puzzled by how an instantaneous decrease or increase in pressure drop affects the system’s flow rate That orifice flow meter measures the pressure difference before and after the throttling element; it’s impossible for the flow rates on those two sides to be different, right? Is the resistance very low, through the amplifier? Is this 20 per 100m value simply something that has been set for this orifice plate specifically for that particular operating condition, at a certain operating point? So, as I understand it, if the system isn’t in a subcritical state, my pump’s flow rate curve shifts forward; can this orifice plate still handle a head pressure of over 100 m? Is the flow rate limit at this time <20 liters? I hope experienced members can give me some advice. I don’t want any low-quality posts, and I’ll do my best to offer rewards! ! !
As a supplementary note, is the design and construction of flow-limiting orifice plates to achieve flow limitation and pressure reduction always based on blocking the flow? As I understand it, only when flow is choked does it qualify as the so-called subcritical flow state, right? Only then can the conditions for limiting traffic be met, right? But in terms of standards, it is necessary to avoid blocking the flow (referenced as choked flow in 20570.15)
This addition is perfect! It just explains both of the author’s scenarios in detail. Whether it is a flow-limiting orifice plate in the return pipeline or in the main pipeline, the key principle is to utilize the characteristic of critical pressure drop to keep the flow rate at the designed value, so that it does not change due to variations in downstream pressure. As mentioned by the original poster, in a return flow scenario, by adjusting the orifice plate pressure drop to match the pump’s 100m head, the flow rate can be maintained at 20 cubic meters per unit time. Additionally, during practical application, it is important to note that for fluids containing particles or with high viscosity, in addition to implementing anti-clogging designs, the flow coefficient of the orifice plate must also be adjusted. It is not possible to use the parameters specific to clean water when selecting the appropriate components; otherwise, the actual flow restriction value may differ from the designed value. The point mentioned by the original poster, where the flow velocity in the throat reaches the speed of sound, is also crucial; it represents the core of critical flow. Once this condition is reached, no changes in the pressure downstream can affect the flow rate upstream. This is precisely why flow control orifice plates can maintain stable flow regulation. When making selections for actual engineering projects, it is best to refer to professional fluid selection manuals or consult experts for parameter verification, in order to avoid design errors.
Essentially, I can only think in terms of conventional approaches, applying Bernoulli’s principles to incompressible fluids. For example, taking a cross-section of a pipe, the flow rate and head pressure (i.e., the total mechanical energy E1) of the fluid within the pipe remain constant. Assuming that internal energy and frictional losses along the pipe are negligible, after passing through a fixed backpressure, the flow rate and head pressure of the fluid will inevitably decrease. As a result, the total mechanical energy E2 will be less than E1. This is similar to how water in a stream flows from a higher elevation to a lower one due to gravity; when it passes through narrow channels or encounters rocks, its flow rate decreases. To some extent, energy is not conserved in such cases, and as long as those narrow channels or rocks exist, each time the stream hits them, its flow rate will decrease. In terms of engineering applications, this is how I understand control valves and flow restrictor orifices; however, it’s difficult to explain this concept using the macroscopic Bernoulli equation. Moreover, it doesn’t make sense either when applied to compressible fluids.