Does the flow rate of a pipe with a nozzle depend on the pressure difference across the nozzle at both ends?

This post was last edited by HEJIYUER on 2020-8-15 at 23:17. It is a dynamic topic regarding the distribution of flow rate/pressure in pipelines. As an example, I take N2 from 6BARA, which is vented through a control valve; there is a flow-limiting orifice plate behind the valve. I analyze how the flow rate changes as the valve moves from fully closed to fully open. It is generally believed that once the absolute pressure downstream of the throttle element is less than half of that upstream, the flow rate depends on the upstream pressure; moreover, the control valve can also change the flow area in order to adjust the flow rate. 1. When the valve is first opened, the flow rate in the pipeline is very low; therefore, the pressure upstream of the orifice plate is not high (unobstructed flow), but the pressure drop across the valve is significant (obstructed flow). An increase in flow rate also leads to an increase in the pressure upstream of the orifice plate. 2. When the flow rate increases, causing the pressure in front of the orifice plate to exceed 2 BARA, both the valve and the orifice plate enter a choked flow state. 3. As the flow rate continued to increase, the pressure in front of the orifice plate exceeded 3 BARA, causing the valve to release from its blocking position and allowing flow to pass, while the orifice plate continued to block the flow. For the above conditions 1/2/3, you can assume a pressure downstream of the valve and use back-calculation to determine a unique flow rate value (i.e., the operating point) that satisfies both the requirements of the control valve and those of the flow restriction orifice. However, when calculating the flow rate, it is necessary to distinguish between \"blocked flow\" and \"unblocked flow\" conditions. The \"nozzle\" you mentioned could be an sonic nozzle used for calibrating flow rates, or it could be a nozzle used for measuring flow rates, with each using its own formula for calculating flow. 　 The flow rate in a pipeline is necessarily the result of a \"compromise\" among various resistance elements, and the magnitude of these resistances determines the proportion of pressure allocated to each of them.

Thank you for your reply; I learned a lot after reading it. I have two more questions I’d like to ask you. First, the pressure at the front of the nozzle at the end of our pipeline can reach up to 0.8 Mpa, while the pressure at the outlet is at atmospheric level; moreover, the flow rate has not reached a choked flow state. Is this phenomenon normal? ； Secondly, it is necessary to determine whether a pipeline with a nozzle at its end can allow for independent control of flow rate and pressure – for example, the flow rate can be adjusted while the pressure remains constant, or the pressure can be adjusted while the flow rate remains constant.

It depends on whether the process parameters before your control valve meet the requirements for operation. For example, if you open the control valve wide, it will cause changes in the process parameters of the system ahead of the valve, which in turn will affect certain parameters downstream of the valve. There is a situation similar to what you described: two control valves, one for flow regulation and the other for pressure regulation. In fact, it is also a comprehensive adjustment based on the operating conditions and changes in the system upstream of the valve. The oxygen supply settings of the oxygen lance system in converter steelmaking can be used as a reference. For reference.

Thank you for your reply. Could you please explain what you mean by the process system parameters before the valve? I have installed a pressure reducing valve at the inlet of the pipeline to maintain a constant pressure before the control valve – is that what you mean? If I want to achieve independent control of flow pressure, would it work to use a pipeline consisting of an electric pressure reducing valve → electric control valve → electric pressure reducing valve → nozzle?

I have seen pneumatic self-regulating valves used in series; it is best not to use pressure reducers in such systems, as the capacity of pressure reducers has certain limitations. It is either prone to leakage or prone to failure. In short, it is necessary to ensure a continuous and sufficient supply of gas. Electric versions are also possible, but the mechanical linkage is rather complicated and prone to failures. Oh, we also need to consider the diameter of the pipe being controlled. It is recommended to consider pneumatic self-regulating control as it is a more reliable option.

This post was last edited by HEJIYUER on 2020-8-18 00:12. 1. What is the structure of your nozzle? Or what criteria were used for the design? 2. According to the conventional gas throttling principle (a control valve can also be considered a throttling element), with a value of 0.8 upstream and 0 downstream, flow restriction will definitely occur, but the flow rate will still increase as P1 rises. As for the flow rate, it depends on the design type of the nozzle and the size of its openings, and it is calculated using formulas specific to nozzles. 3. Is your requirement something like this: to maintain a certain flow rate by using a throttle element, but the flow rate can also be changed by adjusting the opening degree of the upstream valve – thereby changing the pressure upstream of the throttle element – and thus altering the flow rate. 4. If 3 is correct, let me give an example: Reactor pressure at 20 BAR is released to the flare. To prevent the flare from being overloaded due to an excessive instantaneous release volume (when the control valve is fully open), a flow-limiting mechanism must be installed. Set up the relief PIC-PV control, with a flow-blocking orifice plate connected in series afterward; the arrangement order is PV---PT----RO. For this system, it is sufficient to control the pressure upstream of the RO unit (since RO operates on a plug-flow principle); there is no need for flow FIC control. Control valves are installed to adjust the resistance distribution in the pipeline. Flow restriction is achieved through flow control orifice plates, and the design of these orifice plates is intended for only one specific flow rate – that is, the maximum allowable flow rate, such as 4W. You have 20 of pressure drop to distribute. Assuming I allocate it as PV 15 – RO 5, then the orifice plate is sized based on a flow rate of 4W and P1 = 5. As for the control valve, under blocked-flow conditions with a flow rate of 4W, just select an appropriate one. In practice, if you set PIC=5, the maximum flow rate for the entire pipeline is 4W (which corresponds to the design condition of the flow-limiting orifice plate). If you think the flow rate is too high, then reduce the setting to 4; PV will automatically decrease, and the flow rate will also go down. As for the flow rate at P1=4, you can determine it by back-calculating using the flow control orifice plate.

This post was last edited by HEJIYUER on 2020-12-28 at 22:07. Let’s improve together :) – for gases, using PIC to control PV and then connecting a restrictive orifice plate in series allows changing the pressure; by doing so, the flow rate can be altered. Both PV and the orifice plate can be in a restrictive flow state. But liquids are different; since liquid flow obstruction requires vaporization, there can only be one throttling element, and it must ultimately be a throttling component designed for vaporization conditions.