Relationship between flow velocity and operating pressure in pipelines
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Dear experts, could you please explain the relationship between flow velocity within the pipeline and operating pressure? 1. The design pressure for natural gas pipelines is 1.5 Mpa, while the operating pressure ranges from 0.9 to 1.2 Mpa. The maximum flow rate under standard conditions is 35,000 M^3/h. When selecting a flow meter, which pressure should be used to determine its flow range? Should I refer to the given flow rate when selecting the flow range? 2. The faster the flow rate, the lower the pressure. Therefore, when the gas pressure in a pipe increases, the flow rate should decrease. However, this contradicts my understanding that increasing pressure leads to an increase in flow rate. How can this be explained?
1. Under the same conditions, higher pressure results in a faster flow rate. 2. Since your pressure range varies greatly, flow meters of the orifice type probably can’t be used; in that case, pressure doesn’t play a significant role
First, flow rate = flow velocity × inner diameter of the pipe × inner diameter of the pipe × π ÷ 4; therefore, knowing one of these values allows us to calculate the other. But if the pipe diameter D and the pressure inside the pipe P are known, can we calculate the flow rate? The answer is: it is not possible to determine either the flow velocity or the flow rate of the fluid in the pipe. You imagine a valve at the end of the pipe; when it is closed, there is pressure P inside the pipe, but the flow rate inside the pipe is zero. Therefore, the flow rate within a pipe is not determined by the pressure inside the pipe, but rather by the slope of the pressure drop along its length. Therefore, it is necessary to specify the length of the pipe as well as the pressure difference at both ends of the pipe in order to calculate the flow velocity and flow rate of the pipe. From a qualitative analysis perspective, the relationship between pressure and flow rate in a pipeline is direct proportional; that is, the higher the pressure, the greater the flow rate. Flow rate is equal to flow velocity multiplied by the cross-sectional area. For any cross-section of the pipe, the pressure comes from only one end; in other words, the direction of pressure is unidirectional. When the outlet in that pressure direction is closed (the valve is shut), the fluid inside the pipe is prevented from flowing. Once the outlet is opened, its flow rate depends on the pressure in the pipeline. For quantitative analysis, hydraulic model experiments can be conducted by installing pressure gauges, flow meters, or measuring the volume of fluid that passes through. For pressurized pipe flow, it can also be determined through calculation. The steps for calculation are as follows: 1. Calculate the specific resistance S of the pipe. For old cast iron or steel pipes, the Shchelikhov formula can be used to compute the specific resistance, namely s=0.001736/d^5.3, or s=10.3n2/d^5.33; alternatively, relevant tables can be consulted ； 2. Determine the head difference H at both ends of the pipeline, given by H = P/(ρg). If there is a horizontal elevation difference h (meaning the starting point of the pipeline is h units higher than its ending point), then H = P/(ρg) + h. Here, H is expressed in meters ； P: The pressure difference between the two ends of the pipe (not the pressure at a specific section); P is expressed in Pa ； 3. Calculate the flow rate Q: Q = (H/sL)^(1/2) 4. Flow velocity V = 4Q/(3.1416 * d^2) Where: Q —— flow rate, in m^3/s ； H —— Head difference between the start and end of the pipeline, in meters ； L —— the length from the start to the end of the pipeline, in meters.
It depends on the type of flow meter you choose; generally, it’s sufficient to include pressure correction and temperature correction. It’s still more reliable to communicate directly with the flow meter manufacturer
When selecting a flow meter, the worst-case scenario should be taken into account when determining its range. In the case of volume measurement, the range should be chosen based on the conditions under which the highest values are recorded (lowest pressure and highest temperature). Also, consider the measurement accuracy range again; some only use 80% of the range
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For flow meters, it is generally necessary to include both the minimum and maximum flow rates; you can consult the manufacturer for details
If pressure drop is not taken into account, what is the relationship between flow rate and pressure? It’s okay, right!
Apart from professional computing, I think the transmission distance is a limiting factor; perhaps auxiliary devices such as pressure pumps are also needed.
What formula does this calculation software use to perform its calculations? “What does “pressure” refer to?