First of all, flow rate = flow velocity × pipe inner diameter × pipe inner diameter × π ÷ 4; so basically knowing one of the flow rate and flow velocity can calculate the other parameter.
But if the pipe diameter D and the pressure P in the pipe are known, can the flow rate be calculated?
The answer is: the flow velocity and flow rate of the fluid in the pipe cannot be calculated yet. You imagine that there is a valve at the end of the pipe. When it is closed, there is a pressure P in the pipe, but the flow rate in the pipe is zero.
Therefore, the flow rate in the pipe is not determined by the pressure in the pipe, but by the slope of the pressure drop along the pipe. Therefore, it is necessary to specify the length of the pipe and the pressure difference at both ends of the pipe in order to calculate the flow velocity and flow rate of the pipe.
From the perspective of qualitative analysis, the relationship between pressure and flow in the pipe is a positive proportional relationship, that is, the greater the pressure, the greater the flow rate. Flow rate is equal to flow velocity multiplied by the cross section. For any cross section of the pipe, the pressure comes from only one end, that is, the direction of the pressure is unidirectional. When the outlet in the pressure direction is closed (the valve is closed), the fluid in the pipe is in a prohibited state. Once the outlet is opened, its flow rate depends on the pressure in the pipe.
To conduct quantitative analysis, you can install a pressure gauge, flow meter or measure the flow capacity through a hydraulic model experiment. For pressurized pipe flow, it can also be obtained by calculation. The calculation steps are as follows:
1. Calculate the specific resistance S of the pipeline. If it is an old cast iron pipe or an old steel pipe, you can use the Shevelev formula to calculate the pipeline specific resistance s=0.001736/d^5.3 or s=10.3n2/d^5.33, or check the relevant table;
2. Determine the effective head difference H=P/(ρg) at both ends of the pipeline. If there is a horizontal drop h (the starting end of the pipeline is higher than the end by h), then H=P/(ρ g)+h
Where: H: in m; P: is the pressure difference between the two ends of the pipe (not the pressure of a certain section), P is in Pa;
3. Calculate the flow rate Q: Q=(H/sL)^(1/2)
4. Flow rate V=4Q/(3.1416*d^2)
Where: Q——flow rate, in m^3/s; H——water head difference between the beginning and the end of the pipe, in m; L——length from the beginning to the end of the pipe, in m.
Note: The top row is the flow rate. The large data in the middle of the table above is the flow rate. The leftmost row is the nominal diameter of the water pipe.
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