Relationship among pipe diameter, flow, velocity and pressure
Calculation formula of flow and velocity
Flow = flow rate × Inner diameter of pipe × Inner diameter of pipe × π÷4; Therefore, the flow rate and velocity can be basically known to calculate the other parameters.
Calculation of pipeline velocity
But if the pipe diameter D and the pipe internal pressure P are known, can the flow be calculated?
The answer is: the flow velocity and flow rate of the fluid in the pipe can not be calculated yet. You imagine that there is a valve at the end of the pipe. When it is closed, there is pressure P in the pipe, and the flow in the pipe can be zero.
Therefore, the flow in the pipe is not determined by the pressure in the pipe, but by the pressure drop gradient along the pipe. Therefore, it is necessary to specify the length of the pipe and the pressure difference between the two ends of the pipe to calculate the flow rate and flow rate of the pipe.
From the perspective of qualitative analysis, the relationship between pressure and flow in the pipeline is a positive proportion, that is, the greater the pressure, the greater the flow. The flow is equal to the velocity times the section. For any section of the pipe, the pressure only comes from 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 prohibited. Once the outlet is opened, its flow rate is determined by the pressure in the pipeline.
For quantitative analysis, you can install pressure gauges, flow meters or measure flow capacity through hydraulic model experiments. For pressurized pipe flow, it can also be obtained through calculation, and the calculation steps are as follows:
1. Calculate the specific resistance S of the pipe. If it is an old cast iron pipe or an old steel pipe, the specific resistance S=0.001736/d^5.3 or S=10.3n2/d^5.33 can be calculated by using the shevelev formula, or refer to the relevant tables;
2. Determine the acting head difference at both ends of the pipe H=P/(ρg) If there is a horizontal drop h (that is, the starting end of the pipe is h higher than the end), then H=P/(ρg)+h
In the formula:
- H: In m;
- P: Is the pressure difference between two ends of the pipe (not the pressure of a section), P is in Pa.
3. Calculated flow Q: Q=(H/sL)^(1/2)
4. Flow velocity V=4Q/(3.1416*d^2)
In the formula:
- Q – flow, in m^3/s;
- H – head difference between the start and end of the pipe, in m;
- L – length from the beginning to the end of the pipe, in m.
Comparison Table of Pipe Diameter/Flow Rate/Flow
Note: The top line is the flow rate.
The data in the middle of the above table is traffic.
The leftmost row is the nominal diameter of the water pipe.
Question: Which one do I choose for so many flow rates?
Don’t worry… Look at the table below (there is a difference between open system and closed system).
|Pipe diameter (DN)||Recommended flow rate m/s:|
Classification of pipeline pressure grade
Pipeline pressure is generally divided into the following levels:
- Nominal pressure of low pressure pipeline shall not exceed 2.5MPa;
- Nominal pressure of medium pressure pipeline 4-6. 4MPa;
- Nominal pressure of high-pressure pipeline 10-100MPa;
- The nominal pressure of ultra-high pressure pipeline exceeds 100MPa.
Classification of pressure pipes
1. Long distance pipeline is classified as GA, and its level is divided into:
(1). Long distance pipeline meeting one of the following conditions is GA1:
- 1) Pipelines conveying toxic, combustible and explosive gas media with design pressure P>1.6MPa;
- 2) Pipelines transporting toxic, combustible and explosive liquid media, with transmission distance (Note 1) ≥ 200Km and nominal diameter DN ≥ 300mm;
- 3) The pipeline is used to transport slurry medium, with the transmission distance ≥ 50Km and the nominal diameter DN ≥ 150mm.
(2). Long distance pipeline meeting one of the following conditions is GA2:
- 1) Pipelines conveying toxic, combustible and explosive gas media with design pressure P ≤ 1.6Mpa;
- 2) Long distance pipeline beyond the scope of GA1 2);
- 3) Long distance pipeline beyond the scope of GA1 3).
2. Public pipelines are classified as GB, and their levels are:
- GB1. Gas pipeline;
- GB2. Thermal pipeline.
3. Industrial pipelines are GC type; The levels are divided into:
(1). Industrial pipelines meeting one of the following conditions are GC1:
- 1) Pipelines conveying media with extremely hazardous toxicity according to GB5044 Classification of Hazard Levels of Occupational Exposure to Toxicants;
- 2) Pipelines conveying Class A and Class B combustible gas or Class A combustible liquid medium with fire hazard specified in GB50160 Code for Fire Protection Design of Petrochemical Enterprises and GBJ16 Code for Fire Protection Design of Buildings and with design pressure P ≥ 4.0MPa;
- 3) Pipelines conveying flammable and toxic fluid media with design pressure P ≥ 4.0MPa and design temperature ≥ 400 ℃;
- 4) Pipelines conveying fluid medium with design pressure P ≥ 10.0MPa.
(2). Industrial pipelines meeting one of the following conditions are GC2:
- 1) Pipelines conveying Class A and Class B combustible gas or Class A combustible liquid medium with fire hazard specified in GB50160 Code for Fire Protection Design of Petrochemical Enterprises and GBJ16 Code for Fire Protection Design of Buildings and with design pressure P<4.0MPa;
- 2) Pipelines conveying flammable and toxic fluid media, with design pressure P<4.0MPa and design temperature ≥ 400 ℃;
- 3) Pipelines conveying non combustible and non-toxic fluid media, with design pressure P<10.0MPa and design temperature ≥ 400 ℃;
- 4) Pipelines conveying fluid medium with design pressure P<10.0Mpa and design temperature<400 ℃.
Note 1: The transportation distance refers to the direct distance between the production place, the warehouse and the user for transporting commodity medium pipelines.
What is the nominal pressure, working pressure, design pressure and test pressure of the pipeline?
The difference between nominal pressure, working pressure, design pressure and test pressure of pipeline
(1) What is the nominal pressure PN (MPa)?
The pressure strength of the product at the reference temperature is called nominal pressure and is expressed by the symbol PN, which is a combination of letters and numbers used as a reference mark related to the mechanical properties and dimensional characteristics of the piping system components. It consists of the letters PN and followed by the number without a factor, such as nominal pressure of 1.0MPa, recorded as PN10. Due to the different materials of the products, the reference temperature is also different, the reference temperature of cast iron and copper is 120 ℃, the reference temperature of steel is 200 ℃, the reference temperature of alloy steel is 250 ℃. The benchmark temperature of plastic products is 20℃, and the pressure strength of the products at the benchmark temperature is close to the pressure strength at room temperature, so the nominal pressure is also close to the pressure strength of the material at room temperature. The nominal pressure of piping components is shown in Table 1.
Table.1 PN of piping components (nominal pressure)
|DIN series||ANSI series|
Note: Other PN values are allowed when necessary.
(2) Working pressure
It is the maximum pressure specified by the maximum working temperature at all levels of the pipeline transport medium for the operational safety of the pipeline system.
Working pressure is generally expressed in Pt.
Pipes and pipeline accessories in the normal operating conditions of the pressure with the symbol p, this operating conditions must refer to a certain operating temperature, and thus indicate the working pressure of a product should indicate its working temperature, usually in the lower corner of p additional numbers, the number is the highest working temperature divided by 10 integer values obtained, such as the highest working temperature of the medium is 300 ℃, the working pressure of 10MPa, it is recorded as p3010MPa.
(3) Design pressure
Design pressure is the highest pressure set at the top of the pressure vessel, with the corresponding design temperature together as the design load conditions, the value shall not be lower than the working pressure.
In general, the highest pressure to which the system is subjected during the design calculation is selected as the design pressure. Design pressure is generally expressed in Pe.
(4) Test pressure
Before leaving the factory, the pipeline and piping accessories must be pressure tested to check its strength and sealing, the strength of the product test pressure is called strength test pressure, expressed by the symbol Ps, such as test pressure of 4MPa, recorded as Ps4MPa. from the safety point of view, the test pressure must be greater than the nominal pressure.
The nominal pressure of the product according to its definition refers to the benchmark temperature pressure strength, but in many cases, the product is not working at the benchmark temperature, with the change of temperature, the product pressure strength also followed by changes, so the products belonging to a nominal pressure, how much can withstand the working pressure, to be determined by the working temperature of the medium, so it is necessary to know the product at different working temperatures nominal Therefore, it is necessary to know the relationship between the nominal pressure and working pressure of the product at different working temperatures. For this reason, it is necessary to find out the variation law between the pressure strength and temperature of the product through strength calculation. In engineering practice, the working temperature is usually divided into several levels according to the maximum temperature limit of the product, and the allowable working pressure of the product under each temperature level is calculated.
Other material products, the same can be divided into different working temperature levels and calculated in each working temperature allowed to withstand the maximum working pressure. In this way, the nominal pressure, working temperature and maximum working pressure of various products can be converted into a relationship. Other materials can be viewed in the relevant information, not listed here.
(5) The relationship between nominal pressure, working pressure, design pressure and test pressure
Test pressure > nominal pressure > design pressure > working pressure
- Design pressure = 1.5 × working pressure (usually)
- Test pressure = 1.5 × design pressure (hydraulic)
- Test pressure = 1.15 × design pressure (pneumatic)
Source: China Steel Pipelines Manufacturer – Yaang Pipe Industry Co., Limited (www.ugsteelmill.com)