[Science stickers] The relationship between pipe design pressure, working pressure, nominal pressure and pipe series S

1. What is the nominal pressure PN (MPa)? 

The reference value related to the pressure resistance of piping system components refers to the design given pressure related to the mechanical strength of the piping component. 

The nominal pressure is generally expressed by PN. 

2. What is work pressure? 

Refers to the maximum pressure specified by the maximum operating temperature of the pipeline conveying medium at all levels for the safe operation of the pipeline system. 

The working pressure is generally expressed by Pt. 

3. What is design pressure? 

It refers to the maximum instantaneous pressure that the water supply piping system acts on the inner wall of the pipe. Generally, the sum of working pressure and residual water hammer pressure is used. 

The design pressure is generally expressed in Pe.

4. What is test pressure?

The pressure to be achieved for the pressure strength and air tightness test of the pipeline, container or equipment.

The test pressure is generally expressed by Ps. 

The relationship between nominal pressure, working pressure, and design pressure

Nominal pressure is a kind of nominal pressure artificially stipulated for the convenience of design, manufacture and use. The unit of this nominal pressure is actually the pressure, and the pressure is the common name in Chinese, and the unit is "Pa" instead of "N".

Nominal pressure in English is nominal pres-surenomina: l in name or form but not in reality. The nominal pressure of the pressure vessel refers to the nominal pressure of the pressure vessel flange.

The nominal pressure of the pressure vessel flange is generally divided into 7 grades, namely 0.25, 0.60, 1.00, 1.60, 2.50, 4.00, 6.40MPa.   

The relationship between nominal pressure, design pressure and work pressure:

The relationship between the three: nominal pressure ≥ design pressure
               design pressure = 1.5 × working pressure

(According to the standard value of the design internal water pressure in the pressure pipeline in the GB/T50332-2002 Water Supply and Drainage Engineering Pipeline Structure Design Specification)

The maximum allowable working pressure of the pipe, the nominal pressure is corrected according to the temperature and the pressure reduction coefficient

MOP=  PNXfT

In the formula, MOP——Maximum allowable working pressure (MPa);

PN——nominal pressure (MPa);

fT——50-year life requirement, temperature to pressure reduction coefficient

The pipe ring stress σ changes with the pressure P. When σ = design σD, the pipe reaches the maximum value required by the design, and σD does not change under the given conditions of use.

Whether in engineering or in the laboratory, P can be set according to needs; and σD is unique to the material itself and is related to the binding force (strength) between polymer chains;

When external conditions change, such as temperature changes, σD changes.

Cut half of the pipe section along the diameter, and analyze the force on this part of the pipe section. (Note that it is assumed to be a thin-walled pipe) The hydrostatic pressure P in the pipe is perpendicular to the diameter direction and acts evenly on the pipe wall, and its size is indicated by the pressure gauge. Acting on the entire cross section, the force F1 along the diameter side can be approximated as: F1 = P·(dn-en)·L

The ring stress in the pipe wall that is balanced with this hydrostatic pressure is σ, which is not easy to measure directly, but it can be proved to exist through force analysis), and its direction points to the ring direction. Analysis from the figure: the total force F2 acting on the other direction of the cross section is:

F2 ＝ 2 · σ · en · L

When the pressure P is built up inside the pipe, the pipe will produce stress σ, otherwise σ will not appear.

When the pipe is working normally, F2 and F1 act on the pipe section at the same time, their sizes are the same, and the directions are opposite.

then:

P · (dn － en) · L ＝ 2 · σ · en · L

则：σ＝ P（dn－en）/ 2 en

The importance of this relationship is to link the ring stress σ in the pipe wall with the actual engineering parameter P;

The relationship is only an approximation; 

application:

The S series is also based on this.

The tube series "S" is a "priority number system" composed of five priority numbers "5, 4, 3.2, 2.5, and 2". Generally expressed as: "S5, S4, S3.2, S2.5, S2", they are a set of geometric series with a common ratio of approximately "1.25".

The S series is a given value, and its setting follows some mathematical rules. The reason why S is given a specific value is mainly considering that the size of the extrusion die cannot be unlimited.

General: Scalc = dn- en/2 en  

Then calculate the tube value: Scalc =σ/ P

Pressure formula

S (tube series) = dn- en/2 en = (SDR-1)/2 

σS (design stress)=σLPL/C

MOP (maximum working pressure)=2*MRS/C*SDR-1

SDR = Dn / En

PN = 2σS/SDR-1

This article comes from the Internet, only for learning and communication, no commercial purpose.

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