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Update:31-10-2019

Summary:

1. What is the nominal pressure PN (MPa)? The reference […]

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

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

The nominal pressure is generally expressed in terms of PN.

Second, what is work pressure?

It refers to the maximum pressure specified for the maximum operating temperature of the pipeline transport medium for the safe operation of the pipeline system.

Working pressure is generally expressed in Pt.

Third, what is the design pressure?

It refers to the maximum instantaneous pressure exerted by the water supply pipe system on the inner wall of the pipe. The sum of working pressure and residual water hammer pressure is generally used.

Design pressure is generally expressed in terms of Pe.

4. What is the test pressure?

Pipes, vessels or equipment are subjected to pressure and air tightness tests to determine the pressure to be achieved.

The test pressure is generally expressed in Ps.

Relationship between nominal pressure, working pressure, design pressure

Nominal pressure is a nominal pressure artificially defined 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 of Chinese. The unit is “Pa” instead of “N”.

The nominal pressure English is nominal pres-surenomina: l in name or form butnot in reality (nominal, nominally unreal). The nominal pressure of a 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, 6.40 MPa.

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 GB/T50332-2002 Water supply and drainage engineering pipeline structure design specification, the internal water pressure standard value in the design of the pressure pipeline)

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

MOP= PNXfT

MOP in the formula - maximum allowable working pressure (MPa);

PN - nominal pressure (MPa);

fT - 50 years life requirement, temperature versus pressure reduction factor

The pipe ring stress σ varies with the pressure P. When σ = design σD, the pipe reaches the maximum 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, depending on the needs; and σD is unique to the material itself, and is related to the bonding strength (strength) between the polymer chains;

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

The pipe section is cut in half along the diameter, and the section of the pipe section is subjected to force analysis. (Note that it is a thin-walled tube.) The hydrostatic pressure P in the pipe is perpendicular to the diameter direction and acts evenly on the pipe wall, the size of which is indicated by the pressure gauge. Then acting on the entire cross section, the force F1 along one side of the diameter can be approximated as: F1=P·(dn-en)·L

The ring stress in the wall of the pipe that is in equilibrium with this hydrostatic sum is σ, σ is not easy to measure directly, but it can be confirmed by force analysis, and its direction is directed to the hoop. Figure analysis: The total force F2 acting in the other direction of the cross section is:

F2 = 2·σ·en·L

When the pressure P is established inside the pipe, the pipe produces stress σ, otherwise σ does not appear.

When the pipe is working normally, F2 and F1 act on the pipe segment at the same time, and they are equal in size and opposite in direction.

then:

P·(dn-en)·L=2·σ·en·L

Then: σ= P(dn-en)/ 2 en

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

This relationship is just an approximation;

application:

The S series is also based on this.

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

The S series is a given value, its setting, following some mathematical rules, the reason for giving S a specific value, mainly considering the size of the extrusion die can not 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)

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