Determination of pressure and design temperature of piping design

Pipe design pressure is the working conditions, the pipe system may be encountered in the work pressure and work temperature combination of the most harsh conditions of the pressure.
1, the principle of determining the pressure of pipeline design
(1) pipeline design pressure is not less than the maximum working pressure.
(2) the design pressure of the pipeline with the safety relief device shall not be less than the opening pressure (or bursting pressure) of the safety relief device.
(3) all the piping connected to the equipment, the design pressure shall be not less than the design pressure of the connecting device.
(4) the transmission of refrigerant, liquefied gas and other low boiling point of the pipeline, the valve should be closed or media does not flow when the medium may reach the maximum saturated steam pressure as the design pressure.
(5) the design pressure of the outlet pipe of the centrifugal pump shall be not less than the closing pressure of the pump.
(6) the design pressure of the outlet pipeline of the reciprocating pump shall be not less than the setting pressure of the pump outlet safety relief device.
(7) the design pressure of the discharge pipe of the compressor shall be not less than the set pressure of the safety relief device and the pressure drop between the compressor outlet and the maximum flow rate.
(8) a small vacuum pipeline design pressure safety control device for maximum pressure of 1.25 or 0.1 times the value of MPa, and according to the external pressure conditions for vacuum piping design; no safety control device, the design pressure of 0.1 MPa.
The following table is a large engineering company in the domestic pipeline design pressure, for reference study.
Table 1 general conditions of pipeline design pressure

2, piping design temperature
Pipe design temperature is the working conditions, the corresponding design pressure pipe may reach the material temperature.
Process is usually based on normal operating temperature plus a certain margin as the design temperature.
(1) the normal operating temperature of the fluid ≤ 300 degrees Celsius, the margin is not less than 30.
(2) the normal operating temperature of the fluid > 300 degrees Celsius, the margin is not less than 15.
(3) when the fluid temperature is below 0 degrees Celsius, the design temperature is equal to the minimum temperature of the fluid.
(4) outdoor piping shall be in accordance with local minimum winter temperatures. The following table is a large engineering company in the domestic pipeline design temperature, for reference study.
Table 2 general conditions of piping design temperature

3, diameter selection
(1) when the diameter is determined according to the predetermined medium flow rate, the diameter of the lower primary election is adopted:

Form in:
&mdash D; pipe diameter; mm;
&mdash W; mass flow rate of the medium in the pipe, kg/h;
&mdash V0; the volume flow of the medium in the pipe, /h m3;
ρ — the density of the medium in the working condition, kg/m3;
&mdash u; the average velocity of the medium in the pipe, m/s.
(2) when the pressure drop calculation length per 100m control value (Δ Pf100) to select the diameter, the diameter of the initial type:

Form in:
μ — dynamic viscosity of the medium; &bull Pa; S;
Δ Pf100 — 100m calculation length of the pressure drop control value, kPa.
4, pipe material selection
Piping material selection requires designers to use the qualitative and quantitative analysis of engineering, materials science, corrosion and so on. Piping material selection is usually based on the characteristics of the medium (corrosive, toxic, burning, etc.) and operating conditions (temperature, pressure, etc.), in accordance with the principles of both applicable and economic considerations. The commonly used materials of cast iron (grey cast iron, malleable cast iron, ductile iron), ordinary carbon steel, high carbon steel, low alloy steel, high alloy steel, special alloy steel etc.. Commonly used metal materials used in table 3.
Table 3 the use of metal materials

5, the attention of the pipeline material selection
(1) cast iron (gray cast iron, malleable iron, ductile iron), carbon structural steel shall not be used in GC1 grade pressure piping.
(2) seamless steel pipe should be used for GC1 grade pressure piping.
(3) pipeline conveying hypertoxic and combustible medium preferred high quality carbon steel, even if the specification allows the use of carbon steel to as little as possible, not the best.
(4) carbon steel and carbon manganese steel and manganese vanadium steel shall not exceed the maximum working temperature of 425 DEG C, otherwise the possibility of carbide graphite.
(5) metal materials should be avoided in the combination of material and corrosion environment.
Two, flame arrester

1, flame arrester definition
Flame arrester is a kind of safety protection device which allows the fluid to pass through, and can prevent the spread and spread of flame.
2, flame arrester classification
(1) according to the classification performance, divided into resistance type arrester and deflagration detonation resistance type arrester;
(2) according to the site classification, divided into flame arrestor and vent pipe flame arrester;
(3) according to the structure classification, divided into filling type, plate type, metal mesh type, liquid seal type, corrugated type etc..
3, flame arrester set
Pipeline fire resistance should be set in the specified position of the following piping system:
(1) the mixed gas pipeline may cause deflagration or detonation;
(2) a gas pipeline that is capable of self - decomposition to explode and cause fire;
(3) the combustible gas pipeline is connected with the vacuum equipment after the fire (special circumstances can be set to a water seal device);
(4) air vent pipe into the flare head.
4, flame arrester selection
Selection procedure of flame arrester:
(1) according to the site decided to adopt the flame arrester or vent pipe flame arrester.
(2) according to the position of the flame arrester and the direction of the potential ignition source, it is determined that the bidirectional flame arrester is a one-way type flame arrester.
(3) according to the MESG value of the process gas or steam explosion classification, the resistance of the flame arrester is selected, and the clearance of the resistance element is selected.Is less than the MESG value of the process gas or steam.
(4) determine the selection of resistance or resistance deflagration detonation type arrester based on flame propagation velocity at the inlet of flame arrester. If the flame spread to subsonic, with preventing deflagration; if the flame spread to spread sonic or supersonic detonation type, select resistance.
(5) check the pressure drop.
(6) the influence of the position and the size of the pipe and the arrangement of the pipe on the performance of the flame arrester. For example: resistance to detonation flame arrester with source distance is generally not more than 50 times the diameter, otherwise it may develop into a detonation flame arrester failure.
(7) do not recommend 2 or more flame arrester series installation.
Three, safety valve

The safety valve is composed of a spring or by the pilot valve control valve, when the pressure exceeds the set pressure, the valve flap up to discharge protected system overpressure, when the pressure dropped to the back seat pressure, safety relief device can be automatically closed.
1, according to GB/T 12241 "General requirements of the safety valve" classification
(1) direct load type
(2) with power assist device
(3) with supplementary loading
(4) pilot
2, according to the height of the valve plate open classification
(1) full Kai
(2) micro Kai
3, according to the structure of different classification
(1) closed and non closed spring type
(2) with a wrench and without a wrench
(3) with a heat sink and without heat sink
(4) balanced and non balanced
4, according to the balance of internal pressure classification
(1) spring type
(2) lever type
(3) pilot
Safety relief device (safety valve, bursting disc device) setting place:
(1) the design pressure is less than the pressure of the source of the external pressure, the outlet may be shut down or blocked equipment and piping system;
(2) the outlet pipe of the volume type pump and the compressor which may be shut off;
(3) the gas phase pipeline of the distillation tower, which is caused by excessive pressure in the distillation tower, as a result of the interruption of cooling water or reflux, or the excessive heat of the boiler;
(4) equipment and piping systems for the production of overpressure due to non condensing gas volume;
(5) the upstream pipeline of the cut-off valve or the regulating valve in the outlet pipe of the heating furnace;
(6) the pipeline system with the effect of environmental temperature, sun exposure or heat pipe with heat expansion or vaporization is caused by the closing of the valve at both ends;
(7) the heat release reaction may be out of control of the reactor outlet valve upstream of the pipeline system;
(8) the outlet of the steam pipe of the condensing steam turbine;
(9) outlet pipe of steam generator;
(10) export pipeline of low boiling point liquid (liquefied gas, etc.);
(11) outlet piping at the low pressure side of the heat exchanger that may rupture;
(12) the low pressure side of the pressure relief valve group;
(13) the designers believe that other parts of the overpressure may be produced.
Safety valve type selection:
In the selection of safety valve, in addition to determine the nominal pressure, temperature and pressure, spring working pressure level and nominal diameter, secondly, should also be based on the condition of medium selection of safety valve structure and the suitable type of the valve body and the main parts of the material.
Basic principles of safety valve type selection:
(1) when the gas or steam is discharged, the safety valve of the full swing type.
(2) when the liquid is discharged, it is safe to use the full or open type safety valve.
(3) when the water vapor or air is discharged, the safety valve with a wrench can be used.
(4) for the set pressure is greater than 3 MP, the temperature of more than 235 degrees Celsius gas safety valve, you should consider the use of the safety valve with a heat sink, to prevent the release of the media direct erosion spring.
(5) when the discharge medium is allowed to leak into the atmosphere, the safety valve of the open type valve cap is selected, which is not allowed to leak into the atmosphere.
(6) emission is highly toxic, corrosive, extremely dangerous medium, should use the Bellows Safety valve.
(7) the high back pressure of the occasion, the use of back pressure balance type safety valve or pilot type safety valve.
(8) in some important occasions, and sometimes to install the other two safety valve. Two safety valve inlet and outlet valve should adopt mechanical interlocking device to ensure that at any time (including maintenance, maintenance period) can meet the discharge area of the container.
Selection calculation of safety valve
Safety valve selection calculation involves three important parameters: the safety valve set pressure, safety relief volume, minimum discharge area, only these three parameters determine the type of safety valve can be determined.
5, the safety valve set pressure
(1) for the safety relief devices on the pipeline in the independent pressure system, the determination of the relevant pressure should be based on the design pressure of the system, and in accordance with the following requirements.
1) when installing a safety relief device, the safety valve set pressure (or bursting disc device the maximum burst pressure calibration) should not exceed the maximum relief pressure system design, and large discharge pressure should not exceed the design pressure of the system 10% and 20 in kPa.
2) when installing multiple safety relief device, at least one of the set pressure of safety valve (or bursting disc device the maximum burst pressure calibration system) should be less than the design pressure, the safety valve set pressure (or bursting disc device shall not exceed the maximum burst pressure calibration) system design pressure 5%, and safety the larger the maximum relief valve discharge pressure should be not more than 12% or 30 pressure system design in kPa.
3) in order to prevent the occurrence of fire accidents and installation of the safety relief device, and the maximum discharge pressure should not be greater than 16% of the system design pressure.
(2) to prevent liquid pipeline thermal expansion safety relief device, safety valve set pressure (or bursting disc device the maximum burst pressure calibration) should be smaller and less than 120% of the test pressure system pipeline design pressure in value, and the maximum discharge pressure should not exceed the corresponding temperature of pipeline pressure rating of 20% or produced by pressure pipe nominal stress does not exceed the allowable stress value of 20%.
6, safe discharge volume
Safety relief should be determined in accordance with the following provisions:
(1) according to the material balance and energy balance, the calculation of various overpressure conditions of safe discharge.
(2) if a part of the system has several super pressure conditions, it should be calculated separately.The safety relief amount of the super pressure working condition and the maximum value of which is the safety relief amount of the part.
(3) the safety discharge calculation shall be in accordance with the provisions of the 2006 GB/T 20801.6— appendix A, not specified in Appendix A of other overpressure condition safety calculation, can refer to SY/T 1004:3— 2002, SY/T 2002 and HG/T 1004.4— 20570.2— the corresponding provisions 95.
7, minimum discharge area
The minimum discharge area shall be determined in accordance with the following provisions:
(1) according to the safe discharge volume, maximum discharge pressure, discharge fluid temperature, discharge coefficient and the physical properties of the fluid, the minimum discharge area is calculated.
(2) the minimum discharge area of the safety valve and the bursting disc device shall be in accordance with the provisions of 20801.6&mdash GB/T; 2006 annex A.
(3) the actual discharge area of the safety relief device shall be not less than the minimum discharge area.
8, safety valve model
According to the minimum discharge area of the flow channel diameter do, check table 4, can determine the safety valve nominal diameter DN, table 5, can determine the safety valve model.
Table 4 the relationship between the nominal diameter of the safety valve nominal diameter DN and the flow channel diameter do (unit: mm)

Table 5 the model of the safety valve of a valve factory

Process equipment network finishing release, reproduced please indicate the source.