The pressure level of a pipeline refers to the maximum working pressure level that the pipeline system can withstand. This level is usually determined by the following factors: the medium transported by the pipeline, the material of the pipeline itself, and the manufacturing process. Its main purpose is to ensure that the pipeline system can operate safely and reliably without causing damage to production or the environment.
The determination of the pressure level is the core of the pressure pipeline design, and it is also a prerequisite for layout and stress verification, as well as an important factor affecting the infrastructure investment and pipeline reliability of pressure pipelines. The pressure level of the pipeline includes two aspects: the nominal pressure level of standard pipe fittings (expressed in nominal pressure) and the wall thickness level (expressed in wall thickness level). Usually, the pressure level of the pipeline is a parameter determined by the nominal pressure level and wall thickness level of standard pipe fittings.
When determining the pressure level of the pipeline, the following aspects need to be considered: the properties of the medium transported by the pipeline (such as flow, density, viscosity, temperature, pressure and other parameters), the material and process of the pipeline itself (such as the material of the pipeline, internal coating, manufacturing standards, etc.), the external pressure or load that the pipeline system needs to withstand (such as earthquake, wind load, support load, etc.), and the required safety factor and service life (such as service life, stress, fatigue, cracks, etc.). According to these indicators and actual engineering needs, systematic calculations and analyses can be carried out to determine the required pressure level of the pipeline. At the same time, it is also necessary to consider the provisions of relevant national standards and regulations, as well as the actual situation and safety requirements on site.
1. Determination of pipeline pressure level
In engineering, process operation parameters should not be directly used as design conditions for pressure pipelines. Factors such as fluctuations in process operation, the influence of connected equipment, and the influence of the environment should be considered, and a certain safety margin should be given as a design condition based on the process operation parameters. The design conditions mentioned here mainly refer to design pressure and design temperature.
1.1 Determination of design pressure
The design pressure of the pipeline: should not be lower than the pressure under the most demanding conditions composed of internal pressure (or external pressure) and temperature during normal operation.
The most demanding conditions: refers to the conditions that lead to the maximum wall thickness or the highest nominal pressure level of the pipe and pipeline components.
Considering the influence of factors such as the hydrostatic column pressure of the medium, the design pressure should generally be slightly higher than the highest working pressure under the most demanding conditions composed of (or) external pressure and temperature.
The design pressure of the pipeline should take into account the internal and external pressure and temperature factors under the most demanding conditions. For safety reasons, the design pressure is generally slightly higher than the maximum working pressure.
When determining the design pressure, we can refer to the practice of pressure vessels and use the method of increasing the margin coefficient to ensure convenient and safe operation.
a. Generally, the design pressure of pipeline components is determined. Generally, a margin coefficient is added on the basis of the corresponding working pressure. Generally, the design pressure of pipeline components is determined.
The design pressure of all pipelines connected to equipment or pressure vessels should not be lower than the design pressure of the equipment or container, and meet the following requirements:
b. When there is a safety pressure relief device in the pipeline, it indicates that the pipeline may exceed its normal operating pressure during operation. The purpose of setting up a safety pressure relief device (such as a safety valve, bursting disc, etc.) is to automatically release the pressure when the system exceeds its normal operating pressure, so that the hardware of the system such as equipment and pipelines can be protected. At this time, the design pressure of the pipeline should not be lower than the sum of the safety relief pressure and the static pressure of the liquid column.
When no safety pressure relief device is set, its design pressure should not be lower than the sum of the maximum pressure that the pressure source may reach and the static liquid column pressure.
The design pressure of the outlet pipeline of a centrifugal pump without a safety pressure relief device should take the larger of the following two values:
(1) The normal suction pressure of the centrifugal pump plus 1.2 times the rated pressure difference of the pump's outlet.
(2) The maximum suction pressure of the centrifugal pump plus the outlet pressure difference of the pump
c. Pumps with high head in the pipeline, especially reciprocating pumps, often generate a higher sealing pressure in the pipeline and pump before the first shut-off valve within a short period of time after starting. Sometimes this sealing pressure can reach a very large value. At this time, the design pressure of the pump's outlet pipeline should take the maximum sealing pressure value of the pump.
d. The pressure on the vacuum system pipeline is the atmospheric pressure outside it, so its design pressure should be 0.1MPa external pressure;
e. The design pressure of the pipeline connected to equipment such as towers or containers should not be lower than the design pressure of the connected equipment. When there is a high liquid column in the pipeline, the influence of the static pressure head of the liquid should also be considered.
In fact, for the pipeline, its force is more complicated than that of the equipment. This is because in addition to the medium load, it is often subjected to the pipe system force generated by the thermal expansion and contraction of the pipeline. Therefore, the design pressure of the pipeline should generally not be lower than the design pressure of the equipment.
2.Factors affecting the determination of pipeline pressure level
In addition to the above-mentioned design temperature and design pressure, which are the basic parameters for determining the pipeline pressure level, there are some other factors that affect the determination of the pipeline pressure level. Among them, the application standard system, materials, media and operating conditions are all important factors.
2.1 Selection of application standard system
Due to the different nominal pressure level series under different standard systems, the corresponding temperature-pressure gauges are also different. Therefore, even under the same design conditions, if different application standards are selected, the nominal pressure level may be different. Therefore, before determining the nominal pressure level of the pipeline, it is necessary to first clarify the application standard system used.
2.2 Materials used in the pipeline system
The mechanical properties of different materials are different, so their corresponding values on the standard temperature-pressure gauge will also be different. Before determining the nominal pressure of the pipeline, it is necessary to give priority to the material selection of the pipeline and its components, because the selection of materials should be based on the design temperature, design pressure and operating medium.
Under normal circumstances, the material standards used for different components of the pipeline are also different. For example, pipes are generally made of pipes, flanges are mostly made of forgings, and valves are mostly made of castings.
Regardless of the material standards used, they should have the same level of material properties, that is, they should adapt to the same operating conditions and have the same strength; in addition, attention should be paid to the matching between pipes, plates, bars and castings.
The flange pressure-temperature level refers to the highest non-impact working pressure or maximum allowable working pressure that the flange can withstand at different operating temperatures. It is closely related to factors such as the flange material and its high-temperature mechanical properties, the calculation method used by the flange, etc. According to the highest non-impact working pressure of the flange at different working temperatures, the nominal pressure of the flange and flange accessories should be reasonably selected to achieve coordination with the accessories to ensure that the pipeline accessories can operate safely and reliably in the pipeline device. If the bolts and gaskets of the flange joint meet the relevant limiting conditions, and the alignment and assembly of the flange joint meet the good practice, such a flange joint can be applied to the pressure-temperature rating. If the rated value is used for a flange joint that does not meet these limiting conditions, it is the responsibility of the user.
The rated temperature corresponding to the rated pressure refers to the shell temperature of the pressure container of the flange and flange fittings. In general, this temperature is the same as the temperature of the stored fluid. The user is responsible for not selecting the pressure rating according to the storage fluid temperature. When the temperature is below -29℃ (-20℉), the selected pressure rating should not be greater than the rated value at -29℃ (-20℉).
The pressure-temperature ratings of various materials given in the pressure-temperature selection table refer to the maximum non-shock working pressure (in gauge pressure) at the indicated working temperature; for pressures at intermediate temperatures, linear interpolation is allowed.
The working temperature refers to the temperature of the flange metal under pressure. When the working temperature is below 20℃, the maximum non-shock working pressure value of the flange is the same as that at 20℃. For ferritic steel, the maximum non-shock working pressure value at 100℃ can be used up to 120℃; for austenitic stainless steel, the maximum non-shock working pressure value at 20℃ can be used up to 50℃.
When the pressure-temperature rating is used for flange connection, the risk of leakage caused by the forces and moments generated in the pipeline connection should be considered. Therefore, it is recommended not to use threaded flanges when the temperature is higher than 260°C under conditions of rapid temperature changes and thermal cycling.
To ensure that casing and tubing systems function optimally under varying pressure conditions, specific design standards must be adhered to. These standards are determined by international organizations such as the American Petroleum Institute (API) and are essential for guaranteeing both safety and functionality. Some of the key design standards include:
API 5CT Casing Specifications: This standard governs the design and specifications of casing pipes used in the oil and gas industry. It specifies material grades, dimensions, and manufacturing processes that are suitable for different pressure and environmental conditions.
API 5L Tubing Specifications: API 5L specifies the requirements for tubing, ensuring the material strength is sufficient to handle internal pipeline pressures. It includes details on the material grades such as X42, X52, and X60 that offer varying levels of pressure resistance.
Pressure Rating and Material Strength: The selection of casing and tubing materials, as per API standards, is largely based on the maximum pressure the system is expected to handle. For example, materials like J55 steel casing are chosen for moderate-pressure applications, while materials like N80Q are used in higher pressure scenarios.
4.How Pipeline Pressure Levels Affect Design Choices
Pipeline pressure levels directly affect the design of casing and tubing systems. These systems are engineered to withstand specific pressure ranges, and exceeding these pressures can result in failure or catastrophic damage. Pressure determination involves considering factors such as:
Formation Pressure: The pressure exerted by the fluids in the reservoir that could be exerted on the well casing. This pressure must be accurately measured to ensure the casing can withstand it without deformation or rupture.
External Pressure: The pressure exerted on the casing from the surrounding earth formations and fluids. In certain environments, external pressures can exceed internal pressures, making the selection of high-strength steel pipes crucial.
Wellhead Pressure: The pressure at the surface of the well, often influenced by the depth of the well and the characteristics of the fluid within the reservoir. This pressure must be closely monitored to select the appropriate tubing to avoid failure during extraction.
5.Material Selection Based on Pressure Levels
One of the most critical aspects of well design is choosing the correct materials for casing and tubing based on the pipeline pressure levels. The following materials are commonly used in various wellbore applications, each offering different performance characteristics:
J55 Casing: This is commonly used in low-pressure scenarios due to its moderate strength and resistance to corrosion. It is typically used in shallow wells where the pressure is less intense.
N80Q Casing: A higher grade steel, N80Q is designed to withstand higher pressures, especially in more challenging environments like deep wells and high-temperature zones.
Premium Tubing Materials: For extreme pressure conditions, premium materials such as CrMo or corrosion-resistant alloys are used to ensure optimal performance without compromising the well's integrity.
6.Common design standards for pressure piping equipment
The following are common design standards:
1) GB50316-2008 Industrial Metal Pipeline Design Specifications
2) GB50251-2015 Gas Pipeline Engineering Design Specifications;
3) GB50253-2014 Oil Pipeline Engineering Design Specifications;
4) GB50028-2016 Urban Gas Design Specifications;
5) GB50030-2013 Oxygen Station Design Specifications;
6) SH3059-2012 General Rules for the Selection of Petrochemical Pipeline Design Equipment;
7) SH3064-2003 Selection, Inspection and Acceptance of Petrochemical Steel General Valves;
8) HG/T20646 Chemical Equipment Pipeline Material Design Regulations.
FAQs
1.Why is casing pipe pressure important?
Casing pipe pressure is vital to maintaining the structural integrity of the well and ensuring the safe extraction of oil or gas.
2.What is the role of tubing in well design?
Tubing carries extracted oil and gas from the reservoir to the surface, and it must be strong enough to withstand the pressures within the pipeline.
3.What are the best materials for high-pressure wells?
Materials such as N80Q casing and premium alloy tubing are ideal for high-pressure wells, offering durability and resistance to corrosion.
4.How are pipeline pressure levels determined?
Pipeline pressure levels are determined through careful measurement of formation pressures, external pressures, and wellhead pressures, ensuring that the selected casing and tubing can withstand these forces.
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