Design pressure and design temperature of pipeline

March 10, 2020
Industry News

Principles for determining pipeline design pressure

① The design pressure of the pipeline must not be lower than the maximum working pressure.
② The design pressure of the pipeline equipped with the safety relief device must not be lower than the opening pressure (or burst pressure) of the safety relief device.
③ The design pressure of all pipes connected to the equipment shall not be lower than the design pressure of the connected equipment.
④ For pipelines carrying low-boiling media such as refrigerants and liquefied gases, the design pressure shall be the maximum saturated vapor pressure that the media may reach when the valve is closed or the media is not flowing.
⑤ When the passage between the pipeline or pipeline components and the overpressure relief device may be blocked or cut off, the design pressure shall be determined not lower than the maximum working pressure that may be generated.
⑥ The engineering design stipulates that for the pipes that need to calculate the wall thickness, the calculated pressure listed in the "pipe wall thickness data sheet" is the design pressure of the pipeline, and the working temperature corresponding to the calculated pressure is the design temperature of the pipeline.

Selection of pipeline design pressure

① Pressure pipeline with safety valve: p≥ opening pressure of safety valve
② Pressure pipeline connected to equipment without safety valve: p ≥ equipment design pressure
③ Outlet pipeline for centrifugal pump: p ≥ the closing pressure of the pump

Design temperature of the pipeline

The design temperature T of the pipeline refers to the temperature of the pipe material that can be reached under the corresponding design pressure during normal working of the pipe.

Process system professionals choose the design temperature of the pipeline according to the "pressure and temperature combination under the most severe conditions", on the basis of the working temperature of the various working conditions during the normal working process provided by the chemical process specialty.

The pipeline design temperature proposed by process system professionals can be determined by referring to the following principles:

(1) The tube wall temperature heat-transfer calculated or measured at the maximum working temperature of the medium is taken as the design temperature in the normal working process.

(2) When it is not convenient to heat-transfer calculate or measure the wall temperature of the pipe, the highest (or lowest) operating temperature of the medium during normal operation is used as the design temperature of the pipeline.

① Metal pipes

a. For non-insulated pipes with medium temperature below 38 ° C, T = the maximum temperature of the medium.
b. For pipes with medium temperature not less than 38 ° C, T = 95% × the maximum temperature of the medium.
c. For external thermal insulation pipe, T = the maximum temperature of the medium.
d. For internal heat insulation pipe, (lined with heat-insulating material), T = the pipe surface temperature calculated by heat transfer or measured by the test.
e. When the temperature of the medium is not greater than 0 ° C, T = the lowest temperature of the medium.

② Non-metal pipes and non-metal lined metal pipes

a. For pipes without environmental temperature effects, T = the maximum temperature of the medium.
ｂ. For pipes installed at the ambient temperature higher than the maximum temperature of the medium (except for those who have taken protective measures), T = the ambient temperature.
where the ambient temperature is higher than the maximum temperature of the medium (except for those who have taken protective measures), T = ambient temperature.

(3) In the normal working process proposed by the chemical technology professional, the normal working temperature of the medium is added (or subtracted) by a certain margin as the design temperature, and the design temperature can be determined by the following formula.

① When the normal working temperature of the medium is 0 ～ 300 ℃, T≥ the normal working temperature of the medium 30 ℃.
② When the normal working temperature of the medium is greater than 300 ° C, T ≥ the normal working temperature of the medium 15 ° C.

(4) When the temperature of the fluid medium is close to the allowable temperature limit of the selected material, the design temperature should be carefully selected in accordance with the specific situation, so as not to increase investment or reduce safety.

For example, when the calculation result in (3) above will cause the replacement of higher-grade materials, it is allowed to reduce the additional amount of 15 ° C according to the engineering design requirements, but the process must have measures to prevent over-temperature during operation.

(5) When the working pressure and corresponding working temperature have various working conditions or periodic changes, the process system designer should list the data of various working conditions proposed by the specialty of chemical technology and explain it to the specialty of pipe materials.