【Common Issues in Haichuan Pipeline Design】--Should the pressure instrument sampling point be installed separately?

Pressure gauge sampling point: The pressure gauge sampling point refers to the location where the pressure gauge obtains pressure signals from process pipelines or equipment. Its selection must follow the following principles and requirements: Principles for selecting the location – Stable flow pattern of the medium: It should be chosen in sections where the medium flows in a straight line, avoiding areas with bends, branches, dead zones, or places where vortices are likely to form, so as to ensure that the pressure sampling point accurately reflects the actual pressure being measured. Relationship with the temperature sensor: If it is in the same pipe section as the temperature sensor, the pressure sensor should be located on the upstream side of the temperature sensor. Avoid interference: There should be no protruding pipes or components that create resistance on the upstream side of the pressure measurement point (such as thermometer sleeves, valves, baffles, etc.); otherwise, a sufficient length of straight pipe section is required. Pressure measurement points in different media: For liquid media, the measurement point should be located in the lower half of the pipe, at an angle of 0° to 45° relative to the horizontal center of the pipe; it is advisable to avoid taking measurements at the bottom to prevent sediment from blocking the pressure measurement port. Gas medium: The pressure tapping point should be located in the upper part of the pipeline, at an angle of 0° to 45° relative to the horizontal centerline of the pipeline, to prevent liquid from accumulating inside the pressure guiding tube. Steam medium: The pressure measurement point can be located in the upper or lower part of the pipe, at an angle of 0° to 45° relative to the horizontal centerline. Typically, a condensation elbow or condensation tube is required to prevent high-temperature steam from directly impacting the instruments. Treatment for special media: For media containing dust, solid particles, or precipitates, the source components should be installed at an upward angle; on horizontal process pipelines, they should be installed at an acute angle relative to the flow direction. Dust collectors or isolation tanks should be installed if necessary. High-temperature media (temperature above 60°C): The pressure gauge’s sensing element, when installed in situ, should be equipped with an annular elbow or U-shaped condensation elbow to protect the gauge from the effects of high temperatures. Corrosive, viscous, or crystalline media: Measurement is carried out using the isolation method; an isolation tank is installed to fill both the tank and the pressure transfer tube with an isolation fluid, and insulation measures are taken if necessary. Independent installation requirement: For multiple pressure and differential pressure transmitters that utilize the same SIS interlock source, or for the measuring instruments in a safety instrumented system and a basic process control system, it is advisable to locate their pressure-taking points separately, in order to avoid sharing pressure taps, root valves, and pressure guide tubes. The selection of the pressure measurement point must be determined by taking into account specific process conditions, properties of the medium, and the type of instruments, in order to ensure accurate and reliable pressure measurements as well as to facilitate installation, maintenance, and repair.

This is a good topic; such details in pipeline design are indeed easy to overlook. The pressure gauge’s sampling point is designed to be separate, with the main purpose of preventing eddies generated by other components such as thermometers, flow meters, or valves from interfering with pressure measurement and affecting its accuracy. It is recommended to refer to the relevant provisions of HG/T 20570. Generally, sufficient straight pipe sections should be maintained between the pressure measurement point, the temperature measurement point, and the throttling device. If shared measurement points are necessary due to limited space, it is essential to verify that the requirements regarding straight pipe sections ahead and behind are met; it is preferable to use separate measurement points. Additionally, in the case of high-temperature and high-pressure media, having a separate setup also facilitates subsequent maintenance and replacement. What specific layout conflict cases have you encountered during the design process? It can be laid out for discussion together.

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