Discussion on the pressure tapping location for pressure gauges
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The attached diagram shows the pressure tapping locations as specified in the SH3104 instrument installation guidelines. There are two points that I would like to discuss and have clarified: 1. Figures a and c show different pressure tapping locations for gases and steam respectively; so how can we distinguish between gases and steam? Gases such as nitrogen and compressed air fall under the category of gases, and pressure tapping should be done according to figure a. But what about the flue gas from burning materials – does it belong to steam or gases, and should pressure tapping be carried out according to figure a or c? 2. The common practice is to make openings at the top of horizontal pipes, with the pressure gauge positioned vertically upward. What effects might there be if pressure tapping is done at the top of the pipe for different types of media? Especially when the liquid pipeline is not completely filled, will there be a significant difference between the pressure readings taken at the top and those taken according to diagram B?
The different pressure measurement points for steam are determined primarily with the consideration that steam can condense, and it is necessary for it to condense; this helps in determining whether something constitutes steam or not. If the pressure gauge is installed at the bottom, it is mainly the static pressure of the liquid itself that affects the readings; this depends on how high the liquid level is and what the measurement range is. If the pressure is already several MPa and the pipe is small, this amount of error can be ignored, right?
When measuring steam with an opening that faces vertically upward, wouldn’t the condensate have an easier time returning to the main pipe? If we use the range of 0–45°C as shown in part C, wouldn’t the condensate accumulate inside the pressure measurement tube?
Look at the diagram carefully – who said that the steam pressure tapping port has to be oriented vertically upward? There is no major issue going downward; the condensation in the tubes provides adequate insulation, and the pressure of the condensate has little impact. Additionally, when measuring steam differential pressure, it is usually only allowed to be at 45 degrees in the upper part; it cannot be angled downward, as the differential pressure may be very small and could be affected by the height difference of the condensate on both sides.
First of all, thank you for your reply. I now understand that the gas that will condense is steam. However, it’s a bit unclear to me what is meant by the suggestion to measure the steam pressure by allowing condensation at an angle of 45 degrees downward inside the pressure measurement tube, as this would help with heat insulation
The liquid that condenses downward isn’t all inside the tube, right? That means better insulation.
Oh, then will this affect the measured pressure value? It’s as if a water seal is formed, preventing gas from passing through
It’s not a problem whether gas can be used or not; liquids are also pressure-transmitting media. There is simply an additional static pressure equivalent to the height of a water column, so it depends on how high the pressure being measured is and what impact that water-column pressure has. Take another look at my original reply.
It’s still more reliable to use isolation fluid, with the pressure tapping pointing upwards
I looked at it again from top to bottom; I basically understand the principle now. I’ll take another closer look when I’m at the project site next time
In the specifications, the direction of the pressure tap is the same for liquid and gas media, but for steam media it depends on whether pressure or differential pressure/flow is to be measured. The difference between the two is that one covers the range from 45 degrees above the horizontal to 45 degrees below the pipe, while the other covers the range from 45 degrees above the horizontal to the horizontal center line of the pipe. For specific details, you can check the installation manual. And with a proper design, installation drawings will be produced