How to read the PID diagram of metal expansion joint? From Symbol Recognition to Selection Control
What message does the expansion joint symbol on the PID diagram convey? Many engineers get the process flow chart and have a headache when they see a bunch of circles, arrows and dotted lines. However, the selection and installation of expansion joints should start with these small symbols. Today, we will break this matter down clearly, so that you won't be blind again next time you look at the picture.
Alas, two days ago, I met a customer who took the PID diagram of the steam pipeline and asked, saying that there was a ripple with an arrow drawn on the diagram. He thought it was the flow direction of the medium, but it was installed in the opposite direction and almost caused an accident. Actually the arrow refers to the direction of displacement-more on that later. Lay this foundation firmly first.
PID Symbol Analysis of Common Metal Expansion Joints
Axial expansion jointIt is most easily recognized on a PID diagram: a straight line passing through a ripple, which is usually drawn as a saw tooth or wave. This site'sUniversal corrugated expansion jointAndHigh-temperature axial expansion joints are drawn in this way, but the temperature parameters will be marked next to the high-temperature type. What about the horizontal type? A diagonal or curved arrow in the symbol indicates that it can absorb lateral displacement. Such asThe double hinge transverse expansion joint has a small hinge circle in the middle of the symbol, and the corrugations at both ends are connected to the hinge point with dotted lines-if you mix this with the axial type, wait for the pipe to twist to death during installation.
Draw a hinge symbol (a circle with a fork) next to the ripple, sometimes with a tie rod. OurAir-cooled island vacuum pipe double hinge expansion joint on PID is two hinge symbols clamp a set of corrugations. The pressure balanced expansion joint is another way to draw it-a double corrugated structure with a balancing cavity in the middle, often marked "PB" or "P.B." in the symbol. Like this site'sStraight pipe pressure balanced expansion joint andThe pressure balance expansion joint of curved tube uses a balance chamber sandwiched between two corrugations to offset the blind plate force. The symmetry can be seen at a glance on the PID diagram.
Is that the truth? The symbol is the ID card of the expansion joint. Only when you recognize it correctly can you choose it down.
Read Key Parameters from PID Diagram
Optical recognition symbols are not enough. On the PID diagram, a string of numbers will be marked next to the expansion joint, such as DN200, L =300mm, RF and PN16. And guess what?Nominal diameter(DN) determines the interface size. If you buy the wrong flange, you can't install it.Compensation amount(L = xxx mm) is related to how much thermal displacement the expansion energy saving absorbs-the temperature of the steam pipeline rises by 100℃ every 100 meters, and the axial displacement can reach more than ten millimeters. You have to calculate it clearly. The connection method is labeled more directly: RF is flange connection, BW is welding. Almost all the common specifications listed in "Expansion Joint Model and Size" in this site's Q&A can be found in the PID annotation. For example, RF flange matchingUniversal corrugated expansion joint, BW welding fittingExternal pressure single axial expansion joint, check one accurate.
What about that? When you get the PID diagram, you first circle these three parameters with a red pen: DN, L, and connection mode. Compare the product library of this site, and the model can be locked in 10 minutes.
Selection and Comparison of Different Types of Metal Expansion Joints in PID
It is also the expansion joint symbol on the PID diagram, which is used on steam pipes and flue gas pipes, and the selection is completely different. Take a practical case: the main steam pipeline of a thermal power plant has a temperature of 540℃ and a pressure of 10MPa. The axial expansion joint is marked on the PID diagram, and "DN300, L =150mm" is noted next to it. Such operating conditions are preferentially usedExternal pressure single axial type expansion joint orDirect buried (fully buried) type expansion joint-the external pressure structure can withstand higher internal pressure, and is not afraid of condensate corrosion. The flue gas desulfurization pipeline next door has a horizontal symbol with an oblique arrow on the PID diagram. The medium contains SO₂, and the temperature is only 150℃, but do you dare to use stainless steel bellows? Must be onDesulfurization flue gas baffle doornon-metallic expansion joints, orLined with PTFE metal hose, corrosion resistance is the first priority.
For example, in the cement industry, the PID diagram of high-temperature air ducts is often drawnMetal rectangular expansion joint orCement industry metal corrugated expansion joint, the symbol is rectangular frame plus corrugated line. This kind of place has limited space and large compensation, so it can't be stuffed in with circular expansion joints. So you see, the PID symbol only tells you "here you need to absorb displacement", but the medium, temperature, and pressure determine which one to choose.
Common Misunderstandings and Correct Reading Skills of Metal Expansion Joint PID Diagram
The direction of the arrow mentioned at the beginning must be explained thoroughly here. The arrow on the expansion joint, most of the time, representsdisplacement direction, not the medium flow direction. The question and answer of this site "What is the direction of the arrow of the expansion joint" has a detailed explanation: the arrow points to the compression direction or the stretching direction of the corrugation. Think about it, the medium flows along the pipe, and the arrow is usually perpendicular to the pipe. How can it be mixed?
How is the deflector represented on PID? Generally, it is not drawn separately, but "with liner" or "WL" will be noted next to the parameter. The question and answer of this site "The specific function of the expansion joint guide tube" said that the function of the guide tube is to protect the bellows from being washed by high-speed media. If you see the label "Liner DN150" next to the expansion joint in the PID diagram, it means that a guide tube is installed inside, and the flow direction should be consistent when installing.
The difference between the pull rod symbol and the constraint symbol, tsk, novice is most likely to faint. Tie rods are usually drawn on the PID as two parallel lines connecting the ends of the corrugation, while constraints (such as limiting devices) are represented by crossed lines or rectangular boxes. This station's question and answer "The role of expansion joint tie rod" explains it clearly: the tie rod is subjected to internal pressure thrust to prevent the bellows from excessively elongating; Constraints limit the direction or magnitude of the displacement. There was a project in the past, and the draw rod symbol was drawn on the PID diagram. The worker thought it was a restraint and screwed the nut to death. As a result, the bellows could not be expanded and contracted, and the bracket was directly broken. So when you look at the picture, you have to understand the annotation next to the symbol-is it "Tie Rod" or "Limit Stop"? A word difference, a world of difference.
Understanding these, get the PID diagram in the future, from symbols to parameters to selection, in one go. If you are still entangled, directly compare the product information and questions and answers of this site, and basically you will not overturn.