It's also an expansion joint. Why must the one after the pump be taken out separately?
The working conditions faced by the pipeline behind the pump are completely different from those of ordinary pipe sections. The mechanical vibration and pressure pulsation generated during the operation of the pump, coupled with the sudden rise and drop of the medium temperature, if the metal expansion joint is selected according to the conventional idea, the bellows will crack in advance, and the pipe frame will be broken in the worst case. Take a real example: the centrifugal pump outlet of a chemical plant is equipped with a general-purpose corrugated expansion joint, and cracks are found at the root of the corrugated pipe after three months of operation. When I removed it, the guide tube was eroded like a sieve. This is because the flow rate behind the pump is high, the medium contains particles, the guide tube is not thickened, and the bellows is directly exposed to scour. To avoid such pits, you must first understand the core requirements of the expansion joint behind the pump-it must not only absorb axial thermal displacement, but also withstand high-frequency vibration and pressure shock. The "Universal Corrugated Expansion Joint" of this site is fine if it is used in the low-pressure pipe section in front of the pump, but after the pump, you have to consider the "High Temperature Axial Expansion Joint" or "Straight Pipe Pressure Balance Expansion Joint", which is specially designed to cope with high temperature and high pressure.
The working condition after the pump is complicated, how to match the structural design?
Different pump types and different media, the structural differences of expansion joints can make you doubt your life. Let's talk about the high pressure water pump outlet first. When the pressure exceeds 2.5MPa, the number of bellows layers will increase, but when the number of layers is more, the stiffness will be greater, and the ability to compensate displacement will decrease. At this time, "external pressure single axial expansion joint" is a good choice-the external pressure structure makes the bellows only tensile but not compressed, and the fatigue life can be doubled. Let's talk about high temperature media pumps. For example, the outlet temperature of the steam boiler feed water pump can reach above 200℃. Ordinary bellows materials will creep, so heat-resistant stainless steel has to be used, and a guide tube has to be added to protect the inner wall of bellows from being washed by high-speed steam. The "high-temperature axial expansion joint" of this station is equipped with heat-resistant stainless steel and thickened guide tube as standard, but it should be noted that the direction of the guide tube must be consistent with the flow direction of the medium. I specifically wrote about this question in the question and answer. Installing it backwards means not installing it. Finally, mention the situation of limited space behind the pump. Some pump rooms are compact and can't fit long-stroke expansion joints in axial displacement, so they have to use "double hinge transverse expansion joints" or "curved tube pressure balance expansion joints", which can absorb pipe deformation through angular displacement, but don't forget the vibration load of the pump when calculating.
When installing those few screws, remove or not remove? — — The easiest operation to overturn in actual combat
Two days ago, I met a customer. After the expansion joint behind the pump was installed, the transportation screw was not disassembled. As a result, the bellows bulged the next day after the machine was started. Someone asked this question in the question and answer, "Does the screw of the expansion joint need to be removed?"-the answer is that it must be removed, unless it is a model specially designed with a tie rod limit. The vibration behind the pump is large, and the transportation screw is not disassembled, which is equivalent to turning the expansion joint into a rigid body. All vibrations and displacements are held in the pipeline, so it is strange that the bellows is not bad. Another pit is the adjustment of the tie rod nut. The rear pump expansion joint usually comes with a tie rod to prevent the bellows from over-stretching, but the nut cannot be screwed to death. The proper thing to do is to loosen the nut after installation, leaving an allowance for pre-tension or pre-compression. The specific adjustment amount should be calculated according to the design temperature of the pipeline. Don't screw it by hand feel. One more detail: the direction of the arrow on the expansion joint. The arrow points to the flow direction of the medium, and the flow direction of the pipe behind the pump is clear, but some people look at the picture to save trouble and install it backwards. The arrow indicates the installation direction of the guide tube, and the shrinking opening of the guide tube is installed backwards to face the incoming flow, so the resistance increases sharply and the scouring intensifies.
How many of the "three killers" of the failure of the rear expansion joint of the pump have you been recruited?
According to the fault cases dealt with in recent years, the problems of the rear expansion joint of the pump basically can't escape these three categories: First, the fatigue fracture of the bellows. If the pulsating vibration frequency of the pump is close to the natural frequency of the bellows, it will resonate, and the life of the bellows will be shortened from ten years to three months. Solution: When selecting, the manufacturer is required to provide bellows frequency calculation to avoid the pump speed frequency; Or add damping devices. Second, the guide tube falls off or bursts through. As mentioned earlier, the flow rate after the pump is high, and the particulate medium is more serious. The fixing method of the guide tube is very critical-spot welding is easy to take off, and full welding is reliable. The "large-diameter thick-walled expansion joint" of this station has been strengthened in the structure of the guide tube, but if you use thin-walled small factory goods, it is best to disassemble and inspect them regularly. Third, corrosion perforations. The medium after the pump is often corrosive, such as desulfurization slurry and acidic sewage. At this time, ordinary stainless steel can't bear it, so it has to be lined with "PTFE-lined metal hose" or "PTFE compensator". However, it should be noted that the temperature resistance of PTFE lining is limited, and it will age when it exceeds 180℃. In high temperature conditions, metal bellows have to be coated.
There is no standard answer to model selection, but there are five "iron laws"
If we have to condense the selection principle of the metal expansion joint behind the pump into a few sentences, it is:
- For pipelines with large pressure pulsation, multi-wave multi-layer bellows or "straight pipe pressure balanced expansion joint" are preferred, which can balance the blind plate force and reduce the burden on the pump outlet pipe frame.
- If the temperature exceeds 150℃, do not use rubber compensator. The metal bellows must be made of heat-resistant alloy, and the guide bracket should leave enough clearance.
- The medium contains particles or corrosiveness, the material of the guide tube should be higher than that of the bellows, and the thickness should be checked regularly.
- In the case of obvious vibration, vibration damping brackets should be set at both ends of the expansion joint, and it is best to let the manufacturer provide a vibration fatigue life report.
- Never mix the pump rear expansion joint with ordinary pipe sections to save money-the difference you save is not enough for a shutdown and emergency repair.
Write at the end: the metal expansion joint behind the pump is a "small piece", but it affects the "big picture"
An expansion joint in the pump house, which may only account for a fraction of the entire pipeline investment. However, because its failure causes the pump unit to shut down and the pipeline to tear, an unplanned shutdown can lose hundreds of thousands. Therefore, the next time you select a model, spend 10% more time confirming the working parameters-temperature, pressure, medium, vibration frequency, displacement-with the manufacturer-throw these data away and let the manufacturer recommend the appropriate model. Our product line covers more than 20 types, such as "high temperature axial type", "straight pipe pressure balance type" and "double hinge transverse type", but the key is to match your actual working conditions. If you choose the right one, an expansion joint will last for ten years without a problem; If you choose the wrong one, you will have to change it in half a year. This account, can you settle it?