No matter how steady the onshore pipeline runs, it is another matter when it comes to the ship. The metal expansion joint for ships looks about the same length as the expansion joint for land, but if you really choose according to the one on land, you will have to rework it sooner or later.
1. What is the difference between the working environment of ship pipelines and onshore
Salt spray corrosion, continuous vibration, and space grievance, when these three things are put together, the selection logic will all change.
Let's start with salt spray. The concentration of chloride ions in the sea air is more than dozens of times that on land. As long as the corrugated pipe material is not resistant to chloride ion stress corrosion, it can crack fine lines for you in a few months. Let's talk about vibration. The hull engine, the propellers, the waves flapping, these vibrations are all-weather, and the fatigue life of the bellows is consumed much faster on board than on land. Finally, there is space-the pipes in the engine room are so packed that you can't break the flanges back even if they are staggered several degrees, and the expansion joints can't fit in if you don't choose the right structure.
Therefore, the marine expansion joint is never a matter of simple formula, and it has to be recalculated according to the marine working conditions.
2. Clear out the parameters before selecting
Parameter this thing, you fool it, it fools you. Medium temperature, design pressure, axial/transverse/angular displacement, and fatigue life times all directly determine how many layers, materials and guide tubes should be used for bellows.
If the medium temperature of marine steam pipeline exceeds 350℃ all the year round, 316L is not enough to see, so you have to rely on alloy materials with higher temperature resistance. When the pressure is high, the bellows has to be layered, but when the number of layers is more, the stiffness rises again, and the displacement compensation ability decreases-this is a trade-off, not stacking.
There are also displacement types. The requirements for expansion joints are completely different for pipe sections with axial displacement and pipe sections with transverse displacement. If you don't get all the data, you will place an order. If the bellows is installed in less than one voyage, there will be problems, and it will be too late to cry.
3. What kinds of expansion joints are suitable for marine working conditions
Don't expect one expansion joint to dominate the world. In marine pipeline systems, the following categories are most commonly used:
- Universal corrugated expansion joint: Base payment. It is suitable for conventional pipe sections whose temperature and pressure are not extreme, absorbing axial displacement and a small amount of transverse displacement, and has high cost performance.
- Compound hinge transverse expansion joint: The hull is docked in sections or the pipeline runs in complicated places with large lateral displacement, and it is caught. The hinge structure can restrain the axial blind plate force, and the pipeline is not easily pushed open.
- Straight pipe pressure balanced expansion joint: In the scenario where the pressure in the pipe is high and the blind plate force is not allowed to be transmitted to the equipment (such as the pump port and the turbine port), it can eat the pressure thrust by itself, and the equipment flange will not suffer.
- External pressure single axial expansion joint: Special for high-pressure steam pipeline, bellows under external pressure, good stability, not easy to unstable and bulge.
If you choose the wrong structure, you can't clean it up later. You take a general-purpose type to hard-carry the blind plate force of high-pressure steam pipeline, and the flange gasket will be leaked sooner or later.
4. Pits for installation and routine maintenance
No problem with the product, but it is still useless to install and pull the crotch. There are a few details here, and the old master knows them all in his heart:
When do tie rods and nuts adjust?When leaving the factory, the tie rod nut is generally in a locked state, which is used to fix the bellows to prevent transportation deformation. After installation in place, loosen the nut to the designated position according to the design requirements, so that the expansion energy saving can expand and contract freely. It is loose early, and the bellows is pressed and can't move; It's too late, and the displacement is all stuck in the wall of the pipe. If the timing card is not accurate, it means that it is not installed.
How to swing the direction of the guide tube?The function of the guide tube is to reduce the medium flushing the bellows, reduce the flow resistance and wear. The installation direction must follow the flow direction of the medium, and the arrow is clearly marked. If you have to install it backwards, the bellows will wait for it to be worn out by sand.
Do you need to add a protective sleeve to the outside of the bellows?In the marine environment, the surface of corrugated pipe is easily injured by foreign objects and bumped by tools, and the protective sleeve can block most mechanical damage. However, note that the protective sleeve should not be completely closed, and the inspection port must be left, otherwise the surface state of the bellows cannot be seen in daily inspection.
Do you need to pre-stretch the hull when docking in segments?To. When the hull is closed, there will be an installation error in the pipeline. The pre-stretching amount is according to the drawing to compensate for this error. Otherwise, the displacement of the pipeline after thermal expansion and contraction will exceed the design range, and the fatigue life of the bellows will be directly reduced by half.
5. How to judge that the expansion joint should be replaced
Don't wait for the pipe to leak before remembering to change it. The cost of changing it once at sea is enough to buy several pieces of equipment. Usually, the inspection stares at three things:
See if there are stress corrosion cracks in the bellows. This kind of crack is like a branch forking, and you can only find it when you look closely. The salt spray on board is heavy, and 316L material is particularly easy to be recruited in the chloride ion environment. If one is found, the whole batch should be evaluated.
Second, see if there is pitting corrosion on the surface. Small pits the size of needle tips appear on the surface of the bellows, indicating that chloride ions are already gnawing the material. Initial pitting does not affect the use, but will gradually deepen the penetration, the size and density must be recorded, and the changes must be compared regularly.
Third, look at the cumulative displacement times and the design fatigue life. The design fatigue life of marine expansion joints is generally calculated from 1000 to several thousand times (see the parameter table for details), and every start and stop, every thermal expansion and contraction is consuming the life. Installing a counter or ledger record in the cabin is much more reliable than patting your head by feeling.
To put it bluntly, the metal expansion joint for ships is a consumable part, so don't expect it once and for all. Think more about the selection, pay more attention to a few details during installation, and pay more attention to the daily inspection, so that the pipeline can not go wrong at sea. If you want to save trouble by picking one and loading it, the wind and waves at sea won't be polite to you.