How "abnormal" are the working conditions of steam pipes?
Let's talk about the real environment of steam pipelines-high temperature, high pressure, water hammer and corrosion. Four "ruthless characters" take turns. Medium-pressure steam often goes to 350℃, and high-pressure steam goes straight to above 500℃. Wrong choice of corrugated pipe material? 304 can only be the bottom, and 316L is the starting price. If you really want to meet supercritical steam, you have to get Inconel. Let's talk about pressure: the design pressure is between 1.0MPa and 6.4MPa all the year round, and the instantaneous pressure of water hammer can double. There is also corrosion-condensate entrained in steam, oxygen corrosion, stress corrosion cracking, which can make inferior expansion joints scrapped early. Therefore, don't take the universal corrugated expansion joint hard top steam working condition, it must be designed separately.
The core parameters are smoothed out one by one
Design temperature and pressure
These two parameters directly determine the number of layers, wall thickness and material of the bellows. For example, high-temperature axial expansion joints and corrugated expansion joints used in power station industry usually adopt multi-layer thin-walled structures, which not only ensure flexibility but also meet pressure bearing. When selecting, don't just look at the rated value, but leave a margin-instantaneous overpressure of steam pipeline is the norm.
Compensation amount
Thermal displacement calculations can't be lazy. Cold tight pre-tension values must be accurate to millimeters, otherwise the bellows will either crack or deflate after operation. The thermal displacement of steam pipes tends to be larger than expected, especially for long straight sections. If you count 10% less, you could have an accident.
guide tube
This thing is a life-saving component in the steam system. High-speed steam impacts the bellows, and without the protection of the guide tube, the crest of the wave quickly wears out. Before, there was a question and answer that specifically talked about "the specific function of the expansion joint guide tube"-the direction must be consistent with the flow direction of the medium, and installing it backwards is equivalent to suicide. The direction of the arrow is clearly marked, and there are many people who have installed it wrong at the scene.
How to choose common structure types?
Don't blindly put on straight pipe pressure balance type. If the steam pipeline is simple in direction and the fixed bracket is strong enough, it is sufficient to use a general corrugated expansion joint or an external pressure single axial expansion joint, which has low cost and convenient maintenance. But if space is constrained and support thrust is tightly controlled — such as pipes in a turbine room — consider straight-pipe pressure-balanced expansion joints or curved-pipe pressure-balanced expansion joints, which absorb axial displacement without creating blind plate forces. The double hinge transverse expansion joint is suitable for L-shaped or Z-shaped pipe segments, which absorbs transverse displacement more flexibly.
Two days ago, I met a customer who used a sleeve-type pipe expansion joint for the main steam pipeline, and as a result, the leakage was frequent-the sleeve-type seal couldn't withstand the high temperature, and finally the metal corrugated expansion joint was replaced. Therefore, don't be cheap, the working condition determines the selection.
"Pits" in installation and operation and maintenance
Tie rod nut must be removed
After installation, the tie rod for transportation must be loosened or removed, otherwise the expansion joint is equal to display. Previously there was a question and answer "Does the screw of the expansion joint need to be removed"-Answer: It must be removed. Unless it is a pull rod with limit function, it has to be adjusted to the design position. People often don't dismantle it at the scene. As a result, as soon as the pipeline heats up, the bellows directly cracks.
Cold tightness cannot be saved
Steam pipes generally require pre-stretching – the amount of cold tightening is half the amount of thermal compensation. However, many on-site maps skip this step without trouble, resulting in the bellows exceeding the limit when it is hot. If the cold tightening is done well, the life of the expansion joint can be extended by 30%.
Regular inspection to see three points
Whether there are corrosion pits and weld cracks on the surface of the bellows, and whether the guide tube falls off. The life of the expansion joint of steam system is usually designed to be about 15 years, but if the actual environment is harsh, it has to be replaced in 8 to 10 years. Don't wait for a leak to do it, downtime losses are much more expensive than expansion joints.
The selection process is summarized into three steps
First, calculate the thermal displacement, pressure and temperature of the pipeline clearly, and take out a detailed working condition parameter table. Second, select the structure type according to the product catalog-high-temperature axial expansion joint is the first choice for high-temperature steam, large-diameter thick-walled expansion joint for large-diameter steam pipe, and compound straight pipe bypass pressure balance expansion joint for combination that needs to absorb multiple displacements. Third, leave a margin for the material and thickness of the bellows-it is better to spend 20% more budget than save a downtime accident.