Two days ago, I met a customer with a pipe diameter of DN600 and a pressure of 4.0MPa. I asked for a quotation when I opened my mouth. I asked him what the displacement mode was, and he was stunned for a moment and said, "Just the compensation amount is 100mm." This kind of scene is too common in the industry. The specifications of metal corrugated expansion joints are there, but it is good to have two or three out of ten people who really raise the selection conditions according to the specifications.
I can't figure out the standard system, and there are pits behind it
The core standard of metal corrugated expansion joints is GB/T 12777 "General Technical Specifications for Metal Corrugated Pipe Expansion Joints", and the current version is 2019 version. Many people take JB/T 12235-that is the standard of non-metallic expansion joints-to set metal corrugated expansion joints. Both names have "expansion joints", but the technical route, material requirements and test methods are completely different things. It's like taking the frame of a bicycle to install the engine of a car. Can it be matched?
In addition to GB/T 12777, TSG D0001 "Technical Supervision Regulations for Safety of Pressure Pipelines" regulates the installation and use. The flange connection dimensions must also be aligned with the flange standards HG/T 20592. EJMA standard (American Expansion Joint Manufacturers Association Standard) is generally referred to when making design calculations in the industry, and many provisions of GB/T 12777 also originate from EJMA. Placing an order only by staring at a national standard number will have a high probability of problems.
The most easily overlooked in the selection is not the compensation amount, but the displacement mode and pressure thrust
Calculation models and stress evaluation methods are given for axial, transverse and angular displacements respectively. It is also to compensate the displacement of 100mm. Axial type, compound hinge transverse type and straight pipe pressure balance type are completely different ideas in design. If you report a "compensation amount of 100mm", they will supply you according to the axial type by default. Who will absorb the lateral displacement of the pipeline installed on it?
The bellows will generate a huge thrust under the action of internal pressure. DN600, 4.0MPa pipeline, this force can be calculated to tens of tons. When selecting the model, it is necessary to calculate whether the pipe fixing bracket can hold it. Otherwise, the expansion joint is not broken, and the bracket is pushed away first. Who is responsible for this?
The deflector is not an optional accessory. When the medium flow rate is high or contains particulate matter, the guide tube can protect the wave trough of the bellows from being washed and worn. The specification has clear requirements for its size and connection mode. For example, the high-temperature axial expansion joint and the general-purpose corrugated expansion joint of this station will confirm whether to configure the guide tube according to the medium working condition when selecting the type. This is not an option, but a must-answer question.
In the manufacturing and inspection process, the specifications are more stuck than you think
Corrugated pipe material is mainly austenitic stainless steel, commonly used 304, 316L. When the design temperature is lower than-40℃, low-temperature impact test should be done, and not any brand can be used. In terms of welding, the number of longitudinal welds of bellows blanks has an upper limit, the welds must be 100% non-destructive tested, and the welders must have corresponding qualifications. These requirements are listed in GB/T 12777 one by one, but many manufacturers actually implement discounts.
The pressure test is done piece by piece, the test pressure is usually 1.5 times the design pressure, and the holding time is not less than 30 minutes, confirming that there is no visible deformation and leakage. Factory inspection and type test are two different things-the former is done for each piece, and the latter is done only when there are new materials, new structures or process changes. Some manufacturers take a type test report as a universal shield, and the actual factory products have not even completed the airtightness test, which is a typical compliance loophole.
There are more pits in installation than in type selection
There is a calculation formula for the spacing specification of guide brackets, and it is not completed by welding two steel pipes. Pre-stretching or pre-compression, when the deviation between the installation temperature and the design temperature is large, the pre-displacement amount must be corrected according to the actual temperature. Otherwise, the actual displacement of the expansion joint exceeds the design range after it is put into operation, and the bellows will soon be fatigued and cracked.
"Does the screw of the expansion joint need to be disassembled?" When leaving the factory, the pull rod and the nut are in a locked transportation state. After installation, they must be loosened to the specified position, otherwise the expansion joint will not play a compensation role. It is the transportation locking piece to be dismantled, not the tie rod that bears the internal pressure thrust. If this difference is not clear, the equipment will be wasted.
The flow direction of the medium marked on the bellows cannot be installed reversely, which will lead to the instability of the bellows. Don't forget to add temporary constraints during hydraulic test, otherwise the blind plate force can directly pull the expansion joint. These details are all in the specification, but how many of them follow the site installation?
What is the cost of not following the norm
When the expansion joint is selected for the main steam pipeline of a power plant, only the diameter and pressure are reported, and the displacement type is not mentioned. The manufacturer supplies the goods according to the axial type by default. Results the pipeline actually had both axial displacement and lateral displacement. Less than three months after the pipeline was put into operation, cracks appeared in the trough of the bellows, and it was forced to stop the pipeline for emergency repair. The electricity lost in one day's downtime is converted into money, which is enough to buy dozens of qualified expansion joints.
Low cycle fatigue failure is the most common failure mode of metal corrugated expansion joints. There are clear requirements for the design fatigue life in the specification, which is generally not less than 1000 times, but many people don't pay attention to this number at all in actual use. 1000 times is not much, but that is the design life, which is based on the premise of correct selection, correct installation and correct use. Which link goes wrong, the actual life span will be discounted.
The old problem of "re-selection and light installation" has always existed in the industry, and the speed of standard update iteration can't keep up with the complexity of engineering applications. New scenarios such as LNG low-temperature working conditions and large-diameter high-pressure pipelines will only have higher and higher requirements for the understanding and implementation of metal corrugated expansion joint specifications. Don't wait for something to go wrong before you look back at the standards, by then the cost has been paid.