In the energy recovery system of catalytic cracking unit, the repair of the expansion joint at the outlet of flue gas turbine is the key technical work to ensure the long-term operation of the unit. Flue gas turbine outlet flue temperature is high (about 500-650℃), pipe diameter is large, thermal displacement is remarkable, expansion joint is under high temperature alternating stress environment for a long time, weld cracking and bellows damage are common faults. Once a leakage occurs, if it is not dealt with in time, it will affect the power generation efficiency at the least, and lead to unplanned shutdown or even safety accidents at the worst. From fault diagnosis, emergency treatment to permanent repair, this paper systematically explains the professional technical points of flue gas turbine outlet expansion joint repair.
First, diagnose first: find the root cause of the fault
Before repairing the outlet expansion joint of flue gas turbine, the fault type and root cause must be accurately judged.
1. Common fault types and characteristics
| Type of fault | Typical phenomenon | Common Location |
|---|---|---|
| Weld cracking | Visible cracks, accompanied by smoke leakage howling | Butt weld between expansion joint and flue, weld of hinge support |
| Bellows crack | Cracks in crests or troughs | Bellows body |
| Hinge mechanism stuck | The expansion joint cannot rotate normally, and the pipeline stress is abnormal | Hinge plate, pin shaft |
| Integral deformation | The expansion joint has a twisted appearance and is different from the flue | Overall structure |
2. Fault Cause Analysis
According to the actual operation experience of the catalytic cracking unit, the main reasons for the failure of the expansion joint at the outlet of the flue gas turbine include:
Stress Factors: Unreasonable pipeline structure of the system leads to high stress levels. Higher stress at welded joints of pipes is the direct cause of crack propagation until fracture。
Material degradation: Austenitic stainless steel (304, 316H, etc.) used for a long time at high temperatures will experience material degradation. When the use temperature exceeds 480℃, the high temperature durable strength of the material decreases significantly, and the durable limit of 100000 hours is less than 30MPa。
Structural design defects: A total of 6 single hinge expansion joints are designed in the horizontal section from the three-rotating outlet to the hood in a factory, which can only absorb the displacement of the pipeline in the south and north directions, but cannot absorb the displacement in the east and west directions, resulting in large pressure in hot state and cracking of the weld。
2. Emergency treatment: temporary leak plugging without stopping the machine
In an emergency situation where immediate shutdown is not possible, an emergency scheme can be used to deal with flue gas turbine outlet expansion joint leakage.
Common emergency response methods
| Emergency Methods | Applicable scenarios | Operational Points |
|---|---|---|
| Twist seam plugging | Fine crack | The crack is squeezed closed with a twist tool |
| Boxing method | Long cracks or multiple leaks | Wrap a steel box in the leakage area and inject sealant |
| Patch repair welding | Local crack | Clean the crack and attach the steel plate for repair welding |
| Increase tension reinforcement | Weld cracking | Welding stiffener plates around cracks |
Practical case: A crack leakage of about 1m in length occurred in the butt weld between the vertical section of the flue and the expansion joint of the triple rotation of a catalytic cracking unit, and the leakage was plugged by wrapping boxes and adding tension bars on site。 The weld seam of the hinge support of the expansion joint of the other device is cracked, and the expansion joint is integrated with the box to complete the leak plugging. Four DN200 valves are added around the box for pressure relief, and 10mm reinforcing tie bars are added up and down the box。
Precautions
- Emergency response is only a temporary measure and is valid for weeks to months
- Must be permanently repaired during the most recent planned shutdown
- Personnel protection should be done when operating in high temperature environment
III. Permanent restoration scheme
Scheme 1: Weld repair welding and reinforcement
It is suitable for the case where the weld is cracked but the expansion joint body is intact.
Operation Procedures:
- After shutting down and cooling down, polish the cracked area until the metallic luster is exposed
- Use argon arc welding repair welding, and select matching welding materials (such as 316H with ER316H)
- Low current, fast welding, control interlayer temperature ≤150℃
- Sand the weld to smooth
- Perform penetration testing or radiographic testing to confirm no defects
- Increase stiffener plate to disperse stress
Scheme 2: Overall reinforcement of package box
It is suitable for multiple cracks or repeated cracking of welds.
Operation Procedures:
- Make a fitted steel box on the outside of the expansion joint
- The box is welded and sealed with the flue housing
- Set pressure relief valves around the box
- Increase tie bars to prevent hot deformation
Scheme 3: Overall replacement of expansion joint
It is suitable for the situation where the expansion joint body is seriously damaged and repeated repairs are ineffective.
Replacement points:
- Stress check should be carried out in the selection of new expansion joint
- Cold pre-draw strictly according to the design during installation
- Ensure that the welding area between the hinge support and the cylinder is sufficient, and add reinforcing ribs
4. Structural optimization: solving problems from the root cause
For the outlet expansion joint of flue gas turbine which fails repeatedly, simple repair cannot cure the problem, so it must be optimized from the design level.
1. Recalculate the amount of cold pre-pull
According to the transformation experience of a factory, it is an effective means to reduce the thermal stress by recalculating the cold state pre-drawing quantity and canceling the thermal stress。
2. Expansion joint selection and upgrade
| Original structure | Optimized structure | Effect |
|---|---|---|
| Single hinge expansion joint | universal hinge expansion joint | Can absorb multi-directional displacement and reduce axial stress |
| Single hinge | Compound hinge expansion joint | Stronger compensation |
In the overhaul of a device in 2019, the horizontal section of the hood inlet was redesigned, a universal compound hinge expansion joint was added, the pendulum bracket was updated, and the vertical section pipeline was replaced as a whole. Successfully entered the power generation state after the start of construction, and the flue gas line basically did not overtemperature。
3. Add pipeline support
Add a large support (200mm ×200mm section steel) at the level of the elbow at the bottom of the flue to reduce the self-weight stress of the pipeline and avoid cracking caused by self-weight stress。
4. Material upgrade
Consider upgrading the original 304/316H material to a material with higher durable strength at high temperature, taking into account both high temperature strength and anti-sensitization performance。
V. Preventive measures and maintenance suggestions
After completing the repair of the expansion joint at the outlet of the flue gas turbine, the following measures can effectively prevent the recurrence of the problem:
1. Operation Monitoring
- Regularly inspect the expansion joint area to check for abnormal vibration and noise
- Monitor the surface temperature of the expansion joint, and analyze the causes of local overtemperature in time
- Record the displacement of the expansion joint and compare it with the design value
2. Regular inspections
- Permeability testing of expansion joint welds for each planned shutdown
- Check the hinge mechanism for jamming and wear
- Check the bellows for deformation and crack
3. Create a file
- Record the time, location, method, and effect of each repair
- Establish an expansion joint operation ledger to track the deterioration trend
sum up
Flue gas turbine outlet expansion joint repair needs to take targeted measures according to the fault type and severity:
- Emergency treatment: twisting seams, wrapping boxes, repairing welding of patch plates, adding tension bars, suitable for temporary leak plugging without stopping the machine
- Permanent repair: welding repair reinforcement, package box integral reinforcement, expansion joint integral replacement
- Structural optimization: recalculating cold pre-tension, upgrading to universal hinge expansion joint, adding pipeline support, and upgrading materials
Core experience: During the overhaul in 2019, a device successfully realized the power generation of the cigarette hood through measures such as recalculating the cold pre-pull amount, changing the single hinge expansion joint to the universal hinge expansion joint, and increasing the pipeline support. So far, the flue gas line has basically not overheated。 This fully shows that a standardized structural optimization and repair is far more economical and reliable than repeated emergency plugging.