Anti-corrosion repairing method for high-strength anti-corrosion material

Abstract

The invention relates to the technical field of corrosion prevention and repair, and discloses a corrosion prevention and repair method of a high-strength corrosion-resistant material, which comprises the following steps: step 1, cleaning and polishing a tower wall, removing residues, and filling a concave-convex surface until the concave-convex surface is flat; step 2, carrying out primary glass fiber reinforced plastic hand lay-up corrosion prevention on the tower wall, treating a pipe orifice and covering the pipe orifice to the outer flange of the flange; 3, polishing treatment is conducted, specifically, a polishing machine is used for polishing; step 4, secondary glass fiber reinforced plastic hand lay-up corrosion prevention; 5, repairing and accepting the lining, carrying out defect inspection on the lining, and repairing the found defects for no more than two times; 6, after repairing is completed, an operation test is carried out, and it is verified that no leakage exists. The high-strength corrosion-resistant material is combined with the glass fiber reinforced plastic for corrosion-resistant repair, so that the service life can be prolonged by 2-3 years, the production cost is reduced, high-strength stainless steel and the corrosion-resistant glass fiber reinforced plastic are effectively combined, the cost of spare parts is greatly reduced, and more than 1 million yuan can be saved for each crystallizer every year.

Description

Technical Field

[0001] The present invention relates to the technical field of anti-corrosion repair, and specifically to an anti-corrosion repair method for a high-strength corrosion-resistant material. Background Art

[0002] The ammonia desulfurization method is an important technology for removing SO2 from the sintering flue gas of iron and steel enterprises. Its desulfurization efficiency is as high as over 95%, with good desulfurization effect and no secondary pollution. Its by-product ammonium sulfate also has good economic value. However, due to the presence of a large amount of suspended solids, fluoride ions, chloride ions, etc. in the ammonium sulfate mother liquor, and the enrichment of these ions in the ammonium sulfate mother liquor, the chloride ion concentration is as high as 147,900 mg / L and the fluoride ion is 3,041 mg / L, which seriously corrodes the high-strength corrosion-resistant material. Currently, the original tower wall material of the ammonium sulfate crystallizer is 316L with a wall thickness of 16 mm. After operating for 2 years, the wall thickness is only 12 mm, and the surface is uneven, which exacerbates corrosion and results in leakage. Therefore, it is necessary to provide an anti-corrosion repair method for high-strength corrosion-resistant materials to solve the above technical problems. Summary of the Invention

[0003] In view of the deficiencies of the prior art, the present invention provides an anti-corrosion repair method for high-strength corrosion-resistant materials, which solves the problems that a large amount of suspended solids and ions accumulate in the mother liquor, seriously corroding the high-strength corrosion-resistant material and making the surface of the original tower wall of the ammonium sulfate crystallizer uneven.

[0004] To achieve the above objectives, the present invention is realized through the following technical solutions: An anti-corrosion repair method for high-strength corrosion-resistant materials, comprising the following steps: Step 1: Clean and polish the tower wall, remove the residue, and fill the uneven surface until it is flat; Step 2: Conduct the first fiberglass hand lay-up anti-corrosion on the tower wall, process the pipe orifice and cover it to the flange outer edge; Step 3: Conduct a grinding treatment, using a grinder for grinding; Step 4: Conduct the second fiberglass hand lay-up anti-corrosion; Step 5: Conduct an acceptance inspection on the lining repair, conduct a defect inspection on the lining, and repair the discovered defects no more than twice; Step 6: Conduct an operation test after the repair is completed to verify no leakage.

[0005] Preferably, the cleaning in Step 1 includes: successively using a high-pressure water jet of 18 - 22 MPa to remove the residual crystals from the inner surfaces of the crystallizer arch top cover, the tower wall, and the lower reducer, then using an alkaline degreaser with a pH of 10 - 12 for cyclic flushing for 40 minutes, and finally drying with hot air at 80°C until the surface moisture content ≤ 3%.

[0006] Preferably, the grinding in Step 1 includes: The tower wall and the lower reduced-diameter area are sandblasted with 20-mesh carborundum, and the surface roughness reaches Sa3 level. For the range extending 200 mm downward from the joint between the lower reduced-diameter part and the DN600 cylinder, a two-component epoxy putty (viscosity ≥ 5000 cps) is used for stepped filling in 3 layers, and after each layer is cured, it is polished to be flush with the base material.

[0007] Preferably, the nozzle treatment in the second step includes: The inner wall of the nozzle of the crystallizer tower body is chamfered with an R5 mm chamfer, and the flange bolts below DN200 are removed. In the range 200 mm downward from the joint between the lower reduced-diameter part and the cylinder, a transition reinforcement layer with a width of 300 mm is laid, and the transition reinforcement layer contains 2 layers of 45°-angled cross glass fiber cloth.

[0008] Preferably, the first FRP hand lay-up anti-corrosion in the second step includes: A bisphenol A vinyl ester resin (solid content ≥ 65%) is used to impregnate a 0.5 mm medium-alkali glass fiber surface mat to form a sealing layer with a thickness of 1.2 - 1.5 mm.

[0009] Preferably, the grinding in the third step includes: An 80-mesh silicon carbide grinding wheel is used to level the first anti-corrosion layer and eliminate resin protrusions with a height difference > 0.3 mm. For the area where the radius of curvature of the lower reduced-diameter part ≤ 500 mm, a flexible grinding pad is used for surface roughening to Ra25 - 35 μm.

[0010] Preferably, the second FRP hand lay-up anti-corrosion in the fourth step includes: 2 layers of axially wound E-glass fiber cloth (800 g / m²) are added to the straight section of the tower body, and 3 layers of circumferentially wound fiber cloth are added to the lower reduced-diameter area. A FRP reinforcement ring (thickness 12 mm, width 50 mm) is pre-embedded within a range of 200 mm around the nozzle and bonded to the matrix with a resin anchor agent.

[0011] Preferably, the defect inspection in the fifth step includes: A 6 kV electric spark detector is used to conduct a 100% scan on the area with changing curvature of the lower reduced-diameter part. Ultrasonic thickness measurement is carried out on the DN600 cylinder section, and the number of measurement points per square meter is not less than 9. When patching air bubbles, a thixotropic epoxy resin (viscosity ≤ 300 cps) is injected with a syringe, and the thickness deviation after patching ≤ ±0.5 mm.

[0012] Preferably, the integrity of the anti-corrosion layer is jointly determined by electric spark detection and ultrasonic thickness measurement, and the electric spark detection voltage is 3 - 6 kV.

[0013] Preferably, the running test in Step 6 includes: Conduct a hydrostatic test at 1.2 - 1.5 times the design pressure, with a pressure holding time ≥ 2 hours and a pressure drop rate ≤ 1% / h.

[0014] The present invention provides an anti-corrosion repair method for high-strength corrosion-resistant materials, having the following beneficial effects: 1. The present invention combines high-strength corrosion-resistant materials with fiberglass for anti-corrosion repair, which can extend the service life by 2 - 3 years. The repair cost is only 200,000 yuan per unit, significantly reducing the production cost. By effectively combining high-strength stainless steel with corrosion-resistant fiberglass, the spare part cost is greatly reduced, and each crystallizer can save more than 1 million yuan per year.

[0015] 2. The present invention combines two fiberglass anti-corrosion layers with an intermediate grinding and activation process to ensure that the total thickness of the anti-corrosion layer is accurately controlled within the range of 8 ± 0.5 mm. Compared with the conventional single-forming technology, it solves the problems of sagging and bubble aggregation easily occurring in thick-layer construction, and the interfacial bonding strength is increased by more than 40%.

[0016] 3. The present invention introduces a combined mechanism of 6 kV electric spark detection and grid ultrasonic thickness measurement, which can detect micro-defects at the 0.1 mm level. Compared with the traditional visual inspection method, it breaks through the limitations of high undetected rates of hidden delamination and micro-porosity in manual inspection, and the defect recognition accuracy is increased to 99.5%. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 is a flow chart of the anti-corrosion repair method in the present invention; Figure 2 is a construction drawing of the reinforcing ribs for the inner lining of the arched top cover of the crystallizer in the present invention; Figure 3 is a construction drawing of the reinforcing ribs for the inner lining of the tower wall in the present invention; Figure 4 is a position diagram of the combined part of the inclined plate and the tower wall inside the tower in the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0018] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0019] Please refer to the attached Figure 1 - attached Figure 4 , the embodiments of the present invention provide an anti-corrosion repair method for high-strength corrosion-resistant materials, including the following steps: Step 1: Clean and polish the tower wall, remove the residue, and fill the concave and convex surfaces until they are flat. The cleaning in Step 1 includes: for the inner surfaces of the crystallizer arch top cover, the tower wall, and the lower reducer, use a high-pressure water jet of 18 - 22 MPa in sequence to remove the residual crystals, then use an alkaline degreaser with a pH of 10 - 12 to circulate and rinse for 40 minutes, and finally dry with hot air at 80 °C until the surface moisture content ≤ 3%; The polishing in Step 1 includes: Use 20 - mesh emery to perform sandblasting on the tower wall and the lower reducer area, and the surface roughness reaches Sa3 level.

[0020] For the range extending 200 mm downward from the joint between the lower reducer and the DN600 cylinder, use a two-component epoxy putty (viscosity ≥ 5000 cps) to fill in 3 layers in a stepped manner, and polish each layer to be flush with the base material after curing.

[0021] Step 1 of the embodiment of the present invention includes the following contents: High-pressure water jet cleaning: Cleaning range: Cover the inner surfaces of the crystallizer arch top cover (DN5000×H1306 mm), the tower wall (DN5000×H7000 mm), and the lower reducer (DN5000×DN600×H2200 mm).

[0022] Cleaning parameters: Use a high-pressure water jet of 18 - 22 MPa (water temperature 60 - 70 °C), spraying angle 45°, distance from the surface 300 - 500 mm, to remove crystals and corrosion products.

[0023] Chemical cleaning: Use an alkaline degreaser with a pH of 10 - 12 (containing 2% - 5% surfactant) to circulate and rinse for 40 minutes, flow rate ≥ 2 m / s, and replace with deionized water after rinsing until the drainage pH ≤ 8.5.

[0024] Surface drying and sandblasting treatment: Drying control: Use an 80 °C hot air circulation drying system, wind speed 8 - 10 m / s, and the surface moisture content ≤ 3% after drying (detected according to GB / T1725).

[0025] Sandblasting process: Use 20 - mesh emery (Al2O3 ≥ 85%) for sandblasting, pressure 0.7 - 0.9 MPa, spraying distance 150 - 200 mm, the surface roughness reaches Sa3 level (ISO8501-1), and the sandblasting coverage rate ≥ 95%.

[0026] Defect repair and joint treatment: Putty filling: For the range 200 mm downward from the joint between the lower reducer and the DN600 cylinder, use a two-component epoxy putty (viscosity ≥ 5000 cps, compressive strength ≥ 60 MPa) to fill in 3 layers, each layer thickness ≤ 3 mm, and the interlayer interval is 2 hours for curing.

[0027] Flatness control: After filling, use a straightedge to check. The height difference between protrusions and depressions should be ≤ 0.5 mm, and the slope of the joint transition zone should be ≤ 15°.

[0028] Step 2: Conduct the first fiberglass hand lay-up anti-corrosion on the tower wall, process the pipe openings and cover up to the flange outer turn-up. The first fiberglass hand lay-up anti-corrosion in Step 2 includes: Impregnate a 0.5-mm medium-alkali fiberglass surface mat with bisphenol A vinyl ester resin (solid content ≥ 65%) to form a sealing layer with a thickness of 1.2 - 1.5 mm.

[0029] The pipe opening treatment in Step 2 includes: Perform an R5-mm chamfering treatment on the inner wall of the pipe opening of the crystallizer tower body and remove the flange bolts below DN200.

[0030] Within a range of 200 mm downward from the joint of the lower reducer and the cylinder body, lay a transition reinforcement layer with a width of 300 mm. The transition reinforcement layer contains 2 layers of 45° cross glass fiber cloth.

[0031] The second step of the embodiment of the present invention includes the following content: Pre-treatment of pipe openings and joints: Chamfering treatment: Perform an R5-mm mechanical chamfering on the inner wall of the pipe opening of the crystallizer tower body, and the chamfering depth covers 1.2 times the original corroded area.

[0032] Flange treatment: Remove the flange bolts below DN200 and fill the flange gap (width ≤ 2 mm) with silicone sealant.

[0033] Transition reinforcement layer: Within a range of 200 mm downward from the joint of the lower reducer and the cylinder body, lay 45° cross glass fiber cloth (single-layer grammage 400 g / m 2 ) with a sizing content of 70% - 75%.

[0034] Sealing layer construction: Resin preparation: Mix bisphenol A vinyl ester resin (viscosity 450 - 550 cps, solid content ≥ 65%) and methyl ethyl ketone peroxide (MEKP) in a ratio of 100:1.5, and the stirring time should be ≤ 5 minutes.

[0035] Surface mat laying: Use a 0.5-mm medium-alkali fiberglass surface mat (grammage 300 g / m 2 ), impregnate it with resin and then roll to exhaust air to ensure an impregnation rate of ≥ 95%. The final thickness of the sealing layer is 1.2 - 1.5 mm.

[0036] Chopped strand mat reinforcement layer: Laying design: Stack and lay 4 layers of alkali-free chopped strand mats (single-layer grammage 400 g / m 2), with a resin content of 60% - 65% per layer, and the cross-laying method is adopted (the fiber direction difference between adjacent layers is 90°).

[0037] Curing control: Cured between layers to a Barcol hardness ≥ 35 (ASTM D2583), environmental temperature 20 ± 2 °C, relative humidity ≤ 70%.

[0038] Step three: Grinding treatment, using a grinder for grinding. The grinding in step three includes: Level the first anti-corrosion layer using a 80-mesh silicon carbide grinding wheel to eliminate resin protrusions with a height difference > 0.3 mm.

[0039] For the area with a lower variable diameter curvature radius ≤ 500 mm, use a flexible grinding pad to roughen the surface to Ra 25 - 35 μm.

[0040] The third step of the embodiment of the present invention includes the following content: Mechanical leveling: Grinding wheel selection: Use an 80-mesh silicon carbide grinding wheel (grit P80), grinding linear speed 15 - 20 m / s, to eliminate resin protrusions with a height difference > 0.3 mm.

[0041] Curvature area treatment: For the area with a lower variable diameter curvature radius ≤ 500 mm, use a flexible polyurethane grinding pad (hardness Shore A 60 - 70) to roughen it to Ra 25 - 35 μm (GB / T 13288.1).

[0042] Interface activation: Surface cleaning: Wipe with acetone to remove dust, and the volatilization time of residual acetone ≤ 10 minutes.

[0043] Activation treatment: Coat a 5% - 8% benzoyl peroxide (BPO) acetone solution, activation time 10 - 15 minutes.

[0044] Step four: Second fiberglass hand lay-up anti-corrosion. The second fiberglass hand lay-up anti-corrosion in step four includes: Add 2 layers of axially wound E-glass fiber cloth (800 g / m²) to the straight section of the tower body, and add 3 layers of circumferentially wound fiber cloth in the lower variable diameter area; Embed a fiberglass reinforcement ring (thickness 12 mm, width 50 mm) within a range of 200 mm around the pipe orifice, and bond it to the matrix using a resin anchor agent; The fourth step of the embodiment of the present invention includes the following content: Fiber cloth layer increase: Axial reinforcement: Add 2 layers of axially wound E-glass fiber cloth (gram weight 800 g / m 2 , warp and weft density 8 × 8 roots / cm) to the straight section of the tower body, resin content 55% - 60%.

[0045] Circumferential reinforcement: Add 3 layers of circumferentially wound fiber cloth in the lower reduced-diameter area (winding tension ≥ 50 N / bundle), and cure the layer interval to the gel state (no sticking to the fingertip).

[0046] Structural strengthening measures: Installation of strengthening rings: Embedded FRP strengthening rings (thickness 12 mm, width 50 mm) within a range of 200 mm around the pipe orifice, bonded with epoxy resin anchoring agent (shear strength ≥ 18 MPa), and the curing pressure is 0.2 - 0.4 MPa.

[0047] Sagging control: Add 5% gas-phase silica thixotropic agent (specific surface area 200 m 2 / g) between layers, and the thixotropy index of the resin after mixing ≥ 3.0.

[0048] Step Five: Acceptance of the lining repair, conduct defect inspection on the lining, and repair the discovered defects no more than twice. The integrity of the anti-corrosion layer is jointly determined by spark detection and ultrasonic thickness measurement. The spark detection voltage is 3 - 6 kV. The defect inspection in Step Five includes: Use a 6 kV spark detector to conduct 100% scanning on the lower reduced-diameter curvature change area; Conduct ultrasonic thickness measurement on the DN600 cylinder section, with no less than 9 measurement points per square meter; When patching bubbles, inject thixotropic epoxy resin (viscosity ≤ 300 cps) with a syringe, and the thickness deviation after patching ≤ ±0.5 mm; The content of Step Five in the embodiment of the present invention includes: Defect detection: Spark detection: Use a 6 kV DC spark detector (electrode spacing 5 mm) to conduct 100% scanning on the lower reduced-diameter curvature change area, and the moving speed ≤ 0.2 m / s.

[0049] Ultrasonic thickness measurement: Use a pulse reflection thickness gauge (accuracy ±0.1 mm) to detect the DN600 cylinder section according to a 3×3 grid layout, and the total thickness deviation ≤ ±0.8 mm.

[0050] Defect repair: Bubble treatment: For bubbles with a diameter ≤ 5 mm, inject thixotropic epoxy resin (viscosity ≤ 300 cps) with a syringe, and the thickness deviation after patching ≤ ±0.5 mm.

[0051] Delamination repair: For the delaminated area between layers, drill to the bottom of the defect and then inject low-viscosity vinyl ester resin (viscosity ≤ 150 cps), and pressurize to 0.3 MPa and hold the pressure for 30 minutes.

[0052] Step Six: Conduct an operation test after the repair is completed to verify no leakage. The operation test in Step Six includes: Conduct a hydrostatic test at 1.2 to 1.5 times the design pressure, with a holding time of ≥ 2 hours and a pressure drop rate of ≤ 1% / h.

[0053] Step six of the embodiment of the present invention includes the following content: Pressure test: Hydrostatic test: Hold the pressure at 1.2 to 1.5 times the design pressure (highest ≤ 2.5 MPa) for ≥ 2 hours, with a pressure drop rate of ≤ 1% / h, and the temperature difference between the test water temperature and the environment ≤ 15°C.

[0054] Airtight test: Hold the pressure with 0.6 MPa nitrogen, and detect no continuous bubbles with soapy water (the diameter of a single bubble ≤ 3 mm and does not expand within 10 minutes).

[0055] Medium verification: Simulated working condition: Pass in a 15% H2SO4 solution (temperature 80 ± 5°C) and circulate it 3 times (8 hours each time). After the test, detect that the thickness loss rate ≤ 0.1 mm / cycle.

[0056] Electrochemical detection: Measure the polarization resistance (ASTM G59) using a three-electrode system, and require Rp ≥ 1×10 5 Ω·cm 2 .

[0057] Has the following advantages: Structural adaptability: Design a stepped filling and transition reinforcement layer for the DN5000 / DN600 reduced-diameter structure to reduce the risk of stress concentration.

[0058] Process controllability: Achieve an interfacial bonding strength ≥ 15 MPa (ASTM D3164) through two layers of fiberglass paving (total thickness 8 mm) and grinding of the intermediate layer.

[0059] Zero tolerance for defects: Quantify the judgment and repair standards for defects such as bubbles and delamination to ensure that the equipment life after repair ≥ 10 years (refer to NACE SP0592).

[0060] Although the embodiments of the present invention have been shown and described, for those of ordinary skill in the art, it can be understood that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for anti-corrosion repair of a high-strength corrosion-resistant material, characterized in that, It includes the following steps: Step 1: Clean and polish the tower wall, remove the residue, and fill the concave and convex surfaces until they are flat; Step 2: Conduct the first FRP hand lay-up anti-corrosion on the tower wall, process the pipe orifice and cover it to the flange out-turned edge; Step 3: Conduct grinding treatment, and use a grinder for grinding; Step 4: Conduct the second FRP hand lay-up anti-corrosion; Step 5: Conduct acceptance inspection on the lining repair, conduct defect inspection on the lining, and repair the discovered defects no more than twice; Step 6: Conduct an operation test after the repair is completed to verify no leakage.

2. The anti-corrosion repair method of a high-strength corrosion-resistant material according to claim 1, characterized in that, The cleaning in Step 1 includes: For the inner surfaces of the crystallizer arch top cover, tower wall, and lower reducer, use a high-pressure water jet of 18 - 22 MPa to remove the residual crystallized substances in sequence, then use an alkaline degreasing agent with a pH of 10 - 12 for cyclic flushing for 40 minutes, and finally use hot air at 80 °C for drying until the surface moisture content ≤ 3%.

3. The anti-corrosion repair method of a high-strength corrosion-resistant material according to claim 1, characterized in that, The grinding in Step 1 includes: Conduct sandblasting treatment on the tower wall and the lower reducer area with 20-mesh carborundum, and the surface roughness reaches Sa3 level; For the range of 200 mm extending downward from the joint of the lower reducer and the DN600 cylinder, use a two-component epoxy putty (viscosity ≥ 5000 cps) to fill it in 3 layers in a stepped manner, and grind it to be flush with the base material after each layer is cured.

4. The anti-corrosion repair method of a high-strength corrosion-resistant material according to claim 1, characterized in that The pipe orifice treatment in Step 2 includes: Conduct a chamfering treatment with an R5 mm on the inner wall of the crystallizer tower body pipe orifice, and remove the flange bolts below DN200; Lay a transition reinforcement layer with a width of 300 mm within the range of 200 mm downward from the joint of the lower reducer and the cylinder. The transition reinforcement layer contains 2 layers of 45° cross glass fiber cloth.

5. The anti-corrosion repair method of a high-strength corrosion-resistant material according to claim 1, characterized in that, The first FRP hand lay-up anti-corrosion in Step 2 includes: Impregnate a 0.5-mm medium alkali glass fiber surface mat with bisphenol A vinyl ester resin (solid content ≥ 65%) to form a sealing layer with a thickness of 1.2 - 1.5 mm.

6. The anti-corrosion repair method of a high-strength corrosion-resistant material according to claim 1, characterized in that, The grinding in Step 3 includes: Use an 80-mesh silicon carbide grinding wheel to level the first anti-corrosion layer and eliminate the resin protrusions with a height difference > 0.3 mm; For the area where the curvature radius of the lower reducer ≤ 500 mm, use a flexible grinding pad to roughen the surface to Ra25 - 35 μm.

7. The anti-corrosion repair method of a high-strength corrosion-resistant material according to claim 1, characterized in that, The second FRP hand lay-up anti-corrosion in Step 4 includes: Add 2 layers of axially wound E-glass fiber cloth (800 g / m²) to the straight section of the tower body, and add 3 layers of circumferentially wound fiber cloth to the lower reducer area; Embed an FRP reinforcement ring (thickness 12 mm, width 50 mm) within the range of 200 mm around the pipe orifice, and bond it to the matrix with a resin anchor agent.

8. The anti-corrosion repair method of a high-strength corrosion-resistant material according to claim 1, characterized in that, The defect inspection in Step 5 includes: Use a 6-kV electric spark detector to conduct a 100% scan on the curvature change area of the lower reducer; Conduct ultrasonic thickness measurement on the DN600 cylinder section, and the number of measurement points per square meter is not less than 9; When patching bubbles, inject thixotropic epoxy resin (viscosity ≤ 300 cps) with a syringe, and the thickness deviation after patching ≤ ±0.5 mm.

9. The anti-corrosion repair method of a high-strength corrosion-resistant material according to claim 1, characterized in that Jointly determine the integrity of the anti-corrosion layer by electric spark detection and ultrasonic thickness measurement, and the electric spark detection voltage is 3 - 6 kV.

10. The anti-corrosion repair method of a high-strength corrosion-resistant material according to claim 1, characterized in that, The operation test in Step 6 includes: Conduct a hydrostatic test at 1.2 - 1.5 times the design pressure, with a pressure holding time ≥ 2 hours and a pressure drop rate ≤ 1% / h.

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