What Is the Long-Term Performance Difference Between 1.5 mm and 2.5 mm Sheath Thickness for 316 Stainless Steel Heaters in 10% Phosphoric Acid Crude Slurry at 80°C with Abrasive Crystals

The Basic Trade-off in The Service of Phosphoric Acid Production

Wet-process phosphoric acid production produces a crude slurry of 10% phosphoric acid in a mixture of calcium sulphate hemihydrate or dihydrate crystals, unreacted phosphate rock particles and different metallic contaminants. This is a very aggressive environment because of the combination of a mildly acidic medium (pH 1-2) and solid particles which erode the passive coating on 316 stainless steel. Electric immersion heaters in phosphoric acid concentrators and attack tanks are subjected to combined chemical corrosion and mechanical abrasion. The decision of sheath thickness , 1.5 mm versus 2.5 mm , is a long term performance calculation where the additional metal in the thicker wall needs to be balanced against the thermal and mechanical implications of running with a larger cross-section . Unlike clean liquid service where the only worry is corrosion, this application involves monitoring two interacting degradation mechanisms across multi-year campaigns.

Erosion-Corrosion Synergism Dependence on Thickness

The uniform corrosion rate of 316 stainless steel in clean phosphoric acid at 80°C is about 0.1-0.3 mm/year depending on the quantity of fluoride and chloride impurities in the crude acid. By adding abrasive crystals (calcium sulphate hemihydrate crystals) typically 10-100 microns in size and with a Mohs hardness of 2.5-3.0, the degradation mechanism switches from pure corrosion to erosion-corrosion. The protective passive coating is continuously removed by the impinging particles exposing fresh metal to the acid. The erosion-corrosion rate is not just the sum of the pure erosion rate and the pure corrosion rate. Instead, a synergic effect takes place, where the corrosion rate of the continually de-passivated metal is substantially larger than the corrosion rate of a stable passive surface. Published results for 316 stainless steel in 10% phosphoric acid with 5-10% particles at 80°C and a slurry velocity of 1.5 m/s show erosion-corrosion rates of 0.6-1.2 mm per year, which are three to six times the pure corrosion rate . The calculated time to perforation under the continuous erosion-corrosion is 1.3-2.5 years for 1.5 mm wall thickness. For 2.5 mm wall, calculated time is 2.1–4.2 years. However these calculations assume a constant erosion-corrosion rate. In actuality, as the wall gets thinner, the local stress distribution at the surface changes and the erosion rate may increase because a thinner wall deflects more upon particle contact, leading to larger local stresses that fracture the surface more easily. Long term performance statistics from phosphoric acid plants show that the erosion-corrosion rate generally increases by 20-40% when the wall thickness is less than 1.0 mm. So the 2.5 mm wall starts with more metal and takes longer to get into this acceleration regime .

Mechanical Role of Thickness of Walls & Crystal Impact Energy

The abrasive crystals in the crude phosphoric acid slurry do not simply slide over the surface but hit it from different angles depending upon the flow pattern about the heater. In cross-flow, particles in the slurry strike the leading edge of a cylindrical heating tube at nearly normal angles (high impact energy, greatest erosion) and the sides at glancing angles (lower erosion). The impact energy of a calcium sulphate crystal moving at 1.5 m/s is 0.5 to 2 microjoules depending on the particle size. This energy is enough to produce micro-plastic deformation on the surface of 316 stainless steel. The depth of the plastically distorted layer from each hit is generally 1-5 microns. The total removal of material is achieved by a fatigue process with a large number of impacts : repeated impacts produce work hardening , fracture initiation and finally separation of small metal pieces . The material removal rate is inversely related to the hardness of the metal and also depends on the capacity of the underlying bulk material to absorb impact energy without shattering. A thicker wall leads to a bigger elastic foundation under the impacted surface. For 316 stainless steel, the elastic modulus is 193 GPa and the tube wall stiffness in bending is proportional to the cube of the thickness for a given outer diameter. A wall thickness of 2.5 mm has a bending stiffness of about 4.6 times that of a wall thickness of 1.5 mm (2.5³/1.5³ = 4.6). The increased stiffness decreases the dynamic deflection upon impact of particles, and thereby reduces the peak contact stress and erosion rate. Field observations in phosphoric plant heaters indicate that the erosion rates of 2.5 mm wall tubes on the leading edge are often 20-30% less than 1.5 mm wall tubes under the same slurry conditions solely due to the influence of stiffness.

Thermal performance penalty of thicker wall in a fouling service

Phosphoric acid crude slurry is well known to pollute. Calcium sulphate crystals tend to accumulate on hot surfaces . This causes a scale to form which insulates the tube and limits heat transfer . The rate of scale development is dependent on the surface temperature relative to the solubility curve of calcium sulphate. When the sheath temperature is over 90-95°C, the solubility of calcium sulphate hemihydrate drops and its precipitation is accelerated. A 1.5 mm wall heater with a watt density of 6 W/cm2 in 80°C bulk acid has a sheath surface temperature of about 95-100°C, at the threshold for accelerated scaling. At the same watt density, a 2.5 mm wall leads to a sheath temperature of 110-120C, well into the scaling range. The 2.5 mm wall tube will have scale build-up more faster and thicker than the 1.5 mm tube. Once the scale layer is in place, it adds its own thermal resistance, which pushes the sheath temperature even higher in a self-reinforcing cycle. The equivalent wall thickness of 4.0 mm of the effective thermal resistance of a wall of 2.5 mm with 0.5 mm of calcium sulphate scale. This scale not only inhibits heat transport but also produces fissures under the bulk acid chemistry which become locally concentrated and accelerate corrosion. Thus the difference in long term performance of the two thicknesses is vitally dependent on the ability to adjust the scale. The 2.5 mm wall can attain its prolonged erosion-corrosion life in plants using mechanical cleaning (spinners or scrapers) or chemical descaling. The 1.5 mm wall may actually perform better in plants where scale is permitted to collect. The lower surface temperature delays scale formation, and when scale is formed, the thinner base wall maintains higher heat flux through the combined wall-plus-scale layer.

Thermal Cycling and Differential Expansion Stresses

The phosphoric acid concentrators run in batch cycles or with intermittent feed and the heater is thermally cycled from ambient to 80°C and back. Each heating cycle causes differential expansion tensions between the resistance wire, the magnesium oxide insulation and the stainless steel sheath. Since a thicker section gives more resistance to thermal expansion and contraction, the amplitude of these stresses is proportional to the thickness of the sheath. The expansion coefficient for 316 stainless steel for a temperature shift of 80°C is roughly 0.1% (16 × 10-6/°C x 80°C = 0.00128). For a tube of 12 mm outer diameter this expansion is 0.015 mm. A 1.5 mm wall may more easily handle the expansion by elastic deformation than a 2.5 mm wall. The thicker wall is subjected to greater cyclic pressures at the contact between the sheath and the magnesium oxide insulation over thousands of cycles. This can lead to cracking of the insulation which diminishes dielectric strength and finally leads to electrical failure. Long-term performance data from phosphoric plants indicate that 2.5 mm wall heaters have a higher rate of electrical failure (insulation breakdown) than 1.5 mm wall heaters after 3-5 years of service, even when the 2.5 mm wall still has appropriate metal thickness remaining. The thicker wall sacrifices corrosion-erosion life for thermal fatigue life.

Long-Term Performance Comparison Abstract

Performance Metric 1.5mm Sheath 2.5mm Sheath
Erosion-corrosion rate with 1.5 m/s slurry 0.6-1.2 mm/year 0.5-0.9 mm/year (20-30% lower owing to stiffness) 
Time to perforation (Estimated) 1.3 to 2.5 years 2.8 to 5.0 years
Sheath temperature at 6 W/cm² 95-100°C 110-120°C
Scaling rate (calcium sulphate)Moderate (threshold regime) High (accelerated regime) 
Thermal fatigue life (cycles until insulation failure) 4000-7000 cycles 8000-12000 cycles
Typical service life reported for phosphoric plants is2-4 years (scale limited) 3-5 years (thermal fatigue limited)
Best use Plants with descaling capability, batch operationOngoing service with little room for scale
Practical Strategies for Lifetime Extension Beyond Thickness Selection

Sheath thickness alone is not going to resolve the phosphoric acid slurry issue. Three complementary solutions have been demonstrated to extend heater life beyond what would be predicted from thickness alone. 1. Flow management: aligning the heater tubes parallel to the slurry flow instead of perpendicular decreases impact angles and erosion rates by 40-60%. Second, surface hardening: Nitriding or boronizing the surface of 316 stainless steel raises the hardness from 200 HV to 800-1000 HV, greatly reducing erosion rates without increasing thermal resistance. A 1.5 mm tube with a harder surface can be twice as durable as a 2.5 mm tube without tough surface. Third, scale management, with the bulk acid temperature maintained below 75°C or the use of scale inhibitors such as polyacrylates, keeps the sheath surface below the calcium sulphate precipitation threshold so the use of thinner, more thermally efficient walls is possible. Some plants have used 1.2 mm wall heaters with surface hardening and scale inhibition for five or more years with no problems, while untreated 2.5 mm wall heaters failed due to scale-induced overheating.

Conclusion: Performance Difference Explained by Operating Conditions

The long-term performance differential between 1.5 mm and 2.5 mm wall thickness for 316 stainless steel heaters in 10% phosphoric acid crude slurry at 80°C is not a simple benefit for the thicker wall. The 2.5-mm wall gives 1.5 to 2.0 times the erosion-corrosion life in continuous, low-scaling service, but suffers from higher surface temperatures that increase scaling rates and from reduced thermal fatigue life. 1.5mm wall has superior thermal performance, reduced scaling propensity and longer thermal fatigue life, but less metal reserve for erosion-corrosion. For most phosphoric acid plants the optimum thickness is in between these two extremes, around 1.8-2.0 mm, with surface hardening and scale control applied. If a plant with good scale control and continuous operation is forced to pick between the two, the 2.5 mm wall should be selected to maximise erosion-corrosion life. For plants operating in batch mode, with frequent thermal cycling or limited descaling capabilities, a wall thickness of 1.5 mm shall be chosen to avoid scaling and thermal fatigue failures. Surface treatment needs and flow orientation should be a given on the spec, turning a simple thickness selection into a full-fledged slurry handling heater design.