Solar Thermal Molten Salt Pump: CSP Heat Transfer, Thermal Storage, and Steam Generation

Concentrated Solar Power (CSP) plants with thermal storage use molten salt (typically 60% NaNO₃ + 40% KNO₃, "solar salt") as heat transfer and storage medium. Salt is heated to 565°C in the solar receiver, stored in hot tank, pumped through steam generator to produce electricity, and returned to cold tank at 290°C. Pumps handle molten salt at 290–565°C (extreme temperature, corrosive when wet, solidifies below 220°C), thermal oil (for some CSP designs), and steam/water. This guide covers pump selection for CSP molten salt systems.

Process and Pump Services

Critical Service: Molten Salt at 565°C (Extreme Temperature)

Molten salt at 565°C is near the upper limit of common pump materials. The salt itself is not highly corrosive when dry and pure, but any moisture (from air ingress or seal water) causes severe corrosion (nitrate salt + water → nitric acid + metal corrosion). Salt solidifies at ~220°C — any cold spot causes plugging:

• Material: Hot salt (500–565°C) requires austenitic stainless steel 316H (high carbon for high-temp strength), 347H, or nickel alloys (Inconel 600, 625, Hastelloy X). 316L (low carbon) loses strength above 450°C — use 316H or 347H. Cold salt (290°C) can use 316SS or 347SS. NO carbon steel (severe oxidation + salt corrosion above 400°C).
• Thermal expansion: Pump shaft, casing, and piping expand significantly at 565°C (thermal expansion coefficient ~17×10⁻⁶/°C → 500°C rise = 8.5 mm/m expansion). Pump design must accommodate thermal growth — vertical long-shaft pumps with flexible couplings and thermal growth compensation. Horizontal pumps require expansion joints in piping.
• Salt freezing: All wetted parts and piping must be heat-traced or jacketed to maintain >250°C (above freezing point 220°C with margin). Any cold spot (valve, flange, pump casing) causes salt to solidify and plug. Pump must have jacketing or heat tracing. Startup: preheat pump to >250°C before introducing salt.
• Seal: Mechanical seals at 565°C are extremely challenging. Most CSP molten salt pumps use vertical long-shaft turbine pumps installed in the salt tank (sump configuration) — no shaft seal below liquid level, shaft extends above tank with atmospheric seal (gas seal or packing). This avoids the need for a high-temp mechanical seal in salt service. For horizontal pumps, use double mechanical seal with Plan 53B (barrier fluid — cooled oil) and jacketing.
• Bearing: Bearings above salt level (in vertical pumps) are lubricated with oil or grease rated for high ambient temp. Sleeve bearings in salt (if submerged) use salt-lubricated carbon or graphite bearings (salt is a lubricant at high temp).

Critical Service: Vertical Long-Shaft Sump Pump (Tank-Mounted)

The standard CSP molten salt pump is a vertical long-shaft turbine (VLT) pump installed through the tank roof, with impeller submerged in salt at tank bottom. Shaft length can be 10–30 m (tank depth):

• Advantages: No shaft seal in salt (seal is above liquid at atmospheric pressure). Self-draining (when pump stops, salt drains back to tank — no freezing in pump). Easy maintenance (motor and bearing assembly at tank top, above salt). Handles high flow (1000–10,000 m³/h) and high head (100–300 m for receiver feed).
• Design: Multi-stage vertical turbine (1–10 stages) with open or enclosed impellers. Column pipe with shaft inside. Line shaft bearings (carbon/graphite, salt-lubricated). Motor at top (above tank, ambient temp). Thermal expansion accommodated by flexible coupling and column design. Jacketed column (heat tracing) to prevent salt freezing in column during shutdown.
• Material: Wetted parts: 316H/347H for cold salt (290°C), Inconel 625/Hastelloy X for hot salt (565°C). Shaft: 347SS or Inconel 600 (high-temp strength). Bearings: carbon/graphite (salt-lubricated). Motor: standard TEFC (ambient temp at tank top).
• NPSH: Sump pump has flooded suction (tank liquid level above impeller) — NPSH is static head + vapor pressure margin. Tank level must be maintained above minimum NPSH level. Hot salt (565°C) has vapor pressure ~0.1 bar (low — nitrate salts have low vapor pressure), so NPSH is less critical than for water at high temp.

Critical Service: Thermal Oil (Alternative Heat Transfer Fluid)

Some CSP plants (parabolic trough) use thermal oil (e.g., Therminol VP-1, diphenyl/diphenyl oxide mixture) as heat transfer fluid at up to 400°C. Oil is flammable and has low viscosity at high temp:

• Flammability: Thermal oil flash point ~120°C, autoignition ~570°C. At operating temp (300–400°C), any leak causes immediate fire. Zero leakage is mandatory.
• NPSH: Thermal oil at 350°C has vapor pressure ~2–5 bar. Pump must have low NPSHr or be located below oil tank (flooded suction). Standard centrifugal cavitates.
• Pump type: API 610 high-temp centrifugal pump with 316SS/duplex wetted parts. Centerline-mounted (thermal expansion accommodation). Single mechanical seal with Plan 23 (recirculation cooling) or Plan 32 (clean oil flush). Jacketed stuffing box. Magnetic drive for zero leakage (if available for high temp). Explosion-proof motor.
• Design: Centerline support (casing expands symmetrically — prevents misalignment). API 610 BB2 (between bearing) for high temp. Carbon graphite vs. silicon carbide seal faces (high temp). PTFE or flexible graphite gaskets (no paper/composition — burns at high temp).

Material Compatibility

Critical: 316L loses strength above 450°C — use 316H or 347H for hot salt. Duplex forms brittle sigma phase above 300°C — NOT for hot salt. PTFE melts at 327°C — use flexible graphite for high-temp gaskets/seals. Moisture in salt causes severe corrosion — maintain dry nitrogen blanket on tanks.

Common Problems and Solutions

Problem 1: Molten salt freezes in pump (cold spot below 220°C). Salt solidifies at ~220°C — any cold spot (unjacketed valve, flange, pump casing during shutdown) plugs the pump. Solution: jacket/heat-trace all wetted parts and piping (maintain >250°C). Preheat pump to >250°C before startup. Vertical sump pump (self-draining — salt drains back to tank when stopped). Insulate all piping. Temperature monitoring (alarm <250°C). Our vertical sump pump with jacketed column prevents salt freezing.

Problem 2: Hot salt pump material fails (565°C strength loss). 316L at 565°C loses creep strength — shaft/casing deforms over time. Solution: use 316H, 347H, or Inconel 625/Hastelloy X for hot salt service. Material certified for high-temp service (creep rupture data). Stress analysis at operating temp. Thermal expansion accommodation. Our high-temp alloy vertical turbine handles 565°C salt.

Problem 3: Salt corrodes pump (moisture ingress). Dry molten salt is not highly corrosive, but moisture (from air ingress, seal water, rain) causes nitric acid formation and severe corrosion. Solution: maintain dry nitrogen blanket on salt tanks (prevent air/moisture ingress). No water-cooled seals in salt service (use gas seal or dry packing). Vertical sump pump (no seal below liquid). Monitor salt chemistry (water content, pH, corrosion coupons). Our vertical sump pump with gas seal prevents moisture ingress.

Problem 4: Thermal oil pump leaks/catches fire (flammable at 400°C). Thermal oil at 350–400°C autoignites on contact with hot surfaces — any seal leak causes fire. Solution: use API 610 high-temp pump with Plan 23/32 seal (cooled) or magnetic drive (zero leakage). Centerline-mounted (thermal expansion). Explosion-proof motor. Fire detection at pump location. Oil leak detection. Our high-temp API 610 pump with reliable seals handles thermal oil safely.

Problem 5: Vertical pump shaft vibration (long shaft, thermal bow). Long-shaft vertical pumps (10–30 m shaft) can vibrate due to shaft bow (thermal gradient), bearing wear, or misalignment. Solution: precision-aligned shaft coupling. Carbon/graphite line bearings (salt-lubricated, self-lubricating). Thermal growth compensation (flexible coupling). Vibration monitoring (accelerometers on motor and bearing housing). Balanced impellers (hydraulic thrust balance). Our vertical turbine with precision shaft and bearings minimizes vibration.

Selection Summary

1. Cold salt (290°C, tank): Vertical long-shaft turbine (316H/347H, sump, jacketed column)
2. Hot salt (565°C, tank): Vertical long-shaft turbine (Inconel 625/Hastelloy X, sump, jacketed, high-temp)
3. Salt to receiver (high head 100–300m): Vertical turbine multistage or horizontal multistage (high-temp alloy)
4. Salt to steam generator: Vertical turbine (sump) or horizontal (high-temp alloy, high flow)
5. Thermal oil (300–400°C, flammable): API 610 high-temp centrifugal (centerline, Plan 23/32, explosion-proof) or magnetic drive
6. Feedwater (high pressure): Multistage centrifugal (BBTD/ring section, low NPSHr)
7. Condensate: Multistage low-NPSHr or screw pump
8. Cooling water: Split case or vertical turbine (high flow)
9. Salt melt/fill: Small jacketed gear/screw pump (high-temp, batch)
10. Critical: molten salt temp (316H/Inconel, no 316L/duplex at >450°C), freezing (jacketing >250°C, vertical sump self-draining), corrosion (dry N₂ blanket, no water seals), thermal expansion (centerline/vertical, growth compensation), thermal oil safety (zero leakage, explosion-proof, Plan 23/32)
11. Our vertical turbine covers molten salt (sump, high-temp alloy); high-temp API 610 covers thermal oil; multistage covers feedwater; screw pump covers condensate. For CSP molten salt pump selection, talk to our CSP engineers with your salt temp, tank depth, and flow/head requirements.