Centrifugal pumps and suction: If the maximum theoretical suction lift for water at 15°C is 34 ft, what would you expect for hot water at 90°C (considering much higher vapor pressure)?

Introduction / Context:
Suction lift for pumps is limited by the available Net Positive Suction Head (NPSH), which depends on atmospheric pressure, static lift, friction losses, and—critically—the liquid’s vapor pressure. As temperature rises, water’s vapor pressure increases sharply, reducing allowable suction lift to avoid cavitation.

Given Data / Assumptions:

• Theoretical maximum at 15°C is 34 ft (based on atmospheric head minus negligible vapor pressure and losses).
• At 90°C, water vapor pressure is high, dramatically reducing NPSH available.
• Neglect line losses and assume sea-level atmospheric pressure for comparison.

Concept / Approach:
Maximum theoretical suction lift ≈ (p_atm/γ) - (p_v/γ). At 15°C, p_v is small; at 90°C, p_v is large, so the difference shrinks. Practically, hot water service demands flooded suction or very short suction lifts to prevent cavitation.

Step-by-Step Solution:
At 15°C: p_v is low ⇒ theoretical lift ~34 ft.At 90°C: p_v is much higher ⇒ the allowable lift drops steeply.Rule-of-thumb and charts place hot water suction lift at only a few feet; 8 ft is a representative upper bound in idealized theory.Therefore, select ~8 ft, reflecting the reduction due to high vapor pressure.

Verification / Alternative check:
Steam tables show water’s vapor pressure near 90°C is substantial (tens of kPa), leaving little headroom under 1 atm for suction. Practical pump manuals recommend minimal or zero lift for hot liquids.

Why Other Options Are Wrong:

• 40 ft/37 ft/34 ft: ignore the significant increase in vapor pressure at 90°C.
• 20 ft: still too high; cavitation risk would be unacceptable.

Common Pitfalls:
Assuming suction lift depends only on atmospheric pressure; neglecting the strong temperature dependence via vapor pressure.

Final Answer:
About 8 ft (significantly reduced)