Pumping Hot Liquids

What you need to know when pumping hot water.

There are several considerations when pumping liquids at high temperatures. To keep this information practical, we'll focus on the main points of pumping hot water:

• The relationship between pressure and temperature
• Correcting the NPSHr pump characteristic
• Tips for selecting suitable materials for the pump

The relationship between pressure and temperature

Centrifugal pumps are designed to pump liquid, not vapor. Any liquid will turn into vapor at a certain temperature known as the boiling point, which depends on the prevailing pressure. At sea level, where atmospheric pressure averages 1 bar (33.5 feet), water boils at 100°C (212°F).

When the pressure above the liquid decreases, the boiling point also decreases. Conversely, higher pressure increases the boiling point. For instance, at 2 bar (67 feet) pressure, water will boil at 120°C (248°F).

Pressure is crucial when pumping hot liquids. Increasing pressure keeps the medium in a liquid state. Referencing the vapor pressure for water graph, you'll notice the boiling points of water in relation to different pressures.

Correction of the NPSHr pump characteristic

Every centrifugal pump has an NPSHr curve, indicating the required suction-side pressure to prevent cavitation. BBA Pumps tests the NPSHr with water at around 20°C (68°F). However, at different liquid temperatures, this curve changes significantly.

For instance, the available suction head decreases notably when pumping water above 45°C (113°F). Pumping liquid above 80°C (176°F) becomes virtually impossible without pre-pressure.

The graph correcting the NPSHr curve illustrates the standard pump curve adjustments with water temperatures up to 120°C (248°F).

Tips on selecting pump materials

Centrifugal pumps come in various materials like cast iron, bronze, stainless steel, or combinations thereof. Material selection depends on the liquid to be pumped. Each material has its own expansion coefficient; at higher temperatures, volume increases, potentially causing damage to pump parts with minimal clearance. Cast iron is suitable for pumping fresh hot water.

Furthermore, check that pump casing gaskets and shaft seal elastomers can endure the liquid temperature. Many pumps use nitrile rubber (NBR), unsuitable for temperatures above 80°C (176°F). However, PTFE is a sealing material with excellent high-temperature resistance.

For the pump's bearing frame, ensure bearings are lubricated with oil of appropriate viscosity. Standard submersible pumps or direct-coupled pumps aren't recommended for liquid temperatures above 45°C (113°F).

Safety

Always carefully read the pumpset manual for important safety measures when pumping hot liquids.

This general information doesn't cover all details in this pump knowledge area. Therefore, no rights can be derived from this information regarding pumping hot liquids. For critical pump installations or questions about other fluids, please contact us.