Pumping hot liquids  

There are a number of issues that you must consider when pumping liquids at high temperature. In order to keep the information practical, we focus on the main points of pumping hot water:

• The relationship between pressure and temperature
• Correction of 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. We call this the boiling point, and it depends on the prevailing pressure of the atmosphere above the liquid. At sea level, the atmospheric pressure averages 1 bar. Water will then turn into vapor at a temperature of 100 °C.

If the pressure above the liquid decreases, the boiling point will also decrease. Likewise, at a higher pressure, evaporation will start at a higher temperature. For example, water will evaporate at 120 °C if there is a prevailing pressure of 2 bar.

Pressure is therefore a very important factor when pumping hot liquids. By simply increasing the pressure we can keep the medium in a liquid state. In the vapor pressure for water graph, you can see on the horizontal axis the boiling points of water in relation to the different pressures on the vertical axis.

Correction of the NPSHr pump characteristic

Every centrifugal pump has an NPSHr curve. To put it simply: this allows you to read the pressure that a pump needs on the suction side to run cavitation-free. BBA Pumps tests the NPSHr with water at a temperature of approximately 20 °C. Depending on the capacity, the pump will have a certain corresponding available suction height.

Important to know: at different liquid temperatures, the NPSHr curve changes.
This also has to do with pressure. In practice it comes down to this: the available suction head decreases quickly when pumping water with a temperature above 45 °C. Further, from 80 °C suction liquid becomes virtually impossible. The pump then needs pre-pressure!

With the graph correction NPSHr curve you can easily determine how the standard pump curve changes with water temperatures up to 120 °C.

Tips on the selection of pump materials

Centrifugal pumps are available in a range of materials such as:

• cast iron;
• bronze;
• stainless steel; or
• a combination thereof.

Selection of the most appropriate material depends on the type of liquid to be pumped. Each material has its own coefficient of expansion, at higher temperatures the material volume increases. Expansion can cause damage to pump parts where the clearance is minimal. Cast iron is a suitable material for pumping fresh hot water.

In addition, it is important to check that the pump casing gaskets and the elastomers of the shaft seal can withstand the liquid temperature. Many centrifugal pumps are equipped with nitrile rubber (NBR), which is not suitable for temperatures above 80 °C. However, PTFE is a sealing material that offers very good resistance at high temperatures.

For the bearing frame of the pump, the bearings must be lubricated with oil with the appropriate viscosity. For liquid temperatures above 45 °C the use of standard submersible pumps or direct coupled pumps is not recommended.

Safety

Always carefully read the manual supplied with the pumpset. It describes the safety measures that are important when pumping hot liquids.

When sharing this general information, we have not gone into all of the details of this specific area of pump knowledge. As a result, no rights can be derived from the information about pumping hot liquids. If you have a critical pump installation or questions about other fluids, please contact us.