BLOG | Pumping high temperature liquids

Pumping liquids at a high temperature requires extra attention and you need to be aware of all the issues that are involved. High liquid temperature changes the characteristics of the pump, but you also need to consider other factors such as the expansion of materials and operational safety. In this blog we aim to explain this in more detail and focus on the pumping of hot water.

How do we prevent hot water from boiling?

Centrifugal pumps can pump liquid, but not vapor. Under normal conditions (at atmospheric pressure) water boils at 100 degrees Celsius, so how do we make sure water stays in a liquid form above 100 degrees Celsius? The vapor pressure for water graph below shows the phase diagram of water, in which we see a horizontal axis that represents the temperature and a vertical axis that represents the pressure. It shows that if we reduce the pressure enough we can boil water at temperatures below 100 degrees Celsius. However, if we choose to increase the pressure we can make sure water stays in its liquid form, even when the temperature is over 100 degrees Celsius.


Conclusion, by directing the water into the pump under sufficient pre-pressure we can keep it in liquid form and the pump can function without the occurrence of cavitation.

The pump NPSHr characteristic changes at higher temperatures

Providing pre-pressure is not only necessary at an average temperature above 100 degrees Celsius. Above an average temperature of 40 degrees Celsius, the suction height (if applicable) should be seriously considered. At temperatures above 80 degrees Celsius, suction from a lower level is practically impossible and pre-pressure will be required.

Centrifugal pumps are tested with water at 20 degrees Celsius. The pump manufacturer creates the pump performance curve charts based on this temperature. One of those curves is the NPSHr curve, which enables you to determine the available suction head of the relevant pump. However, if you start pumping hot water, the NPSHr curve will change. Simply put, it moves disproportionately with the temperature of the fluid.

With the graph correction NPSHr curve you can easily determine how the standard pump curve changes with water temperatures.

Pump materials

Centrifugal pumps are available in different materials such as cast iron, bronze, stainless steel, etc. The materials are selected depending on the medium to be pumped. Each material has its own coefficient of expansion. The coefficient of expansion is a material property that is indicative of the extent to which a material expands upon heating. When constructing a centrifugal pump, the coefficient of expansion of the selected materials must be considered, otherwise damage may occur, for example in the case of narrow clearances. Also consider the flange connections in the suction and discharge pipes, these are also subject to high temperatures and therefore expansion.

In addition, bearings and gaskets are mounted in a pump, these must be resistant to the average temperature. Pumps that are used for high temperatures are often equipped with a bearing frame/flexible coupling and not close-coupled, this is because the electric motor can potentially overheat due to its proximity to the heat source. For this reason, the use of standard submersible pumps at an average temperature above 45 degrees Celsius is excluded, as the electric motor will overheat and fail. Bearing frames must also be fitted with oil-lubricated bearings because the viscosity of grease reduces at higher temperature and then flows out of the bearing, in which case any lubricating effect is lost.

Many centrifugal pumps are equipped with rubber (NBR) or paper gaskets, however, these materials are unsuitable for temperatures above 120 degrees Celsius. At temperatures above 120 degrees Celsius it is advisable to use PTFE materials.

The mechanical seal also needs extra attention at high temperatures, pay particular attention to the elastomers contained in the mechanical seal, these need to be resistant to the high temperatures.

Safety

Naturally, the hot parts of the pump and the piping system must be properly shielded. There are however additional safety aspects that need to be understood. For this reason, we always recommend to carefully read the safety instructions contained in the pump manual.

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.

Written by Michiel Vaags

Another pump failure that we probably meet is the pump running hot or running at abnormal temperature than it should be. There are several potential causes that can lead to this.

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1&#;&#;Pump Running At Small Flow

If a pump is running at a small water flow, it may experience reduced efficiency and potentially overheating. This can occur when the system resistance is too large, or the valve opening is too small, or the pump simply runs low flow under power frequency.

&#;&#;Solution:

1-Check the reason of excessive system resistance and solve it.

2-Open the valve wide to let the water flows larger.

3-Use frequency conversion device to reduce operating frequency and speed.

2&#;&#;Mechanical Friction

When the friction occurs in the rotating parts such as impeller and pump body, rotor and stator, or bearings, it increases the power consumption of the motor and it overheats.

At this situation, the motor surface is hot and there is noise of friction.

&#;&#;Solution:

Repair or replace the components for friction.

3&#;&#;Low Voltage

When the motor is running at lower voltage than rated, the working current will exceed the rated current under full load which causes the motor temperature rise as well as the cable heat.

&#;&#;Solution:

Increase the power voltage to the rated power range.

4&#;&#;Wrong Wiring(Three phase motor)

The three phase motor has mislead the star wiring to delta wiring.

Normally, there is wiring diagram at the terminal box for three phase motor. It is easy to check if the wiring is wrong.

&#;&#;Solution:

If the wiring is wrong, correct it in time.

5&#;&#;Insufficient Cooling

If the pump is not adequately cooled, it will overheat and cause an abnormal temperature rise. This problem may arise when the motor fan is being sheltered, the cooling fan is covered with dirts and debris, or the ambient temp. is too high.

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&#;&#;Solution:

1-Adjust the installation to ensure sufficient air flow

2-Clean up the cooling fan.

3-Decrease the ambient temperature

6&#;&#;Overloading:

If the pump handles more flow or pressure than it is designed for, it can cause the temperature to rise. This can be due to a blockage in the pump or system, or the valve opens too large.

&#;&#;Solution:

Adjust the valve opening gradually until the linear current is smaller than the rated one.

7&#;&#;Pump Cycles (Pressure problem)

If the pump&#;s setting pressure is not suitable, it will get frequenct start and stop.

The pump stops quickly after it just started, and when the pressure drops to the start pressure, it will start again quickly. After cycles, the motor gets overheat.

&#;&#;Solution:

Adjust the value of stop pressure, ensure:

-System pressure+ Pump Working Pressure&#;Stop Pressure

-System pressure+ Pump Max. Pressure&#;Stop Pressure

8&#;&#;Pump Cycles (System Leakage)

When the system does not use water, the pressure drops after the water pump stops, and the water pump starts again. This is caused by system leakage when the pressure cannot be maintained after pump stops, and the pump starts after the pressure is lost.

&#;&#;Solution:

1-Check the leaks and fix it.

2-Adjust the frequency converter parameters to reduce the pump pressure, and delay the pump start interval.

Those are commonly-reasons that lead to a pump running at abnormal temperature. It is important to identify and address the cause and check it out before getting larger damage and loss.

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