How does temperature affect the selection of centrifugal pumps?

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First, two concepts need to be understood: temperature and absolute zero. Temperature is a measure of the amount of heat present in a gas, liquid, or solid. The common temperature scales that everyone is familiar with are Fahrenheit and Celsius, two systems invented in the 1700s. These two systems differ in important aspects: 1) The freezing point of water is 0 °C or 32 °F. 2) The boiling point of water is 100 °C or 212 °F. Absolute zero is defined as the lowest possible temperature. This is the point in a substance where the atoms are completely at rest, and no thermal energy is transferred. Temperature measurements relative to absolute zero are expressed in kelvins (K) in the Celsius system (0 K = -273.15 °C), and in Rankines (°R) in the Fahrenheit system (0 °R = 459.67 °F). Please note that Kelvin is a ‘degreeless’ unit of measurement. Secondly, it is necessary to understand the relationship between temperature and pressure. Temperature not only affects the stability and efficiency of pump operation but also influences the system pressure. When the temperature of a gas approaches absolute zero, it turns into a liquid. As the temperature continues to rise, the pressure of the gas also increases. If a cryogenic fluid exceeds its boiling point, it will turn into a gas, causing serious pressure and sealing problems. Since different media generate pressure in different ways, a system must be specifically designed based on the properties of the medium to be pumped. If this factor is not taken into account, failures may occur, resulting in significant loss of business time, money, and production resources. In addition, pay special attention to the impact of the user’s on-site ambient temperature. When the pump is installed outdoors or permanently placed in an outdoor area, and temperatures drop significantly at night, the liquid inside the pump may freeze. Problems may arise when these pumps are restarted – damage or wear of components. Therefore, a thermal insulation jacket or a complete insulation layer may be required to insulate or heat the pump. Alternatively, the pump can be mounted on a sled or trolley to allow the operator to move it indoors. The expected temperature range that the pump must handle during operation is crucial for making the right selection; it affects not only the type and design of the pump but also the choice of its components. Although temperature seems to be a familiar concept, in industrial applications it can complicate operations or reduce the reliability of equipment. When selecting a pump, both the ambient temperature at the site and the temperature of the fluid to be pumped must be taken into consideration. 1. Temperature affects centrifugal pumps in the following ways: 1) Impact on structural materials: If certain chemicals are being pumped, corrosion may occur as a result of temperature changes. Compared to liquids at lower temperatures, hot boiling liquids have a greater corrosive effect on materials. Therefore, it is important to check chemical compatibility at the pumping temperature. At extremely low temperatures (such as -150 °C and below), many materials, such as standard carbon steel, become brittle; therefore, systems for pumping LNG or other cryogenic liquids must use specialized component materials. By adding certain elements or forging metals with specific grain structures, it is possible to create parts that can withstand such extreme conditions. 2) Pump components: For packing seals, as the temperature rises, heat is transferred from the seal packing gland to the bearings via the shaft. This will lead to a significant reduction in the bearing’s lifespan, and may even cause the bearing to get stuck. 3) Fluid viscosity: Liquids affected by temperature experience changes in viscosity during pumping. For example, honey becomes thinner when heated, which changes the way it is pumped. Therefore, understanding the effect of temperature changes during pumping on product viscosity will ensure the selection of the correct pump (type and design). 4) Part expansion: At high temperatures, metal parts expand at different rates. This is particularly important during the selection of a pump, and it will affect the structural materials you choose. 5) Insulation or maintaining low temperatures: In some cases, your application may require maintaining a certain temperature. This can be achieved by maintaining flow conditions or using insulation jackets/overall insulation. 2. Temperature affects: 1) the structure of the pump. For example: when the medium temperature exceeds 160 °C, use a pump supported by the pump feet as the center line ； When the medium temperature exceeds 200 °C, a radially split-type structure pump should be used ； When the medium temperature reaches around 400°C and the pumping pressure is not very high, such as in the bottom slurry pumps used in the petrochemical industry, OH2-type pumps are typically chosen in order to minimize potential leakage points ； When the medium temperature reaches around 400°C, and although the pressure is not very high, single-stage or two-stage pumps are insufficient to meet the requirements (such as in the feed pumps for radiant furnaces in the petrochemical industry), BB5 pumps are typically chosen to ensure safety ； When transporting low-temperature media, it is recommended to use the VS6 type vertical cartridge pump, as it facilitates heat retention ； When the viscosity of the medium is high at room temperature, or when it tends to solidify or crystallize easily, it is necessary to install a thermal insulation jacket or provide overall heating/insulation for the pump ； When the viscosity is too high at the pumping temperature, centrifugal pumps are generally not suitable. 2) Material selection. For example: when the pumped medium is at sub-zero temperatures but not extremely low, low-temperature carbon steel or austenitic stainless steel materials are typically used ； When the pumped medium is at sub-zero temperatures and relatively low, austenitic stainless steel materials are typically used ； For high-temperature slurry containing catalyst particles, an OH2-type pump with a lining is used, with the lining and impeller made of wear-resistant cast iron ； When the pumped medium is a high-temperature slurry without catalyst particles, an OH2-type pump with a lining is selected, using CA-6NM material for both the lining and the impeller. 3) Selection of mechanical seals and systems. For example: under normal temperature conditions, Type A seals are usually selected, with a maximum operating temperature of up to 176℃ ； In low-temperature operating conditions, Type B seals are typically selected, with a maximum operating temperature of up to 176℃ ； For high-temperature operating conditions, a C-type seal is usually chosen, with a maximum operating temperature of up to 400℃ ； For pumps whose operating temperature is greater than or equal to the auto-ignition point of the pumped medium, layout methods 2 or 3 are usually chosen ； In high-temperature or low-temperature operating conditions, it is usually necessary to add a PLAN62 seal flushing scheme.