Q. What is the maximum pumped liquid temperature that an end suction centrifugal pump can handle, and what must be done to a pump for it to operate successfully at that temperature?A. Limits are placed on pumped liquid temperatures because of heat that travels from the pump casing through the shaft, motor adaptor or bearing frame. This heat raises the bearing lubricant temperature and can adversely affect internal bearing clearances due to differential expansion. For pumps using close-coupled motors, the motor winding temperature will increase.
Bearing lubricant temperatures above 80-deg C (176-deg F) can cause the lubricants to oxidize and lose their lubricating ability. The degradation of the lubricant will shorten bearing life. It is possible with special bearings and synthetic lubricants to operate above the 80-deg C (176-deg F) limit.
It is also possible to control pump lubricant and bearing temperatures by external cooling with either a cool liquid passed through a finned tube immersed in the bearing lubricant or through passageways designed into the bearing frame, seal chamber or stuffing-box cover. The use of fins on the bearing frame exterior, with air blown over the bearing frame by a fan, is also an effective cooling method.
Temperature limits are also imposed by the materials of construction of the pump. For example, cast gray iron is limited to 175-deg C (350-deg F) due to its mechanical strength, whereas ductile iron (with cooling) has a higher limit of 340-deg C (650-deg F).
For high temperatures (greater than 175-deg C [350-deg F]), flexibly coupled arrangements with centerline mounting of the pump casing is beneficial. This arrangement eliminates the possibility of thermal growth of the casing (in the vertical plane) and thereby minimizes the impact of thermal growth on pump/driver alignment.
Many factors, including pumped liquid temperature, ambient conditions, speed, bearing type, lubrication method, method of sealing, pump design and cooling methods influence the final bearing lubricant temperature. The guidelines in Tables 1.3.6.12a and 1.3.6.12b are based on general experience and are commonly adopted in the pump industry. For temperatures beyond these limits, consult the pump manufacturer. Deviations can be justified based on special design, testing and field experience.
For additional information, refer to the latest edition of ANSI/HI 1.3 Rotodynamic (Centrifugal) Pumps for Design and Application.
Table 1.3.6.12a. Guidelines for minimum and maximum liquid temperature for gray iron, ductile iron, carbon steel, chrome steel, austenitic stainless and duplex stainless steel pumps (°C)
NOTE: The maximum temperatures cited may be higher than the limits imposed by some user specifications. Materials selected for such applications must also be evaluated to match the requirements of the end user.
| Flexibly Coupled Pumps | Close-Coupled Pumps | ||||
| Maximum Temperature | Maximum Temperature | ||||
| Material | Minimum Temperaturea | Without Cooling | With Coolingb,c,d | Without Cooling | With Coolingc |
| Gray Cast Iron | 30 | 175 | 175 | 120 | 175b |
| Ductile Iron | -30 | 175 | 340 | 120 | 175-340b |
| Carbon Steel | -30 | 120 | 425 | 100 | 380 |
| Chrome Steel | -100 | 120 | 425 | 100 | 380 |
| Austenitic | -196 | 120 | 370 | 100 | 380 |
| Duplex | -30 | 120 | 260 | 100 | 260 |
a Minimum temperature depends on pump configuration, sealing arrangement and proven low-temperature ductility of the case material.
b Cooling is generally applied to the bearing assembly to prevent overheating of the lubricating oil.
c Cooling is also applied to the process fluid to prevent flashing at seal faces (flexibly coupled) or other areas of heat load, such as motor windings (close-coupled).
d Recommendations for cooling vary with mechanical seal selection and seal flush piping arrangements.
Table 1.3.6.12b -- Guidelines for minimum and maximum liquid temperature for gray iron, ductile iron, carbon steel, chrome steel, austenitic stainless and duplex stainless steel pumps (°F)
NOTE: The maximum temperatures cited may be higher than the limits imposed by some user specifications. Materials selected for such applications must also be evaluated to match the requirements of the end user.
| Flexibly Coupled Pump | Close-Coupled Pumps | ||||
| Maximum Temperature | Maximum Temperature | ||||
| Material | Minimum Temperaturea | Without Cooling | With Coolingb,c,d | Without Cooling | With Coolingc |
| Gray Cast Iron | -20 | 350 | 350 | 250 | 350b |
| Ductile Iron | -20 | 350 | 650 | 250 | 350-650b |
| Carbon Steel | -20 | 250 | 800 | 212 | 715 |
| Chrome Steel | -150 | 250 | 800 | 212 | 715 |
| Austenitic | -320 | 250 | 700 | 212 | 715 |
| Duplex | -20 | 250 | 500 | 212 | 500 |
a Minimum temperature depends on pump configuration, sealing arrangement and proven low-temperature ductility of the case material.
b Cooling is generally applied to the bearing assembly to prevent overheating of the lubricating oil.
c Cooling is also applied to the process fluid to prevent flashing at seal faces (flexibly coupled) or other areas of heat load, such as motor windings (close-coupled).
d Recommendations for cooling vary with mechanical seal selection and seal flush piping arrangements.
Q. What is the effect of higher elevation above sea level on the NPSH required by a pump? For example, does a pump require more NPSH in Denver than at sea level? If so, is there a simple way of correcting for this?
A. The elevation of a pump installation does not affect NPSH required (NPSHR) by a pump; however, elevation does have an effect on NPSH available (NPSHA). NPSHA is a function of the absolute pressure and the vapor pressure. In an open system, the absolute pressure is a function of the atmospheric pressure, and the vapor pressure varies with the temperature.
When atmospheric pressure is expressed in meters (feet) of water, it has been found that atmospheric pressure is reduced by about 1-m per 1000-m (1-ft per 1000-ft) elevation change. When comparing the NPSHA of two similar installations with the suction open to the atmosphere, the NPSHA at 1524-m (5,000-ft) will be 1.524-m (5 ft) lower than that at sea level. If this reduced NPSHA is still greater than the NPSH required by the pump, the pump will still operate satisfactorily. However, if the NPSHA is lower than the NPSHR by the pump, the system will have to be changed by increasing the pressure at pump suction nozzle or by reducing the suction line loses.
Q. What is water hammer? What causes it? Is it damaging to the pump or system?
A. Water hammer, or hydraulic shock, is a condition that occurs when a column of water suddenly changes velocity. This condition can exist when the power to the driver is lost suddenly, air is completely expelled from the system piping, a valve is closed too quickly or a check valve closes too slowly and allows backflow to occur, which causes the check valve to slam shut. When this occurs, the kinetic energy of the liquid is rapidly transformed into pressure energy, which will cause a sharp rise above normal system pressure. This transformation produces an acoustic pressure wave that propagates upstream within the pipe. The resulting peak pressure can be multiple (10 or more) times higher than normal working pressure. Pressure-containing components will fail if this is not mitigated by some protective device.
It is recommended that specialized engineering services be engaged for suchcalculations, since few pump users or pump manufacturers have the knowledge and experience necessary for this analysis.
Pumps & Systems, August 2008