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 188.8.131.52a and 184.108.40.206b 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 220.127.116.11a. 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|
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 18.104.22.168b -- Guidelines for minimum and maximum liquid temperature for gray iron, ductile iron, carbon steel, chrome steel, austenitic stainless and duplex stainless steel pumps (°F)