Too Hot to Handle?
The basic, simple design has kept pace with the ever-increasing trend toward higher temperature ranges in processes that involve heat transfer media. Today's hot-oil pumps are self-cooling and require no associated cooling, operating at up to 650oF (343oC). Their heat barriers are designed to save energy and minimize product heat loss. Many offer a Frances vane-type impeller that is positively keyed to the shaft, and they also can be made with a balanced impeller that reduces hydraulic thrust. Low stuffing box pressure is possible due to a double wear-ring construction. Other features that can be found on pumps in the current marketplace include an isolated thrust bearing, a contained cover gasket, an adjustable seal cartridge and a compensator for thermal expansion. Standard materials of construction can include low-cost pressure-retaining parts (nodular cast iron). The pumps can be specified with lip seals or mechanical seals. One back-pullout design utilizes a spacer coupling that can minimize maintenance downtime.
Given the potential hazards of handling thermal fluids, care should be taken to ensure the hot oil pump is installed, operated and maintained correctly. The following issues require strict attention and will ensure many successful and safe operating hours:
Foundation and Alignment. Fabricated common base plates that have sufficient foundation bolts, with provision for grouting later, should be used. (Refer to Hydraulic Institute standards.) The service life of the pump motor set is dependent on the correct alignment of the solid couplings. Following elimination of pipe work stresses to the pump casing, a cold alignment check should be undertaken. Recommended total dial-indicator readings (TIR) should be 0.002" max. Following several hours of normal operation at maximum temperature, a hot alignment check should be performed as there could be structure and pump thermal growth. TIR should be maintained at 0.002", and caution should be exercised during this operation.
Piping -- General. Pipe work must be supported adequately and independently to eliminate forces being exerted on the pump flanges. It is recommended that reference be made to the Hydraulic Institute standards. Flexible pipe work connections may be considered, thereby reducing the potential transmission of pipe stresses to the pump bearings and seals.
Suction Pipe Work. Suction pipe work must be kept as short as possible, and it must be of adequate size to minimize friction loss (flow velocity of 6 to10 ft/sec) but of sufficient straight length prior to the suction flange (six to eight times the suction diameter). Ideally, a suction strainer (typically 32 mesh) should be placed temporarily in the line for startup but removed prior to commissioning.
Discharge Piping. Flow control and check valves should be placed in the line. Flow control should never be undertaken on the suction line.
Auxiliary Pressure Connections. To continuously check the pump performance, pressure taps with isolating valves should be placed in the suction and discharge lines as close as practical to the pump.
Drives and Guards. When sizing the motor, consider the maximum liquid viscosity, specific gravity and pump operating range. TEFC motors are required for this equipment, with the fan draft directed toward the pump, to extend the life of the bearing and shaft seals. Open-type coupling guards are required for high temeprature pump/motor installations.
Venting. The radial bearing, lip seals and mechanical seal rely on product lubrication and cooling. Venting of the stuffing box prior to startup is important to avoid premature failure.
Operation. Startup procedures on common pumps are routine; however, the hot oil pumps require certain checks following installation and commissioning such as:
- Re-vent the stuffing box.
- Ensure the required discharge head is being achieved.
- Check that the bearing temperature is not operating above 212oF (100oC).
- Check that the airflow from the TEFC motor is not impeded.
- Check that the pump/motor set is free from debris, which may reduce cooling and increase the surrounding temperature (140oF [60oC] max.).
- Re-check the motor/pump hot alignment.
- Grout in the common base plate.
If you perform these checks routinely, you can increase the pump's service life.