How to Select a Pump
Pump design variations give users the ability to select the industrial thermal pump that best suits the application.
Hot oil pump applications can be solved with a variety of pump products. Some pump companies have product lines specifically for hot oil processing while other pump product lines have adaptations that allow some hot oil use in limited applications.
You may ask, do I need a hot oil pump? Why can't I use a standard pump? In general, if your process temperature application is below 350oF (177oC), a standard industrial pump will be adequate and the most cost effective. This pump will have cast iron construction and an elastomeric, bellows-style mechanical seal with high-grade carbon graphite and ceramic faces and Viton elastomers. The seal will have a 400oF (204oC) rating when used with lubricating fluids.
The hot oil pump range begins at 350oF and continues to 800oF (427oC). This type of pump has an open seal cavity to keep solids from collecting at the seal faces (figure 1). This open design will make the seal temperature the same as the pumpage.
Pumps that are designed specifically for hot oil use will have seal cavity designs that take the seal out of the heat-affected zones. Seals can be adequately cooled up to 400oF below the process temperature. Cooling the seal through pump design allows a lower cost pump/seal package; the alternatives may require a high temperature metal bellows seal or cartridge double-seal pumps with cool flush loops.
Obvious selection criteria limit the pump choices to material issues where the temperature limits are the most dominant. The seal materials must be similar to the standard industrial pump. Pressure-containing components can be iron, ductile iron, carbon steel or stainless steel. Fiber gaskets are not good to use above 550oF (288oC). Graphite foil gaskets are the best for temperatures above 500oF (260oC). Other secondary considerations are pressure requirements, fluid compatibility and safety considerations.
This article will look at three centrifugal pump types of construction that are sold specifically for hot oil pumping, concentrating on the mechanical seal design and cooling aspects. All pumps used for hot oil pumping should have enclosed impellers and materials of construction to handle the temperatures required for the application.
In jacket water-cooled designs, the seal cavity is cooled with a constant flow of water that is usually treated and cooled by an external process (figure 2). The water passage is cast into the seal plate, or inserts are installed into the seal plate to identify the passage. Many water-cooled designs are adaptations of existing industrial pump product lines. This design can be effective at maintaining the lowest seal working temperatures possible if a good source of coolant is maintained and the outlet temperature is well below the vaporization temperature. Calcium carbonate or other contaminants may buildup around the walls of the cavity and reduce the heat transfer and cooling capability, resulting in higher seal temperatures.
All water-cooled pumps have a risk of oil/coolant cross-contamination. This can be potentially dangerous. Wherever vaporization occurs, the steam generated may cause high internal pressures and catastrophic failure. Operating costs from the coolant or water and the energy for cooling are higher than air-cooled designs. The coolant must be cooled or treated before disposal where a cooling processing system is required.
Displacement air-cooled designs have an axial length distance to reduce heat conduction into the seal area (figure 3). This type of design takes the mechanical seal out of the high temperature area close to the hot process oil. Simply moving the seal out of the high heat path is effective and reliable.
If the seal starts to leak a small-to-moderate amount, seal temperatures may rise quickly due to the inability to keep the seal environment cool, and catastrophic seal failure will be sudden. An annular orifice between the pump and seal cavity to reduce this flow should be designed into the pump.
This design is limited to long-coupling pump-and-motor configurations, and most of the pumps have a wet sleeve bearing that is a wear maintenance item.
Seals in this type of pump can be a stack of lip seals or a standard bellows-style seal similar to any industrial pump. Seal materials of choice are carbon, silicon carbide or Viton.
The isolation air-cooled design has all of the benefits from the displacement type and more (figure 4). Isolation reduces the thermal conductivity into the seal area by reducing the heat flux from the pump to the seal environment. This is designed into the pump by using hollow-shaft sleeves and carbon graphite materials that have thermal conductivity values (k value) lower than most metals. Also, the seal environment can be cooled by external airflow to remove additional heat. Heat transfer into the seal cavity is minimized. Process temperatures of 600oF (316oC) with seal temperatures less than 300oF (149oC) are possible.
The result is reduced axial shaft length that allows close coupling to most electric motors. A hot sleeve bearing is not present within the pump to wear out. A close-coupled design takes less space and eliminates shaft-coupling maintenance. Seals are mechanical with the same materials: carbon, silicon carbide or Viton.