Impeller Design Prevents Vapor Lock

Air and gas entrainment in pumped liquids is a long-standing problem for centrifugal pumps. A conventional method of dealing with this issue is using recessed, impeller-style pumps. However, this technology is highly inefficient. Other methods use expensive and environmentally unfriendly defoaming chemicals.

Some facilities use too much defoaming chemical to prevent pumps from vapor locking or to halt flow instabilities. Many operators have problematic pumps that continue to vapor lock. This is a result of too much air entrainment in the process.

Unconventional Solution

One particular centrifugal pump moves liquids containing large amounts of air and other gases. The system can pump liquids containing up to 70 percent gases.

In conventional centrifugal pumping, a liquid's air or gas migrates to the impeller's center. This is because the centrifugal effect imparted by the impeller vanes onto the liquid causes the air or gas to separate from the liquid. The impeller center is an area of low pressure, which leads to gas accumulation. Eventually, the gases increase so much that a bubble forms, preventing the liquid from entering the impeller. The pump essentially vapor locks and no longer performs.

One specially made impeller features teardrop-shaped balancing holes located closer to the center of the impeller, where the gases accumulate. The balancing holes allow gases to pass through the impeller eye. By introducing a lower pressure behind the impeller, the balancing holes allow a removal route for the gases accumulating in the impeller's eye. This concept allows the pump to perform like a normal centrifugal pump while pumping liquids with high gas entrainment. Two different pump models use this design, but each employs a different method to create the lower pressure behind the impeller.

Pump Designs

The first pump design has an external liquid ring vacuum pump mounted next to it. This model has a unique stuffing box arrangement consisting primarily of an expeller inside the stuffing box located behind the impeller, a degas pipe and a recirculation pipe. The vacuum hose from the external vacuum pump connects to the centrifugal pump's degas pipe, which is attached to the stuffing box (see Figure 1). This creates lower pressure behind the impeller, inducing gases to migrate through the balancing holes and into the degas zone, which is the stuffing box area in conventional pumps.

Figure 1. An external vacuum pump is attached to the centrifugal pump. (Images and graphics courtesy of Fluid Process Equipment

The expeller mounted to the shaft inside the stuffing box separates any liquid carryover from the gases. The expeller's centrifugal force moves any liquid carryover into the recirculation pipe. The recirculation pipe is connected to the peripheral portion of the stuffing box where liquid accumulates by the expeller's centrifugal force. The recirculation pipe is routed back towards the centrifugal pump's suction inlet where the liquid carryover returns to the process.

As the gases migrate from the impeller and into the expeller chamber, the expeller's centrifugal force separates the liquid from the gases. Any gases will accumulate in the eye of the expeller just as they did in the eye of the impeller. This is where the removal of the gases takes place. The degas port extends deep inside the pump to the expeller's eye connecting the external vacuum hose to the degas pipe and pulls gases out of the pump and away from the expeller. The degas pipe and recirculation pipe are on opposite sides of the centrifugal pump.

The external vacuum pump discharges its vacuumed gases from the centrifugal pump back into the supply tank. This allows the gas to dissipate. Any liquid carryover goes back into the process.

The pump models that use the special impeller were designed in the 1980s in Karhula, Finland. The design facility has a test stand with a clear tank and a modified pump system with a see-through volute. An air hose is inserted into the tank and discharges near the suction flange, dispersing air in a manner that simulates actual process conditions.

With the degas vacuum pump off, the centrifugal pump is turned on, and it pumps room-temperature water like any centrifugal pump. However, the demonstration begins when air is pushed inside the suction flange. The volume of air gradually increases until the pump vapor locks and stops pumping. The external degas pump is then turned on, and the pump begins working normally. The demonstration continues with air pumped inside the centrifugal pump and the external degas pump continuing to run. The centrifugal pump quickly removes the air bubble from the impeller eye and returns to normal performance.

Comparing Pump Models

The second centrifugal pump model uses the same volute and impeller as the first, but the stuffing box chamber is different. The second model uses a degas rotor instead of an expeller (see Image 1). The degas rotor is similar to the rotor used in a vacuum pump. In fact, the pumps incorporate a miniature vacuum pump inside the stuffing box behind the impeller.

Image 1. Degassing rotor found inside internal degassing pump type 2

The second model has limitations when compared with the first. It can withstand a maximum of 11.5 feet of inlet pressure on the suction flange.

This limitation is governed by the degas rotor capabilities. The pressure inside the degas rotor chamber is directly related to the amount of suction pressure at the suction flange.

The degas rotor cannot perform its degas function if the suction flange is experiencing too much liquid suction inlet pressure. The first model described does not have this limitation.

The second model pump requires less space because the vacuum pump is built into the stuffing box. Both systems can pump from tanks either above or below the pump centerline.

Both designs conform to International Organization for Standardization 5199. Neither pump is of American National Standards Institute design. The volute, impeller, expeller and stuffing box are made from duplex stainless steel grade American Society for Testing and Materials (ASTM) A890 Grade 3A. This metallurgy is superior to 316 stainless steel in both corrosion and abrasion resistance. A890 Grade 3A's closest equivalent in the U.S. and Canada is CD4MCu.

The volute, bearing unit and adapter are the same as those used on a similar process pump. Both models use a single mechanical seal.

These pumping systems cost more than conventional centrifugal pumps of an equivalent size. However, they offer value allowing air- or gas-laden fluids to be predictably and reliably pumped in situations where conventional pumps fail.