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Priming Loss Prevention

Once obtained, priming must be maintained during pump operation. Failing to do so will cause disruptions in the process continuity. The two main causes of priming loss are back-flow and 2-phase mixtures.

Back-flow

Back-flow happens when the pump is places above the surface level of the fluid in the upstream tank (as in the schematic in Figure 1). In this example, as soon as the pump stops, the fluid in the tubes will flow back into the upstream tank. The whole line and the pump will fill with air, requiring a new active priming procedure at the next start-up.

Figure 1: In this system, the pump and suction line will empty as soon as the pump stops, causing loss of prime.

To avoid this problem, a check-valve can be installed in series with the pump (as shown in Figure 2). This type of valve allows the fluid to flow in one way only, blocking the back-flow.

Figure 2: A check-valve in series with the pump will prevent the line from emptying. This way, at the next startup, the pump will already be primed.

2-Phase mixtures

It can happen that the process fluid contains a gaseous phase (such as in the case of foams, highly turbulent tanks, or out-gassing fluids).

For example, in most bio-reactors there is a sparger (see Figure 3) that releases air bubbles to provide oxygen to the cell culture. If these bubbles get sucked into the suction line, they can accumulate in the pump head or in an air trap.

Figure 3: Sparger in a bio-reactor, used to diffuse air in the cell culture to feed the bacteria (source: CerCell)

In semiconductor manufacturing, some etching and cleaning solutions can emit gas due to chemical reactions (such as mixed sulfuric acid and hydrogen peroxide) or high process temperatures (e.g., sulfuric or phosphoric acid in etch applications). Severe out-gassing can cause impeller instability and priming loss.

In re-circulation tanks, it can happen that strong turbulence and splashing lead to the generation of bubbles. If the pump sucks the bubbles in, it could lose priming.

To mitigate the risk of priming loss, Levitronix offers special pump designs featuring

long blades impellers
pump heads with suction port

which are more stable and resistant against 2-phase mixtures. These features are available for most pump sizes, and they can also be used in combination.

In addition to dedicated pump designs, Levitronix recommends hydraulic systems which allow large flow rates to flush the bubbles out of the pump. For example, a less restricted hydraulic circuit or re-circulation bypass lines can help to increase the flow-rate.

Long-Blade Impeller

In 2-phase-flows, the presence of gas reduces the average fluid density, and therefore the pump generates a lower pressure as well. The long blades extend the diameter of the impeller. This causes an increase in the pressure and flow-rate that the impeller can deliver. Therefore, the long blades help to compensate for the pressure loss due to lower density, and the impeller is able to deliver sufficient flow to flush the gas bubbles away.

Figure 4: standard impeller, the blades do not extend beyond the shroud

Figure 5: impeller with long blades. Since they extend the impeller diameter, the pressure generated is higher and compensates for the loss of pressure due to reduced average fluid density.

Pump Head Suction Port

Certain Levitronix semiconductor pumps are available with a suction port feature.

The suction port can be used to extract the gas phase from the pump by connecting it to a Venturi nozzle aspirator (installation as shown in Figure 6). As a result, the pump head will contain mostly liquid and will maintain priming.

Figure 6: Pump head with suction port, installed together with an aspirator (source: Entegris) to remove the gaseous component from the out-gassing process fluid

Re-circulation Bypass

High flow-rates are able to flush the bubbles away, by minimizing their accumulation and therefore preventing loss of priming.

To obtain such larger flow rates, it is convenient to use oversized tubes (lower system load) and pump (a larger pump can easily provide larger flow rates). Since the process requires a specific flow rate, the extra flow rate supplied by the pump can simply be recirculated back to the upstream tank with a bypass (in Figure 7 it is shown the hydraulic setup before and after the upgrade to increase the flow rate running through the pump).

Figure 7: by using a larger pump and oversized tubes, less bubbles will accumulate in the pump, preserving the priming. The extra flow rate can be recirculated into the upstream tank.

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