Minimize flow-through effects of elevated NPSH by controlling diaphragm pump outlet pressure

Multiple-piston diaphragm pumps are often used in buffer dilution and on-demand gradient elution lines feeding chromatography systems in single-use bioprocessing. Diaphragm pumps are well suited for this application and result in accurate buffer concentrations when the system is set up properly. However, if a single-use diaphragm pump has a high net positive suction head (NPSH), excess flow-through may occur, especially at low dosing flow rates, thereby reducing dilution accuracy.

What is NPSH? and how does NPSH affect single-use bioprocessing design?

NPSH is the measure of head pressure available at the pump suction (inlet). If NPSH is too low, cavitation may occur and if NPSH is too high, it may result in unwanted flow-through.

Single-use pumps are used to deliver fluids from a variety of sources, including break bags, header drops, and buffer totes. Sometimes the supply pressure from a header drop can be high, or an elevated tote storage can add several psi to the inlet pressure of these pumps. High feed pressures to most single-use pumps can cause flow-through even when the pump is off. It can also result in less accuracy in the low flow range of the pump. In some system configurations, feed pressures <1 psi can result in this effect, and thus may be a common event in most single use manufacturing settings.

The greater the supply pressure to the pump, the more magnified this “push through” flow phenomenon becomes, limiting pump turndown, decreasing dosing accuracy and ultimately reducing dilution accuracy. High Inlet pressures can also add to the head pressure on a pump, putting the downstream system at risk for bursting.

How to compensate for high NPSH

To prevent high NPSH, the bioprocessing industry has attempted to use stainless steel pressure reducing valves (PRVs) or single-use pinch valves to reduce the pressure of incoming fluids. These two options have led to less than optimal results with pump cavitation still occurring at times. In some instances, there is not yet a clearly defined solution. Some systems may not tolerate a single use pinch valve due to slow response times, leading to no identified solution for single use systems, until now with Equilibar’s SDO product.

Equilibar has discovered that using their patented multiple orifice single-use back pressure regulator (BPR) on the outlet side of a pump is a more effective way of mitigating these problems caused by NPSH. If the BPR is set at a value greater than the supply pressure to the pump (commonly called NPSHa), the pump will operate with the right differential pressure to drive flow and the flow-through problem will resolve. The Equilibar BPR is dome-loaded and controlled by a secondary standard pressure regulator called a pilot regulator. The BPR can be controlled manually or electronically, depending on the application requirements.

Application Highlight – Using Equilibar SDO to control NPSH in buffer delivery

In the example below, an Equilibar SDO single use back pressure regulator is positioned after the buffer delivery pumps and inline mixer to help control differential pressure in the diaphragm pumps.

Positioning the SDO back pressure regulator downstream of the diaphragm pumps and static mixer helps several key aspects of the gradient elution system performance.

• Firstly, the NPSH push-through flow issue can be avoided by setting the upstream pressure of the SDO valve greater than the incoming pump suction pressure. This ensures that the pump’s fluid power can generate flow against a higher head pressure, resulting in proper check valve performance in the diaphragm pump.
• Second, even when the pump is off, the setpoint of the Equilibar SDO regulator will prevent any flow through the system.
• Additionally, the upper and lower flow ranges of the pump will more accurately dose buffer to the column, which should improve performance on the gradient profile and increase precision delivered by this two pump solution.
• The SDO valve is capable of handling any flow range generated by the pump, making it a versatile solution to pair with any pump for manufacturing needs. Read here how an Equilibar valve with extremely wide flow range capabilities can help in sanitary processing.

More benefits to using an Equilibar single-use valve for buffer delivery in bioprocessing

Beyond NPSH issues, the SDO valve also helps dampen pump pressure fluctuation which often arises with positive displacement pumps used in buffer delivery. Dampening pulsations will stabilize pump flow performance and help improve flow meter accuracy due to a normalized pressure environment.

Furthermore, any bubbles that may be present from dissolved gasses or cavitation in the pump will be suppressed by constant pressure control via the SDO valve. This bubble prevention helps improve flow measurement accuracy, which also improves the gradient profile long-term.

These added benefits of better flow stability, increased pump performance, pulsation dampening, and flow meter accuracy create a more stable, precise system. This will lead to improvements in system function, increasing process efficiency and product yield during the ion exchange phase in chromatography.

Contact an Equilibar application engineer for more information.