Bronkhorst

Blog series: How to handle low liquid flows? Part 4

April 21, 2020 Bart de Jong

Part 4: Liquid supply using a pump

In the world of flow control and measurement, we distinguish between ‘low flows’ and ‘high flows’. But what does this really mean? At Bronkhorst, we supply flow meters and controllers in the ‘low flow’ range. Do you know what ‘low flow’ means? In our blog series, we explain the difference and share our tips & tricks for low liquid flow setups. 
 

Flow meter in combination with a pump

Liquid supply – using a pump

To ensure a flow controller has a stable (ultra-)low liquid flow rate, a highly stable inlet pressure is a prerequisite. As you read in Part 3 of our blog series, the presence of dissolved gas in the liquid is the main issue to overcome when using a pressure vessel to generate a stable low liquid flow. If prevention of gas dissolution in the liquid is essential, applying a pump to generate the inlet pressure will be a good choice. And that’s what this fourth part of the low-flow blog series deals with.
 

Coriolis flow meter with pump
Coriolis flow meter with pump

A pump is a device that can provide a continuous, reliable and stable pressure or flow. It will not introduce dissolved gas into the liquid flow because the mechanical action of the pump itself pressurises the liquid. If the pump draws liquid from a separate, non-pressurised vessel, this vessel can be refilled at any time without interrupting the process. It is recommended that such a vessel is positioned at the same level as the pump to prevent vacuum suction with the risk of gas bubbles. This is at the expense of an extra shut-off valve to prevent the vessel from spontaneously emptying into the pump. Make sure that this valve is mounted so that no extra dead volume is introduced in which air can accumulate.

Which pump type is recommended?

A displacement-type pump - either a gear pump or a piston pump - can be used to provide the necessary inlet pressure for the flow controller. In a gear pump, a fixed liquid volume in between interlocking teeth of rotating gears is repeatedly displaced to generate a flow, whereas in a piston pump a liquid flow is generated at subsequent strokes of a piston filling and emptying a fixed volume. Overall, small pumps are favourable, as a pump with a small internal volume reduces the fill and refresh time of the system. Whichever pump type has been chosen, always make sure that the wetted pump materials are compatible with the processed liquid. Because the relevant displaced fixed liquid volume in gear pumps is generally smaller than in piston pumps, gear pumps are the preferred choice if a customer wants to have a reasonably stable flow control for low liquid flows. However, gear pumps are limited to a maximum operational pressure of 10 to 15 bars. Piston pumps can handle higher pressures, from several tens to more than 100 bars - which are frequently occurring process conditions in low-flow applications.

The dual-piston pumps we use at Bronkhorst are shifted 180° in phase, resulting in a very stable pressure/flow delivery which is important in the low-flow range.
However, whether or not this is sufficiently accurate depends on the customer’s application process. The required amount of liquid is accurately dosed, but due to the pumping principle it will be dosed as small pulses. When the process volume is sufficiently large, there will be enough mixing capacity to smoothen the pulses. However, if you want an evenly distributed dosing over a short period of seconds, such a piston pump setup is less ideal and a gear pump may still be the preferred choice. 

Using mass flow meters with a pump to achieve control loops

Bronkhorst mass flow meters, particularly Coriolis-based ones, are ideal for working with pumps to achieve a control loop to establish a stable low flow rate. Another method of creating a stable low flow rate is to use a pressure control setup. Here, the pump speed is controlled by means of a (EL-PRESS) pressure controller to provide a constant and stable inlet pressure. At the same time, flow adjustment is performed with a control valve which is operated by a flow meter like the Bronkhorst mini CORI-FLOW ML120
 

Flow meter liquid supply pressure control setup
Pressure control setup
Flow meter liquid supply direct control setup
Direct control setup

For many low-flow applications, the control loop setup does not have to be as sophisticated as the combined flow & pressure control setup above. The most straightforward way to use a pump for generating a liquid flow is to let it draw the liquid from a vessel and to steer the pump speed with the control signal of the flow meter. In practice, such a ‘direct control’ setup has been applied for the stable dosing of low liquid flows of hydrocarbon compounds without pulsation for high pressure catalysis R&D.

For smooth control behaviour and a stable flow throughout the flow range, a bypass line can be incorporated into the direct control setup with an adjustable restriction - typically a needle valve. Using the bypass method, the speed of the pump will be increased, resulting in a more stable flow/pressure control, allowing the pump to run at a more efficient and stable setpoint for a smoother liquid flow output.

Flow meter liquid supply bypass setup
Bypass setup

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