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How to control liquid and gas flows with greater precision

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This article from Roxspur Measurement and Control outlines how fine control of flow rates in liquids and gases can be achieved.

When setting up any fluid system, whether in a laboratory or a pilot plant, or setting a sample or purge flow, a manual control will be needed to stabilise the flow of liquid or gas from the supply or the pump. Within the range of Platon variable area (VA) flowmeters, NGXV models are supplied with an integral flow control valve for this purpose. Typically a glass tube flowmeter will operate over a 10:1 flow range, and the fine control valve supplied with the frame will easily control all such flows.

Platon also manufactures the Platon FCV - a standalone multi-turn fine control valve - that allows precision adjustment of fluid flows. These are suitable for use on gas flows of up to 100 litres/minute (liquids to 3.5 l/min) in piping systems typically using standard 1/4BSP (G1/4) connections; such installations are common in laboratories and pilot plants or sample/purge flow systems. The Platon FCV gives precision control over a wide flow range, typically 100:1.

In a flow system, the pump or the storage system pressure will vary, either as the demand from other users changes, or as the fluid is discharged. This can lead to a variation of the flow delivered through a simple control valve.

Sampling systems

With sampling systems, the sampled flow conditions might change, or the sample analysis or collection and storage system might provide a variable back pressure, resulting in an undesirable change in the sample flow rate.

Many such site condition changes can be overcome, and a steady flow maintained, by using a simple mechanical flow controller, such as the Platon model MN Flostat.

The MN Flostat is a mechanical device that uses the flow pressure across an orifice to act against a spring-loaded diaphragm within the Flostat; this controls the flow at a value preset using an integral multi-turn needle valve. Typically the MN Flostat is used in series with the Platon NGX flowmeter to show the operator the actual flow rate being pre-set. A differential pressure is required across the Flostat for it to function; the largest MN Flostats suitable for 1/4BSP applications can control gas flows from 8-100 l/min and liquids from 0.25-3.5 l/min, whereas the smallest range units control 0.005-5 l/min gas and 0.002-0.2 l/min water.

Once set, the flow rate will be maintained constant, subject to the differential pressure required being maintained across the Flostat; the constant flow rate will be visible on the NGX flowmeter.

Typical applications

Flostats are suitable for blending or dosing applications to maintain the required delivery of valuable gases or liquids, despite variations in the process line pressure. Similarly, Flostats are used on gas sampling, purging or inert gas tank blanketing systems to maintain a flow rate independent of the back pressure in the delivery vessel. Bubbler type liquid level measurement systems benefit from improved accuracy and lower gas usage when fitted with a Flostat.

Larger Platon Flostats are available - even for as large as 4inch line sizes. Such units have been used to maintain a constant flow of irrigation water into separate branch lines despite large variations in the supply pressure from the reservoir. Other Flostats have been used to control the delivery of aviation fuel into aircraft tanks to a certain safe maximum rate, whatever power the generator driven pump might have been providing. Because the Flostat is a mechanical controller, it requires no power, and there is no problem with using the device in a hazardous area or for the control of flammable liquids. Special versions have been produced for higher pressure applications and for low-pressure-drop applications.

08 March 2007

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