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A control valve is used to control a flow by varying the size of the flow passage as directed by a signal from a controller, such as an on-board PID controller in a flow meter. It is one of the most used accessories in flow control.
Control valves can be furnished as an intergral part of mass flow controllers and pressure controllers or as a separate component used in combination with a flow- or pressure meter. Together with a feedback loop from the mass flow controller or pressure controller, the valve controls the amount of flow passing through to go to an imposed flow- or pressure setpoint.
Depending on the application it is often clear whether your mass flow controller needs a shut-off (open-close) valve or a control valve or a control valve, or whether one needs a normally opened or normally closed valve. Within the group of control valves, there are a number of different valves available, each having their own parameter ranges, advantages, and disadvantages.
In this blog I will highlight some valves and focus on how to cope with higher absolute and differential pressures, and how to get higher flowrates at low differential pressures.
A direct control valve consists of an orifice for controlling the flow and a controlled surface that determines the size of the opening that flow can pass through, and thus determines the amount of flow passing through the valve.
For a valve, the force (F) needed to overcome to open the valve is determined by the orifice diameter size (d) and the pressure difference (Δp) over the valve , (F ~ Δp * ¼ d2). When either the pressure differential or the orifice diameter gets higher, the direct control valve will not open adequately due to this pressure force, which can be > 15 N for a 200 bar differential pressure over a 1mm orifice, pushing the valve shut.
An electromagnetic valve can only exert a force of ca. 5N on its plunger. It could be a possibility to use a stronger coil, delivering a larger magnetic force. However, mass flow controllers often have a limited power supply and the amount of heat that is produced can become a problem as well. Resulting in a limited maximum flow, proportional to pressure and the diameter squared.
In summary, most direct control valves are not suitable for high flows, or to handle high differential pressures or absolute pressures due to these restrictions. The direct control valves could be used for low flows from 1mln/min up to approximately 50ln/min.
The easiest solution to cope with higher pressures is a redesign of the direct control valve. As the orifice size is limited, it can be used for relatively small flows (up to 20ln/min) . To handle the larger pressure differences, up to 200 bar differential pressure (bard), the valve and mass flow controller body have to be more robust. Most valves can not handle a burst of 200 bard; either the sealing material can rupture, or mechanical parts can not handle the sudden force bursts that are possible at 200 bard.
The dimensions of the valve are only slighty larger than for a common valve, and thus the entire mass flow controller. On the other side, low flows are often limited due to leakage through the valve at high pressure differences.
To go to even higher pressures and more flow, up to 200ln/min, we have to take a larger step in changing our mass flow controller. With a so called indirect control valve (figure 1) higher flows and higher absolute and differential pressures can be reached.
It is possible to use larger orifices and reach higher flows with a direct control valve, but to do that, the pressure force in the valve has to be reduced. This can be done with a pressure compensated bellow valve, where the effective orifice for the pressure force has been reduced significantly (figure 2). With a bellow valve, flows of several hundreds of liters per minute can be reached with a minimum pressure difference. However, the absolute pressure is limited due to the design and the valve is much larger and more expensive than a common direct control valve.
Depending on the pressure that you want to put over your mass flow controller and the outlet flow needed, you can either use:
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