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. It is one of the most used accessories in flow control.
An accessory for mass flow controllers
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) 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 today’s 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.
The direct control valve
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. Such a valve is relatively fast, cheap, and uses only little power to control the flow.The disadvantage here is that it can only handle limited pressures and flows.
Let’s take an electromagnetic valve as an example:
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 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.
What alternatives do we have?
- Redesign the direct valve for higher pressures
- Using a 2-phase valve
Using a pressure compensated valve, to reach high flows at low pressures
- High pressure direct control valve
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.
- Indirect control valve, a 2-phase valve
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.
[Figure 1 – Indirect control valve]
An indirect control valve (or 2-phase control valve) consists of:
- a direct controlled pilot valve (A), with the behavior as described before, and without needing any extra power.
- an additional valve in the body; a pressure compensation part (B) to maintain a constant pressure difference (P1 -P2) of only a few bars across the pilot valve. By doing so, both the inlet and outlet pressure may change without having any impact on the valve’s function. The pressure force over the pressure compensated part keeps the valve closed. Only when the top valve opens, the pressure force is brought back to a small enough value to open the valve and control the flow.
So, the indirect control valve consists of two valves in series, where both the pressure drop and the orifice size together determine the resulting flow.
The disadvantages of this valve are its size and the relative high costs. Besides that, a minimal pressure difference is needed to close the pressure compensation part of the valve. Also, the orifices are still limited in size, thus to get to 200 ln/min a minimal inlet pressure of > 150 bara is needed. To get such flows at lower pressures, a whole different kind of valve is needed, like a pressure compensated valve, a bellow valve.
- Pressure compensated valve
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.
Conclusion: Depending on the pressure that you want to put over your mass flow controller and the outlet flow needed, you can either use:
- a direct controlled high pressure valve (up to 200 bara and 20 ln/min), or
- an indirect pressure compensated valve (up to 700 bara or 400 bard and 200 ln/min).
To reach high flows at low pressures, a pressure compensated valve is the best solution.