Select the Best Valve for Your Flow Meter

The direct control valve

A direct flow 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.

Advantage: such a control 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 flow control 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 control 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 flow 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.

What alternative do we have for a direct control valve?

Redesign the direct flow control valve for higher pressures
Using a 2-phase valve, an indirect control valve
Using a pressure compensated valve, to reach high flows at low pressures

Option 1) 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

Option 2) 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.

An indirect control valve (or 2-phase control valve) consists of:

A direct controlled pilot valve (A), with the behaviour 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 (A). 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 (A+B), where both the pressure drop and the orifice size together determine the resulting flow.

More variations on the 2-phase indirect control valve

There are actually 2 more variances of the indirect control valve available:

The first one is adding a third phase to the indirect valve, where the pressure changes from the 2-phase valve is used to open a larger valve to reach higher flows, up to several thousands liters per minute. In this system, the pilot valve is actually parallel to the main flow path. The disadvantage is that this vlave becomes quite large and costly, as well as that a minmum pressure difference is needed to close the piston that controls the main flow.
The other variation of the indirect control valve is one where the pilot valve is again parallel to the main flow path, but where the parallel path only consists of a single valve and a fixed orifice. When the pilot valve is closed, the pressure in the valve builds up to the inlet pressure, which closes of the main flow path by a membrane. By opening the pilot valve, the pressure difference on the membrane can be reduced and flow can be controlled through the larger path. For a membrane valve, smaller pressure differences are needed to function, down to only a few tens of millibars. The disadvantage is that when large membranes are used, the absolute pressures in the system are limited, and when we scale down the membrane, the maximum flow is restricted and the pressure difference needed to close the valve goes up again.

The disadvantages of these valves are the 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.

Option 3) 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.

Pressure compensated 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 control 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 will be the best solution.

What is the best control valve to use?

Summarized below are the valve options discussed, and the flow and pressure range it can be used for.

Valve type	Flow (ln/min)	Pressure (abs / delta)
Direct control valve	0.0007..50	0..1005 / 0..40
Direct control high pressure valve	0.0007..20	40..200 / 1..200
2-phase control valve	0.010..200	4..700 / 4..400
3-phase control valve	50..5000	2..20 / 1..20
Pressure compensated valve	0.1..200	0..10 / 0..5
Membrane valve	5..2000	0..10 / 0..10

Have a look at the control valves we often use to regulate the flow in combination with our GAS flow controllers and LIQUID flow controllers or our electronic pressure controllers.