Myth #9: Servo and proportional valves can control no more than their rated flow

Servo and proportional valves are being used increasingly in critical applications in all technical areas — marine, aerospace, industrial, and mobile. The reason is simple: They offer a superb control. One of the perceived drawbacks, however, is that they are described by a set of performance specifications that require some study to understand. I say perceived drawbacks, because arcane sounding performance specifications describe the very things that give them their control advantages and versatility over other valve types.

Anyone interested in pursuing the exciting field of electrohydraulics is well advised to study and understand these valves and their performance parameters. This month, we will try to peel away one of the layers of confusion, that is, flow ratings of servo and proportional valves, and their supposed differences in efficiency.

Two common misconceptions
For years, servovalves have been offered up as having a particular flow rating. After the invention of proportional valves, which post dates the first servovalves, they too, have been promoted as having specified flow ratings. Servovalves are flow rated at 1000 psi (7 MPa), their flow rating pressure, while proportional valves are flow rated at 145 psi (1 MPa).

Two myths emerge f rom thi s long-held practice. First, there is a prevalent belief that because of their lower flow rating pressure, proportional valves are more efficient than servovalves. Second, a common misunderstanding holds that the rated flow is all the flow that the valve can handle or that they will always deliver the rated flow no matter what the load might be. These conclusions are simply not true and can lead to misapplication of the valves — a situation that all of us in the industry want to prevent.

Background on servovalves
Let us address these two misconceptions, or myths, by starting with some history. Where did the flow rating pressure for servovalves come from? It emerged from the aerospace industry that probably did more to advance the technology than any other. Most aerospace hydraulic systems used pressure-compensated pumps that regulated supply pressure at 3000 psi, and most of the actuators were double-rod cylinders, which have equal area on both sides of the piston.

A condition of maximum power transfer existed from pump to load when the pressure drop across the control valve was 13 of the supply pressure. This left 23 of the supply pressure for the load. (This can be shown using a bit of calculus, which we will not delve into in this discussion.) This simple relationship made the arithmetic and thinking easy. In a 3000-psi system, they simply designed for 2000 psi for the load and then the control valve would take 1000 psi. Therefore, if you rated the valve flow at 1000 psi, you could select the valve without any arithmetic at all. The rated flow in the application was the published rated flow of the valve. Sizing was not at all challenging, but there are at least three problems with this line of reasoning today.

The first problem is that industrial applications are not all standardized on 3000 psi supply pressure. It might be anywhere from 300 psi to 5000 psi or more. The same is true of mobile applications. Second, even in the aerospace applications with their standard 3000 psi, the pressure across the valve in actual operation is never 1000 psi, even though that was the design target. Rather, valve pressure varies from near zero to near the supply pressure, because the flow is never the rated flow. Third, non-aerospace applications rarely use double-rod cylinders, so the flow out the valve’s powered port is not the same as the flow back into the return port. These realities make the sizing process for industrial and mobile applications much more complex, requiring considerable calculation to achieve proper valve and actuator sizes. They also render the rated valve flow essentially meaningless.

Proportional valve background
The history of flow rating pressure for proportional valves is unknown to me, so I must speculate. All valves are affected by the internal flow forces, also called Bernoulli forces, that act on the spool. Furthermore, the amount of force available from the modern proportional solenoid is only slightly greater than the Bernoulli forces. This means the actual spool position will always be less than the commanded spool position.

For example, assume a directacting proportional valve has a solenoid rated at 3 A. If the controller puts out 1.5 A of current into the valve coil, the valve will shift, nominally speaking, about 50%. But if hydraulic power is turned on, the flow force will act to close the valve, that is, move the spool toward center. So the spool will be at a position of less than 50% shift. The flow forces are affected by both the pressure and flow. Knowing these realities, the purveyors of proportional valves chose a lower flow rating pressure, I speculate, so as to stay under the flow force levels that appreciably affect spool position. They chose 10 bar (1 MPa) as a flow rating pressure, which is about 145 psi, or about 17 of that of servo valves.

This raises the question, “Are servovalves somehow immune to flow forces?” The answer is yes and no. Yes, flow forces certainly exist, and they are substantial. However, most servovalves use pilot stages to assist with the shift of the main spool. Proportional solenoids can produce only about 20 or 30 lb of force to maintain the main spool position, whereas the piloting method can produce hundreds of pounds of positioning force.

Most servovalves are available with an internal pilot connection. Therefore, as flow forces commensurately go up with increasing supply pressures, so do the pilot section holding forces. The result is that the piloted valve can maintain spool position much more reliably than the direct-acting proportional solenoid valve can. Large proportional valves are generally pilot operated, and they benefit from the same spool positioning robustness that servovalves do.

In non-aerospace applications, flow rating pressures are essentially arbitrary, bearing little or no resemblance to the flows required in the application. The lower rating pressure for proportional valves was done to avoid high flow forces and has nothing to do with their efficiency. In fact, in a given application, the efficiency will be exactly the same with proportional valves as with servovalves.

When sizing the valve, we calculate the KV of the valve, which is a measure of how much the valve opens. This KV value must be converted to rated flow at 1000 psi in order to choose a servovalve or to 145 psi to choose a proportional valve. The flow and pressure will be exactly the same in the application for the servovalves and the proportional valve. There is absolutely no efficiency advantage of one over the other.

Closing comments
We’ll close this discussion with a few words of caution. If the proportional valve is not pilot operated, it may have a power limiting curve requiring a reduced supply pressure if the valve opening is to be maintained. This means that the application scenario may not be exactly the same when choosing the proportional over the servovalve. The power limitation is especially acute in direct-operated proportional valves.

Also, be aware that many pilotoperated servovalves use flappernozzle or jet-pipe pilot stages that constantly consume flow and hydraulic power. This could be important in applications where power consumption is a critical factor. This, some people will argue, is why they claim that proportional valves are more efficient than servovalves. I concede their point, but grudgingly, and with some qualifications.

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