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Highway hydraulics: flow control

Anyone who has attempted a long journey in a car will notice that the flow of traffic affects how quickly and safely you arrive at your destination. There are various ways that highway angencies seek to control these two elements. Speed cameras slow drivers down to enhance safety, variable speed limits keep the traffic flow at a constant and diversions limit the weight of traffic on and road. Some of these measures are more effective than others.

It would be great if we could accurately predict the time it takes to get from ‘A’ to ‘B’. Unfortunately due to restrictions and accidents our journeys become more difficult and less efficient. Thankfully we do not have to deal with the random variables caused by human frailties when utilising hydraulics within a machine provided the designer uses the best available flow control valves within his system.

In hydraulics it is important to control the flow of oil to augment the safety and efficiency of a machine. The results will vary depending on the accuracy and repeatability of the valves used. It is important to understand the function and operation of the different types of flow regulator before applying them; from the simple needle valve to the proportional pressure compensated priority device. Each type has its place and there are applications that warrant the differing complexity.

We understand that flow takes place from high pressure to low pressure and the amount of flow is dictated by the pressure difference. By introducing restrictions into the line we can control the flow provided the excess flow is given a means of escape.


With a needle valve the flow will not be controlled until the inlet pressure reaches a point at which an upstream relief valve opens or a pump compensator operates to reduce the inlet flow and maintain a balance. The flow across a needle valve depends on the pressure difference across it so changes in outlet pressure will have an impact on the controlled flow.

To overcome this problem compensators of various guises are introduced into the system. There are three main types; Restrictive style, By-pass style and Priority style. The restrictive style flow control consists of a needle valve and compensator element. The compensator can be situated after the needle valve (See figures 1 & 4).



Fluid pressure at the high-pressure side or inlet to the needle valve is sensed on one end of the compensating spool and pressure at the lower pressure side or outlet is sensed by the other end of the same spool. This spool is usually offset to the open position by a spring. The flow through the valve is determined by the position of the needle as the compensator will always ensure that pressure drop across the needle is constant, usually 7 bar. Opening the needle will permit the flow to rise until the pressure drop exceeds the setting of the compensator causing it to shift closing a ring of holes in its sleeve and thus limiting the flow. 

As the flow through the valve will be lower than the flow being delivered by the pump the inlet pressure will rise to a point where an upstream relief will open feeding excess flow to tank or a pump compensator backs off reducing the inlet flow to that required to satisfy the flow setting of the flow regulator. 

The by-pass style of flow control (Figure 2 & 5.) again consists of a needle valve and compensator element but has a third port to allow the excess flow to pass to tank at working pressure as opposed to maximum relief valve setting in the case of the restrictive style. The compensator spool is biased closed by a spring, inlet pressure acts to open the spool against the action of the spring and pressure downstream of the needle valve acts to close the spool in the direction of the spring. Flow through the compensator spool goes to tank. As the pressure drop increases across the needle valve the pressure difference is sensed across the compensator spool until it moves to open the inlet to the tank port. Increase in outlet pressure will tend to reduce the flow across the needle valve but the co-responding change in pressure drop will cause the compensator to restrict the line between the inlet and the tank line. In this way the spool will meter the bypass flow to maintain a constant controlled flow that relates to the force exerted by the spring and the orifice size created by the needle valve. With this type of flow regulator it is important that the tank line pressure is kept to a minimum as this may increase the flow through the regulated line above that required.

The priority style flow control (Figure 3 & 5.) is similar to the bypass style except that it allows the excess oil to be used for other functions even if the working pressure for this function is higher than the controlled flow pressure.

The flow from the pump enters the inlet port and passes across the needle valve, then on through the sleeve, past the compensating spool and out through the regulated port. The passage of oil across the needle valve creates a pressure difference which is sensed across the compensating spool. When the flow is sufficient to create a 7 bar pressure difference across the needle valve the compensating spool will begin to move uncovering the radial holes in the sleeve and opening up a path to the bypass port. Oil will therefore begin to pass to the bypass line. If the flow tries to increase across the needle valve, and so to the regulated port, there will be an increase in pressure difference sensed by the compensating spool causing it to move further against the spring, open further the line to the bypass port and limiting the flow to the regulated line. If the inlet flow falls below the setting of the valve the pressure difference across the needle valve will drop below the 7 bar needed to keep the bypass line open therefore the priority line is always satisfied before the bypass line opens. Changes in operating pressure on either of the two outlets will alter the inlet pressure to the higher of the two pressures (plus the control pressure which is 7 bar). If the working pressure in the regulated line is higher than that in the bypass line then the tendency would be for the flow to try to take the easy way out and flow down the bypass line. This would detract from the flow passing across the needle valve lowering the pressure difference causing the compensating spool to shift, increasing the restriction to the flow to the by-pass and reducing the restriction of the flow to the regulated line. In this way the compensating spool will maintain the regulated or priority flow at a constant level.

If the working pressure in the bypass line is higher than that in the regulated line then there would be a tendency for the flow to increase through the regulated line, increasing the flow would increase the pressure difference across the needle valve and cause the compensating spool to meter the regulated line.

If this is not successful then either an overcenter valve around the motor or a sequence valve before or after the actuator is necessary.

In today’s environment electronic control requirements are becoming more common. To achieve this, the needle valve can be replaced with an electro-proportional valve. With electronic speed feed back, errors in the compensation can be corrected by adding electronic metering. Typical applications include salt or fertiliser spreaders where a relationship between the vehicle speed and the density of spread is required. For high flows it is sometimes necessary to use an electro-proportional pressure reducing valve to pilot open a separate orifice of some kind (figure 8.).

This circuit uses a pilot operated poppet valve to provide the proportionally controlled orifice. Integrated hydraulics' zero differential valves benefit from having a shrouded seat which gives superior flow control as the poppet opens. The poppet is hydraulically balanced so it will open proportionally upon the application of pilot pressure up to 25 bar regardless of system or induced pressure.

Accurate regulation of flow is vital to the safe and efficient operation of hydraulic systems and therefore the machines on which they are used. Each application is different and will demand different control solutions and pressure compensated flow control devises, either mechanically or electronically operated, are designed to offer that accurate, cost effective, reliable and repeatable control.