Flow Regulators Part 2

Priority Flow Regulators

Bypass valves work very well for single systems where all the flow of the pump is to be fed into one system. But what happens when there is more than one actuator in action? Prioritizing flow for different actuator is a part of its regulation as well. Priority type flow regulators supply oil to systems where it is absolutely essential and then divert excess flow to secondary systems which fall lower on the importance list. The excess oil is supplied regardless of the working pressure of the secondary system.

HIC Style Priority Flow Regulator

The logic element used in Priority type Flow Regulators is a 4 Port element where Port 1 is connected to the inlet of the needle valve. The needle valve outlet is connected to Port 4 whose pressure is sensed in the spring chamber harbouring an 80 psi spring. Port 3 goes off the Priority System while Port 2 is connected to the secondary circuits.

The flow to the priority circuit is set on the needle valve. Oil will flow from the Inlet port across the needle valve to Port 4 of the logic element. From here it goes to Port 3 of the element and out to the priority system.  When the flow from the Inlet to the Priority port creates enough pressure drop to match the spring in the logic element (80 psi) the spool moves backward diverting excess flow to the secondary system.

In this way, the priority system is always fed with the required amount of oil before any supply reaches the secondary system. This is possible provided the basic flow rate of the pump is higher than that which is required by the priority system.

If more flow tries to get past the needle valve to the Priority system, there will be a pressure imbalance across the spool. This pressure imbalance will cause it to shift backwards closing the priority line and further opening the Inlet to the Secondary system. This exercise will continue till the flow rate to the Priority system creates just enough pressure drop to counteract the effect of the compensator spring.

Excavators use priority system to ensure oil is always available for steering.

In case the flow into the Inlet port decreases to a level below what is required in the priority, the pressure drop across the spool will fall below that of the compensator spring. This would cause the spring to push the spool forward completely closing off the Secondary system from receiving any oil and diverting all of the incoming oil to the priority port. This would ensure that regardless of the input flow, the secondary system gets oil only after the priority system is COMPLETELY satisfied.

An increase in load in any of the two systems would eventually reflect in the Inlet pressure. Upon increasing the load in the Priority line, the pressure here rises decreasing the flow rate. The pressure in the line is sensed through the orifice plug in the spring chamber causing a force imbalance. This pushes forward the spool and reduces the flow of oil to the secondary system diverting more oil to the priority line.  This occurs till the pressure drop matches the spring force which brings the spool to a state of equilibrium. Thus the flow is brought back up to normal by the metering effect of the spool. Conversely, a surge in flow from the pump will cause the pressure at the inlet to rise. This will create a pressure imbalance across the spool which will push it backwards against the spring force opening up more flow to the secondary line and reducing the flow to the priority bringing it back down to normal. Thus the priority line will be fed with only as much oil as it needs regardless of the working pressure of either the secondary circuit.

Cartridge Style Priority Flow Regulator

Similarly, in cartridge constructions, a compensating spool senses pressure imbalance caused due to increased or reduced flow to the priority line and meters the radial holes at the secondary line. If the flow falls below the requirement of the priority line, the radial holes at the secondary line close completely directing the entire flow to the priority line.

The pump will work at a pressure which is 80 psi more than the higher of the two system lines (secondary or priority) 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.

Circuit schematic for Priority Flow
Regulators used in Fast-Slow Systems

Priority valves can be used for Rapid Speed-Slow Feed systems where the Priority Line is connected back to the Secondary line via a check valve. During the Rapid Traverse, both lines contribute to the flow providing the actuator with increased speed. A solenoid valve at the Priority Port is opened when Slow Feed is required venting the line to tank and cutting off the flow. The system works on the logic of “bleed-off” flow control where the flow to tank is accurately metered rather than the flow to the system. This system works when the flow rate from the pump is constant and not prone to surges.

Electronic and Proportional Systems

As in the case of any hydraulic system, there is always that extra dimension of electronics that can be added to the valves. Electronic systems coupled with flow regulators open up quite a few avenues for customization of flow patterns depending on feedback from the environment.

Low Flow Systems

Needle valves can be replaced by Proportional Flow Control Valves which give the added advantage of in-process flow control. This application is quite popular amongst agricultural equipment manufacturers who need the flow of pesticides and such controlled depending on the speed of the vehicle. This provides for an even spray. The proportional valve directly controls flow to the Outlet or Priority ports in Bypass and Priority type systems respectively. These systems are apt for low flow applications since proportional valves for higher flows turn out to be more expensive.

High Flow Systems

For systems involving the proportional regulation of higher flows, an ingenious concept of a pilot valve is used. Coupling a proportional pressure reducing valve to a fully balanced Overcentre valve (OSIBD Series) we can regulate flows upto 350 lpm!! The OSIBD Series of valves have a fully balanced piston which senses the load pressure on both sides of the poppet. The poppet is now completely under the influence of the Pilot Pressure which is regulated proportionally by the pressure reducing valve from 0-25 bar. A higher electronic signal to this valve would mean an increase in Pilot Pressure which would push back the poppet further and allow more flow to pass. Since the flow required for the pilot port is minimal, cost reduction can be obtained on the system due to the small size of its proportional components.

HIC Style High-Flow Flow-Regulators using Fully Balanced Overcentre Valves, Proportional Valves and Logic Elements. (Click to enlarge)

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.

Link to Flow Regulators Part 1