Flow Regulators Part 1

Processes when allowed to run free unhindered rarely do any good to the designer as well as the user. A lawless system, not unlike a lawless society, can rarely function up to its potential. Checks and balances always have to be maintained to ensure the maximum possible extraction of work from any process. Policemen maintain order in public to ensure that the majority is safe and secure while weeding out the minority that cause problems, traffic lights ensure smooth running of traffic, predators control the population of their prey, sea breezes give costal towns and cities a maritime and pleasant weather while our own moral balance ensure that we don’t veer off course! Each process, in order to be fruitful and productive is gently regulated by employing these checks and balances.

Thankfully, a hydraulic system is more conducive to being regulated and intelligent applications of control points can give an exceedingly precise result as the level of control increases. We have temperature controls, filtration controls, electrical controls, mechanical controls and the most common, pressure and flow controls. While most of the former are either already discussed or will be at a later stage, today we concentrate on the flow control, more specifically, its regulation.

The Control and Regulation of Flow

Flow control boils down to only one particular and very vital concept, the orifice. The orifice controls any and everything about flow in a hydraulic circuit. How much back pressure will be generated in the system due to the flow, what is the speed of the oil flow and what is the basic rate of flow?

Flow control and flow regulation are two separate entities which should not be confused. Where flow control is a predominantly static and crude concept mainly limited to switching flow on and off or setting the flow where parameters do not change by much, flow regulation is a more dynamic exercise with varying entities and effects updated by constantly changing causes. Hence flow can be controlled or regulated by valves as simple as a humble shut off valve or as complicated as a proportional pressure and temperature compensated priority flow regulating valve.

The main reason for using flow regulators is the behavior of fluids passing through the orifice. Fluids flow from an area of higher energy to one of lower energy (height or pressure). Orifices create back pressure upstream due to the fact that all the fluid is not allowed to flow freely. This gives rise to a pressure differential or a pressure drop across the orifice through the restriction creates. Increase in pressure upstream will reduce the pressure drop across the orifice and ultimately the rate of flow passing through it. Increase in pressure downstream will increase the pressure drop and the rate of flow through the orifice. Flow regulators take feedback from the upstream and downstream pressures to regulate orifice size. The size is regulated in such a way that there is always a constant pressure drop across the valve and hence the flow rate remains the same.

Flow regulators come mainly in three basic constructions depending on the system used. There is the simple Restrictive type of flow regulation, the slightly more complicated Bypass type and the Priority type of flow regulators.

Restrictive Flow Regulators

Restrictive type of regulator valves in a hydraulic integrated circuit (HIC) format are made of a needle valve element and a 3 ported pressure compensated orifice regulating logic element. The logic element has compensating spools that shifts backwards and forwards depending on the pressure drop to regulate the flow set on the needle valve element.

HIC Style Restrictive Flow Regulator

The required flow is set across the needle valve. The pressure upstream is sensed at Port 3 of the logic element which pressurizes the spool along with an 80 psi spring. An increased load on the actuator will increase the pressure downstream. This reduces the pressure differential across the valve and lowers the flow rate. The increased downstream pressure is sensed at Port 1 of the logic element which then pushes back the spool closing the radial holes in Port 2. This increases the pressure drop across the valve due to an increased restriction in flow which once again brings the flow rate back up to normal. In this way, the compensating spool always makes sure that there is a pressure drop of 80 psi across the valve which gives us an even flow rate.

Cartridge Style Restrictive Flow Regulator

In the fully cartridge version, the logic element is converted into a pressure sensing spool. An orifice allows for the flow to pass from Port 1 to Port 2. The orifice size is dependent on the adjustable flow range required of the valve. By varying the pressure on the spring by means of an adjuster, the flow required is selected by matching the pressure drop created by the flow to the pressure on the spring. Hence, if an orifice of Dia ‘X’ allows for 3 liters of flow at a pressure drop of 5 bar, a pressure of 5 bar is set on the spring. An increase in pressure at Port 2 will cause an imbalance in the spool pushing it forward towards Port 1 and opening the radial holes further to allow more flow to pass. In this way, the spool meters the flow constantly to provide an even flow rate regardless of pressure.

In addition to the above variations in valve types, we can also add a return check valve for free flow. In the HIC style, it is done by adding a check valve connecting the outlet to the inlet, while a check seat is added in cartridges to permit reverse free flow.

HIC Style Restrictive Flow Regulator with Reverse Check

Cartridge Style Restrictive Flow Regulator with Reverse Check

If most the flow sent out by the pump is consumed in the system, it is well and good. There is not much excess flow that has to be taken care of. But if the flow through the valve is much lower than that of the pump output the inlet pressure will rise to a point where a relief valve upstream cracks open and dumps excess flow to the tank or in the case of a pressure compensating pump, the compensator reduces the output of the pump to match the flow setting of the flow regulator.

Restrictive Flow Regulators in A or B lines need reverse checks. Restrictive Flow Regulators in P lines don't need reverse checks

Bypass Flow Regulators

To overcome the problem of excessive power usage, we use the Bypass type Flow Regulators. In these valves, we have a similar system of a needle valve being used in conjunction with a compensator logic element. This time, however, the logic element is normally closed by the compensator spring and it “tees off” excessive flow to the pump at a minimal pressure drop. Port 1 is connected to the inlet of the needle valve while Port 3 senses the pressure at its outlet. Port 2 bypasses the oil to tank. 

HIC Style Bypass Flow Regulators

During normal working, oil is fed to the system while the rest is bypassed to the tank line at working pressure plus the logic element spring setting (80-100 psi). When the actuator comes on load, the pressure at the actuator end (downstream) starts to increase. The pressure differential across the needle valve is hence reduced. The reduced pressure drop in turn decreases the flow rate through the valve causing the actuator speed to slow down. The increased pressure is sensed at Port 3 of the logic element causing the spool to slowly close the bypass line. This diverts more flow across the needle valve bringing up the flow rate back to normal along with the actuator speed. In this way, the logic element constantly moves forward and back to meter the flow into the cylinder maintaining a constant flow rate regardless of the pressure. A point of caution is that the tank line should have minimum back pressure as any resistance in the bypass line may cause excessive flow being channeled to the actuators.

Inbuilt relief valves save the additional
costs of piping and line bodies

Bypass valves are superior to restrictive type valves because the pump operates at the working pressure of the system (or actuator) plus a nominal 80 psi rather than increase upstream pressure to the one set on the system relief and consume much more power.

As a supplementary system to Bypass type flow regulators, Relief Valves can also be incorporated to allow for pressure relieving. The pressure is sensed in the Outlet line relieving incoming flow to the Bypass line. Popularly used in the case of unidirectional motors who encounter obstructions, the relief valve opens at a pre-set pressure by sensing the load on the outlet. This reduces the cost of additional relief valves downstream providing a compact package while controlling excessive pressure buildup in the system.

Flow Regulators - Part 2