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

NG4-Mini Proportional Valve

Proportional directional control valve
with integrated spool position control 
NG4-Mini from Wandfluh AG

Direct controlled proportional directional control valve with integrated amplifier electronics and spool position control in flange construction NG4-Mini. The valve possesses an integrated position control. With the spool position sensor (LVDT), the actual position of the valve spool is continuously recorded and brought into line with the set-point value transmitted in an analogue manner. Apart from an analogue interface the valve is also available with a field bus interface (CANopen or Profibus DP). The parameterisation takes place through a USB- interface by means of a menu-controlled parametrisation- and diagnostics software. The data are stored in a non-volatile memory. Settings once elaborated can be reproduced and transferred without any problem, also following an electric power failure.


Model Code: BRW.4
♦ Flange construction NG4-Mini
♦ Operating pressure pmax = 315 bar
♦ Maximum volume flow Qmax = 20 l/min
♦ Volume flow levels: QN = 4/8 l/min
♦ Nominal voltage 24 VDC
♦ With integrated spool position sensor (LVDT)
♦ With integrated amplifier electronics (DSV)
♦ Protection class IP 67 

Advantages of the spool position control (LVDT)
♦ Minimal hysteresis
♦ Improved dynamic characteristics

Advantages of the integrated amplifier electronics (DSV)
♦ Intelligent
♦ Compact
♦ Plug & Play

Applications
► Both in industrial - as well as in mobile hydraulics
► Where a high resolution, minimal hysteresis and very good dynamic characteristics are of concern
► Adjustment of the rotor blades of wind power generators
► Machine tool - and paper production machines
► In case of position control systems
► Forestry - and earth moving machines
► Robotics


Further Information
You will find further technical information on the data sheet <<click here>> or on our website. We will be happy to advise you in the selection of the suitable components for your application.

Courtesy: Jürg Schneider, Wandfluh AG.

High Capacity High Low System

Application

A hydraulic circuit may be powered at different times by a high-pressure, low-flow pump and a low-pressure, high-flow pump. This two-pump circuit configuration eliminates the need for a relatively expensive high-pressure, high-flow pump and also saves energy. High Low systems are used with combinations of two (or more) pumps to give high flow at low pressures and high pressures at low flows. The system bypasses the flow from the LP pump(s) to tank at a preset pressure. This allows for pump selection to give, for example, rapid advance and high power compaction with the most economic usage of system components and energy requirements.

Application areas are High Capacity Presses, Cotton Baling Machines, Scrap Baling Machines, Car Crushers.

Operation

Pump inlet into the system is combined to give maximum flow at low pressure. When the load pressure increases to the set level, the high flow (low pressure) pump is bypassed from LP to tank via the dedicated Unloading Valve (2) allowing nearly all the system power to be used for the high pressure pump (See graph for the pressure drop of dumped flow). The system relief valve provides protection by limiting the maximum pressure in the system line.

Features

This is a self contained system including four replaceable cartridges with full adjustment through their respective ranges. The compact nature of the hydraulic system reduces the need for bulky valves and piping. The system can be used for flows up to 400 lpm! (250 lpm high pressure and 150 lpm low pressure pump) Pressures up to 350 bar can be obtained using the system. The HIC comes with an on board pressure switch for electronic signalling of the pressure in the outlet line.

Directional Control Valves - Part 3

More on Coils

Solenoids are devices that are capable of changing electrical energy into mechanical, or linear, energy. The simplest type of solenoids assemblies used in hydraulic valve applications relies on two main aspects for their function: an insulated (or enamelled) wire, shaped into a tight coil, and an armature of varying designs also colloquially called the “iron core” which contains a solid rod or pin of either iron or steel. The iron or steel pin is ferromagnetic, a property that allows it, when exposed to electrical current, to function as an electromagnet. The solenoid uses the magnetic field created from an electrical current as the trigger for the production of a push or pull that drives mechanical action into the pin. Solenoids that rely on electrical current fall into two main categories-- solenoids that rely on AC (alternating current) as the source of power and solenoids that rely on DC (direct current) as the power source. 

While AC and DC solenoids use different types of current, they both work in the same basic manner. When the insulated, coiled wire of the solenoid receives electrical current, the magnetic field produced strongly attracts the pin which pushes the spool inside the valve to change the flow path (“on”). The spool is attached to a compression spring on the opposite side which is compressed till the current is stopped. When the current is turned off, the compressed spring forcefully snaps the pin and spool back into its original position (“off”).

Armature Design:

Two common designs for solenoids are (a) the air gap design; and (b) wet armature design. In the air gap design, the two sections (armature and spool) are isolated from each other using dynamic seals.The downside of this design is that the seal wears off eventually causing leakages from the valve into the armature. The wet armature design is more common with the entire armature assembly submerged in oil from the valve. The solenoid magnetically moves the armature while it is submerged in oil. This design provides a lower leakage due to the absence of wearable dynamic seals. The only sealing in wet armature designs is the O-ring that seals the threaded connection between the iron core and the body and, if present, the one on the manual over-ride push pin which, although a dynamic seal, only acts when the valve has to be manually reset once in a while. (Figure 16) Another advantage of this design is that the movement is naturally dampened by the presence of oil. This provides a smoother, more quite movement with a longer service life. The viscosity of

the oil, however, means that the wet armature design needs 60% more power to actuate. 

Voltage Selection:

There are four main types of voltages used with solenoid valves: 12 VDC, 24 VDC, 110-115 VAC and 220-230 VAC. Although the 115 VAC finds hardly any usage outside North America, the remaining three are quite common in the rest of the world. Now, though the type of current (Direct or Alternating) is the prerogative of the designer, the voltage rating of the solenoids entirely depend on the power required to shift the spool. 

Both the categories (AC and DC) have their own set of advantages and disadvantages. DC solenoids are quieter and require less maintenance than AC coils. On the other hand, they function more slowly than AC  solenoids and are also less powerful than AC solenoids. In AC solenoids, the current that runs through the solenoid starts with a first rush of extremely strong current, then drops to a lower, normal level as the solenoid gap reduces. Thus if the spool gets stuck in the open (full-current) position for too long in the body due to contamination particles in the land areas, it receives too much of this first wave of maximum current and it can permanently damage the device by allowing the coil to burn. By contrast, DC solenoids experience no alteration in currents and do not run the risk of being damaged by the current. Off late, all solenoids are made to function on DC voltage. An AC Solenoid would have a DC Coil rated to a voltage close to the required input AC voltage (195 VDC coil for a 220 VAC input) and a rectifier plug that would convert the incoming AC current into DC (Figure 17). Although DC circuits can be utilized with AC solenoids without a problem, DC solenoids cannot be used on other circuits without becoming noisy and overheated and possibly burning out due to excess current. 

Due to the inherent problems with AC Solenoids, a lot of Original Equipment Manufacturers are rethinking their designs and opting for DC Solenoid Valves. Using the same voltages that are predominantly used in their equipment’s PLCs, it is less of a hassle giving longer service lives free from maintenance issues. 

Conclusion

In conclusion, the selection of a proper directional control valve is as important as its use in the circuit. The above article will help new initiates in hydraulics to select a proper valve for their circuit while refreshing the fundamentals of people already well versed in hydraulics. It’s best to check requirements of the system and decide the best valve for the job. Some of the circuits are highly traditional and outdated and need a major revamp on spool and solenoid selection. Some applications become a lot easier with the help of poppet valves instead of sliding spool valves.

Click Here for Directional Control Valves - Part 1
Click Here for Directional Control Valves - Part 2