Pressure Compensated Flow Control Valve

 

Pressure - Compensated Flow Control Valve:

Introduction:

Any flow control, with pressure compensation feature, will automatically maintain a rate of flow regardless of the pressure changes that are caused by the work (load).

If a cylinder is moving 500 N load at the rate of 0.3 m/min, and suddenly it is required to move 1000 N at the rate of original speed, it means that the volume of oil going to the cylinder must remain the same. But the pressure must be doubled in order to move twice the load.

Pressure compensated flow control valves are designed to accomplish this feature.

Working principle:

To analyze the working principle of these valves, it is necessary to recapitulate the fluid flow law which states that “a fluid rate of flow will be constant through an orifice of a given size as long as the pressure drop remains constant”. This means that the pressure can vary as long as the change is the same.

Flow rate is the measure of flow volume that streams past a measuring point in a given time. Flow rate is measured in liters / minute (abbreviated lpm) or cubic meters per minute. Flow rate has a direct bearing on the speed with which a hydraulic actuator moves a load and is therefore governed by the design concepts of the machine.

Example:

Let p₁ = 70 bar, and p₂ = 60 bar.

The pressure differential is now 10 bar.

Assume the flow to be 40 lpm.

If the pressure p₁ is increased to 90 bar and p₂ is increased to 80 bar, the flow is still 40 lpm because the pressure differential is still the same (10 bar)

This characteristic of fluid flow permits a wide variety of automatic flow control valves. Pressure compensated flow control valve is one such type.

In many hydraulic machines, load on the actuators are fluctuating which results in fluctuating actuator speeds. This is not desirable and it needs a technique or device which automatically maintains constant actuator speed irrespective of the variation in the actuator load.

Since load fluctuation leads to pressure variation at the actuator input line, a pressure compensated flow control valve which controls and maintains a constant flow rate in the actuator line will be a proper solution to maintain constant actuator speed.

Pressure compensated Flow Control valve is designed in such a way that it will maintain a constant flow rate over a limited range of pressure drop.

Principle of operation of the valve:

The volume flow rate for a hydraulic fluid flowing through a flow control valve depends on the pressure drop across the inlet and outlet of the valve. 

Pressure compensated flow control valves are designed such that the pressure drop across its inlet and outlet is maintained constant, thereby ensuring constant flow rate to maintain constant actuator speed.

Construction features:

Pressure compensated flow control valve has two orifices. One of the orifice is either fixed or manually adjusted. Size of the other orifice is a hydraulically controlled variable orifice.

Fixed orifice is used to set the desirable flow rate of fluid for a particular pressure drop (generally 8 bar). The valve is designed such that the hydraulically actuated orifice senses the load pressure and the supply pressure to change the size of the variable orifice to maintain a predetermined pressure drop across the fixed orifice.

Fig. 1 shows a schematic representation of a pressure compensated flow control valve.

 

Flow passes from inlet port A through the pressure compensator orifice then through the manually adjusted fixed orifice 1 which is located on the bottom right. The flow leaves the valve outlet port B. The valve spool, also called as the compensator spool, is located above and it hydraulically meters the size of the second orifice 2.

Pressure compensation is based on the use of pressure positioned variable orifice 2 upstream.

In the normal state of working, the hydraulic forces on the spool will hold the compensator spool in balance, but the bias spring force will force it to the extreme right, thus providing an unrestricted passage (holding the orifice 2 fully open) from valve inlet port A to the fixed orifice 1.

When the fluid enters the inlet port A, it is restricted by the fixed orifice, set manually. This results in the pressure rise in the control chamber C.

The control chamber C is connected to the valve spool on the right side, while the spring chamber S (at the bottom of the spool) is connected to the outlet port B.

To understand the working, following conditions are analyzed:

1.   When the outlet pressure at B is Zero.

2.   When the exit pressure at B rises.

When outlet pressure is zero, since the control chamber C is connected to the right side of the compensator spool, the spring force keeps the valve spool from moving to the left thereby keeping the orifice 2 fully open.

When the pressure in the control chamber is high enough to overcome spring force, the valve spool moves towards left. This leftward movement of the spool reduces the size of the orifice 2 (between the inlet and control chamber) metering the flow to the control chamber (i.e., the spool takes a position such that the orifice 2 allows the same flow as the preset orifice 1). In this condition, there is pressure balance between the control chamber pressure and the spring force.

When the inlet pressure rises (Upstream pressure variation), pressure in the control chamber C also rises, thereby moving the spool to the left to make the orifice 2 size smaller. This reduces the pressure in the control chamber to a value equivalent to the spring force.

When the outlet (exit) pressure at B rises due to the outlet flow moving an actuator (Downstream pressure variation):

The outlet flow from port B when connected to the inlet of an actuator and make it move against a load, the outlet pressure at B vary.

As the pressure at B rises, this would assist the spring to move the compensator spool to move to the right resulting in the increase of the size of orifice 2. The rise in exit pressure at point B will cause the balanced spool to move to the right (assisting the spring force) thus opening the orifice 2 to raise the pressure in the control chamber which is maintained equal with the spring force but the pressure drop across the orifice 1 is maintained equal.

When the outlet pressure at B lowers, the spool moves to the left to close the orifice 2 and thereby reduces the pressure in the control chamber.

Since the valve spool orifice 2 control maintains a constant pressure drop across the fixed orifice 1, the flow control valve maintains a constant flow.

Hence, a pressure compensated flow control valve is a variable resistance valve, which, for a given valve setting, maintains a constant flow across the valve irrespective of the exit pressure at B (the actuator working pressure to match the load resistance).

Fig. 2 represents the schematic diagram of pressure compensation principle to prove that the flow rate is depending on the pressure drop across the control orifice and measuring orifice.

 

Control orifice A₂ and measuring orifice A₁ are connected in series. The compensator spool is pressurized on the right by control chamber pressure p₂ and by exhaust pressure p₃ on the left along with bias spring force F_S.

 For pressure balancing:

Hence the pressure drop across the flow control valve will be:

SUMMARY OF THE WORKING PRINCIPLE (REFER FIG. 2)

The constant pressure differential is maintained as follows.

As the pressure increases at B (which is the pressure required to do the work), the pressure at spring chamber S will also increase, forcing the orifice 2 to open. This reduces the pressure drop across this orifice and increases the pressure in area C. If the pressure at area B decreases, so will the pressure at S. This means that the pressure at point C (which is now greater) will force the spool against the spring, thus closing orifice 2 and reducing the pressure at C and on the area of spool on the right side. This results again in equilibrium. In this manner this spool will be actuated by a difference in pressure at area C and area B and will always take a setting that will keep the pressure differential the same between C and B, thus providing the outlet with a constant volume flow.  

Flow Control Valve

 

FLOW CONTROL VALVE (FCV)

In a Hydraulic system, hydraulic power, P_H (in kW), is a product of system pressure p (N/m²) and fluid flow rate, Q (m³/sec). Mathematically,

From above basic equation, it is clear that modulation of hydraulic power is possible by modulating the system pressure and/or fluid flow rate.

In hydraulic systems, pressure control valves are used to modulate the pressure conditions at various parts. Similarly flow control valves helps in varying/modulating the fluid flow rate.

This post focuses on the function, working principle, types and application of flow control valves used in industrial hydraulic systems.

 

Introduction:

1.   What is the purpose of flow control in a hydraulic system?

Flow control in fluid power (hydraulic) system regulates the flow rate or Liters/minute which may be directed to an actuator such as a hydraulic cylinder or a hydraulic motor. Since the speed of the cylinder piston rod or rotary speed of hydraulic motor shaft is determined by the flow rate through the actuator, Flow (volume) controls may vary or maintain the selected actuator speeds.

 

2.   When is the flow control necessary?

Flow control becomes necessary when the pump delivery rate is more than the actuator requirement. This situation arises when the pump supplies flow for more than one actuator. Typical applications are: regulating cutting tool speeds, spindle speeds, travel rate of vertically moving loads in forklift, hydraulic winch speed to control the rate of hauling of the load, etc.,

 

 3.   How is the flow rate changed?

A flow control valve has a restricted passage whose size is either fixed are adjustable. The restricted passage increases friction to reduce the flow and decreases friction to increase the flow. Thus, flow rate can be changed by changing the size of the restricted passage.

 

Main functions of a Flow Control Valve:

Flow control valves are used to influence the speed of a hydraulic actuator (Cylinder, motor) by changing the opening to flow (decreasing or increasing) at the throttle point.

Flow control valves in hydraulic systems are necessary to control the rate of flow from one part of the system to another.

Flow control valves accomplish one or more of the following control functions;

 

Ø Limit the speed (v) of the linear actuator and hydraulic motors.

                     

Ø Limit the maximum power available to sub circuits by controlling the  flow to them, because

Ø Proportionately divide or regulate pump flow to various branches of the circuit.

  

A flow control valve is designed such that it becomes capable of reducing fluid volume down stream of itself relative to upstream.

Volumetric flow rate, Q, expressed in units of cc/sec or cc/min in SI metric measure is used to calculate the linear speeds of piston rods or rotational speeds of motor shafts.

 

Reference:

 

https://www.fluidpowerworld.com/hydraulic-symbology-204-flow-control-valves/

 

Flow control valve and the method of varying flow depends on the type of design and its location in a hydraulic circuit.

A simplest method of fixing a particular flow rate is with an orifice. An orifice is a small opening incorporated into a cavity.

                  Figure 1 (a)                                              Figure 1 (b)

Fig. 1 (a) and (b) shows the elemental types of fixed orifice. A simple disk with an orifice hole in the center will provide a method of flow control within a ferrule-type fitting as in fig. 1(a) or drilling out a hole in fitting forms another type orifice as shown in Fig. 1 (b). An orifice shall be as short as possible in depth while it is designed to be strong enough to withstand the effects of pressure.

 

Fig. 2 (a) and (b) shows the hydraulic symbols for a fixed flow control valve. Fig. 2 (a) shows the flow path which depicts smooth compression of the fluid. In this design the orifice has certain length in order to provide the gentle passage for fluid flow. Fig. 2 (b) depicts that the orifice is with sharp edge indicating the length of the orifice is minimum.

 

Fixed orifices are normally applicable for factory settings in pumps, manifolds and valves, but they rule out user adjustability.

Factors that determine the flow rate, Q, across an orifice are:

·        Cross sectional are of the orifice (mm²)

·        Shape of the orifice ( round, square, triangle)

·        Length of the restrictor/orifice as shown below

·        Pressure differential (∆p) across the orifice

·        Viscosity of the fluid (cSt) depending on temperature.

A variable orifice flow control valve provides a method to control the size of the gap between the needle and its seat, thus changing the flow rate through itself.

Fig.3 shows a needle valve where in the flow rate is adjusted by turning adjustment stem with a screw driver or a wrench. This design allows the flow to pass through the opening or orifice around the needle.  When the adjusting stem is raised, the orifice is made larger and the restriction to flow is less. When the adjusting screw is lowered the orifice is made smaller and the restriction for flow is greater. Once the adjustment stem is positioned to get the desired flow rate the setting is held by tightening the locknut.

 

The hydraulic symbol shows a diagonal arrow across the restrictor depicting adjustability.

In practice, the variable orifice flow control valves are fitted with an integral check valve such that the flow rate is controlled in only one direction and allows free flow in the reverse direction. In this design, Fig. 4 and Fig. 5 shows a typical designs where in the spool functions as needle during restricted flow and as poppet during reverse flow. In Fig. 3(c) the check valve blocks the flow from A to B there by forcing the fluid to pass through the restrictor. In the reverse flow, from B to A, the fluid unseats the ball poppet and bypasses the restrictor there by ensuring the free flow.

Typical hydraulic circuits incorporating Flow Control valves for speed control of actuators are discussed below:

Meter-in flow control:

Fig. 6 (a) (Reference taken from Yuken Kogyo text book), shows control of actuator speed, in forward direction, using flow control valve in the pressure line (feed line) of the actuator. FCV is located between the Direction control valve and the working port of Piston end. This position ensures that only forward stroke of the actuator is speed controlled. In the reverse direction of the actuator the check valve allows free flow of fluid, bypassing the FCV, thereby ensuring fast retraction.

Flow control valves with integral check valves are used when the reverse flow is not to be restricted.

The type of speed control illustrated in Fig. 6 (a) is known as Meter-in type of control which implies that the fluid flow is metered before it enters the actuator. Fig. 6 (a) depicts that the fluid is metered into the cap end of the cylinder to extend the piston rod at a fixed speed (as set on the variable flow control valve). The displaced fluid from the rod end of the cylinder flows unrestricted to the reservoir.

Take note of the pressure gauge (PG) readings.

PG1 shows the relief valve setting, 10 MPa, the system pressure.

PG2 shows 10 MPa equal to relief valve setting.

PG3 shows 4 MPa, which is equal to the load pressure. Note that the pressure in the system is dependent on the resistance to fluid flow by the load.

PG4 and PG5 indicates zero reading because the fluid is returning to the reservoir without any restriction.

Meter-in type of speed control is highly accurate, and is used where the load on the actuator resists the stroke at all times (no “runaway” situation)

 

Meter-out flow control:

Fig. 6 (b) (Reference: Yuken Kogyo text book), shows control of actuator speed, in forward direction, using flow control valve in the return line of the actuator. FCV is located between the rod end of the actuator and the Direction control valve. This position ensures that only forward stroke of the actuator is speed controlled by metering the exhaust fluid from the actuator. In the reverse direction of the actuator the check valve allows free flow of fluid, bypassing the FCV, thereby ensuring fast retraction.

The type of speed control illustrated in Fig. 6 (b) is known as Meter-out type of control which implies that the fluid flow is metered before it leaves the actuator. Fig. 6 (b) depicts that the fluid is metered out of the rod end of the cylinder to extend the piston rod at a fixed speed (as set on the variable flow control valve). The displaced fluid from the rod end of the cylinder flows restricted by the flow control valve and drains into the reservoir.

Take note of the pressure gauge (PG) readings.

PG1 shows the relief valve setting, 10 MPa, the system pressure.

PG2 shows 10 MPa equal to relief valve setting.

PG3 shows 10 MPa, which is equal to the system pressure.

PG4 shows 12 MPa (observe pressure intensification due to area difference between the piston and rod end). 10 MPa pressure on 100 cm² piston area pushes the piston with 100 kN force. Load pressure 4 MPa on the area 100 cm² offers a resistive load of 40 kN. The net force from the piston is (100 – 40 = 60 kN). Equivalent load from piston to the rod end oil is 60 kN on 50 cm² annular area. Hence the PG4 shows 12 MPa (=60 kN / 50x10^-4 m²), which is more than the system pressure of 10 MPa. Designers shall select the fluid components considering the pressure intensification.

PG5 indicates zero reading because the fluid is returning to the reservoir without any restriction after exiting the flow control valve.

Meter-out type of speed control is highly accurate, and is used where a free falling load or overhauling load tends to get out of control of the actuator. (“Runaway” situation)

  Bleed of flow control:

Fig. 6 (c) shows speed control of the actuator by Bleed-off method. In this method the flow control valve is installed on a bypass line to regulate flow to the tank and control the actuator speed. Compared to meter-in and Meter-out circuits, this method works with small power consumption because the pump’s discharge pressure is fully delivered to the load resistance. Given that the bleed flow is constant, the fluctuation of pump flow determines the actuator speed. i.e., the pump flow directly influences the load and pump’s volumetric efficiency. This circuit does not allow for control of multiple cylinders.

 

Bleed-off speed control method has a power saving advantage, as the pump operates always at the pressure required by the work load, and the excess pump flow returns to the tank via the flow control valve without being pushed over the relief valve.

Bleed – off method is not as accurate as Meter-in, since the measured flow goes to tank and the remaining flow into the actuator.

Observe pressure gauge readings and reason out the validity of the indicated readings.

 

Practice problems:

Identify the flow control valve in the circuit. Discuss the functioning of the circuits shown in Fig. 7, Fig.8 and Fig. 9 identifying the speed control methods adopted in it. Explain how both the forward and return strokes of the cylinders are speed controlled.