SIMULATION ANALYSIS OF SELECTION OF LOAD HOLDING VALVES WITH LOWEST POSSIBLE POWER LOSSES
Dissertation Submitted in Partial Fulfillment of the Requirements for the Degree of
Master of Technology
MINING MACHINERY ENGINEERING
Admission No. – 2016MT001344
Under the guidance of
Under the guidance of
Dr. K. DasguptaProfessor
Dept. of Mining Machinery Engineering
INDIAN INSTITUTE OF TECHNOLOGY (INDIAN SCHOOL OF MINES), DHANBAD
A person cannot go through life without the help and guidance from others. One is invariably indebted, which may be of physical, mental, physiological or intellectual in nature, knowingly or unknowingly. To enlist all of them is not easy. To repay them even in words is beyond our capabilities. It is a pleasure to thank the many people who made this project possible.
With recondite honor, we offer our sincerest gratitude to our supervisor DR.K.Dasgupta, Professor, Department of Mining Machinery Engineering for his enthusiasm, his inspiration and his great efforts to explain things clearly and simply. He took great pain in going through the entire literature, and made immortal comments and acted as a beacon at each and every step in the accomplishment of this project work. We also wish to express my gratitude towards Dr. Niranjan Kumar for providing his selfless help and inputs, thereby adding another brick in the wall in making of this project.
We wish to express our heartfelt gratitude to Dr. L. A. Kumaraswamidhas, course coordinator, Department of Mining Machinery Engineering, for granting me the necessary permission to undertake the above titled project.
We would also like to thank Dr. L. A. Kumaraswamidhas, Head of the Department, Department of Mining Machinery Engineering, for providing excellent experimental and other facilities we needed to produce and complete our project and who has supported us throughout our project with his patience and knowledge.
Lastly, we would like to thank our JRF, especially Mohit sir, for their support in our study and experimental work.
We hereby declare that the dissertation entitled “SIMULATION ANALYSIS OF SELECTION OF LOAD HOLDING VALVES WITH LOWEST POSSIBLE POWER LOSSES” submitted to INDIAN INSTITUTE OF TECHNOLOGY (INDIAN SCHOOL OF MINES), DHANBAD is a record of original work done by us under the guidance of DR K.DASGUPTA, Professor, Department of Mining Machinery Engineering and this work has not been presented anywhere fully or partially for the award of any degree.
Place: (PARWEZ ALAM)
In this project a novel hydraulically implemented regenerative circuit has been discussed. Using MATLAB/Simulink® R2016a software platform the hydraulic system for SELECTION OF LOAD HOLDING VALVES WITH LOWEST POSSIBLE POWER LOSSES. The circuit has been developed in Simulink environment, using Simscape / SimHydraulics library blocks.
This project deals with reducing power losses caused by counterbalance valves. These valves are used in the return line of cylinder and motors to control overrunning, negative loads that work in the direction of movement. The purpose of the valves is to hold a load and to dissipate the energy of the load when it moves so that the overrunning load becomes a controllable load.
Furthermore, the future extension of this project would deal with the study of hydraulic pump loading characteristics. This would provide the energy efficiency of the circuit and the amount of total energy saved after the incorporation of the pilot counterbalance valve to reduce power losses.
Keywords: MATLAB/Simulink; simulation; counterbalance; hydraulic cylinder; hydraulic pump; direction control valve.
SYMBOL UNIT DESCRIPTION
A m2 Effective area of cylinder or motor
CHA m5/N Capacity between cylinder and counterbalance valves
CHR m5/N Capacity between directional valve and cylinder
CR – Cylinder ratio : area downstream/area upstream
F N Force on cylinder
Grelifm5/N s Relief gain of counterbalance valves,dQ/dP1
Gpilotm5/N s Pilot gain of counterbalance valves ,dQ/dPpilotPloadN/m2 Load induced pressure
P1 N/m2 Pressure between cylinder and counterbalance valve
PpilotN/m2 Pilot pressure to open counterbalance valve
PR – Pilot ratio of counterbalance valve
Q m3/s Flow across counterbalance valve
x m/s Cylinder velocity
Energy is the most important factor in almost every field, particularly in the field of machinery/mechanical engineering. The accelerated use of energy resources by the industry has led to an energy crisis. To overcome this energy crisis and to meet the demand for a more environmental, fuel and energy efficient construction machinery various methods have been developed. The Hybrid Power System usually has two energy sources including a combustion engine and an electrical storage device. The fuel economy is increased by operating the engine in an optimum efficiency range with a proper control strategy. Energy saving systems may be broadly classified into electrical, mechanical, and hydraulic systems. These systems use batteries, flywheel and hydraulic accumulator as the energy storage unit.
In particular energy efficiency plays a vital role in Heavy Earth Moving Machinery (HEMM). Earlier, the energy transformation in a machine was done by the means of mechanical components only. As the technology progressed and new inventions were made, a gradual advancement in technology could be witnessed as the mode of these energy transformation shifted from mechanical mode to hydraulics mode. The energy transformation using fluids i.e. incorporation of hydraulic systems has been introduced into the system designs.
At present, in almost every segment of heavy earth moving machinery, use of hydraulic system has increased and will be increasing. The reason behind this is because it has a lot of advantages over mechanical system, for example with the help of hydraulics, power can easily be distributed or it can be applied for one specific operation such as heavy load lifting or lowering operation or multiple task at a time can be carried out very easily.
Other advantages of fluid power have been listed as follows:
Large load capacity with almost high accuracy and precision
Automatic lubricating provision to reduce to wear
Division and distribution of hydraulic force are easily performed
Limiting and balancing of hydraulic forces are easily performed
Hydraulic power systems are designed to perform mechanical work with the help of hydraulic fluid. In fluid power systems, power is obtained by the direct action of a pressurized fluid on fluid cylinder or fluid motor. Hydraulic systems are especially suitable for those operations that are characterized by abrupt loading, frequent stops and starts, reversing and speed variations that cause sharp peak, cyclic and fluctuating power demands. These advantages make fluid power systems very popular.
One of the main advantages of fluid power systems is that they can be designed to lift very heavy objects with the help of mechanical advantages and by using fluid as the transfer medium thus removing complicated mechanical linkages. The hydraulic fluid can be transmitted with the help of flexible hose pipes to perform the required operation. A modern hydraulic system consists of many elements, such as hydraulic pumps, motors, distributors, valves, hydraulic accumulators, filters, hydraulic axes etc. The processes taking place in these systems are fast changing, i.e. working fluid pressure and speed change quickly.
A wide range of fluids are available to be used in the fluid power systems. Working fluid containing gases are also used commonly, for example air, the existence of which in the liquid greatly reduces the speed of sound propagation. Solutions offered to physical processes in hydraulic systems, depending on the characteristics of the system itself, have great practical significance. As mentioned earlier, for the purpose of energy storage we use accumulators in hydraulic systems. The overall efficiency of a very simple pump-controlled hydraulic system under ideal operating conditions is about 70% 2.
Now the important criterion is to increase the efficiency of the machine. The efficiency of the machine can be increased using following two methods viz. maximum utilization of energy by reducing losses or by incorporating energy saving accessories such as accumulators to regenerate energy by reducing the load on the pump used in hydraulic system or by improving efficiency of the individual components working in the system. In this work we study about how the components (counterbalance valves) vary under different conditions. Also, we will be discussing about how the counterbalance valve parameters vary with other parameters related to the hydraulic system.
1.2 COUNTERBALANCE VALVE
Load holding valves do not contribute to power saving. One type of load holding valve the counterbalance valve is an energy destroying element that convert not only the stored potential energy from motors into heat but also works as a resistance when there is no overrunning load or positive load. In all cases the resistance grows with increasing the speed of the motor. For safety reason it is often acceptable to convert the stored potential energy of the motor into heat instead of storing it in an accumulator e.g. the additional losses through the counterbalance valve comprise: the growing losses at increasing speed and the losses when there is no load or a positive load. These losses can be reduced by choosing the right counterbalance valve and by changing the circuit.
Load holding valves are used in the return line of cylinder or motors to keep the motor in a locked position or to avoid overrunning of load. FIGURE 1 shows that pilot to open check valves that keep a cylinder locked when the directional valve is in Centre position. The pilot to open check valves are either fully open or fully closed .therefore they can’t be used to control an overrunning load. Counterbalance valves in contrast FIGURE 2 are modulating valves. Pilot pressure and load pressure can open the valves partially to ensure a smooth lowering of the load. The counterbalance valve can be seen as a relief valve with a setting high enough to keep the highest expected load in position. The recommendation setting is 30% above the maximum induced load pressure.
When pilot to open check valves are open they cause only low pressure losses. When counterbalance valves open, they will full open only at very high pilot pressure, so there needs to be a high pressure on the inlet side of the cylinder to fully open the counterbalance valve in the return line. Only at high positive loads will the counterbalance valve will be fully open. A partially closed counterbalance valves can cause unnecessary pressure drop and power losses.
As mentioned before, counterbalance valves are characterized by a pilot area on which pressure coming from the actuator’s feeding line acts. Such pressure, together with the pressure due to the load, movespilot piston, progressively contrasting the force generated by the setting spring.Hence the combined action of the two pressures is connected to the ratio between the pushing areas onwhich they act. This ratio is known as “Pilot Ratio” (pr), and it is the basic parameter for any counterbalancevalve.
Pilot Ratio (pr) is defined as the geometrical ratio between the area on which the load acts (port 1) and the pilot area (port 3). Thanks to this parameter, it is possible to calculate the values of pilot pressures first opening (Px):
According the Pilot Ratio, counterbalance valves can be divided in 2 types:
-High Pilot Ratio (>6:1): suitable for those applications where the loads are constant (for instance, hydraulic motors) and very stable, where low pilot pressures are demanded in favor of speed and energy savings.
-Low Pilot Ratio (<5:1): suitable for those applications where loads can vary (for instance, trucks cranes) and for those mechanical structures are not stable, where more control and more stability are needed, an higher pilot pressure is required.
When counterbalance valves are installed on hydraulic actuators, to determine the correct value of pilot pressure it is necessary to introduce in the calculation the ratio between the areas of the actuator itself.
Px = (Pt – P1) / (rp+ra)
ra: ratio between the areas of the hydraulic actuator
Because of coupling counterbalance valves with directional control valves, considering the type of spool to use is needed. When the counterbalance valves are in charge of the pressure relief function, it’s essential to make a distinction between “closed-Centre” spool applications and “open-Centre” spool application. Generally, when “closed-Centre” spools are installed, it’s necessary to use compensated counterbalance valves: since these valves are insensitive to back-pressure on return line (A-2), their pressure setting won’t change.
Two examples of compensated valves application are regenerative circuits and circuits in which draining of eventual pressure peaks must be relieved in series by the anti-shock valves installed inside thedirectional control valve.
In case of “open-centre” directional spool application, not-compensated valves are compulsory, in which the spring is connected to the return line (A-2).
Relief compensated type counterbalance valves or CC type
These valve modules have a special configuration of the relief piston that allows the relief opening independently from any back pressure whereas the piloted opening remains subject to additive pressure at port V2.
Single acting counterbalance valve “CC” type
They are employed when it is necessary to relief pressure at the pre-established pressure setting (Pt), without over-pressurizing the system, independently from any back-pressure in the return line. They are normally fitted in conjunction with main control valves having closed centre spools equipped with port relief valves.
Vented type counterbalance valves or CCAP type
These valve modules have a fully vented spring chamber and both the relief opening and the piloted opening are independent from back-pressure at port V2. Venting is often open to atmosphere and, whenever possible, is connected to tank or to a low pressure line.
They must be used only in conjunction with main control valves having closed centre spools and equipped with port relief valves.
Single acting counterbalance valve “CCAP” type ThisThis type of “fully balanced” valves is necessary in a few typical applications and exactly:
a) When the piloted opening determines a reverse flow toward a highly pressurized line (example: regenerative circuits for cylinders, where the oil from the annular chamber is recycled into the line feeding the full bore side, or series type circuits, where the oil unloaded by an actuator is employed to power a second actuator).
b) When pressure surging in the oil return line could cause oscillations of the relief piston which would amplify flow instability and fluctuations.
c) When the pilot opening is controlled through a joystick which delivers a “low pressure signal” and the relief piston needs to maintain stable open positions also with strong pressure fluctuations.
d) When the counterbalance valve is part of a closed loop circuit with pressure upstream and downstream.
1.3 WORKING OF COUNTERBALANCE VALVE
Counterbalance valves are used in hydraulic systems working with overriding (running-away) or suspended load. They are designed to create backpressure at the return line of the actuator to prevent losing control over the load. The following illustration shows a counterbalance valve schematic.
If a directional valve is shifted into position that lowers the load, then the fluid from the rod chamber of the cylinder can exit only if pressure at port P (pilot pressure) and port L (load pressure) create enough force to overcome the spring force and open the valve. In statics, the valve is described with the equationF0+c?x=ppilot?Apilot+pload?Aload?pback?Aback
F0 Spring setting
c Spring rate
x Valve opening
ppilotPilot pressure (pressure at port P)
ploadLoad pressure (pressure at port L)
pbackBackpressure (pressure at return port B)
ApilotValve effective area at pilot port P
AloadValve effective area at load port L
Aback Valve effective area at return port B
Counterbalance valve, classified by type, is an internally-externally piloted valve because both the pilot pressure and the load pressure tend to open the valve
pset + cp?x= ppilot? kpilot+ pload ? pback? kback
pset = F0/Aload
cp = c/Aload
kpilot = Apilot/Aload
kback = Aback/Aload
psetValve pressure setting
cpSpring pressure stiffness (Pa/m)
x Valve opening
The valve displacement is determined
x = (pset? (ppilot?kpilot+pload?pback?kback))/cp
Where xmax is the maximum valve displacement.1.4 PRESSURE LOSSES THROUGH COUNTERBALANCE VALVES.
Usually a counterbalance valve is set at a crack pressure 30% above the highest expected load pressure. The required pilot pressure to open the counterbalance valve can be calculated from the setting of the counterbalance valve, the pilot ratio, the cylinder ratio and load .In figure 2 the pilot pressure is identical to the pressure between the directional valve and the rod end side of the cylinder.the equation (1) shows the force equilibrium on the cylinder equation and the equation (2) show the crack pressure of the counterbalance valves are:
Ppilot . A/CR + F = Pload .A (1)
Ppilot . PR + Pload = S (2)
These can be combined so that from (1) and (2) the required pilot pressure can be calculated.
Ppilot = S-PLOADPR+1/CR (3)
In equation 1 and 2 Pload is the induced pressure.it is identical to the pressure P1 between motor/cylinder and counterbalance valves as long there is no pilot pressure Ppilot. The setting S of the counterbalance valve should be 30% above the highest expected load induced pressure F/A. For very low flows and high loads the required pilot pressure low. For a load induced pressure of 160 bar,a stting of 210bar,a cylinder ratio CR of 2 and a pilot ratio of 3 the required pilot pressure is 14 bar. The required pressure is higher at around 60 bar when there is no load helping with the opening the valve. The required pressure increase further with flow. This increase is difficult to calculate since the opening characteristics of the valves and flow forces, more than the spring rate, affect these pressures.
the actual pilot pressure and pressure between motor/cylinder and counterbalance valve is where the characteristics of the counterbalance valve intersects with the line that describe the force equlibrium.at higher flows the required pressure to move the cylinder increases due to the characteristics of the counterbalance valves. Counterbalance valves with a steep relief curve result in higher required pressures to move the motor/cylinder.
Pressure loss with respect to efficiency:
A high negative load requires low pilot pressure / inlet pressures.it is the purpose of the counterbalance valve to convert energy into heat.
A high positive load requires a high inlet pressure/pilot pressure. This pressure is used to drive the motor and also fully opens the counterbalance valves so that the resistance in the return line is low.
No load is the condition when the counterbalance valves cause the highest unnecessary losses.
2.1Improving System Efficiency by Improving Individual Components
The power efficiency of hydraulic system is affected by both the component and system design. Because of the interest in improving hydraulic system efficiency, individual components (pump, motor, actuator, valve etc.) have been studied extensively by component manufactures and researchers; much progress over the past decade has been made on the improvement of the component efficiency. However, what is more important for system efficiency is how these components are combined to meet the load demands. There are many combinations of components which can be used to accomplish a single task. For example, a variable displacement pump/fixed displacement motor, a fixed displacement pump and motor with a variable speed motor drive or a fixed displacement pump and motor with a flow modulation valve can all be used to vary the rotational speed of a load.
However, the efficiency of each system can be vastly different depending on the loading conditions even though the efficiency of pumps and motors can be very similar. Thus circuit design is the most important factor for power efficiency consideration. Any kind of power transmission technology must be controllable yet efficient. The control of a hydraulic system is achieved by modulating the flow rate of the fluid. Four main methods are used to control flow:
(i)Controlling the power supply unit (engine or electric motor)
(ii)Controlling the displacement of the hydraulic pump
(iii) CBV simulation model
(i)Controlling the Power Supply Unit
The power supply control method changes the delivery flow rate of a fixed displacement pump by changing rotational speed. An energy saving power source proposed by Nakano and Tanaka is shown in Fig. 2.1, in which a fixed displacement pump is driven by an induction motor that uses a frequency converter to control the rotation 7.
Fig. 2.1 Energy-saving power source with inverter-motor drive Nakano and Tanaka, 1988
In this system, the flow rate is nearly proportional to the converter frequency. Because the inertia of the induction motor is so large that it cannot respond rapidly to the demanded input, the pump could not supply the demanded flow rate to the load during the transient. To solve this problem, an accumulator was used to provide supplemental flow to the system in the transient condition. The principle of this system is similar to a pressure compensated pump. The rotational speed of the pump is controlled to supply the necessary amount of oil to the system, and to maintain the system pressure at a certain constant level without the use of a relief valve.
In order to maintain the system pressure at a constant value, the system pressure is sensed and fed back to a frequency converter controller by which the rotational speed of the induction motor is controlled. When compared with the conventional constant flow hydraulic power source such as shown in Fig. 2.2, the use of the frequency converter drive demonstrated a 36% saving of the total power.
Fig. 2.2 Power Loss of a Valve Controlled Hydraulic System
Fig. 2.2 Power Loss of a Valve Controlled Hydraulic System
Pump-controlled systems are the preferred hydraulic power drive systems for applications in which large horsepower is required. The actuator (motor or cylinder) in a pump-controlled system is controlled by adjusting the displacement of the pump which is driven by a constant rotational speed power source. The advantage of these kinds of systems is high efficiency because there are no “system dependent” losses (pressure and flow losses) in the system. However, a limitation of pump-controlled systems is that one pump can only control one load although a pump can supply flow to many actuators. Pump-controlled systems can appear in two forms, one is an open circuit shown in Fig. 2.3(a), and the other is a closed circuit shown in Fig. 2.3(b) , commonly defined as a hydrostatic system in which the return fluid is ported directly back to the inlet of the pump rather than through a reservoir. The advantages of open circuit pump control systems are simple configuration and higher capacity of heat dissipating; on the other hand a closed circuit pump control system is characterized by the reduced system size and oil volume. Hydrostatic systems contain a fixed displacement motor and a replenish circuit which is used to keep a minimum pressure in each line and supply supplemental fluid to each line due to the leakage.
When compared with valve-controlled systems, pump-controlled systems have higher system efficiency; however their dynamic performance is often poor. This is the result of two factors: (1) the natural frequency is reduced by a factor of because only one line between the pump and actuator is controlled; thus the trapped oil spring rate is one half of that of the valve-controlled system (2) if the length of the line between a valve and actuator is same to the length for a pump, the compressed fluid volume is larger with a pump than that of a valve 8.
Fig. 2.3 Typical pump controlled hydraulic system Merritt, 1967
3. CBV simulation model
This section describes the lumped parameter approach utilized to simulate the reference CBV of fig 4. The model is able to study
Fig. 1. Reference CBV symbol (ISO 1219-1) 5, and simplified cross section of the reference CBV.
Fig. 4 Geometrical and force balance details of the poppet and check valve body of the CBV.
the poppet model, which calculates the flow areas between the directional valve and the actuator ports, accounting for the flow forces, leakages and friction phenomena involved with the pop-pet motion;
The check valve model, which simulates the opening of the CBV when it operates as check valve, accounting for the friction and internal leakages;
the poppet/check valve contact model which simulates the relative motion of the poppet and the check valve body and their mutual contact. This portion of the model also accounts for the calculation of the poppet/check valve relative position and, therefore, of the flow area used to calculate the flow and the flow force, given by Eqs 5, 6, 7
Fig. 7. Semplified test circuit used to characterize the CBV (ISO 1219-1).
At each time step and in each sub-model, the fluid properties (density, bulk modulus, and kinematic viscosity) are evaluated in each part of the model as a function of the instantaneous pressures evaluated by the three chambers.
2.2 THERE ARE DIFFERENT WAY TO REDUCE THE LOSSES THAT CAN BE CONSIDERED.
1. Using counterbalance valves with a higher pilot ratio except that these types of counterbalance valves may become unstable in the application 8
2. Using counterbalance valves with a flatter relief curve so that the losses don’t increase as much with increasing flow. 8
3. Using additional counterbalance valve that open and reduce the resistance in the return line at low or positive load when instability to occur 8