ENERGY LOSSES IN HYDRAULIC SYSTEMS AND THE TRANSITION TO LOAD SENSITIVE SYSTEMS

ENERGY LOSSES IN HYDRAULIC SYSTEMS AND THE TRANSITION TO LOAD SENSITIVE SYSTEMS

ENERGY LOSSES IN HYDRAULIC SYSTEMS AND THE TRANSITION TO LOAD SENSITIVE SYSTEMS

POWER (ENERGY) LOSSES in the Hydraulic System

In the hydraulic circuit diagram created in Figure 1, the most ideal state is in question. A fixed flow pump sends its flow directly to the cylinder. The load driven by the cylinder creates the load resistance (pressure) ps on the piston side of the cylinder. Here is a point that should never be forgotten; pressure is formed according to the action-reaction principle. In other words, there is pressure that occurs at the pump outlet in response to the effect (ps pressure) created by the load resistance. Therefore, if there is resistance, there is pressure.
Since the pump sends its entire flow to the cylinder
QP = Qs and pP = ps
is happening. (Losses in pipes are neglected here.)
In this case, if the total power usage is written:
vppvsSSpη600pQη600pQPP × isim = × Lincoln ==

-----------------------

Power used
Here, the lost power does not appear at all, as can be seen. The power drawn by the pump from the electric motor is used exactly in the system without any loss. This is the ideal state to be achieved.
While the system is working in this way, an orifice should be placed between the pump and the cylinder to narrow the section, as shown in Figure 2. What kind of changes will occur in this situation?

If the necessary precautions have been taken for the pressure increase in the circuit shown in Figure 2 (ie the pump, pipe and circuit elements are resistant to the pressure to be formed, the electric motor has the characteristics to meet the power created by the high pressure); Since the pump is a constant flow pump, the pump will send its entire flow to the cylinder again. But unlike Figure: 1, a pressure loss Δp will occur in the placed orifice. If another basic principle of hydraulics is remembered here; In hydraulic systems, oil passes through a certain cross section at a certain pressure loss at a certain flow rate. Here, the biggest phenomenon of hydraulic systems and a concept of Δp pressure loss that must always be considered is encountered. The pump flows into the cylinder; however, the load resistance and Δp formed in the orifice

sends if he can overcome the pressure loss. As it is accepted that every precaution has been taken here, the pump sends its entire flow to the cylinder.
So,
Qs = QP = Qhv and pP = ps + Δp
is happening.
If the total power loss is examined:
PS Phv
Power used Lost power
As can be seen from here, speed adjustment valves used in hydraulic systems cause lost power and thus heat. Well here, although the section is reduced; If the pump is still sending all its flow to the system; How will it be possible to adjust the speed of the hydraulic cylinder? In addition, the pump outlet pressure must be controlled. Based on these two requirements, a pressure relief valve should be installed in the pump outlet line as shown in Figure 3.

In this pressure relief valve, a value slightly higher than the pressure required in the system is set. Thus, the Δp pressure loss in the speed regulating valve is also taken under control. In this case, as long as the cross section in the speed adjustment valve is not changed, the flow rate corresponding to the Δp pressure loss that occurs will pass through the speed adjustment valve. The remaining flow will return to the tank through the pressure relief valve over the set pressure value.
Here the goal has been achieved. It will now be possible to adjust the flow rate by changing the cross-section on the speed adjustment valve. In this case, a part of the pump flow rate goes to the cylinder over the speed adjustment valve and the remaining part goes to the tank through the pressure relief valve. So:

QP = Qhv + Qbv and Qhv = Qs
If it is taken into consideration;
QP = Qs + Qbv and pp = ps + Δp = pbv
is happening.
If the total power loss is examined:
-------------- ------------- ----------------------
Ps Phv Pbv
-------------- ------------------------------------ -
Power used Total lost power
emerges.
When this formula is examined above; especially in hydraulic systems;
a) Speed ​​regulating valves
b) Pressure relief valves
it appears to cause lost power, hence heat. If; In any system with speed adjustment, if a manual contact is made to the speed adjustment valve and the pressure relief valve, it will immediately be felt that these valves are much more heated than the other elements.
The above-mentioned formula is applied to a hydraulic system below in order to make a better sense of the power and lost powers used. In the example given, slide motion in a machine tool application is analyzed.

0,25 Kw 0,53 Kw 4,12 Kw
5% * PP 11% * PP 84% * PP
Power used at speed regulating valve Pressure relief valve
lost power lost power
If the results in this formula are examined; It is seen that the most power usage is formed by the flow through the pressure relief valve. Then comes the power loss caused by the flow passing through the speed adjustment valve. In the given example, the power drawn by the pump from the electric motor is 4.9 Kw, while the required power consumption is 0.25 Kw, which makes up only 5% of the total power drawn.
Therefore, in hydraulic systems;
a) Discharging from pressure relief valve to tank
b) Flow rates passed through speed regulating valves at high pressure losses
Significant power losses occur. These power losses turn into heat and cause the oil to heat up, the warming of the oil causes the viscosity of the oil to decrease, the decrease of viscosity causes the pressure loss in the hydraulic elements to increase, and the increase of the pressure losses causes power losses as seen in the examples above.

TRANSITION TO PRESSURE SENSITIVE VARIABLE FLOW PUMPS:
As can be seen in the example examined, the most important part of the power losses was the power losses caused by the flows discharged from the pressure relief valves. Here, hydraulic equipment manufacturers have researched for years to reduce or eliminate this power loss. In the end, they decided to place the pressure relief valves used in the systems over the pumps. Figure 4.
Burada pompanın regülasyon yapısı incelendiği zaman 3 ana eleman göze çarpmaktadır.
1- Servo valve with pressure adjustment
2- Cylinder that tries to reduce the displacement angle of the pump.
3- Spring trying to increase the displacement angle of the pump
The servo valve number 1 is a 3-way 2-position pressure balance valve. The pressure value set on one side and the pump pressure on the other side are effective. If attention is paid; As long as the pump outlet pressure (pP) is smaller than the set pressure value (pp setting), the back of the cylinder is connected to the tank, so the spring no.3 creates the max displacement angle. In this case, the pump sends its maximum flow to the system.
When the electric motor is started; The pump will want to send its max flow rate to the system. However, due to the narrowing of the cross section in the speed regulating valve, the pressure at the outlet of the pump will start to increase. This pressure also acts on the no.1 servo valve. Increased pressure adjusted on servo valve pp. When it reaches the set value, the servo valve slide will change position, and due to the changed position, pressurized oil will come from the pump outlet line to the cylinder no. This pressurized oil will start to reduce the displacement angle of the pump by moving the cylinder.
So how long will the decrease in flow continue? This flow reduction will end when the flow delivered by the pump is equal to the flow through the speed regulating valve. Only the flow passing through the speed regulating valve; It should be kept in mind that it is dependent on the pressure loss occurring in the speed regulating valve and the cross section of the valve.
In summary; Pressure sensitive pumps send the flow required by the system to the system. For this regulation to take place, the pressure at the outlet of the pump must reach the set pressure (pp. Setting). So
QP = Qhv = Qs and pP = ps + Δp = pP. Setting
is happening.

If the total power loss is examined
Ps Phv
Power used Total lost power
When the above formula is examined; It is seen that the power loss caused by the flow under pressure through the pressure relief valve in systems with constant flow pump is eliminated. With the use of pressure sensitive pumps in hydraulic systems, one of the most important power losses is eliminated. In variable flow pump applications; Pressure sensitive pumps have a very good place for themselves.

TRANSITION TO LOAD SENSING VARIABLE FLOW PUMPS:
With the use of pressure sensitive pumps, the biggest power loss factor is eliminated. But it still maintains the power loss condition in the system caused by the pressure loss Δp. The biggest reason for this is; The pump cannot detect what kind of pressure changes in the system after the speed adjustment valve. This causes the Δp pressure difference to follow a continuously changing course depending on the system pressure. Here, after the speed adjustment valve, the pressure changes in the system must be transmitted to the pump in some way.
The mechanism used in pressure sensitive pumps is also used in load sensitive pumps. The only difference here is; It is the change of pressure balance in servo valve no. Pp pump on one side of servo valve

pressure, ps system pressure and Δp set value are effective on the other side. When the pressure balance is established here:
pp = ps + Δp setting or pp - ps = Δp setting
consists of.
Δp = pp -ps
Considering that:
Δp = Δp setting s
emerges.
Thus; The pressure loss Δp in the speed regulating valve, which cannot be controlled, is fixed to a certain value.
Here, too, when the electric motor is started, the pump will want to send its maximum flow to the system. However, due to the narrowing of the cross section in the speed regulating valve, the pressure at the outlet of the pump will start to increase. This pressure; When ps reaches the sum of the system pressure and the Δp set value, the servo valve slide no.1 will change position, and pressurized oil will begin to come to the cylinder no.2 due to the changed position. This pressurized oil will start to reduce the displacement angle of the pump, ie the flow rate, by moving the cylinder. This flow rate reduction, as in pressure sensitive pumps; The flow sent by the pump will continue until it is equal to the flow passing through the speed adjustment valve at the ayarp set value.

In summary; load sensitive pumps send the flow rate required by the system to the system at a predetermined and fixed pressure loss. So;
Qp = Qhv = Qs and pP = pS + Δp = ps + Δp adjustment
is happening.
If the total power loss is examined

---------------- -------------------
Ps Phv
Power used Total lost power
Although there appears to be a loss of power here, it remains at negligible levels as this is due to a controlled and fixed Δp pressure loss.

There are two kinds of costs caused by power losses in hydraulic systems. First; excess power drawn directly from the electric motor when power losses occur, secondly; the power consumed to cool the heat resulting from these lost powers. It can be clearly understood how important the resulting power losses are.
• Load sensitive variable flow pumps in systems with large pressure variations and different flow rates;
Pressure sensitive variable variable flow pumps in systems where there are no large pressure variations, but where different flow rates are required,
its use will contribute to significant amounts of energy savings.

ABBREVIATIONS USED
F: System resistance (kg)
vs: Roller advance speed (mm / sec)
Ds: Cylinder piston diameter (mm)
As: Cylinder piston area (cm2)
ps: System pressure (resistance) (bar)
Qs: Flow from the cylinder (lt / min)
Ps: Power used in the system (Kw)
pp: Pump outlet pressure (bar)
Qp: Pump flow (lt / min)
Pp: The power drawn by the pump from the electric motor (Kw / h)
pp adjustment: Pressure value set in pressure sensitive variable flow pump (bar)
Δp setting: Δp value set in flow rate sensitive variable flow pump (bar)
Δphv: Pressure loss in speed regulating valve (bar)
dhv: Cross-section diameter (mm) set at the speed regulating valve
Qhv: Flow rate passing through the speed regulating valve (lt / min)
Phv: Power loss in the speed regulating valve (Kw / h)
Thv: Heat (kcal / h) caused by loss of power in the speed regulating valve
pbv: Pressure relief valve pressure set value (bar)
Qbv: Flow rate passing through the pressure relief valve (lt / min)
Pbv: Power loss in pressure relief valve (Kw / h)
Tbv: Heat (kcal / h) caused by loss of power in the pressure relief valve
ηv: Total efficiency factor between pump and electric motor; 0.85 may be taken.
REQUIRED FORMULAS
Cylinder piston area:
400DΠA2ss × =
Roller advance speed:
sssA3Q500v × Çevir =
Power drawn by the electric motor
vppη600pQP × Çevir =
Flow through speed regulating valve
() () [] 44,1pΔ7,0DQ5,0hv2shv × Çevir =
Heat generated
Thv = Phv × 860
Strength achieved
F = As × ps

Source: I. NATIONAL HYDRAULIC PNEUMATIC CONGRESS AND EXHIBITION