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Hydraulic pressure balances

1 - Introduction

It can be seen that the flow which passes through a restrictor changes according to the pressure difference at its terminals.

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Q=K * S * √∆p

Q = Flow in l/min

K : Shape coefficient of the orifice (take 1 if unknown)

S = Hole section in mm²

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Proportional directional valves work like restrictor, since they allow part of the flow to pass according to the instruction they receive.

Therefore the flow in a proportional directional valve is not conserved and changes according to the load of the receiver.

The installation of a pressure balance also called compensator can correct this problem.

Pressure balances can be installed:

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2 - Pressure balance in parallel with the directional valves

The pressure balance is installed in parallel with the directional valves. It behaves like a pressure limiter whose adjustment value changes according to the load on the receivers.

Balance de pression en parallele

In the Fig. A :

Rep 1 : Heat engine..

Rep 2 : Fixed displacement pump.

Rep 3 : Main pressure relief valve.

Rep 4 : Parallel pressure balance.

Rep 5 : Proportional directional valve.

Rep 6 : Proportional directional valve.

Rep 7 : Circuit selector.

Rep 8 : Hydraulic motor.

Rep 9 : Hydraulic motor.

The pump driven by the combustion engine delivers 100 l/min. The pressure relief valve protecting the pump is set at 200 bars.

The flow from the pump is directed to the proportional directional valves.Directional valves receive a 10% deposit. The flow passing through the directional valves drives the hydraulic motors and compresses to 100 bars for the Rep 8 motor and 50 bars for the Rep 9 motor.

The circuit selector valve directs the pressure of the most loaded engine (100 bars) to the spring of the pressure balance Rep 4. The pressure balance is at this moment 100 bars of hydraulic pressure + 10 bars of spring. The excess flow 65 l/min returns to the tank via the pressure balance.

It can be seen that the pressure difference (Δp)  at directional valve terminals is different. 10 bars are observed at the terminals of the directional valve Rep 5 and 60 bars at the terminals of the directional valve Rep 6. This explains the difference in flow through the directional valve.

 The pressure balance makes it possible to maintain a Δp  constant at the terminals of the directional valve which has the most loaded movement. The speed of the most heavily loaded motor remains constant to the detriment of the other receiver. This assembly is reserved for systems that do not work with simultaneous movements.

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3 - Distributor pre-compensated

The pressure compensators are installed in series before the directional valves. They behave like pressure reductions whose value changes according to the loads of the receivers. The parallel pressure balance is retained to avoid rolling excess flow into the pressure relief valve.

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Balance de pression en serie precompensee

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In the Fig. B :

Rep 1 : Heat engine..

Rep 2 : Fixed displacement pump.

Rep 3 : Main pressure relief valve.

Rep 4 : Parallel pressure balance.

Rep 5 : Proportional directional valve.

Rep 6 : Proportional directional valve.

Rep 7 : Circuit selector.

Rep 8 : Hydraulic motor.

Rep 9 : Hydraulic motor.

Rep 10 : Pressure balance in serie.

Rep 11 : Pressure balance in serie.

 

 

 

 

 

 

 

The pump driven by the combustion engine delivers 100 l/min. The pressure relief valve protecting the pump is set at 200 bars.

The flow from the pump is directed to the proportional valves.

Directional valves receive a 40% deposit. The flow passing through the pressure balances and the directional valves drives the hydraulic motors by compressing to 100 bars for the Rep 8 motor and 50 bars for the Rep 9 motor.

The circuit selector valve directs the pressure of the most loaded engine (100 bars) to the spring of the pressure balance Rep 4. The pressure balance is at this moment 100 bars of hydraulic pressure + 10 bars of spring. The excess flow of 20 l/min returns to the tank via the pressure balance.

At the same time, the pressure of the load of each hydraulic motor is brought back to control on the pressure balances installed in series before each directional valve.

Pressure compensators behave like pressure reducers whose setting value changes depending on the receiver load. Load + 5 bars of spring.

It can be seen that the pressure difference (Δp)  at the directional valve terminals is identical. (5 bars) The flow through the directional valves is the same and the motors run at the same speed.

This assembly is recommended for systems that operate with simultaneous movements.

This assembly works as long as the pump manages to saturate the directionals with oil. If the flow rate of the pump is no longer sufficient (lower thermal regime, increase in the setpoint of the directional valve, etc.) the (Δp)  will not be maintained at the terminals of the directional valves, the speeds of the receivers will not be maintained and will remain random.

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4 - Post-compensated directional valve

The pressure compensators are installed in series after the directional valves. They behave like sequence valves whose value changes according to the loads of the receivers. The parallel pressure balance is retained to avoid rolling excess flow into the pressure relief valve.

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Balance de pression en serie postcompensee
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In the Fig. C :

Rep 1 : Heat engine..

Rep 2 : Fixed displacement pump.

Rep 3 : Main pressure relief valve.

Rep 4 : Parallel pressure balance.

Rep 5 : Proportional directional valve.

Rep 6 : Proportional directional valve.

Rep 7 : Circuit selector.

Rep 8 : Hydraulic motor.

Rep 9 : Hydraulic motor.

Rep 10 : Pressure balance in serie.

Rep 11 : Pressure balance in serie.

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The pump driven by the combustion engine delivers 100 l/min. The pressure relief valve protecting the pump is set at 200 bars.

The flow from the pump is directed to the proportional valves.

Directional valves receive a 40% deposit. The flow passing through the directional valves and the pressure balances drives the hydraulic motors by compressing to 100 bars for the Rep 8 motor and 50 bars for the Rep 9 motor.

The circuit selector valve directs the pressure of the most loaded engine (100 bars) to the spring of the pressure balance Rep 4. The pressure balance is at this moment 100 bars of hydraulic pressure + 10 bars of spring. The excess flow of 20 l/min returns to the tank via the pressure balance.

At the same time, the pressure of the load of each hydraulic motor is brought back in control on the pressure balances installed in series after each distributor.

Pressure compensators behave like sequence valves whose setting value changes depending on the receiver load. Load + 5 bars of spring.

It can be seen that the pressure difference (Δp)  at the directional valve terminals is identical. (5 bars) The flow through the directional valves is the same and the motors run at the same speed.

This assembly is recommended for systems that operate with simultaneous movements.

The notable difference with the pre-compensated directional valves is that the speed of the receivers will remain controlled even if the pump does not manage to saturate the directional valves with oil. (Fig.D)

 
Balance de pression en serie postcompensee 2
 

In Fig. D, the internal combustion engine has slowed down and the flow rate of the pump can no longer saturate the oil consumption requested by the directional valves. The parallel pressure balance Rep 4 closes, the pressure at the pump outlet tends to decrease (108 bars observed in M1). The opening pressure of the pressure balances is always the same (load pressure + 5 bars of spring). We see that the (Δp)  at the directional valves terminals will decrease but will remain the same on both circuits. The flow through the directional valves will be reduced while keeping the same setpoint proportions. The speed of the receivers remains under control. This concept is called "flow sharing"

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