What Is a Hydraulic Differential Valve?by Pauline Gill
Hydraulic differential valves are used to detect differential pressure situations in compound hydraulic systems. The most common use is detecting malfunction or failure of one of the two or more hydraulic circuits in automotive braking systems. Hydraulic differential valves accomplish this task by sensing the difference in pressure between the two systems and alert the driver or operator of an impending problem in stopping the vehicle. Differential valves are also used to control the differential pressure across a fixed restriction such as an orifice or flow section to control the flow of a fluid.
Modern automotive brakes use dual master cylinders to actuate independent front and rear braking circuits, or diagonal braking circuits. Additional to this may be anti-lock braking functionality. Most vehicles monitor the status of the two independent primary master cylinder circuits. When brakes are depressed, the two independent pressures are compared in a hydraulic differential valve, which has a small limit switch between them. The two sides are isolated from each other by high pressure seals. A differential pressure between the two circuits would signify a malfunction in one of the redundant circuits, and the switch would be actuated, turning on a brake malfunction light in the driver dashboard. In such an event, the car's brake system should be checked.
Hydraulic differential valves can also be used to establish a flow though a flow restrictor by maintaining a constant differential pressure across the restrictor. For example, it may be necessary to control the flow of hydraulic fluid through a hydraulic motor powering a conveyor belt in an industrial application. If the plant has many hydraulic users, flowing pressure throughout the plant can vary widely, which would result in fluctuations of the conveyor belt speed. A hydraulic differential valve would closely modulate the flow through the motor by adjusting itself to maintain a constant differential pressure across a fixed restrictor prior to the motor inlet, which would maintain a constant speed in the motor independently of motor load.
Hydraulic differential valves may also be used in conjunction with hydraulic cylinders on jacks, lifts, cranes or hoists to keep the mechanism at a fixed position or to lift a specified amount of weight.
Hydraulic differential valves are also used to protect mercury-filled and other liquid-filled pressure measuring columns from overflowing one side. A hydraulic differential valve is connected to both pressure taps. If the differential process pressure being measured is less than the setting of the valve, it is allowed to be applied to the differential column for a reading by the technologist. However, if the pressure difference is too great, the hydraulic differential valve seals off both column pressure taps until the situation is alleviated.
Hydraulic differential valves can be used to prevent suddenly too hot or too cold showers when another person turns on another faucet, alleviating at least some domestic squabbles.
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