What is an Unloading Valve? - Types, Principle

What is an Unloading Valve?

According to various sources, an unloading valve is a device that controls hydraulic pressure in a fluid power system. It plays a crucial role in ensuring that pressure is relieved when necessary, preventing potential damage to pumps and other components. Unloading valves can be pivotal in applications where careful pressure management is essential.

Unloading Valve

Imagine a system equipped with two pumps, both designated for specific tasks. Once these tasks are completed, it becomes necessary to maintain the desired pressure in the system while only operating one pump. The other pump must redirect its delivery to the tank at a low pressure, which is precisely the function of an unloading valve.

Principle

An unloading valve operates as a pressure control valve. It primarily functions based on hydraulic force rather than a spring force. When the hydraulic pressure reaches a threshold that exceeds the spring force, the valve spool shifts to manage fluid flow appropriately.

Purpose

The main purpose of an unloading valve is to relieve excess pressure in a system. When the delivery from the pump is not in use and is low pressure, the unloading valve routes it back to the tank. Control of the unloading valve may be achieved through a specialized cock or a pilot valve.

Types of Unloading Valves

Unloading valves can be categorized into two distinct types:

• Direct operated pressure Unloading Valve
• Pilot operated pressure Unloading Valve

Direct Operated Unloading Valve

A direct-operated unloading valve features a spool that remains closed due to a spring's force. Under normal conditions, this spool prevents the flow from the inlet to the tank port. When the pressure from the pump reaches the spring's threshold, the system bypasses the tank, allowing for the proper management of surplus fluid.

Pilot Operated Unloading Valve

This type of unloading valve includes an additional unloading spool, a component not present in pressure relief valves. In the event of built-up pressure within the pilot section, fluid flow is initiated towards the tank, causing an imbalance in the poppet. This imbalance ultimately leads to the relief of excess pump flow into the tank.

Double Pumps with Unloading Valve

One of the primary applications of an unloading valve is within dual pump circuits. It combines a high-pressure, low-flow pump with a low-pressure, high-flow pump to create efficient operations. The unloading valve ensures fluid is managed effectively, allowing for adjustments in pressure while maintaining optimal functionality.

Advantages

• The setting pressure of an unloading valve is typically lower than that of a relief valve.
• Recirculation with a second pump in the circuit helps to prevent the oil from becoming excessively hot.
• This practice prolongs the lifespan of O-rings by averting overheating of the oil.
• Utilization of unloading valves results in lower energy consumption.

Reference: Fluid Power Control Systems by MD Faiyaz Ahmed.