Inst ToolsInst ToolsInst Tools
  • Courses
  • Automation
    • PLC
    • Control System
    • Safety System
    • Communication
    • Fire & Gas System
  • Instrumentation
    • Design
    • Pressure
    • Temperature
    • Flow
    • Level
    • Vibration
    • Analyzer
    • Control Valve
    • Switch
    • Calibration
    • Erection & Commissioning
  • Interview
    • Instrumentation
    • Electrical
    • Electronics
    • Practical
  • Q&A
    • Instrumentation
    • Control System
    • Electrical
    • Electronics
    • Analog Electronics
    • Digital Electronics
    • Power Electronics
    • Microprocessor
  • Request
Search
  • Books
  • Software
  • Projects
  • Process
  • Tools
  • Basics
  • Formula
  • Power Plant
  • Root Cause Analysis
  • Electrical Basics
  • Animation
  • Standards
  • 4-20 mA Course
  • Siemens PLC Course
Reading: What is an Unloading Valve? – Types, Principle
Share
Notification Show More
Font ResizerAa
Inst ToolsInst Tools
Font ResizerAa
  • Courses
  • Design
  • PLC
  • Interview
  • Control System
Search
  • Courses
  • Automation
    • PLC
    • Control System
    • Safety System
    • Communication
    • Fire & Gas System
  • Instrumentation
    • Design
    • Pressure
    • Temperature
    • Flow
    • Level
    • Vibration
    • Analyzer
    • Control Valve
    • Switch
    • Calibration
    • Erection & Commissioning
  • Interview
    • Instrumentation
    • Electrical
    • Electronics
    • Practical
  • Q&A
    • Instrumentation
    • Control System
    • Electrical
    • Electronics
    • Analog Electronics
    • Digital Electronics
    • Power Electronics
    • Microprocessor
  • Request
Follow US
All rights reserved. Reproduction in whole or in part without written permission is prohibited.
Inst Tools > Blog > Control Valves > What is an Unloading Valve? – Types, Principle

What is an Unloading Valve? – Types, Principle

An unloading valve is a pressure control valve that works on the principle of the hydraulic force as opposed to a spring force.

Last updated: September 28, 2022 2:34 pm
R Jagan Mohan Rao
Control Valves
No Comments
Share
6 Min Read
SHARE

In certain specific applications, there is a need for completely unload pumps flow to the tank instead of relieving it over a relief valve. This can be done using unloading valves.

Contents
Unloading ValvePrinciplePurposeTypes of Unloading ValvesDirect Operated Unloading ValvePilot Operated Unloading ValveDouble Pumps with Unloading ValveAdvantages

Unloading Valve

Unloading Valve
Image Courtesy: Mechatronics Control

Let us imagine a case of a system, where there are two pumps. Both pumps are required to perform specific work.

After completion of the job, there is a need to maintain the required pressure in the system by operating only one pump and the delivery of the other pump must be sent to the tank at low pressure. This job is done by an unloading valve.

Principle

An unloading valve is a pressure control valve that works on the principle of the hydraulic force as opposed to a spring force.

When pressure builds to the point, where hydraulic force is greater than that of the spring force, then the valve spool is shifted.

Purpose

The unloading valves are used for relieving the extra pressure in a system, that is at low pressure and connecting it to the tank when the delivery of the pump is not used.

The unloading valve may be controlled by a special cock or a pilot valve.

Types of Unloading Valves

The unloading valves are classified into two types.

They are as follows.

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

Direct Operated Unloading Valve

A direct-acting unloading valve consists of a spool held in the closed state by a spring.

The spool blocks flow from the inlet to the tank port under normal operating conditions.

High-pressure fluid from the pump exerts a force against the pilot as it enters from the external pilot port.

Direct Operated Unloading Valve

When the system pressure increases to the force of the spring setting the fluid bypasses the tank.

When the pressure goes above the spring setting, the spool opens fully to dump the surplus fluid into the tank at little or no pressure.

Pilot Operated Unloading Valve

Unloading spool is the addition in a pilot-operated unloading valve, it is not found in the pressure relief valve.

Without the unloading spool, this valve would function the same as any pilot-operated relief valve.

Pressure buildup within the pilot section would open a certain amount of fluid flow to the tank. It makes unbalances the poppet, allowing it to open and relieve excess pump flow to the tank.

Pilot Operated Unloading Valve

Unloading spool receives a signal through the remote-pilot port when the pressure in the working circuit goes more than its setting.

Simultaneously, fluid pressure on the spring-loaded ball in the pilot section starts to open it.

Pressure drop on the front side of the unloading spool brings down back force and pilot pressure from the high-pressure circuit forces the spring-loaded ball completely off its seat

Double Pumps with Unloading Valve

The primary use for an unloading valve is associated with a dual pump circuit. A high-pressure, low-flow pump along with a low-pressure, high-flow pump is used double pump operated circuit.

An un-loading valve is used with two pumps, create high discharge flow, and the other one able to create a high line pressure with low oil discharge.

double-pumps unit unloading valves

Deliveries of both pump are discharged into the circuit until the pressure approaches the setting of the unloading valve.

At this stage, fluid from the high pressure-low flow pump is passed through CV1 to the cylinder through the directional control valve but not allowed by CV2.

In a particular application, sheet metal punch press in which the hydraulic cylinder must extend rapidly over a length with low-pressure but high-flow requirements. This occurs under no load.

However during the punching operation for short motion, the pressure requirements are very high, but the flow requirements are low as the cylinder travel is small. 

It eliminates the requirement of having a very expensive high-pressure, high-flow pump.

As and when the punching operation initiates, the increased pressure opens the unloading valve to unload the low-pressure pump.

The purpose of relief valve is to protect the high-pressure pump from high pressure buildup at the end of cylinder stoke and when the directional control valve (DCV) is in its spring-centered mode.

The check valve protects the low-pressure pump from high pressure due to the high-pressure pump, which occurs during punching operation, at the ends of the cylinder stroke, and when the directional control valve (DCV) is in its spring-centered mode.

Advantages

  • The pressure of unloading valve is typically less of the relief valve.
  • The oil getting hot can be avoided by re-circulation with second pump in circuit. 
  • Life span of O-rings can be extended by avoiding oil becoming hot.
  • Less energy consumption.

Reference: Fluid Power Control Systems by MD Faiyaz Ahmed.

If you liked this article, then please subscribe to our YouTube Channel for Instrumentation, Electrical, PLC, and SCADA video tutorials.

You can also follow us on Facebook and Twitter to receive daily updates.

Read Next:

  • Fluid Power Systems
  • What is Pressure Control Valve?
  • How to Select Directional Valves
  • Level Control Valve with Float
  • Locked Open and Closed Valves
Don't Miss Our Updates
Be the first to get exclusive content straight to your email.
We promise not to spam you. You can unsubscribe at any time.
Invalid email address
You've successfully subscribed !

Continue Reading

Globe Valve Body Design
Self Actuated Pressure Control Valve Functional Testing
What is ON/OFF Valve ?
Control Valve Tips and Tricks
Deluge Valve Operation
What is a Pilot Valve? When do we use a Pilot Valve?
Share This Article
Facebook Whatsapp Whatsapp LinkedIn Copy Link
Share
Leave a Comment

Leave a Reply Cancel reply

Your email address will not be published. Required fields are marked *

Stay Connected

128.3kFollowersLike
69.1kFollowersFollow
210kSubscribersSubscribe
38kFollowersFollow

Categories

Explore More

How to Protect Pumps from Cavitation? – Reasons, Solutions
Gas Valve Sizing
What is Reducing Valve ?
PID Controller Action : Directing Acting & Reverse Acting
Compare Solenoid Valve Manual Reset and Solenoid Manual Override
Pneumatic Control Valve Actuators
What is Directional Control Valve (DCV)?
Solenoid Actuated Valves

Keep Learning

Valve Technician Interview Questions

Valve Technician Interview Questions

3-way solenoid valves control instrument air pressure

Solenoid Valve’s Energized or De-energized State ?

How is pressure drop calculated through a valve?

How is Pressure Drop Calculated through a Valve ?

Digital Control Valve Working Principle

Digital Control Valve Working Principle

Types of Failures in Control Valves

Types of Failures in Control Valves

Solving Critical Control Valve Hunting

Solving Control Valve Hunting: Instrument Troubleshooting

Malfunctioning Inlet Check Valves

Malfunctioning Inlet Check Valves caused chaos at Compressor stop/trip

Classification of Solenoid Valves

Classification of Solenoid Valves

Learn More

PLC Water Project

Smart Spray System using Wecon PLC

Testing and Validation in PLC Development

Testing and Validation in PLC Development

Single Port Globe Valve

How Sliding Stem Valves Work ?

DCS Vs PLC

Difference between DCS & PLC Systems

Integral controller eliminates offset error

How Integral Controller Reduces offset error ?

Level Transmitters Auto Calibration

DP Level Transmitter Auto Calibration

PLC Count values higher than 999

PLC Count values higher than 999

Calibration Instruments

Basics of Calibration

Menu

  • About
  • Privacy Policy
  • Copyright

Quick Links

  • Learn PLC
  • Helping Hand
  • Part Time Job

YouTube Subscribe

Follow US
All rights reserved. Reproduction in whole or in part without written permission is prohibited.
Welcome Back!

Sign in to your account

Username or Email Address
Password

Lost your password?