Hydraulic Pressure Relief Valve


The need for Pressure Relief Valve (PRV)
  • A pressure relief valve (PRV) is one of the most important types of safety valves. These types of valves set a limit on the rise of pressure within a hydraulic line or system. 
  • In normal operation, the valve is closed and no fluid passes through the PRV. But if the pressure in the line exceeds the limit, the valve opens to relieve the pressure. 
  • This protects the expensive machines such as pumps, motors, and actuators from getting damaged due to extensive pressures. 
  • Without a relief valve, pressure can continue to grow until another component fails and pressure is released.

The need for Pressure Relief Valve (PRV)
Types of Pressure Relief Valve
  1. Direct Acting PRV
  2. Pilot Operated PRV
Direct Acting Pressure Relief Valve
  • Direct-acting PRV is held closed by the direct force of a mechanical spring. The spring force holding the valve closed is opposed by the system hydraulic pressure. 
  • The cracking pressure is the pressure at which the valve will open. The cracking pressure is set by changing the tension of the mechanical spring by adjusting the nut or knob. 
  • As long as the system operates at or under the cracking pressure, the valve remains closed. If the system pressure even increases slightly greater than the cracking pressure, the valve opens and excess oil starts trickling through the valve. 
  • The pressure at which the valve fully opens is called the full relief valve pressure and is greater than cracking pressure. When the system pressure reaches full relief valve pressure, the valve opens fully and the oil is discharged from the outlet port back to the reservoir.  


Pilot Operated PRV
  • Pilot operated PRV are typically smaller than direct-acting PRV are capable of handling higher pressure and flow. 
  • This valve has two stages: 
  1. Main valve: The first stage is composed of the main valve with a main poppet and spring capable of handling the maximum pressure rating of the valve.
  2. Pilot Poppet valve: The second stage is composed of much smaller direct acting PRV which consists of the pilot poppet, pilot spring, and an adjustment knob. This relief valve is usually mounted in cross-way above the main valve body.
  • As long as the line pressure is less than relieving pressure set on the control knob, the pilot poppet will remain closed. Since the pilot poppet valve is closed, the pressure in the main valve chamber is the same as the line pressure. Since these pressures (main valve chamber pressure = line pressure) are equal, there is no pressure drop from one side to the other and the main poppet also remains closed.
  • When the line pressure slightly increases greater than the relieving pressure, the pilot poppet moves to the open position. This allows the fluid to flow from the pressure side through the orifice across the pilot valve to the tank.
  • Once the pilot valve is opened, there is a pressure drop across the main valve poppet with higher pressure on the pump sideline.  This causes the main poppet to move, allowing full flow through the relief valve. A vice versa process occurs when the valve closes. 


Comments

Post a Comment

Popular posts from this blog

Washing Machine: An example of mechatronics system

Image
The washing machine is an example of a  sequential control system.  A system operating with sequential control is one where a number of preset operations are performed in sequence.  The control may be implemented as  event-based,  where the next action cannot be performed until the previous action is completed.  Another mode of sequential control is termed  "time-based",  where the operations are sequenced with respect to time. Example: An example of a time-based sequential control system is shown in the following Figure 1. The pump (fill valve) is switched ON for a particular time to fill enough water into the tank to fill it to approximately the correct level. Following this, the pump is switched OFF.   Figure 1: Time-based sequential control Among these two modes,  event-based control  is intrinsically  more reliable  than time-based control.  An example of an event-based sequential control syste...
Read more

Proximity sensors

Image
Proximity sensors Proximity sensors are pilot devices used to detect the presence of an object without making any physical contact and are widely used in industrial automation for the detection of objects at a position. Proximity sensors are a non-contact type of solid-state electronic devices and are normally of eddy current type or inductive, capacitive or photoelectric proximity sensor. Inductive proximity sensor An inductive proximity sensor is a non-contact proximity sensor that can detect ferrous objects that contain iron, ideally mild steel thicker than one millimeter. Construction: An inductive proximity sensor is a non-contact proximity sensor that detects ferrous objects that contain iron, ideally mild steel thicker than one millimeter.  An inductive proximity sensor consists of four major components: a ferrite core with coils , an oscillator , a detector or signal conditioner , and an output amplifier all packed in a compact housing.   The oscillator creates a sy...
Read more

Piezoelectric Pressure Sensor

Image
Piezoelectric Pressure Sensor A piezoelectric sensor is a device that uses the piezoelectric effect to measure changes in pressure, force, strain or temperature by converting them to an electrical charge.  The word piezo is derived from the Greek word piezein which means “Press” or “Stress”. The piezoelectric effect is observed in various materials available in different forms such as crystalline, ceramics, lead-free piezoceramics, semiconductors, and polymers. The  Lead zirconate titanate and quartz crystal are the common types of piezoelectric materials used in piezoelectric pressure sensors.  The piezoelectric material can be referred to as a bidirectional transducer as it generates an internal voltage resulting from an applied pressure or mechanical force due to piezoelectric the effect, contrarily,  it can also generate internal pressure or mechanical strain when an electric field is applied due to the inverse piezoelectric e...
Read more