Relief Valves

Overview

Relieve valves, also known as pressure relief valves, are a type of safety valve used to control or limit the pressure in a system. The reason why it is categorized as a type of safety valve is because pressure that is built up excessively in an instrument or equipment can lead to failure or even fire. The relief valves allow these pressurized fluid to flow out of the system through an auxiliary passage. 
 

Components of Relief Valves

Relief valves are usually made up of a ball, spool or poppet opposed by a spring, and installed into a ported body or cavity. A poppet is basically a cone or disc shaped object that is positioned conversely to the machined seat. When the spring pressure forces it to close, very low leakage will occur. A spool is a steel, cylindrical rod with notches or metering grooves is opposed by spring pressure as well. A spool valve is more prone to leakage as opposed to a poppet valve, but the spool is superior in terms of metering.
 

Where Are Relief Valves Used

Pressure Relief valves are typically installed in a hydraulic system after the pump. Following the pump, pressure relief valves are most frequently mounted in a hydraulic system. As the pressure relief valve opens to bleed fluid to the reservoir, this location is able to provide the most immediate and direct response, thereby reducing pressure equal to its spring environment. As the pressure triggered by a downstream load or backpressure rises high enough to push the poppet or spool open against its spring, the pressure relief valve may open.
 

How Does Relief Valve Work

The pressure relief valves are designed to open at a predefined set pressure to protect pressure vessels as well as other equipment from exceeding their design limit due to pressure. When the set pressure is surpassed, the pressure relief valve is forced to open so that a portion of the pressurized fluids can get out through the auxiliary route. This is often referred to as the “past of least resistance”. 

In systems that contain flammable fluids, the fluid that is diverted out of the pressure relief valves is usually directed through a piping system known as the relief header to a central in which the fluid is burnt and the produced combustion gases are dispersed into the atmosphere. 

In non-hazardous systems, on the other hand, the diverted fluid is usually released into the atmosphere through a proper discharge pipeline designed to prevent rainwater ingress which can influence the set lift pressure, and positioned in a way not posing any danger to the users. After the fluid is diverted, the pressure will stop building up inside the vessels. And once the reseating pressure of the valve is reached, the pressure relief valve will close. 
 

What Happens in High-Pressure Gas Systems

So what happens if you are dealing with a high pressure gas system? In this case, it is recommended that you have the outlet of the pressure relief valve positioned out in the air. This means that the outlet will be connected to the pipelines, and the pressure will build up in the downstream of the pressure relief valve as it opens every time, which also means that the reseating will not be necessary when the set pressure is reached. 

Moreover, the so-called “differential” pressure relief valves are generally used in these kinds of systems. What this implies is that the pressure will only work on an area that is smaller than the opening area of the pressure relief valve. When the valve is opened, the pressure has to go down substantially before the pressure pressure relief valve closes, and the outlet pressure of the valve can make the valve stay open easily. One other thing you need to keep in mind is that if there are other pressure relief valves connected to the piping system of the outlet, they might open as the pressure in the exhaust pipes increases. This often leads to undesired consequences. 

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