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Table of Contents

Table of Contents


Pressure
Release
Relief Valve [
3
1]

Image of a pressure relief valve.Image Modified


What are Relief Valves?

A relief valve  valve is a type of safety valve designed to open at a preset pressure, and discharge fluid until the pressure drops to an acceptable levels level [12]. The valve's valves release of elevated liquid, gas, or steam pressures acts to prevent damage to the system. 


Components of a Relief Valve

Relief valves are comprised of three main elements:

Valve element - typically

, a valve, a sensing element, and reference force element.

Valve Element

Pressure relief valves often use a spring loaded poppet valve as the valve element [

1]
  • Sensing element - typically a diaphragm or piston [1]
  • Reference force element -
    commonly a spring [1]

    2]. The spring and upstream pressure apply opposing forces on the valve. When the force of the upstream pressure becomes greater force than the spring force, the poppet moves away from the valve seat, allowing fluid to pass through the outlet. As the upstream pressure drops below the set point, the valve will close.

    Sensing Element

    Piston style designs are often used when higher relief pressures are required [2]. Pistons however, tend to be more sluggish than diaphragm designs, as a result of the friction from the piston seal. In low-pressure applications or when high accuracy is required, the diaphragm style is preferred [2]. Diaphragm relief valves essentially eliminate friction using a thin disc element, typically made of an elastomer, to detect pressure changes [2].

    Reference Force Element

    The reference force element of a relief valve is commonly a mechanical spring [2]. This spring exerts a force on the sensing element which acts to close the valve. Pressure relief valves are typically designed with an adjustment, allowing the user to adjust the relief pressure set-point by changing the force exerted by the reference spring [2].


    Generic Pressure Release Valve Diagram [
    2
    3]

    Diagram Outlining the Components of a Generic Pressure Release Valve.Image Modified




    Types of Relief Valves

    Conventional Spring Loaded

    With a conventional spring loaded relief valvvalve, the bonnet, spring, and guide are exposed to the released fluids. If the bonnet is vented to the atmosphere, the relief-system backpressure back pressure decreases the set pressure [4]. If the bonnet is vented internally to the outlet, the relief-system backpressure back pressure increases the set pressure. The conventional spring-loaded valve is used in noncorrosive services and where backpressure systems and in which back pressure is less than 10% of the set point . [54].

    Conventional Spring Loaded [
    4
    5]
    Image Removed

    Diagram Outlining the Components of a Conventional Spring Loaded Relief Valve.Image Added



    AdvantagesDisadvantages
    • Most reliable type (if properly sized and operated)
    • Versatile
    • , as it can be used
    in
    • for many
    services
    • applications [
    4
    • 5]
    Relieving pressure
    • Pressure relief is affected by back pressure
    Susceptible
    • Prone to chatter if the built-up back pressure is too high [5]


    Balanced Spring Loaded

    The balanced spring-loaded valve incorporates a means to protect protects the bonnet, spring, and guide from the released fluids and minimizes the effects of backpressureback pressure. The disk area vented to the atmosphere is exactly equal to the disk area exposed to backpressureback pressure [4]. These relief valves can may be used in corrosive or dirty service and services, with variable backpressure. back pressure [54].

    Balanced Spring Loaded [
    4
    5]
    Image Removed

    Diagram of a Balanced Spring Loaded Relief Valve.Image Added



    AdvantagesDisadvantages
    • Relieving pressure is not affected by back pressure
    • Can handle higher built-up back pressure [5]
    • Protects spring from corrosion [
    4
    • 5]
    • Bellows are susceptible to fatigue/rupture [5]
    • May release flammables/toxics to atmosphere [5]
    Requires
    • Require a separate venting system

    Pilot Operated

    The pilot - operated relief valve is combined with and controlled by an auxiliary pressure pilot [4]. The resistance force on the piston in the main valve is assisted by the process pressure through an orifice [4]. The net seating force on the piston actually increases will increase as the process pressure nears the set point . [54]

    Pilot Operated [
    4
    5]
    Image Removed

    Diagram Outlining the Components of a Pilot Operated Relief Valve.Image Added



    AdvantagesDisadvantages
    Relieving pressure
    • Pressure relief is not affected by
    backpressure
    • back pressure [5]
    • Can operate at up to 98% of set pressure [5]
    • Less susceptible to chatter (
    some models)Pilot is susceptible
    • model dependant) [5]
    • The pilot is prone to plugging
    • Limited chemical and high temperature use by “O-ring” seals [5]
    Vapor
    • Vapour condensation and liquid accumulation above the piston may cause problems [5]
    • Potential for back flow



    Design Parameters

    Relief Pressures

    The expected relief pressure is an important consideration when determining the ideal product for the application.

    Flow Requirements

    The maximum flow rate required by the application, and the possibility for flow

    Operation of a Relief Valve

    Suppliers

    rate variation are important considerations [2]. The porting configuration and effective orifices must also be taken into account [2].

    Fluid Used 

    The chemical properties of the fluid to be used with the relief valve must be considered to determine the ideal materials for the application. Care must be taken when selecting materials that will be in contact with the fluid [2].

    Size & Weight

    Where space is limited, it is important to consider factors such as size and weight. Material, in particular that of the relief valve body components, will affect the weight [2]. Other important considerations include the port (thread) sizes, adjustment styles, and mounting options, all of which will impact the weight [2].

    Materials

    There are a wide range of materials available to handle various fluids and operating environments [2]. Common pressure relief valve component materials include brass, plastic, aluminum, and various grades of stainless steel [2]. Springs are often made of music wire (carbon steel) or stainless steel [2]. More details on common pressure relief valve component materials are outlined below. 

      • Brass - Suitable for most common applications and economical [2]
      • Aluminum - Often specified when weight is an important consideration [2]
      • Plastic - May be considered when cost is a primary concern or if a throw away item is required [2]
      • Stainless steel - Often chosen for corrosive fluids, if fluid cleanliness is a consideration, or for high operating temperatures [2]

    Another important material consideration is the compatibility of the seal material with the fluid and the operating temperature range [2]. Buna-N is a common seal material, however some manufacturers may offer seals in Fluorocarbon, EPDM, Silicone and Perfluoroelastomer [2].

    Temperature

    The relief valve materials must be able to function as intended at the expected operating temperature. More specifically, the primary concern is with the elastomer, which must function properly throughout the expected temperature range [2]. The operating temperature may also affect the flow capacity, or even the spring rate in extreme conditions [2].



    Contributors:

    Contributors Summary
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    Faculty Advisor: Mary Robinson