Routing Of Flare And Relief Valve Piping: An article

The purpose of this article is to provide a brief idea of Flare Piping and Relief Valve piping highlighting the important points.

What are Relief Events?

Relief events are obligatory events which prevents efficiency or performance and increase cost but must be met considering safety of the operating plant. Examples of potential relief events are

  • External fire
  • Flow from a high-pressure source
  • Heat input from associated equipment
  • Pumps and compressors
  • Ambient heat transfer
  • Liquid expansion in pipes and surge

Potential Lines of Defense

To act against the above-mentioned potential relief events the following defense methods are followed.

  • Inherently Safe Design
  • Low-pressure processes
  • Passive Control
  • Overdesign of process equipment
  • Active Control
  • Install Relief Systems

What is a Relief System?

A relief system is an emergency system used to safeguard plants during relief events by reducing pressure or discharging gas during abnormal situations. The relief system consists of

  • A relief device, and
  • Associated lines and process equipment to safely handle the material ejected

Why Use a Relief System?

Installing relief systems in operating plants is a must from process and technical safety points as

  • Inherently Safe Design simply can’t eliminate every pressure hazard
  • Passive designs can be exceedingly expensive and cumbersome
  • Relief systems work!

Code Requirements for relief system design

General Code requirements include:

  • ASME Boiler & Pressure Vessel Codes
  • ASME B31.3 / Petroleum Refinery Piping
  • ASME B16.5 / Flanges & Flanged Fittings

Relieving pressure shall not exceed MAWP (accumulation) by more than:

  • 3% for fired and unfired steam boilers
  • 10% for vessels equipped with a single pressure relief device
  • 16% for vessels equipped with multiple pressure relief devices
  • 21% for fire contingency

Locating Reliefs – Where?

Relief valves are normally located at

  • All vessels
  • Blocked in sections of cool liquid lines that are exposed to heat
  • Discharge sides of positive displacement pumps, compressors, and turbines
  • Vessel steam jackets

Choosing Relief Types

Normally one of the below-mentioned two devices are used as relief system in process industries

  • Relief Valves
  • Rupture Devices

Spring-Operated Valves

Conventional Type:

Fig.1: Conventional type PRV
Fig.1: Conventional type PRV

Balanced Bellows Type:

Fig.2: Bonnet Bellow type PRV
Fig.2: Bonnet Bellow type PRV
Pressure Safety Valve
Fig.3: A Typical Pressure Safety Valve

Pros & Cons of Conventional Valve


  • Most reliable type if properly sized and operated
  • Versatile — can be used in many services


  • Relieving pressure affected by back pressure
  • Susceptible to chatter if built-up back pressure is too high

Pros & Cons of Balanced Bellows Valve


  • Relieving pressure not affected by back pressure
  • Can handle higher built-up backpressure
  • Protects spring from corrosion


  • Bellows susceptible to fatigue/rupture
  • May release flammables/toxics to the atmosphere
  • Requires a separate venting system

When to Use a Spring-Operated Valve

  • Losing entire contents is unacceptable
    • Fluids above the normal boiling point
    • Toxic fluids
  • Need to avoid failing low
  • Return to normal operations quickly
  • Withstand process pressure changes, including vacuum

When to Use Both Types

  • Need a positive seal (toxic material, material balance requirements)
  • Protect the safety valve from corrosion
  • System contains solids

A Special Issue: Chatter

  • Spring relief devices require 25-30% of maximum flow capacity to maintain the valve seat in the open position
  • Lower flows result in chattering, caused by rapid opening and closing of the valve disc
  • This can lead to the destruction of the device and a dangerous situation

Chatter – Principal Causes

Valve Issues

  • Oversized valve
  • Valve handling widely differing rates
  • Relief System Issues
    • Excessive inlet pressure drop
    • Excessive built-up backpressure

Rupture Devices

  • Rupture Disc
  • Rupture Pin

Conventional Metal  Rupture Disc

Rupture Disc
Fig.4: Conventional Rupture Disc

Conventional Rupture Pin Device

Conventional Rupture Pin Device
Fig.5: Conventional Rupture Pin Device

Difference between a PSV and PRV

  • Pressure Relief Valve (PRV) opens gradually in relation to the pressure, on the other hand when the pressure reaches a certain value a Pressure Safety Valve or PSV opens suddenly to release the overpressure.
  • PRV is normally used for liquid systems while PSV for gaseous systems.

Few more useful Resources for you…

Pre-Commissioning and Commissioning Checklist for Flare PackageOpens in a new tab.
Routing Of Flare And Relief Valve Piping: An article-Part 1Opens in a new tab.
Flare systems: Major thrust points for stress analysisOpens in a new tab.
Stress Analysis of PSV connected Piping Systems Using Caesar IIOpens in a new tab.
Articles related to Process DesignOpens in a new tab.
Piping Layout and Design BasicsOpens in a new tab.
Piping Stress Analysis BasicsOpens in a new tab.
Few Job Opportunities for youOpens in a new tab.

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Anup Kumar Dey

I am a Mechanical Engineer turned into a Piping Engineer. Currently, I work in a reputed MNC as a Senior Piping Stress Engineer. I am very much passionate about blogging and always tried to do unique things. This website is my first venture into the world of blogging with the aim of connecting with other piping engineers around the world.

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