Session: FSI-01-01 Thermal Hydraulic Phenomena with Vessels, Piping and Components-1

Paper Number: 123460

123460 - A Review of Relief Valves in Unsteady Flow - Behavior, Analysis, and Design 

BACKGROUND

Relief valves perform an often simple, but critical function – protection from high pressures. An “ideal” relief valve might be said to completely prevent all transient pressures above some set maximum, while relieving a minimal amount of process fluid. A large variety of designs of such valves exist, each with their own design considerations.

Relief valve designs and selection are often carried out in a steady-state context, but they are inherently transient in nature – often opening rapidly to relieve high pressures quickly, and perhaps reclosing just as quickly. This rapid valve action generates surge – an often-large transient rise in pressure that travels through the fluid as an acoustic wave.

Depending on the system, the event that opens the valve, and the valve design – the system-level effects of this transient surge can easily exceed the pressures the relief valve was intended to prevent. The very device selected to protect the system can be the cause of catastrophic failure.

SYSTEMS VULNERABLE TO TRANSIENT RELIEF VALVE ISSUES

A discussion of common system configurations most vulnerable to issues related to poor relief valve specification is presented, with common mitigation techniques covered in brief.

PHYSICS AND ANALYSIS

The presence, severity, and mitigation of transient relief valve issues is typically analyzed with computer simulation. Basics of these simulation techniques and models are discussed.

The simplest relief valve model may simply be a rupture disk – an impediment to flow that is instantly removed in the mathematical model at bursting pressure. On the other end of the spectrum of complexity are valves such as surge anticipation relief valves. Such valves are pilot operated, priming the valve to open when pressure at a remote location drops below a setpoint, foreshadowing a significant pressure surge from wave reflection. The valve from then on may follow a flow coefficient curve – opening until overpressure, and not reseating until blowdown pressure, both of which might be different than the set pressure. The valve might also be rate-limited in either opening or closing, or both – meaning it cannot respond instantly to the pressure conditions.

Even the more complex model is relatively simple by necessity. Real relief valves are more complex and detailed than these models can practically cover. However, this does not mean that the models are inadequate for practical analysis that provides actionable conclusions. This paper discusses the models with simulation examples and points out how they may differ from behavior in the field.

 

 

BASIC DESIGN PRINCIPLES

A relief valve designed to prioritize steady operation may perform exceptionally poorly under surge conditions. Some designs perform better than others at reducing transient surge, but the selection of a surge relief valve cannot be made without analysis of the system as a whole. A relief valve that relieves surge appropriately in one system may cause dangerous pressure waves in another. The basics of different designs and their behavior during a transient is discussed.

It is a misconception that surge relief requires an exceptionally complicated valve, perhaps needing regular maintenance or sophisticated systems with, for example, nitrogen or other instrument gas. Relatively simple and robust mechanical designs can excel at surge mitigation if appropriately designed.

SURGE-AWARE RELIEF VALVE SELECTION

Guidelines for selection of relief valves are presented here. Namely, if a surge-aware design should be specifically considered, and how to select an appropriate valve for the system and surge event in question.

Presenting Author: Mark Dudley Applied Flow Technology

Presenting Author Biography: Mark Dudley is an Engineering Software Developer at Applied Flow Technology in Colorado Springs, CO. His work at Applied Flow Technology is focused on implementing computational methods for piping systems, both incompressible and compressible. His contributions consistently revolve around numerical schemes for both heat transfer and slurry systems in steady and transient applications, as well as response analysis for surge-prevention components. Mark has obtained a Bachelor of Science in Engineering Physics and a Master of Science in Mechanical Engineering from the Colorado School of Mines. His masters research focused on Multiphysics computational modeling for desalination systems.

Authors:

Mark Dudley Applied Flow Technology
Jans Schreuder Mokveld Valves
Dylan Witte Brown and Caldwell
Devin Rorabaugh Applied Flow Technology
Scott Lang Applied Flow Technology