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

Paper Number: 122657

122657 - A Review of Check Valves in Unsteady Flow - Behavior, Analysis, and Design 

BACKGROUND

Check valves are ubiquitous in many systems. Generally, an “ideal” check valve completely prevents reverse flow and does not negatively affect steady operation.

While a typical check valve will completely prevent sustained reverse flow for any negative pressure gradient, check valves are inherently transient devices in nature – the valve will close freely when reverse flow is encountered. During this transient event, fluid decelerates through the check valve, and closure begins as zero velocity is approached. Fluid deceleration continues during closure, and some reverse flow is unavoidable.

This transient closure is a concern because the rapid closure of the valve against a non-zero flow – check valve slam – will generate a transient pressure surge. The increase in pressure can easily exceed operational limits, and travels through the fluid as an acoustic wave, potentially damaging not only the check valve but equipment throughout the system.

 SYSTEMS VULNERABLE TO SLAM

A discussion of common system configurations most vulnerable to check valve slam is presented, with common mitigation techniques covered in brief.

PHYSICS AND ANALYSIS

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

A simple dynamic check valve model is to provide a fully open loss coefficient and velocity at which the valve instantly closes. Real valves do not instantaneously close, so it may seem that such a model cannot be accurate. However, the model replicates the effects of real slam quite well, as shown with comparisons to field data.

More complex physical models are also an option, but these require simplifications and a high level of detail. The physics of the model may be superior, but the uncertainty in the added parameters often means the complex and simple models are equally viable.

HISTORY AND APPLICATION OF CHARACTERISTICS

The reliability of the simple model depends directly on the accuracy of the reverse closure velocity. There is a relationship between the closure velocity and the strength of the fluid deceleration – a faster change in fluid velocity results in a higher closure velocity and therefore greater slam. This functional relationship between deceleration and closure velocity is the check valve’s surge characteristic.

Different valve designs have different characteristic curves, generally determined from analysis on a test stand. There are few such curves the authors are aware of. These curves were generated primarily in the 1980s and 1990s, with various papers and publications re-publishing the same curves. Misunderstandings over the years have confused the data in some respects, perhaps leading to the conclusion that more test data is available than really exists.

This paper aims to clarify the history of this data, to provide a comprehensive summary of all the published curves the authors are aware of, and to clarify when and how particular curves should or should not be used for analysis.

BASIC DESIGN PRINCIPLES

A check valve designed to prioritize steady operation may perform exceptionally poorly under surge conditions. Some general designs perform far better than others in reducing surge pressures. The basics of different check valve designs and their behavior during a transient is discussed.

It is a misconception that any check valve designed primarily for surge will sacrifice performance in other areas. It is possible to design valves that perform well both for surge and for typical operation, and the fundamental design principles for such a valve are covered here.

SURGE-AWARE CHECK VALVE SELECTION

Guidelines for selection of check 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.

Additionally, the effects of selection of sub-components such as check valve springs is discussed.

Presenting Author: Scott Lang Applied Flow Technology

Presenting Author Biography: Scott Lang is an Engineering Software Development Specialist at Applied Flow Technology in Colorado Springs, CO. He supports Applied Flow Technology by researching and developing numerical methods for modeling a variety of fluid systems. Of particular interest to him is the prediction of turbomachine behavior under steady, transient, or performance-impacting conditions, and the general analysis of incompressible and compressible transient fluid flow. He holds a Bachelor of Science in Engineering with Mechanical and Electrical specialties from the Colorado School of Mines and is a member of the Engineering Honor Society Tau Beta Pi.

Authors:

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