Safety Moment #4: Excess Flow Valves

Excess Flow Valves

The material shown here has been extracted from the ebook 52 Process Safety Moments and in the book Plant Design and Operations.
 


Contents

  • Introduction
  • Operation
  • Concerns
  • Further Information

Introduction

The purpose of an Excess Flow Valve (EFV) is to stop the flow of gas or liquid through a pipe if the flow rate exceeds a specified value. Typically, they are spring-operated, although some use a magnetic device.

They are often found in the following applications.

  • Hoses that are used for loading/unloading tank and rail cars — if there is a large leak from the hose or if the hose becomes disconnected, the EFV will close and minimize the size of the spill (but not necessarily stop it altogether).
  • The loading and unloading systems for ammonia tanks.
  • Level gauges between the isolating valves and the pressure vessel on which the instrument is located.
  • Domestic gas lines between the main header and the local gas meter. If there is a large leak at the customer site the valve will close and restrict (but, once more, not totally stop) the flow of gas.

EFVs are sometimes used in non-safety situations where they can help prevent excess consumption of the fluid being transferred. For example, in consumer service EFVs are sometimes used to limit the total flow to a single user from a header.

Operation

The basic principle of the spring-operated type is as follows:

  • The fluid can move in either direction through the valve. It is important to understand that they are not check valves.
  • Although the fluid can flow in either direction, the valve will generally only work for one direction of flow. Hence the body of valve will have an arrow stamped on it showing which way it should be installed.
  • The valve is spring loaded. During normal operation the pressure in the fluid is sufficient to keep the valve open. However, if the fluid velocity exceeds the prescribed value the internal pressure drops and the spring forces the valve closed.
  • The valve will remain closed until the pressure between the upstream side and downstream side of the valve equalize. To allow for this the disk is drilled with a small opening. (The existence of the weep hole means that the valve will not provide a tight shutoff.)
  • Once the pressures have equalized, the valve will automatically re-open. However,

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Copyright © Ian Sutton. 2018. All Rights Reserved.