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Relief Valves: "What Can Go Wrong" Scenarios - Part 1

Nov 08 2010 01:30 PM | pleckner in Safety and Pressure Relief -----

What can go wrong in a chemical facility? Plenty! A report in the August 2000 issue of CEP1 shows that operator error or poor maintenance was the leading of cause of accidents for unfired pressure vessels eight years running.

The Problem

Accidents not only damage equipment but also cause injury or even death to plant personnel. To reduce the number of incidents of accidents, it is the job of the Process Engineer to analyze the process design, determine the "what can go wrong" scenario and either find a way to "design" out of it or provide protection against catastrophic failure in the event an accident does occur, i.e. install a relieving device such as a relief valve and/or rupture disk.

For the purposes of this discussion and those that will follow, a "what can go wrong" scenario is defined as an action that will cause a vessel containing a gas or liquid to overpressure, leading to a catastrophic failure of that vessel if it were not for the presence of a relief valve or rupture disk. To find these potentially deadly incidences, the Process Engineer goes through a type of "self HAZOP" (Hazardous and Operability Study), analyzing the process to determine what these scenarios are. For each credible scenario identified, the Process Engineer performs calculations to determine the amount of vapor or liquid that must be relieved from the vessel in order to prevent the overpressure from occurring (the relieving load).

Fair warning, this discussion and those that will follow in future installments assume the reader is at least somewhat knowledgeable in Process Design. Safety analysis should never be left up to the junior Process Engineer unless closely supervised. So if you Mr., Mrs., Ms. Reader become confused or somewhat lost in some of the terminology used, I sincerely apologize. I welcome your very direct and specific questions about anything I write and would be happy to help you understand what I am saying. This is an extremely important function for Process Engineers.

The Checklist

Since there are many potential causes of failure, it would be nice to have a checklist to make the analysis organized and somewhat standard. After all, those of us in Process Design may be working on a project for a chemical plant today and end up on a project for a pharmaceutical plant tomorrow (it happens, believe me). A pretty good checklist is given by Table 2 in Section 3 of The American Petroleum Institute (API) publication, "Guide for Pressure-Relieving and Depressuring Systems"2, better known as API Recommended Practice 521 (or just API 521). For those not familiar with API, this is the organization in the United States that sets the standards by which codes (laws) are followed. API publishes the Recommended Practices 520 and 521, among others. A condensed version of the API checklist is presented in Table 1 below.

Table 1: API RP 521 Scenario Check List

API RP 521 Item No.

Overpressure Cause


Closed outlets on vessels


Cooling water failure to condenser


Top-tower reflux failure


Side stream reflux failure


Lean oil failure to absorber


Accumulation of noncondensables


Entrance of highly volatile material


Overfilling Storage or Surge Vessel


Failure of automatic control


Abnormal heat or vapor input


Split exchanger tube


Internal explosions


Chemical Reaction


Hydraulic expansion


Exterior fire


Power failure (steam, electric, or other)

In this installment, I want to establish a framework for analyzing a given process. In future installments, I will begin to tackle the scenarios (Overpressure Cause) themselves in some detail. The ultimate goal is for the Process Engineer to identify credible "what can go wrong" scenarios, perform relieving load calculations to prevent catastrophic failure and eventually size the relieving device and system.

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