E2G | The Equity Engineering Group, Inc. 1

INSIGHTSE2G INDUSTRY

Fall/winter 2015 | Volume 2

Fall/winter 2015 |

Volume 2

E2G Industry Insights

Corporate Headquarters20600 Chagrin Boulevard, Suite 1200Shaker Heights, OH 44122

Satellite O�cesHouston, TXThe Woodlands, TXVictoria, TXAlberta, Canada

216.283.9519www.EquityEng.com

API 520 Part II Updates to the 3% rule – Why all the fuss?

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PROCESS TECHNOLOGY

It took 12 years, but the API Pressure Relieving Systems

(PRS) Subcommittee finally published the long-awaited 6th Edition of API 520, Part II “Sizing, Selection, and Installation of Pressure Relieving Systems.” E2G’s Phil Henry has been the task force chairman of API 520 since 1996 and cannot remember any standard within the API PRS Subcommittee being so difficult to get published.

WHY THE WAIT?

The new guidance related to the infamous 3% rule on inlet pressure drop to pressure relief valves (PRVs) has become a political and often times contentious debate between the industry pressure relief experts and the Occupation Health and Safety Administration (OSHA). OSHA is concerned about industry safety and was under the impression that API was “relaxing” the rules on PRV installation, thus, jeopardizing safety to save on repair costs. In reality, the industry represented by most of the major oil companies has safely allowed inlet pressure drops in

API 520 PART II UPDATES TO THE 3% RULE – WHY ALL THE FUSS?PHILIP HENRY

Process Technology & Process Safety Management

Business Unit – Director Principal Engineer

CONTACTFor more information on API 520 part II updates to the 3% rule, please contact Philip Henry.

Email: PAHenry@EquityEng.com

excess of 3% of the PRV set pressure on installations based on experience, engineering judgement, and an “engineering analysis” as far back as the 1970s.

Confrontations between OSHA and the Oil Industry have been at a fever pitch during the last decade during which OSHA has initiated their National Emphasis Program (NEP), designed to audit refineries and petrochemical facilities on compliance with the OSHA CFR 1910.119 legislation on Process Safety Management (PSM).

Pressure relief system design and documentation is a major component of PSM’s rules on Mechanical Integrity and a primary focus of OSHA’s NEP audits. Many refineries were being heavily fined with “willful violations” of Recognized and Generally Accepted Good Engineering Practice (RAGAGEP) if relief valve installations showed inlet pressure drop in excess of 3%, even though safety was not being compromised.

the API Pressure Relieving Systems (PRS) Subcommittee has published the 6th Edition of API 520, Part II “Sizing, Selection, and Installation of Pressure Relieving Systems.”

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PROCESS TECHNOLOGY

WHAT ARE WE WORRIED ABOUT?

Valve Instability! If the pressure drop in the inlet piping to a pressure relief valve is too high, there is the possibility the valve will cycle or chatter when the pressure at the inlet to the valve goes below its blowdown pressure (reseat pressure). Therefore, inlet lines that are too small, too long, or contain too many fittings or valves need to be avoided.

PRV chatter is a phenomenon where, for various reasons, the valve opens and closes in a rapid fashion. This can result in reduced PRV capacity and/or high piping vibration/loads. There have been incidents in industry where PRV chatter has led to PRV damage, flange leaks, and piping failures.

HOW ABOUT A HISTORY LESSON?

We believe the inlet pressure drop limitation first appeared as good engineering practice for the installation of PRVs in a paper by Sylvander and Katz from 1948. As part of a University of Michigan research effort, API commissioned the work to provide the industry with good design guidance on installation of PRVs.

“Conventional spring-loaded relief valves have the rate of flow affected by the inlet pressure drop, since the pressure at the inlet to the valve will then be below the vessel pressure. If an excessive pressure drop occurs through the inlet piping, the valve tends to close prematurely. As

the valve closes, pressure within the protected equipment builds up rapidly and the valve is forced open again. This action results in what is commonly referred to as “chattering.” Chattering creates serious vibrational disturbances and can result in damage to the relief valve parts and possible failures of piping connections.”

The study further recommends that PRV installations provide a safety margin between blowdown and inlet loss. For the 1940s vintage safety valves analyzed in the Sylvander and Katz work, the typical blowdown value was 4%. Therefore, it was recommended to keep the inlet pressure loss at or below 3% of the valve’s set pressure to maintain a 1%

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safety margin for valve stability. It was not until 1963 that API introduced this guidance into the 2nd Edition of API 520 Part II:

“For gas, vapor, or flashing-liquid service, it is recommended that the inlet piping between the protected equipment and the inlet flange of the pressure relief valve be designed so that the total pressure loss in the line shall be the sum total of the inlet loss, line loss, and valve loss and shall not exceed 3 percent of the set pressure, in pounds per square inch gage, of the valve.”

Recognizing that the original basis for the 3% rule in the 1948 University of Michigan study was for relief valves with a typical blowdown of 4%, and that modern ASME certified relief valves provide typical blowdowns in the range of 7 to 10% of set pressure, many oil companies were accepting inlet losses above the 3% guidance for existing installations (many installations were designed before codification of the 3% rule) without experiencing any instability problems. In fact, a 5% inlet loss criterion was often allowed since this still provided at least a 2% safety margin between the inlet loss and the 7% blowdown typical for modern day PRVs.

In 1994, API supported the concept of exceeding the 3% limit on inlet pressure drop; however, it was recommended to do this in conjunction with an engineering

analysis. In the 4th Edition of API 520 Part II, the following statement was added:

“An engineering analysis of the valve performance at higher inlet losses may permit increasing the allowable pressure loss above 3 percent.”

Although no additional guidance was given as to what the engineering analysis should look like, conventional wisdom was that the engineer should work with the valve manufacturer to understand the blowdown characteristics of the installed PRV and to assess the potential for valve instability with increased inlet losses.

JOINT INDUSTRY PROJECT

RESULTS:

Flow testing results of a recent Joint Industry Project (JIP) sponsored by API and the industry and conducted over the last 10 years showed very little correlation between PRV stability and inlet pressure drop.

Some installations that had pressure drops in excess of 3% of the valve’s set pressure were stable. Other valves where the inlet losses were below 3% became unstable. Some members felt the JIP was a failure because of this. However, E2G believes the lack of any correlation between pressure drop and valve stability proves that focusing on a hard fast 3% rule is insufficient. A second JIP sponsored by API, which includes more focused testing, is currently underway.

WHAT IS THE SOLUTION?

This brings us to today’s environment where the battle between OSHA, API, and Owner/Users has, at times, resulted in costly fines to the Industry. It is not uncommon for $70,000 fines to be levied for each installation where calculated inlet pressure drops of 3.1%, 3.5%, 3.7 %, etc., went unmitigated.

Where inlet pressure drop is calculated to be in excess of 3% of the valve’s set pressure, the Owner/User is left with several options.

Costly inlet piping modifications are one way of addressing the problem.

However, further “Engineering Analysis,” as presented in API 520 Part II, can now be performed that will assist the user in determining which installations truly require costly capital expenditures and which ones can be mitigated by further analysis.

The 6th Edition of API 520 Part II was finally released in March of 2015 and provides more details on this engineering analysis. It consists of a thorough review of past inspection and operating records to determine if there is any historical evidence of chatter. A new Force Balance method (incorporating inlet pressure drop, outlet pressure drop, and PRV opening and closing characteristics) has been added that shows excellent correlation with the JIP flow test data when predicting whether a valve is stable or not.

PROCESS TECHNOLOGY

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Also, an acoustic interaction check is now recommended, particularly for liquid lines where the majority of actual chattering valves have been experienced.

E2G CAN HELP!

Before any capital dollars are spent rerouting or replacing inlet piping to PRVs to mitigate a 3% issue, Owner/Users should be performing an engineering analysis to determine if modifications are necessary. Please contact E2G for assistance.

REFERENCES

1. Sylvander and Katz “The Design and Construction of Pressure Relieving Systems,” University of Michigan Press 1948.

2. “API RP 520 Part II 2nd Edition,” American Petroleum Institute 1963.

3. “API RP 520 Part II 4th Edition,” American Petroleum Institute 1994.

4. “API RP 520 Part II 6th Edition,” American Petroleum Institute 2015.

5. “ASME Section VIII, Division 1 1986 Edition,” American Society of Mechanical Engineers 1986.

PROCESS TECHNOLOGY

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INSIGHTSE2G INDUSTRY

Fall/winter 2015 | Volume 2

Fall/winter 2015 |

Volume 2

E2G Industry Insights

Corporate Headquarters20600 Chagrin Boulevard, Suite 1200Shaker Heights, OH 44122

Satellite O�cesHouston, TXThe Woodlands, TXVictoria, TXAlberta, Canada

216.283.9519www.EquityEng.com