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Why Valve Qualification Programs?

As the valve world continues to turn and tilt its production towards new countries and their untested manufacturing plants, the potential backlash is a rise in quality issues. In decades past, when bottom-dollar pricing was not the chief procurement driver, higher quality cast steel valves, produced in the United States, Great Britain, and Japan could be counted upon. Unfortunately, the global economic realities of the past 10 years have caused many of these companies to either cease production in their native countries or go out of business altogether.

The commodity steel valve industry with its lower profit margins is undergoing a period of significant change as manufacturers struggle to remain competitive. As a result, virtually every major commodity valve manufacturer has turned to countries such as China and India, with their inexpensive labor markets, in an effort to maintain market share. This relocation to new manufacturing sites has made many users uneasy with the potential of poor quality in the valves they purchase.

To confirm the quality and repeatability of these products, many of the major end-users are requiring that the manufacturers qualify their products in accordance with API document, RP591, “User Acceptance of Refinery Valves”. RP591, which is only a recommended practice – not a standard, requires that candidate valves undergo a rigorous series of tests and inspections, including nondestructive evaluations, critical dimension measurements and stem-to-wedge strength tests on gate valves.

The current third edition, with several substantive changes, was approved and published in September of 2003 and published in early 2004. One of the key revisions is that the qualification is now “manufacturing facility specific” instead of brand specific. In other words, the qualification no longer applies just to a manufacturer by name, but also to a specific manufacturing plant and/or foundry.

Just what does API RP591 mean to the end-user and the manufacturer? To the end-user, requiring a manufacturer to undergo RP591 qualification, affords his company some assurance that the advertising and marketing claims of superior quality and API 600 compliance by a valve manufacturer are not just hollow words. To the manufacturer, it is an opportunity to assess the quality, and to a limited sense, the repeatability of their product. If the valves do very well in the testing, there is also the opportunity to help gain a position on a coveted end-user Acceptable Manufacturer List (AML).

RP591 History

To better understand why API RP 591 “Process Valve Qualification Procedure” exists today, we need to look back at the history of the domestic commodity cast steel business over the past 30 years. The story begins with the worldwide petrochemical and refinery construction boom of the mid to late 70’s. While this profitable period was a tremendous economic shot-in-the-bank account for the leading domestic valve manufacturers such as Crane, Powell, Walworth, Lunkenheimer, Jenkins and others, it drastically depleted the commodity steel valve inventory that would normally be consumed for new U.S. construction projects and MRO use.

This situation cracked open an economic door that foreign valve manufacturers had been trying to walk through for many years. Due to acute shortages, domestic manufacturers began bringing in licensed products from Eastern Europe and the Far East. Additional non-affiliated offshore manufacturers also began to make a play on the ripe US market.

Unfortunately, some of the products that were imported did not fare too well in service and a pattern of failures prompted some end-users to address the issue. Here-to-fore, the only testing and inspection that was performed by a user on commodity valves was the cursory API 598 hydrostatic test block. The comfort zone that the over-engineered, rock-solid American valves had provided the industry for over 50 years had now disappeared amidst a rash of leaking castings, stem breakage and seating failures.

In 1979-80, Shell Oil Company became the first refiner to address the qualification testing issue with a fairly thorough test procedure that included a variety of dimensional examinations, operability tests, hydrostatic tests, NDE and chemical analysis of key materials. Through information shared at API Refining Meetings, the major end-users discussed the need for an official valve qualification document. Two men led the valve quality charge in API at that time, Harry Howarth of Mobil and Curt Ball of Exxon.

In 1985, Ball & Howarth presented a draft document to the valve industry that eventually would become the backbone of the API RP591 document. Although it took five years of work group negotiation and open meeting haggling, the first edition of API RP591, “User Acceptance of Refinery Valves”, was published in 1990. The second edition was approved and published in 1995 with only a few changes.

Document Overview

RP591 can be divided into two distinct parts: 1) the manufacturer’s documentation and quality requirements and 2) the actual qualification testing procedure. The first part, which deals primarily with the manufacturer’s quality program, is basically a paraphrase of the 19 tenants of ISO 9001/API Q1.

The actual qualification testing program begins in section 6.0 of the document with requirements for documentation to be provided by the manufacturer to the testing agency and included in the test report. This data includes drawings, requested welding procedures, casting and/or forging source information, closure torques, rim pull calculations, and the location of final assembly and testing of the valves.

The first requirement for the manufacturer is to select a mutually acceptable testing facility to perform the inspection. To be eligible for consideration as an API RP591 testing facility, the lab must have a degreed mechanical or metallurgical engineer on staff overseeing the testing.

The inspection process begins with the testing facility randomly selecting valves from an inventory of like valves in pressure class, material and size from manufacturer or distributor stock. The valves are then tagged and heat and serial numbers are recorded. They are then shipped to the testing facility for the actual inspection process.

Prior to disassembly the valves are subjected to hydrostatic pressure tests in accordance with API 598. All optional API 598 closure tests are performed as well. For all seat tests, the recommended closure torque values provided by the manufacturer are used, and the actual closure torques are measured via a calibrated torque wrench. A device to connect the torque wrench is attached to the center of the stem shaft or gear operator. If the torque recommended by the manufacturer is inadequate to prevent leakage, the torque may be increased by a maximum of 25%. All valves are tested in the stem horizontal position.

Visual Inspection & Material Tests

Following hydrotesting, the visual inspection process is begun. After as-assembled dimensions are recorded, the valves are disassembled for detailed component inspection. Over 40 different items are visually inspected and/or measured. The visual inspection includes everything from handwheel construction to stem cylindricity and straightness.

After all the measurements have been taken and conditions recorded, paint and sealants are removed from the body, bonnet and covers (as applicable) and the castings are visually inspected per MSS SP55. Forgings are inspected to confirm they are free of laps and seams.

A key requirement of RP 591 is chemical analysis and hardness testing of all key components on a minimum of five sample valves.

To confirm that valve handwheels are in conformance with API 600 and will not fail during the rigors of severe field use, two tests are performed. The first test involves striking the handwheel between spokes with a 3 lb. or 10 lb. hammer, depending upon valve NPS. The second test requires the wheel to be subjected to a torque test at 300% of the manufacturers design torque. Any damage is to be reported.

Stem Shaft/Closure Element Strength Test

One of the prime motivators for the creation of RP 591 30 years ago was a series of stem to wedge failures encountered by several end-users. The “Stem Pull Test” as it is generally called, is performed on a tensile test machine. The Closure member and stem are both held in special fixtures to enable them to be properly gripped by the tensile test machine. The stem is then “pulled” and the actual load and point of failure is recorded. The manufacturer is required to provide expected stem shaft to closure element failure calculations to the testing facility, so that the fixtures may be properly designed and built.

The goal of the stem pull test is to insure that the first point of failure in a stem to closure element break occurs outside the body of the valve. Because an outside-the-pressure-containing-area failure provides the valve owner a section of stem to grab in case emergency opening procedures are required following a stem to closure element failure. Failure inside the pressure containment area means that the line will have to be de-energized and the valve disassembled for stem removal and disc opening. The stem pull is also the most exciting part of any RP591 test, especially when a large diameter stem snaps apart in the tensile machine!

Nondestructive Evaluation (NDE)

The most telling aspect of the RP 591 inspection process has to be the NDE phase, particularly the radiography. All accessible pressure containing welds are radiographed in accordance with table 341.3.2 of ASME B31.3, using the acceptance criteria for normal fluid service conditions. Pressure containing welds that cannot be radiographed are examined by either magnetic particle or liquid penetrant in accordance with ASME B16.34. Sections of cast valves, as identified in B16.34, from four valves or 25% of the sample lot (whichever is larger) are examined by radiography in accordance with B16.34 as well. Details of any discontinuity as well as sketches illustrating the film locations are included in the report.

One of the key revisions in the 3rd edition of RP 591 is the requirement that changes in sources of pressure-containing forgings or castings require that additional testing (generally radiography) be performed. Items that require complete requalification of a valve include “any design change that will reduce the strength or impair operability of the valve or a change in location of manufacture”.

What RP 591 Doesn’t Require

While RP 591 is an extensive document, there are additional tests that are not found within its pages. The issue of fugitive emissions is not addressed and many end-users will request that some form of fugitive emissions testing be performed in conjunction with the 591 testing program. Some users also require a submerged helium shell test to further insure casting quality. As far as metallurgical examinations go, detailed testing of castings is not required. However, some users who have experienced casting imperfections are requiring metallographic examination of castings, as well as other more thorough metallurgical examinations plus additional radiography.

Also absent from the 591 document is a requirement for testing valves in different positions, such as the stem horizontal with horizontal flow configuration, to determine that the valve operates in all positions. This multi-directional test requirement was a part of some users testing requirements prior to the publication of RP 591.


RP 591 testing is not cheap for the manufacturer. However the economic benefits of gaining a spot on an active AML can be great. Even if the testing reveals inadequacies, this data can help the manufacturer to improve their product, by pointing out specific areas of needed improvement, which can translate to more satisfied customers and greater sales.

API RP 591 is an excellent tool to determine a”snapshot” view of a manufacturer’s valve quality. Users should also maintain a record of valve performance gathered from their own in-service observations as well as information provided from their valve repair and modification contractor(s). This valuable data can help insure that the RP 591 qualified valves are maintaining their pedigree.

Almost every major commodity valve manufacturer has moved their production location during the past three years, or is considering doing so in the next 12 months. The economics of a global economy are demanding it. These changes mean that past performance data cannot necessarily be counted on to confirm current valve quality. This makes the time and effort of implementing some form of examination program, such as RP 591 testing, a worthwhile endeavor that could potentially save lives and property.

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