Further to my earlier November, 2019 Post regarding API Vs ASME (Effective Vs Actual Area) Pressure Relief Valve (PRV) Sizing, I thought it an idea to complement that article with some other, often misread or misunderstood arguments, concerning PRV selection and compliance to API Standard 526.
API-526 Currently in its Seventh Edition, dated September, 2017. (Henceforth API--526 in this article).
API-526 covers “Flanged Steel Pressure-relief Valves” and is described as a Purchase Specification/Standard for PRV Users.
API-526 Provides PRV selection data such as, pressure/temperature ratings, inlet x outlet size configurations - with respective (effective) orifice areas, main materials of construction, centre to face dimensions and temperature limits by material etc.
Generally, API-526 is considered complimentary to the requirements of ASME VIII.
API-526 is the only such PRV standard available and is used globally.
The ideology of API-526 as a User Standard, is to allow interchangeability of fit (piping installation), form and function between different manufacturers PRV designs stated as conforming to API-526.
API-526 is specifically and only for ASME B16.34 Flanged Spring and Pilot Operated Pressure Relief Valves only.
It does not cover threaded, welded or other type of connection or flange standards other than ASME (B16.34), nor inlet sizes below 1”
Some Misconceptions Explained,-
Can API-526 Pressures & Temperatures be Exceeded?
Yes. Many PRV manufacturers can meet and exceed the size, pressure, temperature, back pressure etc., limits stated in API-526, using modified or custom designs. Likewise, some designs cannot meet those published in API-526. Not all sizing software will tell you either. Consult the manufacturer and ask the question.
Agreement between purchaser/end user and manufacturer needs to be obtained on such understanding, since any such issue is outside the scope of API-526. Any future replacement (especially by others) may be problematic if not recorded. (Note: No law or jurisdiction is broken here).
In API-526 Large Orifice Size Spring Operated PRV’s have Lower Pressure Limits. Why?
This is a legacy from the historic days of API-526 when major manufacturers at the time, agreed on offerings to satisfy then industry requirements with the American Petroleum Institute (API).
Basically, high pressure, large orifice spring operated PRV’s would be impractical and spring design problematic.
Pilot Operated PRV’s can Satisfy Full Flange Ratings for ALL Orifice Sizes
The pilot, acting as an on/off controller for the main valve, does not change in size with any orifice size. Full pressure rating of a given flange, is possible across the D to T range of orifice sizes.
Much higher outlet flange ratings are also a standard feature of pilot operated PRV’s, and the pilot operated design does not require a balancing bellows for any imposed back pressure condition.
Centre to Face Dimensions Differ
Centre to face dimensions between spring operated and pilot operated PRV’s are different in the main. Caution must be exercised when changing from a spring operated PRV to a pilot operated PRV and vice-versa. Some manufacturers can satisfy both.
Total Inlet Flange Thickness Dimension “X” or “s” (etc.) is not an API-526 Requirement
Manufacturer catalogued “X” or "s" (for example) dimension (PRV inlet total flange thickness incl. raised face), is stated in manufacturer catalogues in order to determine inlet flange bolt or stud length and is NOT an API-526 requirement.
it actually differs between manufacturers whom use full nozzle designs (mainly spring PRV’s), since this dimension is unique for each maker.
Essentially the manufacturer unique “X” or “s” dimension is necessary information for the user/installer as non-standard length bolting is required and should appear on drawing submittals.
In 2017 API-526 Added Restricted Lift Requirements for Spring Operated PRV’s
In API-520/526 the orifice areas differ some 21 to 78% in the D to T range and not uniformly. A calculated orifice area may fall in the lower size availability of a selected orifice resulting in an “oversized” PRV which would have excess flowing capacity.
Dependent on the application and other end user preferences, this “oversize” may cause unstable PRV operation during opening, such as chattering, long blowdown, increased potential for acoustic fatigue in outlet piping etc.
Example: Calculated Area 3065 mm² (4.75 in²), requires selection of the next available API Orifice Area “P” 4116 mm² (6.38 in²). Here the selected P Orifice Area is some 34 % oversized for the system fluid capacity. The rated capacity can be reduced by restricting the lift of the spring operated PRV.
Current API-526 7th Edition (2017) section 7.7 Introduced use of Restricted Lift for spring operated PRV’s.
API-526 Does state that this is allowable on the proviso that the manufacturer performs specified flow and operation laboratory testing to ASME VIII and is accredited by the National Board (NBBPVI).
Many manufacturers have already done this. You can always check a specific manufacturers PRV “UV/NB” Accreditation by accessing the NB on-line “Red Book” NB-18 here,-https://www.nationalboard.org/SiteDocuments/NB18/NB18.pdf
(Note: An alternate method of restricting capacity, is to utilise a custom orifice size, specifically sized for the application. This is not mentioned in API-526 but allowable under ASME VIII if certain conditions are satisfied).
What Are “Lightweight” CL.300 Inlet Flanges? – Spring Operated PRV’s only
When observing the API-526 flange pressure ratings for spring operated PRV’s, you will notice two CL.300 inlet flange entries. The first CL.300 is restricted to the maximum pressure rating of the CL.150 rating. It is often referred to as a “Lightweight CL.300 Flange” and has had (parenthesis) added by API to indicate a lower rating than standard.
It is by all accounts a CL.300 flange in shape and form, but limited to the maximum CL.150 rating. (Also called by some as a "Cosmetic CL.300 Flange).
It was decided to include these lightweight CL.300 flanges across the D to T orifice PRV selections, in order to give the specifier an option to use a smaller inlet size (than the one with the next “full” CL.300 rating) where the application allowed this.
The last statement is not true for some sizes, and the real reasons long lost in the sands of time. The most plausible explanation is that manufacturers designs at the time were more unique to each & every size and rating to give best price offering. Nowadays a “fit all” design is better utilised for the modern economics of manufacture. Hence. The lightweight CL.300 remains to this day.
This anomaly did not extend to Pilot Operated Pressure Relief Valves which were added to API-526 at a much later date. Pilot Operated PRV’s do not have this issue and feature full flange rating across the orifice D to T size scope.
Bellows (Outlet) Pressure Rating – Spring Operated PRV’s
Bellows design PRV’s within API-526 pressure/temperature listings, have lower outlet pressure ratings (emphasised by parenthesis). This is due to a nominal bellows design limitation – another API legacy from the old days.
The bellows generally having a lower pressure rating capability, dictates the complete PRV's maximum outlet flange pressure rating.
The bellows material is not specifically advised in API-526, but it standardises all potential bellows material pressure ratings at 38 °C (100 °F). This is extended further in recent editions of API-526 with the introduction of Annex C which provides factors to be used to calculate bellows pressure rating at elevated temperatures, for the majority of bellows materials available.
Note: This information should be used where the manufacturer can actually provide a bellows design for a specified material, size and pressure of PRV required. Again, some manufacturers can exceed the published limits and not all can meet those tabled.
Spring Materials – Spring Operated PRV’s
Most manufacturers have changed from Carbon Steel to Chrome Steel as their standard spring material. This provides the flexibility of extending the upper temperature range from 232 °C (450 °F) to 343 °C (650 °F) for their basic offering. This is within API-526 (Section 8.2). Note that API-526 maintains its pressure/temperature charts with a 450 °F upper temperature limit for carbon steel springs.
Materials Below -29 °C (–20 °F) & Low Temperature Selection
For temperatures below -29 °C (-20 °F) API-526 defaults to Austenitic Stainless Steel as the PRV body material..
For more economic PRV selections and offerings, most manufacturers offer low temperature impact tested/certified, carbon steel body material, where the application/temperature permits. This is allowed by API-526.
API-526 specifies materials against operating temperatures. Flange ratings should be selected according to the relieving temperature. Some applications may also involve rapid cooling during relief, requiring low temperature material. These temperatures should all be considered for the material selection makeup of the PRV.
Historically, API-526 never considered ambient temperatures. These may sometimes be covered by the design temperature of the plant/application. Ensure spring and bolting (studs and nuts) materials are suitable for these temperature extremes.
2½” (DN65) Size Flanges
API-526 Removed 2½” sized flanged connections in 1984 and replaced with 3” (DN80) size. Most manufacturers can still provide 2½” when required, for example a replacement/MRO PRV’s.
RELEVANT NOTES
ASME VIII “UV/NB” Code Stamped PRV’s
A PRV having pressure exceeding API-526 limits, can be “UV/NB” Code Stamped if applied for by the manufacturer and accredited accordingly.
API-526 is not a requirement of ASME VIII. Conversely, a more basic, semi nozzle, non-adjustable PRV can satisfy ASME requirements and become accredited to be “UV/NB” Code Stamped. ASME VIII Is concerned about the design and demonstrated function of PRV’s rather than the fit and form.
API Stamp
Unlike certain in-line process valves, there is no API Stamp for Pressure Relief Valves.
“Borderline” Pressure/Temperature Maximum Inlet Flange Rating
Should you come across an application where the set pressure literally sits on the inlet flange pressure/temperature limit, it more than likely means you are obliged to select the next higher flange rating.
Note that the PRV will not be experiencing just the set pressure, but the overpressure as well during opening, upon which flange rating selection should be made.
In such cases where the borderline flange is preferred, it is worth reviewing an allowance specified in the flange standard used for API-526.
ASME B16.34 (2017) Section 2.5.1 “Safety Valves, Relief Valves or Rupture Disc Operation. Under conditions of safety valves, relief valves or rupture disc operation, pressure may exceed the rated pressure for a valve furnished under this standard by no more than 10% of that defined by the pressure-temperature rating. Such conditions are necessarily of limited duration. Damage that may result from pressure excursions in excess of the aforementioned is solely the responsibility of the user.”
Sizes larger than 8” x T x 10”
Many manufacturers offer sizes greater than “T” orifice, such as 10” x V x 14” up to 20” x 24” size. Although these PRV;s may look similar, they are not API-526 listed and orifice sizes, centre to face dimensions etc., vary between manufacturers. Consider these sizes as unique as there is no user standard.
Small ½”, ¾” Sized PRV’s (Generally Threaded Connection PRV’s)
All manufacturers complement their standard API-526 range with a smaller, generally threaded (as standard, optionally flanged or welded connections) PRV. These are essentially the simpler design used for thermal relief applications. They are also available with 1” and 1½” inlets and can also have API designated effective areas. However, these are not API-526 PRV’s and are quite often bundled together within a purchase requisition. There is no similar user standard for these designs. Consequently, differing dimensions and design features vary between manufacturers.
API-520 Effective Vs ASME Actual Area Sizing
See my earlier post in these FAQ'a for the details.
