Magnetic Level Indicator Compliance to ASME B31.3: Conformity Considerations(PDF)
Michael J. Bequette, P.E., VP of Engineering SOR Controls Group
Emily Rose Giunta, Level Partner Success Manager SOR Controls Group
Magnetic Level Indicators (MLIs), also known as magnetic level gauges (MLGs), are a proven and reliable method for viewing and monitoring process fluid levels. Due to their robust and versatile construction, MLIs provide superior personnel protection from hazardous process conditions and are able to easily integrate additional instrumentation, making them a superior alternative to sight glasses and reflex gauges. Plant process piping connects directly to the MLI, requiring its design and construction to comply with the standards of ASME B31.3 – Process Piping, hereafter referred to as B31.3.
Drawing on over forty engineering societies, industries, government bureaus, institutes and trade associations, the American Standards Association created Project B31 to be solely administered by the American Society of Mechanical Engineers (ASME) in 1926. Section B31.3 of the B31 code was designated in 1973 to be adopted as ANSI B31.3-1973 and has since been revised many times, with the most recent version released in 2018. B31.3 defines the rules for design, fabrication, assembly, examination and testing, as well as the material characteristics and components for process piping. Products conforming to the scope of B31.3 are found in many industries around the globe but are most common in process piping facilities, such as cryogenic, chemical processing, pulp and paper, pharmaceutical plants and especially petroleum refineries.
B31.3 Fluid Service
Physical application parameters are the driving factor behind all plant piping system design decisions. These include, but are not limited to, design pressure, design temperature, piping loads and intended service. This service, referred to as fluid service in ASME B31.3, is defined by the user and serves as the basis of engineering design for all instrumentation and equipment within the piping system – including magnetic level indicators.
Section B31.3 defines the following fluid services:
- Normal Fluid Service
- Category D
- Category M
- Elevated Temperature Fluid Service
- High Pressure Fluid Service
- High Purity Fluid Service
These fluid service categories group similar piping systems by accounting for factors such as fluid properties and operating conditions. Typically, external engineers will assume Normal Fluid Service, unless a category is specifically identified by the owner of the piping system. The majority of ASME B31.3 is written to provide design requirements for systems falling within Normal Fluid Service; however, the code does include provisions and rules of conformity for the other fluid service categories.
In addition to general design requirements, B31.3 also defines necessary values for design calculations, such as temperature and pressure ratings, tolerances, allowances, required safety factors and material stress values. Many component designs such as flanges and couplings, are defined by other ASME standards, and a list of these is referenced in B31.3 Table 326.1. The code includes this data to ensure proper engineering design calculations take the maximum pressure and temperature of the piping system into consideration. For an MLI, B31.3 Section 304.1.2, Straight Pipe Under Internal Pressure, provides the required formula(s) to determine proper vessel wall thickness. Typically, vessel wall thicknesses of MLIs conform to standard pipe schedules – ranging from schedule 10 to 160.
B31.3 Weld Design
Weld design is another crucial part of ASME chamber compliance. B31.3 allows welded joints for all metallic materials when performed using qualified weld procedures by certified welders, and with conformance to the rules in B31.3
Chapter V. Fluid service also affects which welded joint types are permissible in a system. For example, socket welds greater than 2 nominal pipe size should not be used in systems with severe cyclic conditions, or if there is risk of crevice corrosion or severe erosion. A visual representation of common magnetic level indicator chamber weld types is shown in Figure 1.
Figure 1. Common Chamber Welds
MLIs usually require the use of branch connections to facilitate connection to the main chamber. In order to conform to B31.3, branch connection design may require reinforcement. This is a critical design area to investigate when assessing whether a unit is constructed to meet code requirements. Branch connections with listed fittings, unlisted fittings, and pipe to pipe connections all fall under the scope of B31.3. Due to the necessity of drilling into the chamber run pipe, the original material ratings may no longer be valid. ASME code provides the equations required for strength calculations, as well as the specific instances where pipe to pipe welds are permissible; however, these instances are often not applicable to MLI construction since section 304.3.5 states that such branch connections should be avoided when the branch pipe size approaches the chamber run size. It can be assumed that the branch connection is strong enough when using a listed branch fitting, B16.11 tee or when welding a listed threaded/socket welded coupling or half coupling to the main pipe – provided that the fitting allows for fully penetrated groove welds, per Section 328.5.4. Furthermore, extruded outlets must also follow the same design criteria with regards to strength and reinforcement, as covered in Section 304.3.3.
Although Section II and Chapter V of B31.3 address many scenarios involving welds and branch connections, the code does not include details for every possible application. For instance:
- The minimum distance between branch welds
- The intersection of branch and circumferential welds
- Whether heat affected zones (HAZs) are allowed to overlap
The first scenario is mentioned in B31.3, but only briefly – Section 304.3.3 states that the distance between the centers should be at least one and a half times the connections’ average diameter, but to otherwise consult Pipe Fabrication Institute ES-7.
The other two examples are not addressed outright by B31.3; however, the majority of users who require compliance to ASME code and manufacturers who build per ASME have their own weld standards to dictate such requirements. That said, current industry standards prohibit the use of intersecting branch and circumferential welds to ensure proper distance between HAZs and protect the integrity of the material. Therefore, it is a safe assumption that any chamber displaying either of these latter conditions was not built to ASME B31.3 compliance. Illustrated examples of non-compliant branch connections can be seen below in Figure 2.
Figure 2. Non-Conforming Welds
B31.3 Materials
Obtaining materials that conform to B31.3 is another challenge when manufacturing code compliant chambers, as there are a variety of alloys and grades available on the market. Understanding the differences and overlaps between ASTM and ASME codes helps to prevent costly mistakes. In many cases, ASTM and ASME material are identical, with some even carrying a dual certification. On one hand, ASTM defines the material specifications and acceptable testing methods used to determine required physical characteristics. ASME, on the other hand, selects ASTM materials that will sufficiently perform in the defined service. It is important to note that this relationship is unidirectional, as there are some ASTM materials that are not acceptable by ASME code.
Temperature ratings, both minimum and maximum values are another material consideration that must be evaluated when designing a B31.3 compliant magnetic level indicator. Although the material may be occasionally used outside of the standard temperature range, this will often require additional testing and Section II and Appendix A of the code should be thoroughly reviewed to ensure conformity.
In addition to the factors mentioned above, some materials are also subject to manufacturing process requirements. Some of the most common requirements are due to welding effects. To achieve quality weld joints and/or maintain material strength, many materials require pre-weld heat treatment, post-weld heat treatment or both. Moreover, material-dependent interpass temperatures may need to be maintained. Welding procedure specifications (WPSs) are instrumental to ensure repeatable welds that conform to B31.3. WPSs are established through Procedure Qualification Records (PQRs), which document the welding procedures used, the code-required testing of the procedure on sample materials, or coupons, as well as the test results.
B31.3 Testing
During the course of manufacturing, code compliant magnetic level indicators require inspection and non-destructive examination (NDE). Section B31.3 contains specific testing methodology and acceptance criteria based on the fluid service of the system where the MLI will be installed, which is defined in Table 341.3.2. NDE methods for B31.3 compliant MLI chamber welds include visual examination, magnetic particle examination, liquid penetrant examination, radiographic examination, ultrasonic examination and in-process examination; however, magnetic particle testing is usually not applicable because magnetic materials are unusable for the majority of MLI components. In addition, all personnel performing the examinations must be qualified and certified to carry out the assessments. Definitions of NDE methods and an example of how NDE varies by fluid service can be found below in Tables 1 and 2 respectively.
Table 1. Non-Destructive Examination Methods
Table 2. Fluid Service Non-Destructive Testing Requirements
Regardless of fluid service, B31.3 always requires hydrostatic testing to ensure that no leaks occur (there are some exceptions to this rule, but the majority do not apply to MLI chambers). The code specifies in Section 345.5.4.2 that the hydrostatic leak tests are to be performed at no less than 1.5 times the design pressure, multiplied by the allowable stress at test temperature, divided by allowable stress at component design temperature, see Equation 1 below. B31.3 also requires that the test pressure be held for a minimum of 10 minutes. After pressure is removed the unit is evaluated for leaks.
For example, testing a float to 1.5 times a chamber pressure rating of 5000 psi requires a test pressure of 7500 psi. A float designed to support such a pressure would need to include more robust support rings and wall thickness, making the float heavier and thereby increasing the minimum Specific Gravity (SG) needed for operation. Most MLI manufacturers design floats with built-in safety factors: therefore, testing above the float design pressure is still a necessary process, but testing to 150% of the chamber rating is unnecessary. Typically, testing at 1.1 or 1.2 times the float’s rated pressure is sufficient. Industry standards reveal that most manufacturers test MLI floats at 110% – 130% of the rated float pressure and separately from the chamber.1
1Information sourced from a variety of manufacturers including: SOR Inc., Jerguson, Orion, Jogler and ABB/Ktek.
Conformance Quick Guide
There are many considerations when designing and manufacturing ASME B31.3 compliant magnetic level indicators. Awareness is imperative for preventing mistakes impacting time, cost and personnel safety. When reviewing if an MLI complies to B31.3, the following reference graphics and flowchart can act as a quick check for conformity:
Citations
ASME B31.3 – 2018 Edition: Process Piping. American Society of Mechanical Engineers, 2019.
Atlas Magnetic Level Indicator. Orion Instruments. (2022, August). Retrieved March 17, 2023, from https://www.orioninstruments.com/sites/default/files/downloads/ori-142_orion_atlas_mli.pdf
Installation, Operation, & Maintenance Instructions. MAGNICATOR. Jerguson. (2017, March). Retrieved March 17, 2023, from https://5bb63a30-1ae0-467f-a18f-ea6c24e5f38a.filesusr.com/ugd/1bdacb_ee61d78f0a2c4299a12b8f980ae9c33d.pdf
Jogler MLG Component Improvements. Jogler. (2017, November 14). Retrieved March 17, 2023, from https://static1.squarespace.com/static/5304df70e4b0658094bde8e2/t/5a32d4f9e2c4833cb8161c65/1513280762306/JOG_TechnicalNotes-MLG-improvements-FINAL-1.pdf
Operating Instruction: KM26 Magnetic Level Gauges. ABB . (2020, October 12). Retrieved March 17, 2023, from https://library.e.abb.com/public/a3bc1d10a56f41ad9d3c8ffb0bdefb4f/OI_KM26_EN_RevJ_2020.pdf
Michael Bequette, P.E. – VP of Engineering SOR Controls Group
Michael Bequette has dual undergraduate degrees in Electrical Engineering and Theoretical Physics from Kansas State University. He has a Master’s Degree in Electrical Engineering from the University of Kansas, and a Master of Business Administration from Park University. Michael has 29 years of experience in the oil and gas space, as well as aerospace, glass, pulp and paper and water/wastewater. Michael is a licensed professional engineer in multiple states, holds 4 patents for fiber optic product development and capacitive fault location and is a senior member of IEEE.
Emily Rose Giunta – Level Partner Success Manager SOR Controls Group
Emily Rose Giunta is a graduate of the Missouri University of Science and Technology with a Bachelor of Science in Engineering Management. She has a Master’s Degree in Business Administration from the University of Missouri – Kansas City. Before coming to SOR, she worked as an Application Engineer in the rotating equipment industry. Emily Rose is the Level Partner Success Manager at SOR and is responsible for mechanical level and magnetic level indicator product success. She has also served as a Product Manager and an Application Engineer during her time with SOR.
