"Can Magnetic Level Gages (Indicators) be installed on my boiler?"

Magnetic level indicators (MLIs) offer superior visibility and reliability compared to traditional glass gages, especially in challenging environments. But can they be used on boilers? The answer is yes — but only within specific ASME Code requirements. This guide explains when MLIs are permitted, their limitations, and how to properly specify them for boiler applications.

The Short Answer

Yes, magnetic level indicators can be installed on boilers. However, they must meet seven specific ASME Code rules. Violating any of these rules can result in code non-compliance, equipment rejection during inspection, or unsafe operation.

ASME Code Requirements for MLIs on Boilers

If you're considering an MLI for boiler service, these rules are non-negotiable:

1. Maximum Operating Pressure: 900 PSIG (6.2 MPa)

MLIs are permitted only on boilers with a maximum operating pressure of 900 PSIG or less. Above this pressure, accuracy can deviate during frequent cycling. Many power generation boilers operate at 1,500-2,500 PSIG, making MLIs unsuitable for these applications. This is the most restrictive requirement and eliminates MLIs from high-pressure utility and industrial boiler service.

Why This Limit Exists: At higher pressures with frequent load cycling, the float response and magnetic indication drift from actual water level. This drift creates false readings, which is unacceptable for boiler safety.

2. Indication Scale Limitations

The MLI indication scale must be positioned between:

• Upper limit: 2 inches above the danger point (as determined by boiler manufacturer)

• Lower limit: The low side of the outside diameter of the steam connection exiting the drum

This establishes a narrow "safe zone" where the MLI can reliably indicate. The danger point is typically the lowest water level the boiler can safely operate at before tube damage occurs. The upper limit prevents overfill carryover.

Practical Impact: On a typical small boiler, this might mean only 8-12 inches of actual viewing range — significantly less than glass gages or electronic sensors.

3. Switches & Transmitters Prohibited

External switches or transmitters cannot be attached to a magnetic level gage for control functions. This means you cannot use the MLI to automatically control feedwater valves or trigger alarms. If you need automation, you must use a separate electronic level transmitter.

Why This Rule: MLIs operate on float position. Attaching external controls creates complex dependencies that can fail or create hazardous conditions.

4. MLI Does NOT Replace Required Glass Gage

This is critical: ASME Code requires a direct-reading gage glass as your primary boiler water level indication. An MLI can be installed in addition to a glass gage, but it cannot replace it. You must have both.

Practical Impact: You need at least two separate level indication systems — a glass gage (independent, passive) and an MLI (for supplemental visibility).

5. MLI Cannot Function as Water Column

A magnetic level indicator is not a water column assembly. You cannot combine an MLI with a water column that houses gage glass, isolation valves, and drain valves. The MLI must be a standalone installation with its own connections.

Why: Water columns are complex assemblies with multiple functions. Combining components increases failure modes and violates ASME design requirements.

6. Isolation & Drain Valves Required

Every MLI on a boiler must have:

• Upper isolation valve (at steam connection)

• Lower isolation valve (at water connection)

• Drain valve (at bottom)

These are the same isolation requirements as glass gages — they allow maintenance and emergency isolation if the MLI fails.

7. Reference ASME Code Subsections

The specific code language is found in ASME Boiler & Pressure Vessel Code, Section 1:

• Subsection PG-12: Boiler inspection and certification

• Subsection PG-60: Level indication and control requirements

If your boiler is constructed after 2021, these requirements may have evolved — always consult the current code or a certified boiler inspector.

MLI vs. Glass Gage vs. Electronic Sensors for Boilers

Aspect	Glass Gage	MLI	Electronic Transmitter
ASME Required	✅ Yes (primary)	❌ No (supplemental only)	❌ No (backup)
Max Pressure	3,000+ PSIG	900 PSIG	3,000+ PSIG
Visibility	Good	Excellent	Remote only
Automation	Manual	Not permitted	✅ Yes
Service Life	5-10 years	10-15 years	5-8 years
Cost	$1,500-$3,000	$2,500-$5,000	$2,000-$4,000
Code Compliance	Primary indication	Supplemental only	Secondary

When MLIs Are Suitable for Boilers

Magnetic level indicators work best for:

• Industrial watertube boilers at 600-900 PSIG

• District heating boilers at low to medium pressure

• Backup instrumentation on older boilers (when glass gage visibility is poor)

• Standby or emergency boilers with infrequent cycling

• Food & beverage processing boilers where visibility is critical

MLIs do NOT work for:

• Utility power generation boilers (typically 1,500+ PSIG)

• High-pressure industrial steam boilers

• Applications requiring level control automation

• Primary level indication (must be secondary to glass gage)

Compliance & Documentation

When installing an MLI on a boiler:

1. Verify with manufacturer that your boiler is rated for 900 PSIG or less

2. Have plans reviewed by a professional engineer or certified boiler inspector

3. Document installation with pressure settings and isolation valve specifications

4. Keep brochure on file: "Understanding the Use of Magnetic Level Gages and ASME Code Compliance on Power Boiler Applications"

5. Schedule inspection before placing boiler in service

Non-compliance can result in:

• Equipment rejection during inspection

• Insurance coverage denial

• Operational shutdown

• Safety liability

Learn More

For detailed technical guidance on MLI selection, code compliance, and proper installation on your specific boiler, contact Clark-Reliance's engineering team.