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MIDEL 7131
Synthetic Ester Transformer Fluid — Pricing & Guide
Buy MIDEL 7131 Synthetic Ester Transformer Fluid | Pricing & Fast Delivery

Get instant freight-included pricing on MIDEL 7131 synthetic ester transformer fluid. Available in drums, totes, and bulk.

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​Shell MIDEL 7131 is the world's most widely used synthetic ester dielectric fluid — available to order online with real-time delivered pricing. Ship anywhere in the US. Order online in 60 seconds.

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What Is Synthetic Ester Transformer Fluid?

Synthetic ester transformer fluid is a high-performance dielectric insulating liquid engineered from polyol esters — chemically distinct from both mineral oil and natural esters. Unlike petroleum-based transformer oil, synthetic esters are manufactured through a controlled chemical process that produces a product with exceptionally consistent properties, batch after batch.

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Synthetic esters were developed specifically to overcome the limitations of mineral oil in applications where fire safety, environmental protection, or extreme temperature performance are critical. Today they are used in power transformers, distribution transformers, traction transformers, offshore platforms, underground vaults, and data center infrastructure worldwide.

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The most recognized synthetic ester on the market is MIDEL 7131, originally developed by M&I Materials in the 1970s and now manufactured and distributed globally by Shell Lubricants following their acquisition of the MIDEL brand in January 2024.

MIDEL 7131 Overview - 

Product Identity

  • Manufacturer: Shell Lubricants (formerly M&I Materials / MIDEL & MIVOLT Fluids Ltd.)

  • Chemical type: Synthetic organic ester (pentaerythritol fatty acid esters)

  • CAS No.: 68424-31-7

  • Product code (Shell): 007A3822

  • Current SDS: OSHA 29 CFR 1910.1200 compliant, Version 1.3, revised April 2025

Why MIDEL 7131 Is the Benchmark

MIDEL 7131 has been installed in transformers across 70% of countries worldwide and has maintained a flawless fire safety record since commercial introduction in the 1970s. It is the only synthetic ester with decades of real-world installation data across utility, industrial, marine, traction, and renewable energy applications.

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When engineers specify synthetic ester, MIDEL 7131 is almost always the reference product — and increasingly the default choice for new fills, retrofills, and high-risk installations.

MIDEL 7131 Specifications

Property
Value
Test Method
Appearance
Pale amber liquid
—
Flash point
≥ 250°C / 482°F
ISO 2719
Fire point
> 300°C
IEC 61039
Pour point
≤ -45°C / -49°F
ISO 3016
Auto-ignition temperature
> 320°C / 608°F
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Kinematic viscosity @ 40°C
≤ 35 mm²/s
ISO 3104
Density @ 20°C
970 kg/m³
ISO 3675
Vapor pressure @ 20°C
< 0.5 Pa
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Biodegradability
Readily biodegradable
OECD 301F
Fire classification
K-class
IEC 61039
NFPA Rating (Health / Fire / Reactivity)
0 / 1 / 0
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Transport classification
Not regulated (49 CFR / IATA / IMDG)
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Buy MIDEL 7131 — Instant Delivered Pricing

TransformerParts.com is the only online source where you can get freight-included pricing on MIDEL 7131 in under 60 seconds. Enter your container type, quantity, and delivery ZIP code — and get a real delivered price with no phone call required.

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We ship from multiple distribution centers nationwide to get you the most competitive freight cost to your location.

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Available container types:

  • Drums (approximately 55 gallons)

  • Totes (275–330 gallons)

  • Bulk tanker (contact us for large-volume bulk pricing)

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Synthetic Ester vs. Mineral Oil vs. Natural Ester

Choosing the right dielectric fluid depends on your transformer's application, location, age, and risk profile. Here's how synthetic ester compares to the alternatives:

Factor
Mineral Oil
Natural Ester (FR3)
Synthetic Ester (MIDEL 7131)
Flash point
~145°C
~330°C
≥ 250°C
Fire point
~160°C
> 360°C
> 300°C
Fire classification
Not K-class
K-class
K-class
Biodegradability
Not readily biodegradable
Readily biodegradable
Readily biodegradable
Pour point
~-40°C
~-21°C
≤ -45°C
Cold weather performance
Good
Limited
Excellent
Moisture tolerance
Low
Very high
High
Oxidation stability
Good (inhibited)
Moderate
Excellent
Cellulose insulation life extension
Baseline
Up to 5–8x
Up to 5–8x
Retrofill compatibility
N/A
Limited (flush required)
Excellent
Typical price range (per gallon)
$7–$13
$12–$20
$40–$70

When to Choose Synthetic Ester Over Mineral Oil

Synthetic ester makes the most sense when one or more of the following applies:

  • The transformer is indoors, in a basement, underground vault, tunnel, or other location where a mineral oil fire would be catastrophic

  • The installation is near environmentally sensitive areas — rivers, wetlands, drinking water infrastructure

  • The transformer operates in cold climates where natural ester's limited pour point is a concern

  • You need K-class fire classification for building code compliance or insurance requirements

  • You're retrofilling an existing transformer and need a fluid compatible with existing mineral oil residues

  • The asset has long service life requirements and insulation life extension is a priority

Why MIDEL 7131 Is the Benchmark

Natural ester (FR3, BIOTEMP) and synthetic ester (MIDEL 7131) are both K-class, biodegradable alternatives to mineral oil — but they're not interchangeable in every application:

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  • Cold climates: MIDEL 7131's pour point of -45°C dramatically outperforms natural esters (-21°C), making it the clear choice for northern utilities and outdoor installations

  • Retrofilling from mineral oil: Synthetic ester is far more tolerant of residual mineral oil, allowing retrofills without aggressive flushing

  • Oxidation stability: Synthetic ester maintains stable properties in free-breathing transformers far better than natural esters, which can oxidize in open systems

  • Long-term consistency: As a manufactured product, synthetic ester has batch-to-batch consistency that bio-based natural esters can't always match

Fire Safety Advantages

K-Class Classification

MIDEL 7131 meets IEC 61039 K-class classification, which requires a fire point above 300°C. Mineral oil does not qualify. This classification directly impacts what fire suppression systems are required, what building code restrictions apply, and how insurers assess risk for transformer installations.

100% Fire Safety Record

Since commercial introduction in the 1970s, MIDEL 7131 has never been involved in a transformer fire. That's not a marketing claim — it's a documented, industry-recognized record spanning 50 years of global deployments including naval vessels, underground transit systems, high-rise buildings, and offshore platforms.

Relevant for Navy and Marine Applications

MIDEL 7131 is specified and used in naval and marine applications where NAVSEA and ship repair contractors require K-class dielectric fluids. It ships with full OSHA-compliant SDS documentation and is non-regulated under 49 CFR, IATA, and IMDG for transport purposes.

Environmental Benefits

Readily Biodegradable

MIDEL 7131 achieves >60% biodegradation in 28 days under OECD 301F testing — meeting the regulatory threshold for "readily biodegradable." In a spill event, this dramatically reduces environmental remediation costs and regulatory liability compared to mineral oil.

Reduced Containment Requirements

Many jurisdictions allow reduced or eliminated secondary containment requirements for transformers filled with readily biodegradable K-class fluids like MIDEL 7131. This can materially reduce installation costs, especially in sensitive locations.

Regulatory Profile

  • Not listed under CERCLA / SARA hazardous substances

  • No SARA 311/312 hazards

  • California Prop 65: clean

  • Clean Water Act Section 311: no listed hazardous chemicals

  • TSCA inventory: all components listed

Insulation Life Extension

One of the least-discussed advantages of synthetic ester is what it does to your transformer's paper insulation. Cellulose insulation in transformers degrades primarily through moisture and heat. MIDEL 7131 actively absorbs moisture from the cellulose into the fluid — with a saturation limit of approximately 2,700 ppm — dramatically slowing insulation degradation.

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Studies have shown that ester-filled transformers can achieve 5 to 8 times the insulation life of equivalent mineral oil transformers, deferring capital replacement costs and reducing lifecycle total cost of ownership.

Cold Weather Performance

With a pour point of -45°C, MIDEL 7131 is one of the most cold-weather-capable dielectric fluids available. It starts reliably in sub-arctic conditions where natural esters would gel, and its viscosity at low temperatures remains within operational limits for transformer starting and loading.

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This makes synthetic ester the preferred choice for northern utilities, Canadian operations, mountainous terrain, and any application where winter startup reliability is non-negotiable.

Retrofilling Existing Transformers with MIDEL 7131

MIDEL 7131 is one of the most widely used retrofill fluids in the industry. Unlike natural esters, which typically require aggressive flushing to remove mineral oil residues, MIDEL 7131 is highly tolerant of residual mineral oil — simplifying the retrofill process and reducing downtime.

The Basic Retrofill Process

  1. Drain the existing mineral oil under vacuum

  2. Perform a vacuum dry-out to remove moisture from the insulation

  3. Fill with MIDEL 7131 under vacuum

  4. Verify dielectric strength and moisture content post-fill

  5. Update transformer nameplate and documentation

What to Watch For

  • Seals and gaskets: MIDEL 7131 is compatible with most seal materials used with mineral oil, but confirm with your transformer OEM

  • Existing oil labeling: the transformer nameplate and records should be updated to reflect the fluid change

  • Inspection intervals: establish a new baseline DGA and moisture reading after retrofill

Applications

Where MIDEL 7131 Is Specified

  • Utility substations — particularly indoor, vault, or environmentally sensitive locations

  • Navy and marine — ship repair, naval vessel maintenance, NAVSEA-governed procurement

  • Traction / rail — light rail, subway, high-speed rail traction transformers

  • Renewable energy — wind turbine transformers, solar facility step-up transformers

  • Data centers — mission-critical power infrastructure where fire risk is unacceptable

  • Mining — underground mining transformers where fire in enclosed spaces is catastrophic

  • Industrial facilities — refineries, chemical plants, manufacturing with on-site substation transformers

  • High-rise buildings — building code compliance for indoor transformer installations

Ordering MIDEL 7131 from
TransformerParts.com

How It Works

Ordering transformer fluid shouldn't require a phone call, a quote request, or a three-day wait. TransformerParts.com gives you real delivered pricing in under 60 seconds:

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  1. Go to the Oil Store — Click the button below

  2. Enter your delivery ZIP code — freight is calculated in real time

  3. Enter your gallonage — Totes, drums, and pails

  4. Select a product tile — After prices generate, select a product tile by clicking on it

  5. Send yourself a quote or continue to checkout

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We ship from distribution centers across the US to minimize freight cost to your location.

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Frequently Asked Questions

What is MIDEL 7131 made of?

MIDEL 7131 is a synthetic organic ester made from pentaerythritol fatty acid esters (CAS 68424-31-7). It is a manufactured product — not derived from vegetable or animal sources — which gives it highly consistent properties and excellent oxidation stability.

Who makes MIDEL 7131 now?

MIDEL 7131 was developed by M&I Materials Ltd. and sold under the MIDEL brand for decades. In January 2024, Shell Lubricants acquired the MIDEL and MIVOLT businesses. The product is now manufactured and distributed globally as Shell MIDEL 7131.

Is MIDEL 7131 the same as FR3?

No. FR3 (made by Cargill) is a natural ester derived from soybean oil. MIDEL 7131 is a synthetic ester manufactured from pentaerythritol esters. Both are K-class and biodegradable, but they have different cold-weather performance, oxidation stability, and retrofill compatibility characteristics.

Does MIDEL 7131 require special handling or permits to ship?

No. MIDEL 7131 is not regulated as a dangerous good under 49 CFR (US), IATA (air), or IMDG (sea). No hazmat placarding or special permits are required. It ships as standard freight.

Can I mix MIDEL 7131 with mineral oil?

MIDEL 7131 is highly tolerant of mineral oil residues, which is why it's a popular retrofill fluid. However, mixing intentionally is not recommended — it will dilute the fire safety and biodegradability properties of the synthetic ester. For retrofills, drain as thoroughly as possible before filling.

What SDS documents are required for MIDEL 7131?

The current US SDS is Shell MIDEL 7131, SDS Number 800010065925, Version 1.3, revised April 2025, written to OSHA HazCom Standard 29 CFR 1910.1200. This document is available from Shell's product portal and can be provided with your order.

What is the shelf life of MIDEL 7131?

MIDEL 7131 has excellent oxidation stability and long shelf life when stored in sealed containers away from direct sunlight and temperature extremes. Opened containers should be resealed promptly to minimize moisture ingress and air exposure.

Can I get NET-30 terms?

Yes. TransformerParts.com offers NET-30 credit terms to approved business accounts. You can apply during checkout or contact us directly. We typically respond to credit applications within one business day.

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