The core reasons why 3A unions do not have a sealing ring + structural advantages + applicable/prohibited scenarios

3A unions are not "sealless," but rather abandon traditional rubber/PTFE seals, employing a metal-faced hard seal combined with a precision conical interference fit to achieve a seal. This is a dedicated design for hygienic applications such as food and pharmaceutical manufacturing, and is also a mandatory requirement for clean fluid connections in 3A certification. The following will explain in detail the reasons for the absence of seals, structural advantages, applicable scenarios, and prohibited scenarios, aligning with the actual needs of hygienic industrial applications.

I. Why don't 3A unions need traditional sealing rings?

The core reason is that the sanitary requirements of 3A certification conflict with the cleanliness deficiencies of traditional sealing rings. 

Furthermore, 3A unions achieve ringless sealing through their precise structure, not by omitting the sealing design. Specifically, there are two 

points:

1. Traditional sealing rings pose a "contamination hazard" in sanitary conditions, violating 3A core standards.

3A certification is the core standard for sanitary fluid equipment in industries such as food, pharmaceuticals, and daily chemicals. 

The core requirements are a flow channel without dead corners, no easily damaged/residual parts, and the ability to withstand CIP/SIP online 

cleaning/sterilization. Traditional rubber/PTFE sealing rings have inherent defects:

The contact surface between the sealing ring and the joint is a sanitary dead corner, easily harboring residual media and bacteria, and cleaning

 fluid cannot completely rinse it away, becoming a source of microbial contamination;

The sealing ring is made of organic material, which is susceptible to contamination during SIP high-temperature sterilization (121~135℃) 

or long-term contact with acids and alkalis. When exposed to organic solvents, the seals are prone to aging, deformation, and shedding, 

contaminating the fluid medium.

The seals are consumable parts and need to be replaced regularly. The disassembly and assembly process can easily introduce external 

contamination, and the waste seals generated after replacement will incur consumable costs, affecting the continuous operation of the production line.

2. 3A Unions Achieve "Sealing Without a Ring" Through Precision Structural Design

The sealing of 3A unions relies on a hard metal seal and a tapered interference fit, eliminating the need for additional sealing components. 

The core structure consists of a male tapered end, a female tapered end, and a lock nut:

Both the male and female ends are precision-machined 304/316L stainless steel tapered surfaces (tap angles are typically 15°/30°),

 polished to sanitary grade precision (Ra≤0.8μm);

When the lock nut is tightened, the male and female tapered ends create an interference fit, and the microscopic protrusions on the metal 

surfaces interlock to form a hard metal seal, preventing fluid leakage;

Some high-end models feature mirror polishing (Ra≤0.4μm) or hard alloy welding on the tapered surfaces to improve the fit and wear 

resistance of the sealing surface, enhancing the sealing effect.

Simply put: 3A unions are not "no seal," but rather use structural seals instead of component seals, which meet both sealing requirements 

and 3A sanitary standards.

II. Core Benefits of the 3A Union Joint with No Seal Ring Structure

This structure is tailor-made for hygienic cleanroom environments, offering benefits concentrated in three core dimensions: cleanliness, 

durability, and operability. It fully meets the stringent requirements of food and pharmaceutical production lines, specifically:

1. "Ultimate Cleanliness": No dead corners, compliant with 3A/FDA certification, eliminating media contamination

The flow channel features a smooth, one-piece conical transition, eliminating dead corners such as seal ring grooves and steps, ensuring no 

media residue. CIP cleaning solution can be used for complete rinsing, and SIP high-temperature steam can sterilize without dead corners;

The absence of organic seals avoids media contamination caused by seal ring aging, shedding, and swelling, meeting the contact requirements 

of high-cleanliness fluids such as food, pharmaceuticals, and water for injection. It can directly pass hygienic certifications such as 3A, FDA, 

and EHEDG.

2. "Ultra-long Durability": Metal hard seal, temperature/pressure/corrosion resistant, no vulnerable parts

The sealing surface is made of 304/316L stainless steel (or Hastelloy/hard alloy weld overlay), with an extremely wide temperature range

 (-20~450℃), capable of withstanding SIP high-temperature sterilization and low-temperature fluid transportation, without aging failure issues;

The interference fit metal sealing surface has good pressure resistance (standard nominal pressure PN10~PN25, high-pressure models up to 

PN40), and is resistant to acid, alkali, and organic solvent corrosion, suitable for various clean process fluids;

No vulnerable parts such as sealing rings, no need to replace sealing components for life, only periodic inspection of conical surface wear is 

required, significantly reducing equipment maintenance costs and production line downtime.

3. Easy to Operate: Quick assembly and disassembly, high repeatability and sealing accuracy, suitable for frequent production line maintenance.

Utilizes a quick-assembly structure with a locking nut and conical surface, requiring no special tools for assembly and disassembly. 

This is more efficient than flange or welding connections, meeting the needs of frequent cleaning and maintenance in food and pharmaceutical 

production lines.

The conical surface is a precision-machined standardized structure. After disassembly, simply tightening restores a consistent interference fit, 

ensuring high repeatability and sealing accuracy, eliminating the problem of reduced sealing performance after disassembly and reassembly 

associated with traditional sealing rings.

4. Compact Structure:Small size and light weight, suitable for installation in confined cleanrooms. Without sealing ring grooves or additional 

sealing components, the overall structure of the connector is simple and compact. Its size is significantly smaller than ordinary unions with sealing rings,

and its lighter weight makes it suitable for installation environments with limited space and dense piping in cleanrooms of food and pharmaceutical 

production lines.

5. Strong Compatibility:Standardized conical design allows for seamless connection with other 3A sanitary fittings.

The conical dimensions and locking method of the 3A union are standardized 3A certified designs, allowing direct connection with the conical 

interfaces of 3A sanitary elbows, tees, valves, and pumps without the need for additional adapters, ensuring overall sanitary consistency of the pipeline.  

III. Applicable Scenarios for 3A Union Joints without Sealing Rings

The core compatibility lies with sanitary cleanroom environments that meet 3A certification requirements, and fluid transport pipelines with 

high demands for sealing cleanliness, temperature resistance, and ease of assembly and disassembly. These are primarily found in the food, 

pharmaceutical, daily chemical, and fine chemical industries. Specific scenarios include:

1. Pharmaceutical and Biotechnology Industry (Core Main Application)

Sterile Drug Delivery: Preparation and delivery pipelines for injections, oral solutions, and biological vaccines;

High-purity Water Delivery: Piping connections for pharmaceutical-grade pure water (PW), water for injection (WFI), and ultrapure water;

Sterile API Production: Clean production pipelines for antibiotics, biological proteins, and pharmaceutical excipients, compatible with CIP/SIP

online cleaning/sterilization.

2. Food and Beverage Industry

Aseptic filling production lines: conveying and filling pipelines for milk, yogurt, juice, and aseptic beverages;

High-end alcoholic beverage/condiment production: aseptic fermentation broth and finished product pipelines for red wine, white wine, 

soy sauce, and vinegar;

Dairy product processing: clean production pipelines for cheese and cream, avoiding flavor contamination caused by residual media.

3. Daily Chemical and Cosmetic Industry

High-end aseptic skincare product production: clean conveying pipelines for serums, creams, and facial masks;

Medical personal care product production: production pipelines for disinfectants, medical hand sanitizers, and aseptic laundry detergents.

4. Fine Chemical Industry

Food-grade additive production: clean conveying pipelines for citric acid, xylitol, and food flavorings;

High-purity reagent delivery: high-purity acids and alkalis, and organic solvents for laboratories and fine chemicals 

(no risk of swelling of organic seals).

General Applicable Conditions

Fluid Medium: Clean, particle-free, and abrasive-free liquid/gas (avoid scratching metal sealing surfaces);

Operating Parameters: Temperature -20~450℃, Pressure PN10~PN25 (standard models), capable of withstanding frequent CIP/SIP;

Installation Requirements: Sanitary piping connections requiring frequent disassembly, cleaning, and maintenance, such as valves, 

pump bodies, and filter inlets and outlets.

IV. Prohibited Scenarios for 3A Union Joints without Sealing Rings

This structure relies on a precise interference fit between the metal conical surface and the seal. It has inherent limitations such as easy 

scratching of the sealing surface, limited sealing effect at low pressure, and incompatibility with media containing particles. Therefore, 

its use is strictly prohibited in non-sanitary conditions, media containing abrasives, low-pressure large-diameter applications, and scenarios 

involving severe vibration, specifically as follows:

1. Pipelines containing solid particles/abrasives/impurities (Core Prohibited Scenarios)

The metal hard sealing surface is a precision polished surface. If the medium contains solid particles such as sand, iron filings, or powder, 

it will quickly scratch the conical surface, destroying the interference fit seal and leading to permanent leakage. Examples include:

Ordinary industrial water, cooling water, and wastewater pipelines;

Fluid pipelines containing particles such as chemical raw materials, ore slurries, coatings, and inks;

Unfiltered tap water and groundwater pipelines.

2. Non-Sanitary Industrial Applications with No Cleanliness Requirements

The precision machining and sanitary polishing of 3A unions result in costs significantly higher than ordinary unions 

(3-5 times that of ordinary carbon steel/stainless steel unions). Using them in non-sanitary applications is "overkill" and completely unnecessary, 

such as:

Ordinary process pipelines in metallurgical, mining, and steel plants;

Building water supply and drainage, municipal pipe networks, and HVAC pipelines;

Non-clean raw material transport pipelines in ordinary chemical and petrochemical industries.

3. Low-Pressure (PN≤4), Large-Diameter (DN≥80) Fluid Pipelines

The sealing effect of metal hard seals relies on the interference fit created by the pre-tightening force of the locking nut. In low-pressure, 

large-diameter applications:

The large contact area of the large-diameter conical surface requires extremely high pre-tightening force to achieve a seal. Manual tightening 

is difficult to achieve, easily leading to low-pressure seal failure;

Under low-pressure conditions, the fluid sealing force is insufficient to enhance the adhesion of the metal sealing surface, significantly 

increasing the risk of leakage. 4. Operating Conditions with Severe Vibration, Pipeline Displacement, and Frequent Impacts

The sealing of the 3A union relies on the static interference fit of the conical surface. If the pipeline experiences severe vibration or axial/radial 

displacement due to pump vibration, equipment operation, or thermal expansion and contraction, gaps may appear at the contact surfaces 

of the conical surfaces, compromising the sealing effect. Examples include:

Pipelines near the inlet and outlet of vibrating equipment such as pumps, fans, and compressors;

Long-distance outdoor pipelines with large temperature differences (thermal expansion and contraction causing pipeline displacement);

Hydraulic and pneumatic pipelines of engineering machinery and mobile equipment.

5. Special Operating Conditions with Low Temperature Cryogenic (<-20℃) and Ultra-High Pressure (>PN40)

Low Temperature Cryogenic Conditions: The stainless steel conical surface will shrink and deform below -20℃, leading to increased interference 

fit clearance and seal failure;

Ultra-High Pressure Conditions: The locking nut and conical surface structure of the conventional 3A union cannot withstand high pressures 

above PN40, easily resulting in nut breakage and conical surface deformation. Special high-pressure flanges/welded connections must be used.

6. Temporary pipelines/pressure testing pipelines requiring quick sealing and low preload

3A unions require even tightening of the lock nut to ensure a complete fit of the conical surface. Uneven tightening can lead to poor fit of the 

conical surface in some areas, resulting in leakage. Therefore, they are not suitable for pressure testing pipelines or temporary maintenance 

pipelines that require quick disassembly and assembly.

V. Core Summary

1. The Nature of the Seal-less Fitting:The 3A union uses a metal hard seal to replace the traditional sealing ring, achieving a seal through a 

precision conical interference fit. It is not seal-less, but it meets the 3A sanitary grade requirement of no dead angles.

2. Core Structural Advantages:Extreme cleanliness (no dead angles, CIP/SIP compatible), durability (no vulnerable parts, temperature and 

corrosion resistant), convenience (quick installation and disassembly, high accuracy for repeated sealing).

3. Core Applicable Scenarios: 3A sanitary grade cleanroom conditions in food, pharmaceutical, daily chemical, and fine chemical industries; 

suitable for clean fluid pipelines without particles, under medium to high pressure, and requiring frequent disassembly and cleaning.

4. Core Prohibited Scenarios:Pipelines containing solid particles/abrasive media, non-sanitary grade general operating conditions, 

low-pressure large-diameter pipelines, pipelines subject to severe vibration, and low-temperature/ultra-high-pressure pipelines.

In short: The 3A union is a "dedicated connector" for sanitary grade cleanroom conditions. In non-sanitary conditions or conditions containing 

particulate media, using a regular union with a sealing ring or flange connection is the optimal choice.