What are the details of the D-type and S-type connection methods for stainless steel pipe fittings

In the fields of building water supply and drainage, fire protection engineering, and industrial fluid transportation, thin-walled stainless steel pipe fittings have gradually become the mainstream choice due to their advantages such as corrosion resistance, long service life, and environmental friendliness. D-type and S-type pipe fittings, as the two core types of press-fit connections, have design differences that directly affect the stability and applicable scenarios of the piping system.

First, the structural differences of stainless steel pipe fittings:

1) D-type stainless steel pipe fittings (equal diameter stainless steel pipe fittings) have no extended straight section at the socket end, using a single press-fit connection technology. During installation, the stainless steel pipe is inserted into the fitting to the positioning step, and a special press-fit tool is used to compress only one side of the U-shaped groove to form a hexagonal cross-section. While this design simplifies the operation process, its tensile strength is relatively weak, and the sealing ring may undergo slight deformation due to uneven stress during the press-fit process, leading to a risk of leakage after long-term use.

Typical scenarios: DN15 straight pipe connections for sinks, toilets, etc. in home renovations, or scenarios requiring consistent pipe diameter in hot water circulation systems. For example, a high-end residential project used D-type elbows to connect hot water pipes. By reducing abrupt changes in pipe diameter, the hot water delivery efficiency was improved by 15%, while noise was reduced by 5 decibels.

2) S-type stainless steel pipe fittings (reducing stainless steel pipe fittings) have an extended straight section at the socket end, forming a double-press structure. During installation, both sides of the U-shaped groove need to be pressed simultaneously, causing the sealing ring to radially shrink and fill the gap, forming a double sealing barrier. Its tensile strength is more than 40% higher than that of the D-type, and the extended straight section design can effectively disperse stress and prevent the pipe from loosening due to vibration or thermal expansion and contraction.

Typical scenarios: High-rise building fire sprinkler systems, industrial steam pipelines, and other high-pressure scenarios. For example, in a fire protection project of a commercial complex, the S-type reducing tee smoothly transitioned the DN50 main pipe to the DN25 branch pipe, ensuring uniform water flow distribution. Testing showed it could withstand a pressure of 1.6 MPa, far exceeding national standards.

Second, the connection strength of stainless steel pipe fittings:

1) Pull-out test of stainless steel pipe fittings: S-type has a significant advantage.

According to the standard "Stainless Steel Press-fit Fittings" (GB/T 19228.1-2024), the pull-out strength of S-type stainless steel pipe fittings under a double-press-fit structure is 1.8 times that of D-type stainless steel pipe fittings. Taking a DN25 pipe as an example, S-type stainless steel pipe fittings can withstand a tensile force of 2.5kN without falling off, while D-type stainless steel pipe fittings show signs of loosening at 1.4kN.

2) Sealing verification of stainless steel pipe fittings: Double insurance for greater reliability.

The sealing groove of S-type stainless steel pipe fittings is engraved with anti-slip texture, and together with a EPDM rubber ring, it forms a double guarantee of mechanical interlocking and elastic sealing after pressing. Actual tests show that under a water pressure of 2.5MPa, the leakage rate of S-type pipe fittings is less than 0.01% after 24 hours, while the leakage rate of D-type stainless steel pipe fittings may rise to 0.1% under long-term vibration.

Third, Application Scenarios of Stainless Steel Pipe Fittings

1) D-Type Stainless Steel Pipe Fittings: A "Lightweight Choice" for Home Renovation

Kitchen and Bathroom: D-type fittings directly connect to DN15 water pipes at water points such as sinks and toilets, improving installation efficiency by 30%.

Hot Water Circulation System: D-type elbows maintain consistent pipe diameter, avoiding heat loss due to reducing fitting diameters.

Water Purification System: D-type stainless steel fittings connect pre-filters and point-of-use drinking water systems, ensuring stable water flow and preventing filtration efficiency degradation.

2) S-Type Stainless Steel Pipe Fittings: A "Heavy-Duty Bearer" in Industrial Engineering

Fire Sprinkler System: According to the "Technical Specification for Fire Water Supply and Fire Hydrant Systems," sprinkler branch pipes and main pipes must use S-type reducing tees to ensure uniform water coverage.

Steam Pipelines: Reinforced S-type elbows have a 20% thicker wall, capable of withstanding pressures above 1.6MPa, preventing bursting risks.

Food Processing: Mirror-polished S-type stainless steel fittings (Ra≤0.8μm) meet sanitary standards, preventing microbial adhesion.

Fourth, Recommendations for Stainless Steel Pipe Fittings

Clarify Working Requirements: For home renovations or small projects, prioritize D-type stainless steel pipe fittings to reduce costs. For high-pressure scenarios such as high-rise buildings and industrial equipment, S-type stainless steel pipe fittings are mandatory.

Pay Attention to Pipe Diameter Matching: For pipes below DN100, press-fit connections are recommended. For pipes above DN100, grooved or flanged connections are required.

Strictly Control Installation Quality: For S-type stainless steel pipe fittings, ensure the transition slope at diameter reduction points is ≤1:50 to avoid stress concentration. For D-type stainless steel pipe fittings, pipe clamps must be installed every 3 meters to prevent pipe deformation.

Conclusion: The difference between D-type and S-type stainless steel pipe fittings essentially lies in the balance between "convenience" and "stability." Press-fit stainless steel pipe fittings extend the pressure range to PN25, and their optimized dimensions better meet engineering requirements. Whether for efficient home renovations or safety-critical industrial projects, scientifically selecting D-type or S-type stainless steel pipe fittings is essential to building a fluid transport system that can withstand the test of time.