I. Core Structure and Working Principle

1. Structural Composition

Outer Sleeve: Q235B carbon steel (tensile strength ≥375MPa), inner wall chrome-plated (thickness ≥20μm) for anti-wear;

Inner Sleeve: 20# steel (yield strength ≥245MPa), surface sprayed with molybdenum disulfide (friction coefficient ≤0.15);

Sealing System: V-type fluororubber ring (temperature -20℃~200℃) + graphite packing (temperature ≤650℃) combined sealing;

Balance Mechanism: Built-in pressure balance bellows (304 stainless steel) to offset medium pressure thrust.

2. Thrust-Free Principle

The internal pressure balance of bellows cancels out medium thrust, so external supports bear no axial force. Formula:

F 

thrust

 =P×A 

inner sleeve

 −P×A 

bellows

 =0

(

P

 = medium pressure, 

A

 = effective area)

II. Technical Parameters and Performance Indicators

Parameter Type Conventional Specifications High-Temperature Custom Specifications

Nominal Diameter (DN) DN50~DN1200 DN200~DN800

Design Pressure ≤1.6MPa (PN16) ≤2.5MPa (with reinforcing ring)

Compensation Axial ±50~±200mm Axial ±80~±150mm

Temperature Range -40℃~150℃ (rubber seal) -40℃~600℃ (graphite seal)

Friction Resistance ≤0.05DN (N) ≤0.08DN (N)

III. Application Scenarios and Advantage Comparison

1. Typical Applications

Long-distance Heating Networks: DN600 steam pipeline (2.0MPa, 300℃), single compensator with ±120mm compensation, reducing fixed support load by 90%;

Petrochemical Units: Reformer inlet/outlet pipelines (550℃, 0.8MPa), equipped with thermal shields to prevent seal aging;

Municipal Engineering: Inverted siphon crossing rivers (DN300, burial depth 10m), avoiding displacement failure from soil pressure.

2. Comparison with Traditional Compensators

Type Thrust-Free Sleeve Compensator Axial Bellows Compensator

Thrust Balance Mode Internal bellows self-balancing Dependent on fixed supports for thrust

Compensation Larger (±200mm vs ±80mm) Smaller

High-Temperature Resistance Better (600℃ vs 450℃) General

Cost Advantage More economical for large DN (>500) More economical for small DN (<300)

IV. Key Installation Techniques

Coaxiality Control

Installation deviation ≤1mm/m, calibrated with laser aligner to avoid sleeve eccentric wear (wear >0.5mm/year requires maintenance);

Guide support spacing ≤3D (D = pipe diameter), gap ≤2mm to prevent radial displacement.

Pre-Stretching Process

Calculate pre-stretching amount based on design temperature:

ΔL 

pre-stretch

 = 

2

ΔL 

total

 × 

T 

operating

 −T 

ambient

T 

installation

 −T 

ambient

Example: DN300 pipeline (ΔLtotal=100mm, operating temp 250℃, installation temp 20℃), pre-stretch = 50×(20-20)/(250-20)=0mm (no pre-stretch at room temperature).

Sealing System Debugging

Fill with sealing grease (bentonite-based, dropping point ≥300℃) before initial operation, and refill after 24 hours;

Pressure test at 1.5×design pressure (hold for 30min, leakage ≤0.1L/min).

V. Maintenance and Inspection Specifications

Routine Inspection Items

Sleeve displacement marks: Record weekly, check guide supports if deviation >10% of rated value;

Packing temperature: Inspect thermal insulation (rock wool thickness ≥50mm) if exceeding operating temperature by 20℃.

Periodic Maintenance Cycles

| Item | Cycle | Operation Details |

|----------------------|-------------|---------------------------------------|

| Sealing Grease Replacement | Annual | Clean sealing cavity, replace graphite packing |

| Non-Destructive Testing | Every 3 years | Ultrasonic thickness measurement (replace if sleeve wall thinning >15%) |

| Pressure Test | Every 5 years | 1.25×design pressure, hold for 1 hour |

VI. Industry Standards and Selection Calculations

1. Execution Standards

Domestic: GB/T 12777-2019 General Technical Conditions for Metal Bellows Expansion Joints (reference clause 5.3);

Mechanical Industry: JB/T 12622-2016 Thrust-Free Sleeve Compensators.

2. Selection Calculation Example

Given: DN500 steam pipeline (L=80m, ΔT=220℃, P=1.6MPa)

Thermal expansion: ΔL=12×10⁻⁶×80×220=211.2mm

Selection: 2 thrust-free sleeve compensators (each ±120mm compensation), with balance bellows effective area ≥0.2m² to offset thrust (F=1.6×π×(0.5/2)²=0.314MN).

VII. Failure Modes and Solutions

Failure Phenomenon Possible Cause Solution

Sleeve Sticking Guide support failure Replace supports, apply MoS₂ lubricant

Abnormal Leakage Sealing grease drying out Inject high-pressure liquid sealing grease (viscosity ≥1000cSt)

Balance Bellows Rupture Overpressure operation Install pressure relief valve (set at 1.1×design pressure)

Thrust-free sleeve compensators significantly reduce pipeline system reliance on fixed supports through pressure self-balancing design, especially suitable for large-diameter and high-pressure conditions. When selecting, focus on medium temperature, compensation amount, and installation space. It is recommended to collaborate with design institutes for mechanical calculations to ensure long-term reliable operation.