Details

1. Introduction
In critical piping networks (especially high-consequence process and nuclear piping), snubbers are the last line of defense during dynamic events. During normal operation they must allow slow thermal growth and contraction without imposing excessive restraint forces. But during an earthquake, water hammer, vibration burst, or other transient, they must lock almost instantly and behave like a rigid support—preventing large pipe displacements, protecting nozzles, supports, welds, and connected equipment.

That dual behavior is exactly why snubbers are periodically tested: a snubber can still “look fine” externally while internally developing high drag, excess lost motion, delayed locking, or out-of-band drift, any of which can compromise restraint performance when it matters most.

The Snubber Test Bench Facility is purpose-built to execute these required in-service performance tests on hydraulic and mechanical snubbers, using servo-hydraulic actuation, high-accuracy displacement measurement, and automated data acquisition/software. The system runs standardized test modules, calculates results automatically, generates test graphs and purchaser-format reports, and provides a structured QA workflow for acceptance, review, and record retention.

2. Purpose and Scope
2.1 Purpose
• Perform repeatable, instrumented in-service tests for snubbers installed in a plant.
• Produce traceable test reports with graphs, computed parameters, acceptance criteria comparison, and remarks/review blocks.
• Reduce operator dependency by using software-controlled motion profiles and automatic calculations.

2.2 Snubber Types Covered
• Hydraulic snubbers (velocity-based locking characteristics)
• Mechanical snubbers (acceleration-based locking characteristics)

2.3 Test Modules Provided
The facility executes two main software workflows:
1.  Free Operability Test module
Covers: Free Operability, Drag Force, Lost Motion

2.  Sensitivity Test module
Covers: Lock Sensitivity (locking parameter) and Drift Speed

3. System Overview (What the Facility Includes)
3.1 Major Sub-Assemblies (Typical Layout Blocks)
• Hydraulic Power Pack & Chiller System 
• Loading Structure / Test Frame
• Data Acquisition & Control Console (rack + PC + printer + UPS)

Overall GA sub-part dimensions (as provided):
• Hydraulic Power Pack & Chiller System: 1900 × 1750 mm
• Loading Structure: 3800 × 1400 × 1290 mm
• Data Acquisition System with wheel: 2000 × 600 × 800 mm



3.2 Core Functional Elements
• Servo-hydraulic loading actuator provides controlled displacement/velocity profiles for all tests.
• Load measurement (load cells) captures frictional/locking loads and drift loads.
• High-resolution displacement measurement captures stroke, lost motion, drift displacement.
• DAQ + control software runs pre-programmed test sequences, records data, generates plots and reports.

3.3 Safety & Repeatability Features (Test-Relevant)
• Adjustable mounting and load frame supports multiple snubber envelopes.
• Positive hydraulic clamp locks the moveable beam and remains locked even without hydraulic pressure (safety during setup and testing).
• Controlled software motion limits and procedural “end-before-stroke-end” protection during drift.

4. Technical Specifications (Detailed)
Note: Where a value depends on the snubber under test (rated load, stroke, etc.), it is shown as “snubber-dependent”.

4.1 Performance and Capability Table

  

    
Parameter
    
Specification / Capability
  

  

    
Maximum test load required
    
200 kN (200,000 N)
  

  

    
Hydraulic system maximum working pressure
    
350 bar
  

  

    
Standard operating pressure at servo valve inlet
    
300 bar
  

  

    
Working pressure used for sizing (after pressure drops)
    
~230 bar
  

  

    
Servo actuator (loading cylinder) – static force
    
0 – 200 kN
  

  

    
Servo actuator – dynamic force
    
+200 kN
  

  

    
Total actuator stroke
    
250 mm
  

  

    
Cylinder bore / rod
    
130 mm / 90 mm
  

  

    
Test speed range
    
0 – 12.57 mm/s
  

  

    
Servo valve
    

      NG-6; working pressure up to 350 bar; flow capacity 30 LPM;
      digital onboard electronics and position feedback
    
  

  

    
Main hydraulic pump
    

      Fixed displacement axial piston pump; 300 bar; 20 LPM; fluid ISO VG 32
    
  

  

    
Main motor
    

      11 kW, 4-pole, 1440 RPM, 415 VAC
    
  

  

    
Hydraulic reservoir
    

      400 L, SS-304; ~1000 × 710 × 600 mm; bottom slope
    
  

  

    
Offline cooling pump
    

      Vane pump, ~10 bar, ~47 cc/rev
    
  

  

    
Air-cooled heat exchanger
    

      10 kW, air-cooled type
    
  

  

    
Load measurement
    

      Load Cell A: 250 kN universal; Load Cell B: 10 kN universal
    
  

  

    
Displacement measurement (primary)
    

      Absolute digital linear encoder; scale 250 mm; linearity <0.01% FS
    
  

  

    
Displacement measurement (secondary)
    

      Linear variable displacement transducer; range 40 mm; linearity <0.01% FS
    
  

  

    
DAQ hardware
    

      Controller: 16 DI / 16 DO / 8 AI (4–20 mA) with accessories
    
  

  

    
Software
    

      LabVIEW licensed + complete testing software development;
      automated report generation
    
  

  

    
Load frame
    

      200 kN load rating adjustable beam frame
    
  

  

    
Beam locking
    

      Positive hydraulic clamp; locked without hydraulic pressure
    
  


4.2 Instrumentation (Typical Examples)
• Pressure gauge/transmitter ranges appropriate for high-pressure testing (e.g., 0–400 kg/cm² class instruments).
• Temperature monitoring (transmitter, gauge, switch) for oil health and safe operation.

6. Test Philosophy, Definitions, and Data Channels
6.1 Test Coordinate Definitions (Standard)
Three standard positions are referenced for most tests:
• A = Retracted Position (Compression end)
• B = Mid Position
• C = Extended Position (Tension end)
A → C is defined as 80% of total snubber stroke for Free Operability and Drag Force tests.

6.2 Direction Convention
• Compression direction: C → A
• Tension direction: A → C
(Tests and results are always recorded in both directions unless specifically not applicable.)

6.3 Data Channels Captured (Typical)
Depending on snubber type and test module, the system records:
• Displacement (absolute encoder, LVDT as applicable)
• Load (load cell(s))
• Velocity (derived or directly logged where applicable)
• Acceleration (particularly for mechanical snubbers / derived from motion profile)
• Time stamps and test metadata (operator, tag, etc.)

6.4 Automated Reporting
Reports are generated in a structured format including:
• Summary page: acceptance criteria vs actual observed values + remarks
• Graph pages per test: compression + tension plots
• Operator and reviewer sign-off blocks

7. DETAILED TEST DESCRIPTIONS (WITH MAXIMUM DETAIL + GRAPH PLACEHOLDERS)
7.1 Test Module A: Free Operability + Drag Force + Lost Motion
(Executed from “Free Operability Test” software module)

A.1 FREE OPERABILITY TEST (Compression & Tension)
A.1.1 Objective
To verify that the snubber can move smoothly through its operating travel window under controlled low speed without abnormal sticking, binding, or discontinuities, and to confirm that the system achieves the defined stroke coverage requirement (80% stroke).

A.1.2 Test Setup (Mechanical + Mounting Discipline)
1. Adjust moveable plate / fixture to match snubber length and clevis geometry.
2. Mount snubber with no clearance/tolerance between clevis and clevis mounting plate.
3. Ensure load cell chuck nut and mounting fasteners are fully tightened.
4. Ensure the locking valve / lock state in software is set correctly as per the test step (locked/unlocked as required).

A.1.3 Positioning and Motion Profile
• Bring snubber to mid position (B) (as marked on snubber body).
• Define working endpoints A and C such that A→C is 80% of total stroke.
• Execute the free-operability travel at controlled low speed:

Step sequence:
1. From B, extend to C (tension direction) as required to reach the defined endpoint.
2. Move from C → A (compression direction) and record displacement vs time.
3. Move from A → C (tension direction) and record displacement vs time.

Speed requirement:
• Maintain low speed < 1 mm/s during recorded travel.

A.1.4 Data Recorded
• Displacement vs time for compression (C→A)
• Displacement vs time for tension (A→C)
• Actual stroke covered in each direction (mm)
• Any software flags/remarks (if motion irregularities detected)

A.1.5 Calculations and Outputs
Software computes:
• Recorded travelled stroke in compression direction (mm)
• Recorded travelled stroke in tension direction (mm)
• Confirms stroke coverage meets the test requirement (80% stroke band)

A.1.6 Acceptance / Interpretation
• Free operability is considered satisfactory when:
  ▹ Movement is smooth and continuous (no stick-slip artifacts beyond acceptable)
  ▹ Travelled stroke equals required stroke coverage
  ▹ No abnormal discontinuities (flat segments, sudden jumps not explained by fixture geometry)

A.1.7 Graph Placeholders 
• Free Operability (Compression): Displacement vs Time (C → A)



• Free Operability (Tension): Displacement vs Time (A → C)



A.2 DRAG FORCE MEASUREMENT (Average Resistance in Compression & Tension)

A.2.1 Objective
To quantify internal friction / resistance during slow travel. Drag force is a key health indicator for snubber internals (seal friction, contamination, wear, misalignment).

A.2.2 Principle
During the free-operability motion, the system records load. The average resistance force during controlled travel is treated as drag force.

A.2.3 Test Execution Logic
Drag is measured during the same recorded motion windows:
• Drag in compression is measured during C → A
• Drag in tension is measured during A → C
Important recording rule:
The force used in calculations should exclude:
• force required to bring snubber to mid position, and
• force during unrecorded movements from mid position to extremes (only the specified recorded window is used).

A.2.4 Data Recorded
• Load vs time (compression and tension)
• Displacement vs time (used to validate motion window)
• Average load over recorded travel window (kg or N, as configured)

A.2.5 Calculations
Software computes:
• Average drag force in compression
• Average drag force in tension

A.2.6 Acceptance Criteria
Typical requirement used in report evaluation:
• Drag force should be less than 2% of snubber rated load (OBE / Level-B)
(Reported and compared direction-wise.)

A.2.7 Graph Placeholders
• Drag Force (Compression): Load vs Time (C → A)



• Drag Force (Tension): Load vs Time (A → C)



A.3 LOST MOTION TEST (Axial Lost Motion in Compression & Tension)

A.3.1 Objective
To measure axial slack/backlash (“deadband”) before meaningful restraint load develops. Lost motion is critical because excessive lost motion reduces restraint effectiveness under dynamic loading.

A.3.2 Test Setup
• Snubber positioned at mid position (B).
• Ensure correct load cell selection and signal stability.
• Ensure snubber is in the correct mechanical state (unlocked / free state per procedure).

A.3.3 Motion Profile (Typical)
• Apply a small controlled movement around the mid position:
  ▹ Move approximately 5 mm in compression at low speed (typically < 1 mm/s)
  ▹ Move approximately 5 mm in tension at low speed (typically < 1 mm/s)
• Record load response and displacement for both directions.

A.3.4 Data Recorded
• Load vs time
• Displacement vs time
• Derived relationship of “load rise onset” vs displacement in each direction

A.3.5 Computation Logic (Software)
Software determines:
• the displacement span over which the developed load remains below a threshold percentage of rated load (commonly 0.1% of rated load in the method), and reports that as axial lost motion.

A.3.6 Acceptance Criteria
Typical acceptance criterion used in reports:
• Lost motion should be < 1 mm (direction-wise evaluation)

A.3.7 Graph Placeholders
• Lost Motion (Compression): Load/Displacement vs Time



• Lost Motion (Tension): Load/Displacement vs Time



7.2 Test Module B: Sensitivity (Locking) + Drift Speed
(Executed from “Sensitivity Test” software module)
This module validates dynamic behavior: the snubber must lock within a defined parameter band (velocity for hydraulic, acceleration for mechanical) and then drift within an acceptable speed band under sustained load.

B.1 SENSITIVITY + DRIFT TEST (HYDRAULIC SNUBBERS)

B.1.1 Objective
• Measure locking velocity (sensitivity) in compression and tension.
• Measure drift speed after locking under sustained load.

B.1.2 Acceptance Targets (Typical)
• Locking velocity range: 2 – 6 mm/s
• Drift speed range: 0.2 – 2 mm/s

B.1.3 Test Setup (Load and Pressure Readiness)
1.  Bring snubber to mid position (B).
2.  Set system pressure corresponding to required test load for the snubber.
3.  Confirm that software motion parameters are loaded for the hydraulic snubber test profile.

B.1.4 Motion Profile (Typical Lock + Drift Sequence)
For locking in tension (same concept applies in compression with reversed direction):
1.  Move from mid position in tension direction at < 1 mm/s for ~5–10 mm (pre-conditioning window).
2.  Suddenly increase speed to ~10 mm/s (≈600 mm/min) for ~20–25 mm to initiate lock event.
3.  Maintain applied load and allow drift for ~45 seconds.
4.  Ensure test ends ~10 mm before stroke end to avoid snubber damage.

B.1.5 What “Locking” Looks Like in Data
• Locking is characterized by a sharp rise in load / change in response during the speed step.
• Software detects and reports the locking parameter (velocity) at the event.

B.1.6 Drift Speed Measurement
• During sustained load hold after lock, displacement continues slowly.
• Software computes average drift speed over the drift window.

B.1.7 Graphs Generated (Hydraulic)
• Velocity vs time
• Load vs time
• (Often combined overlays where the lock event and drift segment are visually clear)

B.1.8 Graph Placeholders (Hydraulic)
• Sensitivity/Lock + Drift (Compression): Velocity vs Time + Load vs Time



• Sensitivity/Lock + Drift (Tension): Velocity vs Time + Load vs Time



B.2 SENSITIVITY + DRIFT TEST (MECHANICAL SNUBBERS)

B.2.1 Objective
• Measure locking acceleration (sensitivity) in compression and tension.
• Measure drift speed under sustained load post-lock.

B.2.2 Acceptance Targets (Typical)
• Locking acceleration: ≤ 0.02 g
• Drift speed range: 0.4 – 2 mm/s

B.2.3 Test Setup
1.  Bring snubber to mid position (B).
2.  Set system pressure corresponding to test load.
3.  Ensure mechanical snubber test profile is selected in software.

B.2.4 Motion Profile
Same staged profile concept as hydraulic, but evaluated parameter is acceleration:
• Slow movement at <1 mm/s for a short distance (pre-conditioning)
• Sudden increase to induce locking event
• Sustained load drift window (~45 sec) with stroke-end protection (~10 mm before end)

B.2.5 Graphs Generated (Mechanical)
• Acceleration vs time
• Load vs time
• Optional derived velocity/displacement segments depending on report format

8. Test Report Package (What You Get Per Snubber)
For every snubber tested, the system produces a complete record package typically structured as:

1.  Comprehensive Test Report (Summary Page)
• Snubber designation / ID, test metadata
• Acceptance vs actual values for:
  ▹ Free operability (stroke)
  ▹ Drag force (compression & tension)
  ▹ Lost motion (compression & tension)
  ▹ Sensitivity (locking parameter)
  ▹ Drift speed (compression & tension) 
• Remarks / reviewed by / signature blocks

2.  Graph Pack Pages (typically 1 page per test group)
• Lost motion page (compression + tension)
• Free operability + drag page (compression + tension)
• Sensitivity + drift page (compression + tension)

Example: Comprehensive Test Report 



9. Typical Step-By-Step Operator Workflow (Practical)

9.1 Pre-Test Checks
• Verify hydraulic oil level, temperature, cooling readiness.
• Verify sensors: load cell zero, displacement encoder status, DAQ communication.
• Confirm correct fixture configuration and alignment.

9.2 Mounting
• Adjust moveable beam/plate to snubber length.
• Ensure no clearance at clevis interfaces.
• Tighten chuck nuts / locking fasteners fully.
• Engage beam locking clamp for safe setup.

9.3 Run Tests (Recommended Sequence)
1.  Free Operability + Drag Force
2.  Lost Motion
3.  Sensitivity + Drift Speed

9.4 Review and Sign-Off
• Verify report values vs acceptance.
• If any parameter is out-of-band, mark for review and disposition (repair/overhaul/retest).

10. Manual Test Provision (Contingency / Offline Mode)
The facility includes a manual provision using hand pumps and manual valve configuration for certain operations (movement and unlocking). This allows contingency operation for basic movement/measurement when automation is unavailable, while still maintaining controlled test discipline.