1. Foundation Types
Reciprocating compressors generate large dynamic internal forces from reciprocating component acceleration and piston rod compression forces. Foundations must provide a stable base that dissipates these vibratory forces, prevents excessive settlement, and maintains alignment for 30+ years.
Foundation Purpose
- Support: Carry static weight of compressor, driver, and auxiliaries
- Alignment: Maintain frame and coupling alignment over time
- Energy dissipation: Provide paths for vibratory forces into ground
- Crack prevention: Resist dynamic stresses without cracking
Common Foundation Configurations
Shallow Foundation
Block Foundation
Driver and compressor mounted directly on concrete block. Best for good soil, >5,000 HP units.
Shallow Foundation
Skid on Block
Equipment on steel skid, skid grouted to concrete block. Standard for <5,000 HP packaged units.
Deep Foundation
Block on Driven Piles
Concrete block supported by steel piles driven to bedrock. Required for poor/weak soils.
Deep Foundation
Skid on Piles
Steel skid mounted directly on driven piles, no concrete block. Fastest installation for weak soil.
Selection by Horsepower
| Driver HP | Recommended Foundation | Min Block Depth | Notes |
|---|---|---|---|
| <2,500 HP | Skid on block | 4-5 ft | Standard packaged units |
| 2,500-5,000 HP | Skid on block | 6 ft minimum | Evaluate block mount for high vibration |
| >5,000 HP | Block mount (no skid) | 6-8 ft minimum | Direct mount preferred for stiffness |
Industry Recommendation: For engine and compressor frames over 5,000 HP, block-mounted foundation (no skid) is recommended practice. The skid introduces flexibility that can cause vibration problems at higher power levels.
2. Foundation Design Rules
Industry best practices and ACI 351.3R provide rules of thumb that have proven successful for reciprocating compressor foundations. These guidelines help achieve the target 30-year no-maintenance service life.
Mass Ratio Requirements
Foundation Mass Ratio:
W_foundation / W_machine >= 3.0 (minimum)
W_foundation / W_machine >= 5.0 (preferred for recip)
Where:
W_foundation = weight of concrete block (lbs)
W_machine = total mounted equipment weight (lbs)
= compressor + driver + piping + bottles
Example:
Compressor: 118,000 lbs
Driver: 198,000 lbs
Piping & bottles: 72,000 lbs
Total machine: 388,000 lbs
Required foundation: 388,000 x 5 = 1,940,000 lbs
At 150 lb/ft3: 12,933 ft3 concrete minimum
Geometric Proportions
Key Dimensional Ratios:
1. Height/Width Ratio (prevents rocking):
H_crankshaft / W_foundation <= 0.65
Where H_crankshaft = height from base to crankshaft centerline
W_foundation = foundation width (or pile group width)
2. Width vs. CG Height:
W_foundation >= 1.0 to 1.5 x H_CG
Where H_CG = vertical distance from base to machine CG
3. Foundation Oversizing:
Length: 1-2 ft longer than equipment/skid
Width: 1-2 ft wider than equipment/skid
4. Center of Gravity Alignment:
Machine CG must be within 5% of foundation CG
(Prevents differential settlement and torsional vibration)
Dynamic Load Factors
Reciprocating Equipment De-Rating:
Static design parameters for reciprocating machinery
must be reduced by 50% compared to static equipment:
- Ultimate base resistance: reduce 50%
- Factored base resistance: reduce 50%
- Shear strength: reduce 50%
Dynamic Load Limit:
Max dynamic load <= 1.5 x max static load
(Prevents fatigue in concrete and rebar)
Settlement Limits:
Maximum settlement: 0.50 inches
Preferred settlement: 0.40 inches or less
Differential settlement: minimize by CG alignment
Soil Bearing Capacity
Bearing Pressure Limits:
Target: q_actual < 1,500 psf
Maximum: q_actual < 2,000 psf (absolute limit)
q = (W_foundation + W_machine) / A_base
Where:
q = bearing pressure (psf)
A_base = foundation base area (ft2)
If soil capacity insufficient:
- Add driven piles to bedrock
- Increase foundation footprint
- Consider deep foundation design
Design Priority: The primary failure mode for reciprocating machinery foundations is cracking. Design focuses on preventing crack formation and growth under dynamic loads, achieved through adequate mass, proper rebar density, and correct anchor bolt design.
3. Concrete and Reinforcement
High-strength concrete with dense rebar reinforcement is essential for managing the cyclic stresses from reciprocating equipment. The tensile strength of concrete controls crack formation.
Concrete Specifications
Concrete Requirements (ACI 318):
Compressive strength: f'c >= 4,000 psi @ 28 days
(higher is better for crack resistance)
Tensile strength: f't ≈ 0.1 x f'c
(tensile controls cracking in foundations)
Density: 150 lb/ft3 (reinforced concrete)
Modulus of elasticity: E = 57,000 x sqrt(f'c)
E ≈ 3.6 x 10^6 psi for 4,000 psi concrete
Rebar Specifications
| Location | Bar Size | Spacing | Notes |
|---|---|---|---|
| Top 3 ft of block | #8 (1" dia) | 8" horizontal | Dense grid, high stress zone |
| Top 3 ft - vertical | #6 (3/4" dia) | 8" centers | Vertical cage members |
| Below 3 ft depth | #8 horizontal | 16" centers | Reduced density acceptable |
| Around anchor bolts | #6-#8 | 6" centers | 3D cage required |
| Equipment pedestals | #6-#8 | 6-12" horizontal | 6"/9"/12" vertical spacing |
Rebar Material:
Standard: ASTM A-615 Grade 60
Yield strength: 60,000 psi minimum
Rebar density target:
Top 3 ft of block: ~1% by volume
(Critical for crack growth management)
Cover requirements:
Top and bottom: 3 inches minimum
Sides: 2 inches minimum
Connection method:
Tie rebar intersections (do not weld)
Welding requires special procedures and verification
Pour Requirements
- Monolithic pour: Main block must be single continuous pour to avoid cold seams
- Vibration: Minimum 3 persons conducting vibration during pour
- Aggregate: Size must allow flow through dense rebar (consult concrete supplier)
- Water content: Strict water-cement ratio control per ACI 318
- Testing: 4 specimens per truck, test at 7, 14, 21, and 28 days
Crack Management: Cracks will form at anchor bolt terminations regardless of design. Dense rebar stops crack growth - cracks grow until they encounter the first rebar course and typically stop. A 3D rebar cage around each anchor bolt is essential.
4. Anchor Bolt Design
Anchor bolt design is a critical aspect of foundation engineering. Old L-type and J-type bolts are the primary cause of foundation cracking. Modern straight bolts with bottom disk termination are required.
Anchor Bolt Requirements
Material Specification:
Standard: ASTM A-193 Grade B7
Material: AISI 4140 alloy steel, rolled threads
UTS: 125 ksi minimum
Yield: 105 ksi minimum
Nuts: ASTM A-194 Grade 2H, heavy hex
Washers: Double spherical, high tensile
Diameter:
Minimum: 1.125" for equipment and skid bolts
Preferred: Largest size compatible with frame holes
Compressor main bolts: typically 1.5" - 1.75"
Anchor Bolt Length
Length Requirements:
Minimum free length: 12 x bolt diameter
Preferred: Extend to lower 1/3 of block
Best practice: Extend to mat (entire depth in compression)
Typical lengths:
Internal anchor bolts: 36" minimum
Skid perimeter bolts: 36" minimum
Miscellaneous bolts: 18" minimum
Block-mounted equipment: 60-92" (to lower 1/3)
Termination:
Use heavy hex nut on bottom disk (NOT L or J bolts)
Disk size: 5" diameter x 3-1/8" thick typical
Enclose in 3D rebar cage
Anchor Bolt Tensioning
Preload Requirements:
Tensioning method: Hydraulic (torque is inaccurate)
Target preload: 70-80% of yield strength
Holding force per bolt (with chock friction):
F_hold = Preload x coefficient of friction
Coefficient of friction:
New dry chock: mu = 0.30
Oily chock: mu = 0.12 (use for design!)
Example (1.75" A193-B7 bolt):
Tensile area: 1.90 in2
Yield: 105 ksi
Preload = 0.70 x 105,000 x 1.90 = 139,650 lbs
Holding force (oily): 139,650 x 0.12 = 16,760 lbs
Holding force (dry): 139,650 x 0.30 = 41,900 lbs
What NOT to Use
| Bolt Type | Problem | Alternative |
|---|---|---|
| L-bolt (bent) | Induces cracking, pulls out over time | Straight bolt with disk |
| J-bolt (hooked) | Same as L-bolt, straightens with retensioning | Straight bolt with disk |
| Short bolts | Insufficient stretch, rapid preload loss | 36" minimum, lower 1/3 of block |
| Bonded bolts | No stretch capacity, rapid preload loss | Canister sleeve (debonded) |
Canister Sleeve Design: Wrap anchor bolts in tape or encase in plastic tubes to prevent bonding to concrete. This allows free bolt stretch. A two-piece replaceable canister design allows the upper bolt to be replaced if threads are damaged.
5. Grouting and Equipment Mounting
Epoxy grout has replaced traditional cementitious grout as the standard for reciprocating equipment. It provides superior strength, chemical resistance, and vibration damping.
Epoxy Grout Specifications
Minimum Properties:
Compressive strength: >= 12,500 psi @ ambient
Tensile strength: >= 2,000 psi @ ambient
Flexural modulus: >= 1.6 x 10^6 psi
Thermal expansion: <= 23 x 10^-6 in/in/degF
Grout cap thickness:
Minimum: 4 inches
Application: Pour after chipping concrete to top rebar
Corner radii:
All items embedded in epoxy: 3/8" minimum radius
(Prevents stress concentrations)
Expansion Joints
- Required in epoxy grout cap to control thermal stress
- Fill with elastomeric sealant after grout cures
- Prevents oil and foreign material intrusion
- Follow grout manufacturer recommendations for spacing
Chock Mounting
| Application | Chock Type | Notes |
|---|---|---|
| Compressor to skid | Composite (epoxy/steel) | Thermal insulation, adjustable |
| Compressor to block | Soleplate + shim packs | SS shims, max 1/4" thick, max 3 shims |
| Engine to skid | Adjustable wedge | Steel acceptable (frame isolates heat) |
| Skid to foundation | Full bed epoxy grout | Gravity boxes for flow verification |
Oil Protection
Oil Contamination Prevention:
Epoxy grout is susceptible to oil damage:
- Oil carries sulfate, phosphate, chloride ions
- Attacks concrete matrix and rebar
- Softens epoxy grout
- Magnifies crack tip pressure (hydraulic effect)
Protection methods:
1. Impervious epoxy grout cap on all horizontal surfaces
2. Sheet metal or thermoplastic oil pans under equipment
3. Elastomeric seals in anchor bolt relief pockets
4. Sealed expansion joints
5. Drain lines at low points of oil pans
Cementitious Grout: Not recommended for large reciprocating compressors. It cracks and falls apart in vibratory service. Use only with pollution containment skid to prevent oil absorption. Epoxy grout is the standard.
6. Vibration and Dynamic Design
Foundation natural frequencies must be separated from compressor operating frequencies by at least 20%. The goal is to place foundation modes either below or above the operating speed range.
Natural Frequency Targets
Frequency Placement (Concrete Block Foundations):
Horizontal modes (translation, torsion):
Target: 300-500 cpm (5-8 Hz)
Below operating speed range
Vertical and rocking modes:
Target: 1250-1450 cpm (20-24 Hz)
Above operating speed range
Minimum separation:
Natural frequency >= 1.20 x operating speed, OR
Natural frequency <= 0.80 x operating speed
Pile foundations:
Horizontal modes typically tuned ABOVE operating speed
(More rigid system than block on soil)
Damping Values
| Material | Damping Range | Design Value |
|---|---|---|
| Reinforced concrete | 7-10% | 7.5% |
| Steel skid with equipment | 2-4% | 2.5% |
| Soil (varies widely) | 5-30% | Per geotech report |
Dynamic Loads
Forces to Consider:
1. Driver dynamic (roll) torque
2. Crankshaft inertia unbalance forces
3. Crankshaft inertia unbalance moments
4. Vertical crosshead forces
5. Time-domain gas forces at each throw
6. Time-domain inertia forces at each throw
NOT required in foundation analysis:
Pulsation forces in piping
(These are handled in piping analysis)
Analysis approach:
Apply forces as forcing functions at their frequencies
Sweep through operating speed range
Identify and eliminate resonances
Vibration Limits
Typical Vibration Acceptance Criteria:
Foundation block top: 0.10-0.15 in/s peak
Compressor base: 0.15-0.20 in/s peak
Compressor bearings: per OEM specification
Engine bearings: per OEM specification
Units:
Velocity (in/s or ips) preferred for machinery
Displacement (mils p-p) for low frequency modes
Rigid body motion of block:
Acceptable if within soil and piping stress limits
Main concern is transmitted vibration to surroundings
Skid vs. Foundation Flexibility: The more flexible the foundation (weak soil), the stiffer the compressor skid must be. A concrete block foundation can be flexible and resonant if soil is insufficient. Always include soil stiffness in dynamic analysis.
7. Soil Analysis and Testing
Geotechnical analysis is essential for foundation design. Standard core-sample testing is NOT sufficient - specialized dynamic property testing is required.
Required Soil Properties
Critical Dynamic Properties:
1. Shear wave velocity (V_s)
2. Damping coefficient
3. Unit weight (gamma)
4. Poisson's ratio (nu)
5. Shear modulus (G)
Testing method:
Cross-hole seismic testing (preferred)
Provides accurate values at each soil layer
Testing depth:
Shallow foundations: 4 x foundation equivalent radius
Plus verification of no reflective layer below
Note:
Textbook soil stiffness ranges are NOT accurate enough
Site-specific testing is required
Soil Types and Concerns
| Soil Type | Characteristics | Foundation Concerns |
|---|---|---|
| Wet clay | Expands/contracts, low bearing | Settlement, seasonal movement |
| Dry sand | No cohesion, moderate bearing | Vibration transmission |
| Clay-sand mix | Good stiffness when moist | Best shallow foundation soil |
| Rock/shale/limestone | Extremely dense, high bearing | Excellent bedrock for piles |
When Piles Are Required
- Soil bearing capacity insufficient for foundation weight
- High water table (above foundation base)
- Excessive dynamic response on soil (rigid body rocking)
- Significant settlement expected (>0.5 inches)
- Poor soil properties extend deep, bedrock accessible
Pile Spacing Requirements:
Center-to-center spacing: 2.5-3.0 x pile diameter
Drive from center outward in groups
All piles to approximately same tip elevation
Pile group effects must be considered (4+ piles)
Pile location priorities:
1. Under crosshead guide supports
2. Below compressor crankcase
3. Two piles below each cylinder head end
(Future head end support provision)
Pile spacing guideline:
Approximately equal to distance from compressor
centerline to top of skid
Local Expertise: Select a local geotechnical consultant with knowledge of local soil and climate conditions. Seasonal moisture variation affects soil properties significantly.
8. Common Failure Modes
Understanding failure modes helps identify critical design requirements and maintenance needs.
Foundation Cracking
| Cause | Location | Prevention |
|---|---|---|
| Anchor bolt tension | Below bolt termination | 3D rebar cage, bottom disk, long bolts |
| L/J bolt straightening | Around bolt bend | Use straight bolts only |
| Oil penetration | Existing cracks | Epoxy cap, oil pans, seals |
| Dynamic stress | High stress areas | Dense rebar (1% by volume) |
| Sharp corners | Re-entrant corners | 45-degree bevels, 3" min each side |
Settlement and Movement
- Uniform settlement: May be acceptable if within limits, affects alignment
- Differential settlement: Causes misalignment, piping stress - CG alignment prevents
- Rigid body rocking: Block oscillating on weak soil - piles required
- Seasonal movement: Expansive clay, moisture changes - engineered backfill
Anchor Bolt Problems
- Preload loss: Temperature cycling, grout creep - long bolts with high initial preload
- Thread damage: Corrosion, over-torque - canister sleeves, SS protection
- Machine sliding: Oily chocks reduce friction - design for oily mu (0.12)
- Bolt fatigue: Insufficient preload, cyclic loading - hydraulic tensioning to 70% yield
Grout Deterioration
- Oil softening: Epoxy grout absorbs oil - containment and sealing
- Thermal cycling: Expansion joint cracking - proper joint design and sealant
- Cementitious cracking: Vibration damage - use epoxy grout
- Grout curl: Edge lifting - pin grout to concrete per manufacturer
30-Year Design Life: Achieving maintenance-free operation requires getting all details right: adequate mass, proper rebar, correct anchor bolt design, good grout, and oil protection. A single weak point can cause premature failure.
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