Steel Pipe

Steel pipe remains the predominant piping material for HVAC hydronic heating, chilled water, steam, and condensate systems due to its strength, pressure capability, temperature resistance, and cost-effectiveness for medium to large diameter applications.

Carbon Steel Pipe Types

Black Steel Pipe

Uncoated carbon steel pipe for closed-loop hydronic systems, steam distribution, and condensate return.

Applications:

• Hot water heating systems (up to 250°F typical)
• Chilled water systems (40-55°F)
• Steam distribution (low to high pressure)
• Condensate return piping
• Closed-loop glycol systems

Advantages:

• Lower cost than galvanized or stainless
• Suitable for closed systems with controlled chemistry
• Excellent weldability
• High strength-to-weight ratio
• Available in all standard schedules and sizes

Limitations:

• Requires water treatment in hydronic systems
• Not suitable for open systems or potable water
• Surface rust forms if exposed to moisture during storage
• Requires proper pipe preparation before installation

Surface Condition:

• Mill scale coating from manufacturing
• Oil film may be present (removed before installation)
• Interior surface smooth but unprotected

Galvanized Steel Pipe

Carbon steel pipe with zinc coating applied by hot-dip galvanizing process.

Coating Process:

• Pipe immersed in molten zinc (840°F)
• Zinc metallurgically bonds to steel surface
• Typical coating thickness: 1.4-3.9 mils
• Forms iron-zinc alloy layer plus pure zinc outer layer

Applications:

• Condensate return systems (protects against low pH)
• Outdoor exposed piping
• Corrosive environments
• Dual-temperature systems with seasonal changeover
• Makeup water lines

Temperature Limitations:

• Maximum continuous: 390°F
• Zinc coating degrades above 390°F
• Not recommended for high-temperature steam (>250°F)
• Extended exposure above 200°F accelerates coating deterioration

Compatibility Considerations:

• Avoid contact with copper in wet systems (galvanic corrosion)
• Use dielectric fittings when connecting to dissimilar metals
• Alkaline water (pH > 12) attacks zinc coating
• Acidic condensate (pH < 7) protected by zinc sacrifice

Installation Requirements:

• Field threading removes zinc coating at joint
• Regalvanizing compound or zinc-rich paint at threads
• Welding destroys coating locally (requires field repair)
• Grooved connections preserve coating integrity

Stainless Steel Pipe

Corrosion-resistant steel alloys containing minimum 10.5% chromium.

Common HVAC Grades:

Advantages:

• Excellent corrosion resistance (no treatment required)
• Smooth interior surface (low friction factor)
• No degradation from temperature cycling
• Long service life (50+ years)
• Maintains appearance in exposed applications
• Resists scale formation

Cost Considerations:

• Material cost 4-8 times carbon steel
• Reduced installation labor (no painting required)
• Lower maintenance costs (no treatment system)
• Life-cycle cost advantage in critical applications

Welding Requirements:

• TIG welding preferred for thin wall
• Purge gas (argon) prevents oxidation
• Avoid carbon steel contamination
• Passivation after welding restores corrosion resistance

Schedule Ratings and Dimensions

Pipe schedule indicates wall thickness for given nominal diameter. Higher schedule numbers represent thicker walls.

Standard Schedules

Schedule 40 (Standard Weight):

• Most common for HVAC hydronic systems
• Adequate for typical pressures (up to 300 psi)
• Threaded, welded, or grooved connections
• Cost-effective for general applications

Schedule 80 (Extra Strong):

• High-pressure applications (300-600 psi)
• Steam systems above 150 psig
• Threaded connections requiring additional strength
• Areas subject to mechanical abuse

Schedule 10:

• Thin-wall pipe for low-pressure applications
• Welded or grooved connections only (not threaded)
• Weight and cost savings
• Large diameter chilled water systems (6-inch and larger)

Schedule 5:

• Very thin wall, stainless steel only
• Press-fit or orbital welded connections
• Reduces material cost for stainless applications
• Limited to low-pressure systems (<150 psi)

Wall Thickness Comparison

Representative wall thickness for common sizes (Schedule 40 vs. Schedule 80):

Weight Considerations

Pipe weight affects:

• Structural support requirements
• Hanger spacing
• Installation labor
• Transportation costs
• Seismic bracing design

Water-filled weight calculation:

• Empty pipe weight (from schedule tables)
• Water weight = 0.3405 × D² lb/ft (D = inside diameter in inches)
• Insulation weight (typically 0.5-2.0 lb/ft depending on thickness)
• Total dead load for hanger design

Pressure Ratings

Working Pressure by Schedule

Allowable working pressure depends on:

• Pipe schedule (wall thickness)
• Material specification (tensile strength)
• Operating temperature
• Safety factor
• Code requirements

ASME B31.9 Pressure Rating (Carbon Steel, <250°F):

Typical HVAC Design Pressures:

• Chilled water systems: 125-150 psi
• Hot water heating (low temp): 30-50 psi
• Hot water heating (high temp): 125-150 psi
• Low-pressure steam: 15 psig
• High-pressure steam: 150-250 psig
• Condensate return: 15-50 psi

Safety Factors:

• ASME B31.9 includes built-in safety factors
• Design pressure set at peak system pressure plus surge allowance
• Pressure relief devices prevent overpressure
• Hydrostatic test pressure: 1.5 × design pressure

Temperature Effects on Pressure Rating

Material strength decreases at elevated temperatures, reducing allowable pressure.

Allowable Stress Reduction (Carbon Steel):

Design Implications:

• High-temperature hot water (350-400°F) requires pressure derate
• Steam systems above 250°F use reduced allowable stress
• Thermal expansion forces may govern over pressure stress
• Consult ASME B31.1 for power piping above 250°F

ASTM/ASME Material Specifications

Carbon Steel Specifications

ASTM A53 - Pipe, Steel, Black and Hot-Dipped, Zinc-Coated, Welded and Seamless:

• Most common specification for HVAC piping
• Grade A: Lower strength (48,000 psi tensile)
• Grade B: Standard for HVAC (60,000 psi tensile)
• Type E: Electric resistance welded (ERW)
• Type S: Seamless
• Type F: Furnace butt welded (sizes up to 4 inch)

ASTM A106 - Seamless Carbon Steel Pipe for High-Temperature Service:

• Grade A, B, or C (B most common)
• Seamless construction (no weld seam)
• Higher temperature ratings than A53
• Preferred for steam systems above 250°F
• Better quality control than welded pipe

ASTM A135 - Electric-Resistance-Welded Steel Pipe:

• Grade A or B
• Large diameter applications (12 inch and up)
• Longitudinal weld seam
• Lower cost than seamless for large sizes

ASTM A795 - Black and Hot-Dipped Zinc-Coated (Galvanized) Welded and Seamless Steel Pipe for Fire Protection Use:

• Similar to A53 but with modified testing
• Listed for fire sprinkler systems
• May be specified for hydronic systems
• Grade A or B

Stainless Steel Specifications

ASTM A312 - Seamless, Welded, and Heavily Cold Worked Austenitic Stainless Steel Pipes:

• Most common stainless pipe specification
• Grades: TP304, TP304L, TP316, TP316L
• Seamless or welded construction
• Annealed condition (solution heat treated)

ASTM A778 - Welded, Unannealed Austenitic Stainless Steel Tubular Products:

• Welded and drawn, not annealed
• Lower cost than A312
• Limited to lower temperatures
• Press-fit system applications

ASTM A409 - Welded Large Diameter Austenitic Steel Pipe for Corrosive or High-Temperature Service:

• Sizes NPS 14 and larger
• Longitudinal or spiral weld
• Grades same as A312

ASME Code Applications

ASME B31.9 - Building Services Piping:

• Applies to most HVAC piping systems
• Covers design, materials, fabrication, installation, testing
• Temperature limit: -20°F to 366°F (some exceptions to 450°F)
• Pressure: Variable based on application

ASME B31.1 - Power Piping:

• High-pressure steam systems (typically >250°F, >150 psig)
• More stringent requirements than B31.9
• Required inspections and documentation
• Higher safety factors

ASME B36.10M - Welded and Seamless Wrought Steel Pipe:

• Dimensional standard (not material specification)
• Defines nominal sizes, schedules, wall thickness
• Outside diameter constant for given nominal size
• Inside diameter varies with schedule

Connection Methods

Threaded Connections

NPT (National Pipe Thread) - ASME B1.20.1:

• Tapered thread (3/4 inch per foot taper)
• Mechanical seal plus thread sealant
• Size limitation: NPS 2 and smaller for steel
• Schedule 80 required for sizes above 2 inch if threaded

Thread Engagement:

• Hand-tight plus 2-3 turns with wrench
• Minimum engagement: 4-5 threads
• Thread compound or PTFE tape on male threads only
• Avoid over-tightening (cracks fitting or galls threads)

Installation Considerations:

• Threading removes material (reduces wall thickness)
• Galvanized pipe loses coating at threads
• Field threading requires proper tools and cutting oil
• Thread inspection for debris and damaged threads

Welded Connections

Butt Weld (Groove Weld):

• Full penetration weld joint
• Strongest connection method
• Requires beveled pipe ends (typically 37.5° bevel)
• Backing ring or consumable insert for consistent penetration
• X-ray or ultrasonic testing for critical applications

Socket Weld:

• Pipe inserted into fitting socket
• Fillet weld at fitting face
• Easier to fit up than butt weld
• Limited to 3 inch and smaller, <600 psi
• 1/16 inch gap at bottom of socket (expansion allowance)

Welding Procedures:

• Qualified welders (per ASME Section IX)
• Procedure Qualification Record (PQR) for critical systems
• Purge interior for stainless steel welds
• Post-weld cleaning and inspection
• Pressure test after welding complete

Grooved Mechanical Connections

Coupling Types:

• Rigid coupling: No movement (replaces welded joint)
• Flexible coupling: Allows angular deflection and pipe movement
• Combination systems use both types strategically

Groove Formation:

• Roll grooving: Deforms pipe to create groove
• Cut grooving: Machined groove (for thick wall or stainless)
• Factory-grooved pipe available
• Groove dimensions per AWWA C606

Advantages:

• Fast installation (no welding or threading)
• Can be disassembled for maintenance
• Vibration isolation with flexible couplings
• Visual inspection of gasket placement
• No hot work permit required

Applications:

• Large diameter piping (4 inch and larger)
• Areas requiring periodic disassembly
• Seismic installations (flexible couplings)
• Fast-track projects

Design Considerations

Corrosion Protection

Water Treatment (Black Steel):

• pH control: 7.5-9.5 optimal
• Oxygen scavengers (closed systems)
• Corrosion inhibitors (molybdate, nitrite)
• Biocides for microbiologically influenced corrosion (MIC)
• Glycol systems: Inhibited glycol required

External Corrosion:

• Insulation with vapor barrier prevents condensation
• Coating systems for exposed pipe
• Cathodic protection for buried piping
• Avoid copper contact in wet environments

Galvanic Considerations:

• Steel is anodic to copper (corrodes in wet couples)
• Dielectric fittings isolate dissimilar metals
• Stainless steel can be anodic or cathodic (depends on grade and conditions)
• Zinc (galvanized) sacrificial to both steel and copper

Thermal Expansion

Linear expansion coefficient:

• Carbon steel: 6.5 × 10⁻⁶ in/in/°F
• Stainless steel (304/316): 9.6 × 10⁻⁶ in/in/°F

Expansion calculation: ΔL = α × L × ΔT

Where:

• ΔL = change in length (inches)
• α = coefficient of thermal expansion (in/in/°F)
• L = pipe length (inches)
• ΔT = temperature change (°F)

Expansion Compensation:

• Expansion loops (U-shaped or L-shaped)
• Expansion joints (bellows or slip type)
• Offsets in piping layout provide natural flexibility
• Anchors and guides control expansion direction

100-foot run example:

• Hot water heating (70°F to 180°F, ΔT = 110°F)
• Carbon steel: ΔL = 6.5 × 10⁻⁶ × 1200 × 110 = 0.86 inches
• Chilled water (70°F to 42°F, ΔT = 28°F)
• Carbon steel: ΔL = 6.5 × 10⁻⁶ × 1200 × 28 = 0.22 inches

Friction Losses

Hazen-Williams C-factor for steel pipe:

• New black steel: C = 130-140
• Average service black steel: C = 100-120
• Galvanized steel: C = 120-130
• Stainless steel: C = 140-150

Design Recommendations:

• Use C = 100 for long-term design of black steel hydronic systems
• Account for scale formation over system life
• Water treatment maintains higher C-factor
• Stainless steel maintains constant friction factor

Support and Hanger Spacing

Maximum hanger spacing (ASME B31.9, steel pipe, Schedule 40):

Hanger Types:

• Clevis hangers: Individual pipe support
• Trapeze hangers: Multiple pipes, concentrated load
• Spring hangers: Vertical movement compensation
• Constant support hangers: Minimal load variation during movement
• Riser clamps: Vertical pipe weight support

Insulated Pipe:

• Support at pipe surface (not insulation)
• Insulation shields prevent crushing
• Saddles distribute load over pipe circumference
• Increased spacing may apply for thick insulation

Joining to Equipment

Connections to rotating equipment (pumps, chillers):

• Flexible connectors isolate vibration
• Flanged connections typical at equipment
• Grooved connections alternative to flanges
• Support piping independently (not from equipment)
• Maintain clearance for service access

Thermal considerations:

• Allow pipe movement without stressing equipment
• Expansion compensators near equipment if needed
• Anchor points beyond flexible connector
• Guides prevent lateral loading on equipment nozzles

Code and Standard Compliance

Installation Requirements:

• Maintain slope for drainage (1/4 inch per 10 feet minimum)
• Air vents at high points
• Drain valves at low points
• Isolation valves for service and maintenance
• Clearances for insulation and maintenance access

Testing Requirements:

• Hydrostatic test: 1.5 × design pressure, 2 hours minimum
• Test medium: Water (glycol systems test with water, then drain and fill)
• Visual inspection during test (all joints exposed)
• Documentation of test pressure and duration
• Repair leaks and retest affected sections

Documentation:

• Material certifications (mill test reports)
• Welding procedure specifications
• Welder qualifications
• Pressure test records
• As-built drawings showing routing and connections

Steel pipe selection involves balancing initial cost, service life, maintenance requirements, and system performance. Carbon steel Schedule 40 provides economical performance for most HVAC applications with proper water treatment. Stainless steel offers superior corrosion resistance and life-cycle value for critical or aggressive-environment applications. Schedule selection depends on pressure requirements, connection method, and mechanical loading conditions.