Tanks and Storage

Receiver tanks and distribution systems are critical for delivering stable, quality compressed air.

Why Use Receiver Tanks?

Receiver tanks serve multiple essential functions in the compressed air system.

Tanks serve multiple essential functions:

1. Heat Dissipation

Hot compressed air from the compressor cools in the tank, allowing moisture to condense and separate before entering the distribution system.

2. Condensate Collection

Tanks provide a collection point for water and oil that separate as air cools. Proper drainage removes these contaminants.

3. Air Storage

Gases can be compressed and stored - unlike liquids. Stored air provides:

• Buffer for demand spikes
• More stable system pressure
• Reduced compressor cycling

4. Pressure Stabilization

Tanks dampen pressure fluctuations, providing more consistent pressure to equipment.

Tank Configurations

Without Pressure Differential

Problem: The tank is just part of the piping - a "bubble" in the line. The compressor directly controls system pressure, and there's no stored energy.

With Pressure Differential (Capacitance)

Solution: By regulating output below tank pressure, we create stored energy (capacitance). The tank can supply air during demand spikes without immediate compressor response.

Storage Capacity Calculation

Storage Units Formula

$$\text{Units} = \frac{P_{tank} - P_{output}}{P_{atm}}$$

Example:

Tank: 1000 gallons @ 105 PSIG, Output: 80 PSIG

$$\text{Units} = \frac{105 - 80}{14.7} = 1.70$$ $$\text{Effective Storage} = 1000 \times 1.70 = 1{,}700 \text{ equivalent gal}$$ $$= \frac{1{,}700}{7.48} = 227 \text{ ft}^3$$

This can run the equivalent of a 50 HP compressor for approximately 1 minute during a demand spike!

Pump-Up Time Formula

To calculate how long it takes to fill a tank:

$$T = \frac{V \times (P_2 - P_1)}{7.48 \times P_a \times C}$$

Where:

• $T$ = Time (minutes)
• $V$ = Tank capacity (gallons)
• $P_2$ = Final pressure (PSIG)
• $P_1$ = Initial pressure (PSIG)
• $P_a$ = Atmospheric pressure (14.7 PSIA)
• $C$ = Compressor capacity (CFM)

Sizing Guidelines

Application	Tank Size Rule
General	3-5 gallons per CFM of compressor output
High demand cycling	5-10 gallons per CFM
Load/unload compressors	Larger tanks reduce cycling

Distribution System

Piping is Energy Transmission

$$E_{transmitted} = E_{input} - E_{losses}$$

Sources of Loss

Factor	Effect
Friction	Molecules collide with pipe walls
Turbulence	Caused by fittings, valves, direction changes
Leaks	Direct loss of air
Pressure drop	From undersized piping

Best Practices

1. Size piping adequately - Future demand often exceeds initial estimates
2. Use ring configuration - Provides two paths to any point
3. Minimize fittings - Each elbow ≈ 25 pipe diameters of equivalent length
4. Slope pipes toward drains - Allows condensate to flow to collection points
5. Take air from the top - Condensate settles at the bottom

Wet vs Dry Receivers

System Configuration

Two-tank system (recommended):

   Compressor → Aftercooler → WET TANK → Dryer → DRY TANK → Distribution
                     │            │          │        │
                     ▼            ▼          ▼        ▼
                 Separator    Primary    Treatment  Stable, dry
                              moisture    complete   air storage
                              removal

Wet Receiver (Primary Receiver)

Location: Immediately after aftercooler, before dryer

Functions:

1. Radiant cooling surface (additional heat dissipation)
2. Condensate collection point
3. Dampens pulsations from compressor
4. Provides buffer before treatment equipment

Parameter	Typical Value
Location	After aftercooler, before dryer
Size	1 gallon per CFM (minimum)
Pressure	Full compressor pressure
Condensate	Heavy - requires auto drain

Dry Receiver (Secondary Receiver)

Location: After dryer, before distribution

Functions:

1. Stores treated air for demand peaks
2. Stabilizes system pressure
3. Provides capacitance for control
4. Reduces compressor cycling

Parameter	Typical Value
Location	After dryer, before distribution
Size	3-10 gallons per CFM
Pressure	Below tank pressure (regulated)
Condensate	Minimal - should be dry

Single vs Dual Tank Systems

Configuration	Pros	Cons
Single tank (wet)	Lower cost, simpler	Dryer works harder, less storage
Dual tank	Better moisture control, more storage	Higher cost, more space
Distributed storage	Point-of-use capacity	Requires multiple drains

Best Practice

Use a wet receiver before the dryer and a dry receiver after. This protects the dryer and maximizes usable storage.

Tank Sizing Formulas

CAGI Sizing Rule

For load/unload compressors:

$$V = \frac{C \times P_a}{(P_1 - P_2) \times N}$$

Where:

• V = Tank volume (gallons)
• C = Compressor capacity (CFM)
• Pₐ = Atmospheric pressure (14.7 psia)
• P₁ = Cut-out pressure (psig)
• P₂ = Cut-in pressure (psig)
• N = Allowable cycles per hour (typically 4-10)

Example:

• 100 HP compressor producing 450 CFM
• Cut-in: 100 psig, Cut-out: 110 psig
• Target: 6 cycles per hour

$$V = \frac{450 \times 14.7}{(110 - 100) \times 6} = \frac{6,615}{60} = 110 \text{ gallons}$$

VSD Compressor Sizing

VSD compressors need less storage since they modulate:

$$V_{VSD} = 1-3 \text{ gallons per CFM}$$

Demand Event Sizing

For systems with large intermittent demands:

$$V = \frac{(Q_{demand} - Q_{supply}) \times t \times 14.7}{P_1 - P_2}$$

Where:

• Q_demand = Peak demand flow (CFM)
• Q_supply = Compressor output (CFM)
• t = Duration of demand event (minutes)

Example: 30-second blow-off using 200 CFM

• Compressor provides 100 CFM
• Allowable pressure drop: 10 PSI

$$V = \frac{(200 - 100) \times 0.5 \times 14.7}{10} = \frac{735}{10} = 73.5 \text{ gallons}$$

System Capacitance

Total system storage includes all piping:

$$V_{system} = V_{tanks} + V_{piping}$$

Where:

$$V_{piping} = \frac{\pi \times D^2 \times L}{4 \times 231}$$

(D in inches, L in inches, result in gallons)

ASME Pressure Vessel Code

ASME Section VIII

ASME Boiler and Pressure Vessel Code, Section VIII governs compressed air receiver design and construction.

ASME Code Requirements:

┌─────────────────────────────────────────────────────────┐
│  ASME SECTION VIII - PRESSURE VESSELS                  │
├─────────────────────────────────────────────────────────┤
│  Division 1: General requirements (most common)        │
│  Division 2: Alternative rules (high-stress design)    │
│  Division 3: Alternative rules for high pressure       │
└─────────────────────────────────────────────────────────┘

Key ASME Requirements

Element	Requirement
Material	ASME-approved materials (SA-516, SA-283, etc.)
Design pressure	MAWP clearly stamped
Welding	Certified welders, documented procedures
Inspection	Third-party inspection (Authorized Inspector)
Testing	Hydrostatic test to 1.3× MAWP
Documentation	Manufacturer's Data Report (U-1 form)
Nameplate	ASME U-stamp with all required data

ASME Nameplate Data

ASME U-Stamp Nameplate:

┌────────────────────────────────────────┐
│              [ASME U STAMP]            │
│                                        │
│  MAWP: ______ PSI at ______ °F         │
│  MDMT: ______ °F at ______ PSI         │
│  Serial No: ______________             │
│  Year Built: ______                    │
│  Manufacturer: ________________        │
│  National Board No: ___________        │
└────────────────────────────────────────┘

MAWP = Maximum Allowable Working Pressure
MDMT = Minimum Design Metal Temperature

Non-ASME Vessels

Exempt from ASME code:

• Vessels ≤ 5 cubic feet AND ≤ 250 psig
• Vessels ≤ 1.5 cubic feet with no pressure limit

Code Compliance

Using non-ASME vessels above exemption limits:

• Violates OSHA regulations
• Voids insurance coverage
• Creates liability exposure
• May incur regulatory fines

Pressure Relief Requirements

ASME Relief Valve Sizing

$$A = \frac{Q}{C \times K \times P_1 \times \sqrt{\frac{M}{T}}}$$

Where:

• A = Required orifice area (in²)
• Q = Required relieving capacity (CFM)
• C = Coefficient (356 for air)
• K = Valve coefficient (typically 0.975)
• P₁ = Set pressure + overpressure (psia)
• M = Molecular weight (29 for air)
• T = Temperature (°R)

Relief Valve Requirements

Parameter	Requirement
Set pressure	≤ MAWP of vessel
Capacity	≥ Compressor output at MAWP
Type	ASME-certified, UV-stamped
Testing	Annual inspection recommended
Discharge	Piped to safe location

Relief Valve Installation

Correct installation:

         ┌─── Relief valve (no shutoff between valve and vessel)
         │
         ▼
    ┌─────────┐
    │ ░░░░░░░ │
    │ ░VESSEL░│
    │ ░░░░░░░ │
    └─────────┘

NEVER install:
- Shutoff valve between relief and vessel
- Reducer below relief valve size
- Relief valve in discharge piping only

All pressure vessels require:

• Relief valve rated for maximum working pressure
• Pressure gauge for monitoring
• Regular inspection and testing

Safety

Relief valves protect against over-pressurization. Never block or disable them.

Inspection Requirements

Frequency Guidelines

Jurisdiction	Typical Requirement
OSHA (general)	Initial + as needed
State/local	Often annual internal + external
Insurance	Per insurer requirements
National Board	Follow NB-23 guidelines

Inspection Points

Component	What to Check
Shell	Corrosion, pitting, bulging
Welds	Cracks, corrosion at heat-affected zones
Nozzles	Corrosion, thread damage
Relief valve	Operation, corrosion, proper set pressure
Drain	Function, corrosion
Supports	Corrosion, structural integrity
Nameplate	Legible, attached

Thickness Testing

Ultrasonic thickness (UT) testing determines remaining wall:

$$\text{Remaining life} = \frac{t_{actual} - t_{minimum}}{\text{Corrosion rate}}$$

Where:

• t_actual = Current measured thickness
• t_minimum = Minimum required by code

Silencing

Compressed air exhausting to atmosphere creates significant noise. Solutions:

• Exhaust silencers/mufflers
• Diffusers
• Gradual pressure release

Condensate Management

Drain Types

Drain Type	Operation	Pros	Cons
Manual	Operator opens valve	Simple, cheap	Requires attention, often forgotten
Timer	Opens on schedule	Automatic	Wastes air if timer misadjusted
No-loss (float)	Opens when liquid present	No air loss	Mechanical parts can fail
Electronic level	Sensor triggers drain	No air loss, reliable	Higher cost

Timer Drain Waste

Timer drain air loss calculation:

If drain opens 10 sec every 5 min at 100 psig:
- Air loss per cycle ≈ 0.5 CFM × 10 sec = 0.08 CF
- Cycles per hour: 12
- Air loss per hour: 0.96 CF
- Annual air loss: 8,400 CF
- Annual cost @ $0.25/1000 CF: ~$2,100

Solution: Install no-loss drain

ROI

No-loss drains typically pay for themselves in 6-12 months through reduced air loss.