Pulsation Analysis & Control

1. What is Pulsation?

Pulsation is the pressure wave phenomenon generated by the intermittent flow of gas through a reciprocating compressor. Unlike centrifugal compressors that deliver continuous flow, reciprocating machines deliver gas in discrete pulses as each piston completes its stroke.

Key Concept: Every time a discharge valve opens, a pressure pulse is sent into the piping system. These pulses travel as acoustic waves through the gas, reflecting off pipe ends, tees, and restrictions, creating complex pressure patterns.

Pulsation Characteristics

Frequency

RPM-Based

Fundamental frequency equals compressor speed times number of compression events per revolution.

Harmonics

Multiple Frequencies

Pulsation contains energy at 1X, 2X, 3X, and higher multiples of the fundamental.

Amplitude

Pressure Variation

Typically 5-15% of mean pressure if uncontrolled, reduced to 2-5% with proper dampening.

Pulsation Frequency

Fundamental Pulsation Frequency: f = (RPM × N) / 60 Hz Where: RPM = Compressor rotational speed N = Number of compression events per revolution = 1 for single-acting, single-cylinder = 2 for double-acting cylinder 60 = Seconds per minute Example: 900 RPM, double-acting cylinder f = (900 × 2) / 60 = 30 Hz

Understanding Harmonics

Pulsation is not a pure sine wave at a single frequency. Because the valve opening/closing creates a non-sinusoidal pressure pulse, the resulting waveform contains energy at multiple frequencies called harmonics:

Harmonic	Frequency	Example (30 Hz fundamental)	Relative Amplitude
1X (Fundamental)	f	30 Hz	100% (reference)
2X (2nd Harmonic)	2 × f	60 Hz	40-60%
3X (3rd Harmonic)	3 × f	90 Hz	20-40%
4X (4th Harmonic)	4 × f	120 Hz	10-25%
5X, 6X...	5 × f, 6 × f...	150, 180 Hz...	Decreasing

Why Harmonics Matter: Even if piping avoids the fundamental frequency, it can still resonate at a harmonic. The 2X and 3X harmonics often carry enough energy to cause damaging vibration. Pulsation analysis must check all significant harmonics, not just the fundamental.

Multi-Cylinder Effects

When multiple cylinders share common piping, their pulsations interact:

In-phase cylinders: Pulsations add together, increasing amplitude
Opposed cylinders (180° apart): Pulsations partially cancel at 1X but reinforce at 2X
Unequal cylinders: Create complex beat frequencies

Balanced-opposed compressor configurations help reduce 1X pulsation but do not eliminate 2X and higher harmonics. Proper bottle sizing and piping design remain essential.

2. Effects of Pulsation

Uncontrolled pulsation can cause serious problems ranging from nuisance vibration to catastrophic piping failures. Understanding these effects emphasizes the importance of proper pulsation control.

Equipment Damage

Effect	Cause	Consequence
Pipe vibration	Pressure waves excite pipe natural frequencies	Fatigue failure at welds, supports
Valve damage	Pressure reversals cause valve flutter	Broken valve plates, springs
Instrument error	Pressure gauges see oscillating pressure	Erratic readings, control instability
Flow measurement error	Orifice meters affected by pulsation	Custody transfer inaccuracies
Capacity loss	Interference with valve operation	Reduced throughput, efficiency

Safety Concern: Severe pulsation-induced vibration has caused piping failures resulting in fires, explosions, and fatalities. API 618 pulsation studies are mandated specifically to prevent these catastrophic failures.

Signs of Excessive Pulsation

Visible pipe movement: Piping shaking or swaying during operation
Noise: Hammering, knocking, or rumbling sounds
Premature valve failure: Valves wearing out faster than expected
Loose connections: Bolts, clamps, or instruments working loose
Cracked welds: Fatigue cracks at pipe welds or branch connections
Erratic instruments: Pressure gauges bouncing, flow meters unstable

3. Pulsation Bottles

Pulsation bottles (also called dampeners or surge volumes) are pressure-rated vessels installed at compressor suction and discharge to attenuate pulsation before it enters the piping system.

How They Work: The large volume of the bottle allows pressure pulses to expand and dissipate. The sudden increase in flow area creates an acoustic impedance mismatch that reflects pulsation energy back toward the source rather than transmitting it to the piping.

Bottle Locations

Location

Suction Bottle

Dampens pulsation traveling back into suction piping. Prevents interference with upstream equipment.

Location

Discharge Bottle

Primary pulsation control. Dampens pressure waves before they enter discharge piping.

Sizing Principles

Bottle volume is the primary sizing parameter. Larger volumes provide better dampening but increase cost and space requirements:

Rule of Thumb - Bottle Volume: V_bottle ≈ 10 to 25 × V_displacement Where: V_bottle = Pulsation bottle volume V_displacement = Cylinder swept volume per stroke Larger multipliers for: - Lower operating pressure - Longer piping runs - More sensitive downstream equipment - Higher pulsation attenuation required

Application	Volume Multiplier	Notes
Standard gas gathering	10-15×	Typical field installations
Pipeline compression	15-20×	Long runs, custody transfer
Process/plant	20-25×	Sensitive equipment downstream
Low pressure suction	20-30×	Large volume at low density

4. Internal Design Features

Modern pulsation bottles contain internal components that enhance their effectiveness beyond simple volume dampening.

Baffles

Internal plates that divide the bottle into compartments:

Redirect gas flow to increase path length
Create multiple expansion chambers
Break up organized pressure wave patterns
Provide structural support for the vessel

Choke Tubes

Restrictive passages between bottle compartments or at the outlet:

Create acoustic resistance to pressure wave propagation
Convert pulsation energy to heat through throttling
Sized to attenuate specific frequency ranges
Balance between dampening and pressure drop

Design Trade-off: Choke tubes improve pulsation control but add pressure drop. Typical choke tube velocity is 50-100 ft/s, balancing dampening effectiveness against energy loss.

Outlet Nozzle Location

The position of inlet and outlet nozzles affects performance:

Opposite ends: Maximum path length, best dampening
Same end: Compact but reduced effectiveness
Radial outlets: May cause standing waves at certain frequencies

5. Acoustic Resonance

Acoustic resonance occurs when pulsation frequency matches the natural acoustic frequency of the piping system. This can amplify pulsation to dangerous levels even with properly sized bottles.

Speed of Sound in Gas

Acoustic Velocity: c = 68.1 × √(k × T / MW) ft/s Where: k = Specific heat ratio (Cp/Cv) T = Temperature (°R = °F + 459.67) MW = Molecular weight (lb/lb-mol) Example: Natural gas (MW=18, k=1.27) at 100°F c = 68.1 × √(1.27 × 559.67 / 18) c = 68.1 × √39.5 = 1,350 ft/s

Pipe Resonant Lengths

Piping can resonate when its length equals certain fractions of the acoustic wavelength:

Resonant Pipe Lengths: Half-wave (λ/2): L = c / (2 × f) Quarter-wave (λ/4): L = c / (4 × f) Where: L = Pipe length (ft) c = Speed of sound (ft/s) f = Pulsation frequency (Hz) Resonance occurs when pipe length matches L or multiples

Lengths to Avoid

Compressor Speed	Fundamental (Hz)	λ/2 Length (ft)*	λ/4 Length (ft)*
300 RPM (DA)	10	67	34
600 RPM (DA)	20	34	17
900 RPM (DA)	30	23	11
1200 RPM (DA)	40	17	8

*Based on c = 1,350 ft/s (natural gas at 100°F). DA = double-acting.

Resonance Avoidance Zone

Resonance doesn't occur only at the exact calculated length. Due to end effects, temperature variations, and acoustic uncertainties, pipe lengths should avoid a zone around resonant lengths:

±20% Rule: Avoid pipe lengths within 20% of calculated resonant lengths. For example, if λ/2 = 25 ft, avoid lengths from 20 ft to 30 ft. This margin accounts for acoustic uncertainties and provides a safety factor against operating condition changes.

When evaluating existing piping:

<10% of resonant length: High risk - expect resonance problems
10-20% of resonant length: Moderate risk - monitor closely, may need modification
>20% from resonant length: Acceptable - should operate without resonance issues

Harmonic Resonance: Resonance can also occur at 2X, 3X, 4X... the fundamental frequency. Check resonant lengths for all significant harmonics, not just the fundamental.

6. Multi-Stage Systems

Multi-stage compression requires pulsation bottles at each stage. The sizing follows a logical pattern based on gas density and volume at each pressure level.

Bottle Sizing Hierarchy

Key Principle: The first stage suction bottle is the largest; the final stage discharge bottle is the smallest. Each successive stage handles higher pressure gas at lower actual volume.

Stage	Pressure	Gas Volume	Bottle Size
1st Stage Suction	Lowest	Highest	Largest
1st Stage Discharge	Low-Medium	High	Large
2nd Stage Suction	Medium	Medium	Medium
2nd Stage Discharge	Medium-High	Lower	Smaller
Final Discharge	Highest	Lowest	Smallest

Interstage Considerations

Cooler piping: Often the longest runs, most susceptible to resonance
Scrubber integration: Interstage scrubbers can serve as pulsation volumes
Isolation: Each stage's bottles should isolate it from adjacent stages
Pressure rating: Bottles must be rated for maximum possible pressure

7. API 618 Requirements

API Standard 618 (Reciprocating Compressors for Petroleum, Chemical, and Gas Industry Services) specifies requirements for pulsation and vibration analysis.

Design Approach Levels

Approach	Analysis Required	Application
Design Approach 1	Simplified analysis	Low-risk, standard installations
Design Approach 2	Analog or digital simulation	Moderate complexity systems
Design Approach 3	Full acoustic/mechanical study	Critical service, complex piping

Acceptable Pulsation Levels

API 618 Pulsation Limit: Peak-to-peak pulsation ≤ 7% of line pressure (Design Approach 1) Or calculated per: ΔP_allowable = a × (ID)^(-0.5) Where: ΔP = Allowable peak-to-peak pulsation (psi) a = Constant from API 618 tables ID = Pipe inside diameter (inches) More stringent limits for sensitive equipment

When Studies Are Required

New installations: All significant compressor installations
Modifications: Speed changes, cylinder changes, piping modifications
Problems: When excessive vibration or pulsation is observed
Capacity changes: Operating conditions significantly different from design

Study Deliverables: A pulsation study typically provides pulsation bottle sizing, orifice plate specifications, recommended pipe support locations, pipe spans to avoid, and operating speed restrictions if any.

8. Troubleshooting Pulsation Problems

When pulsation problems occur in existing systems, systematic troubleshooting can identify causes and solutions.

Diagnostic Steps

Step	Method	What It Reveals
1. Visual inspection	Observe piping during operation	Location and severity of vibration
2. Speed variation	Change RPM if possible	Resonance vs. forced response
3. Pressure measurement	Dynamic pressure transducers	Pulsation amplitude and frequency
4. Frequency analysis	FFT of pressure signals	Dominant frequencies, harmonics
5. Pipe survey	Measure pipe lengths, supports	Potential resonant lengths

Common Solutions

Add pipe supports: Reduce span lengths to raise natural frequency
Install orifice plates: Add acoustic resistance at strategic locations
Modify pipe routing: Change lengths to avoid resonance
Add volume: Install additional dampening volume (field bottles)
Change speed: Operate at RPM that avoids resonance
Internal bottle mods: Add baffles or choke tubes to existing bottles

Professional Analysis: Significant pulsation problems typically require analysis by specialists with acoustic modeling software. Field modifications without proper analysis can make problems worse or shift them to other locations.

Intermittent Flow

Pulsation Bottles

API 618

Contents

This guide covers: