Overview

Progressive pigging is a controlled cleaning method for pipelines with significant internal deposits that reduce the available flow diameter. Introducing a full-size cleaning pig too early can cause excessive resistance or dislodge enough material to obstruct the line. Instead, progressively larger pigs are used to remove deposits in stages.

Internal restrictions may be concentric (deposits distributed around the pipe circumference) or nonconcentric (deposits concentrated along one side, often the bottom). The procedure below addresses concentric deposits. It is general guidance, not a universal operating procedure; qualified pigging personnel should evaluate the line and adapt the sequence to actual conditions.

Illustrative example

Consider a nominal 16-inch pipeline with concentric mineral-scale deposits that leave an estimated 8-inch effective internal diameter. This example illustrates the sequence; actual starting pig sizes must be determined from field measurements and operating conditions.

1. Assess and prove the line

Determine the effective internal diameter by inspecting accessible pipe ends or fittings and reviewing available line data. Because deposit thickness can vary along the route, a light-density, line-size foam swab may be used, where appropriate, to help assess the restriction.

For this example, run a 16-inch light-density swab. It may compress through restricted sections and expand in areas with less buildup while collecting loose debris. Record the run time, operating pressure and pig velocity. Promptly measure the recovered swab’s outside diameter before it expands toward its original size. Treat this measurement as an estimate of the most restrictive passage, not a substitute for verifying line conditions.

2. Begin progressive cleaning

• Bare foam pig. Start with an 8-inch medium-density bare foam pig, followed by a 16-inch light-density swab where the line and operating conditions permit. The swab helps maintain movement through sections with a larger internal diameter. Repeat as needed until the smaller pig returns with minimal wear.
• Criss-cross foam pig. Next, run an 8-inch medium-density criss-cross foam pig, again followed by a 16-inch swab when appropriate. Its coated pattern provides additional cleaning action. Repeat until pig wear and recovered debris indicate readiness to proceed.
• Criss-cross wire-brush pig. Then run an 8-inch medium-density criss-cross wire-brush pig, followed by a 16-inch swab if appropriate. Brush contact provides more aggressive removal of suitable deposits. The number of passes depends on deposit characteristics, line length, pig condition and operating response.

3. Increase pig diameter in stages

Once the initial cleaning stage is complete, increase the pig diameter and repeat the bare, criss-cross and wire-brush sequence as appropriate. Depending on the line and deposit conditions, diameter increases may range from approximately ¼ inch to 2 inches, or follow the next practical nominal pig size. Continue until the pipeline’s intended internal diameter is reached and the cleaning objectives are met.

4. Complete the final cleaning pass

Finish with a line-size medium-density bare foam pig to sweep remaining loose material from the pipeline. Establishing a routine maintenance-pigging program can help limit future deposit accumulation and reduce the need for extensive progressive cleaning.

Operational considerations

Before each run, confirm pig compatibility, launcher and receiver arrangements, pressure limits, available flow, debris-handling capacity and a recovery plan. Monitor differential pressure and pig movement throughout the operation. Stop and reassess if conditions indicate a potential blockage or an unsafe operating condition.

Overview

Pipeline drying may range from removing standing water with a criss-cross foam pig to performing multiple brush-pig and swab runs to address moisture retained in rust, mill scale and debris. The sequence below describes a general commissioning approach and must be adapted to the pipeline and its required dryness specification.

1. Verify piggability

Following hydrostatic testing, confirm that pigs can travel through the line. If passage has not been established during filling and dewatering, consider a light-density swab to prove the line. Inspect the retrieved swab before proceeding.

2. Remove free-standing water

A standard cup pig, disc pig or foam pig may be used for initial dewatering. Criss-cross foam pigs are commonly selected for their flexibility, handling and cost. The original guidance estimates that an initial run may remove approximately 85%–95% of the water, with two or three additional passes used as needed. A dewatered pipeline is not necessarily dry; moisture can remain within deposits and on pipe surfaces.

3. Use a dry propelling medium

Before further cleaning and drying, remove residual moisture with swabs as appropriate and ensure the propelling medium is sufficiently dry. Nitrogen or suitably dried and filtered air may be used, subject to the project requirements. Air compressors drawing ambient air should have appropriate moisture-removal equipment to avoid reintroducing water into the pipeline.

4. Clean and dry the pipe wall

Brush foam pigs may loosen rust and mill scale, while swabs help remove fine debris and residual moisture. The original procedure describes alternating groups of three brush pigs and ten swabs, at approximately 7–10 feet per second. Actual run speeds and quantities must be selected for the pipeline, its internal condition and the required dew point.

Repeat cleaning and swabbing as needed while monitoring the condition of recovered pigs and measuring dryness. A pig that appears dust-dry does not, by itself, establish a particular dew point; use calibrated measurements to verify that the specified acceptance criterion has been achieved.

5. Complete the final sweep

A medium-density bare pig may be used for a final wipe and debris sweep. Where compatible with the project requirements, a nitrogen blanket may help preserve the dried condition until the line is placed into service.

Technical and safety considerations

Drying acceptance criteria, pig selection, gas handling, pressure limits and verification methods should be established by qualified pipeline personnel. This document provides general information only and does not replace a project-specific commissioning procedure.

Why water lines require cleaning

Steel, cast-iron and ductile-iron water mains can develop tuberculation—deposits associated with corrosion and mineral accumulation along the pipe wall. These deposits may contribute to discolored (“red”) water, unpleasant odors, restricted flow and increased pumping costs.

Pigging can remove deposits and improve hydraulic performance. The original guidance identifies Hazen–Williams C-values above 120 as a potential outcome; actual results depend on pipe condition and the effectiveness of cleaning. Cleaning may offer an alternative to replacing otherwise serviceable piping.

Selecting a cleaning approach

The appropriate cleaning method depends on the internal condition of the pipeline and the severity of the deposits. Light buildup may require only a light-density foam swab. More substantial deposits may require progressive pigging, beginning with pigs sized to the effective internal diameter and gradually increasing pig diameter, density or configuration toward the full line size.

Recommended planning and cleaning sequence

1. Prepare and notify. Coordinate any service interruption with affected customers. Identify and inspect lateral connections and in-line valves.
2. Isolate and assess. Isolate the section to be cleaned using the appropriate valves. Establish the available pressure, flow direction and flow rate in accordance with the operator’s procedures.
3. Prove the line. Begin with a light-density swab to assess passage through the line and establish baseline travel time and differential pressure.
4. Clean progressively. Have experienced pigging personnel select the sequence, quantities, sizes and styles of pigs based on the line’s condition. Periodically run a line-size swab to assess cleaning progress.
5. Inspect and flush. Inspect retrieved swabs for damage and evidence of remaining deposits. Once cleaning is complete, thoroughly flush the pipeline, including laterals and service connections, under the water utility’s approved procedures.

Important considerations

Avoid excessive cleaning that could expose pipe surfaces to additional corrosion. For potable-water service, follow all applicable utility requirements for water quality, disinfection, flushing, sampling and return to service. These recommendations are general guidance, not a substitute for a site-specific cleaning plan.

General recommendation

Conventional butterfly valves are generally not suitable for pig passage because the valve disc and associated components obstruct the pipeline bore. Pigs Unlimited International, LLC does not recommend attempting to run standard pipeline pigs through butterfly valves.

Why butterfly valves present a risk

A pig may become lodged against the valve disc or be damaged while passing through the restricted opening. An unsuccessful attempt can damage the valve, pig or pipeline and may require an extensive recovery operation.

Specialized applications require engineering review

The original guidance describes an exceptional procedure using a specially designed pig. Such an operation should not be attempted based solely on general written guidance. Before considering it, qualified personnel should evaluate:

• Valve configuration and condition. Identify the number, type and condition of butterfly valves and verify the manufacturer’s restrictions.
• Pipeline characteristics. Confirm pipe material, diameter, length, geometry, deposit thickness and deposit hardness.
• Operating conditions. Evaluate available pressure, flow and velocity against the complete system design and approved limits.
• Pig compatibility. Consult the pig manufacturer and valve manufacturer regarding a purpose-designed solution; do not substitute a standard foam or other conventional pig.
• Monitoring and contingency planning. Develop an approved method for monitoring pig travel and differential pressure, including a recovery and shutdown plan.

Safety and responsibility

Do not partially close or otherwise manipulate a butterfly valve to force pig passage without a specifically engineered and approved procedure. Pressure changes, unexpected pig movement and valve damage can create serious hazards. Seek assistance from personnel experienced in specialized pigging applications.

Pigs Unlimited International, LLC does not recommend pigging through conventional butterfly valves. Any proposed exception requires independent technical evaluation and approval by the pipeline operator and other responsible parties.

Overview

A pipeline pig may stop moving for several reasons. Identifying the likely cause is essential to selecting a suitable recovery method and reducing the risk of damage to the pig or pipeline. Two common situations are loss of sealing contact and a physical obstruction. The guidance below describes potential recovery approaches that may help avoid excavation or other invasive intervention.

These recommendations are general information only. Any recovery action must be reviewed and authorized by qualified pipeline personnel in accordance with site procedures, equipment ratings and applicable safety requirements.

1. Pig stopped because of loss of seal

A pig may lose its seal because of excessive wear during a long run, abrasive conditions, dual-diameter sections, or damage from partially closed valves or debris. When sealing contact is lost, the propelling medium may bypass the pig rather than move it forward.

• Adjust propelling-medium volume. Where operating limits and system conditions allow, increasing the flow volume may provide sufficient effective force to move the pig despite some bypass.
• Relieve pressure and allow the pig to recover. After safely isolating or depressurizing the system as required, allowing a flexible pig to rest may help it recover its original shape. The original guidance suggests approximately 15 minutes as an illustrative recovery period; actual behavior depends on pig material and conditions.
• Consider a line-size foam swab. In some circumstances, a light-density, approximately 2 lb/ft³ foam swab may help reduce bypass around the stopped pig and restore movement. Launching a second pig introduces additional risks and should occur only after confirming the location, line condition, compatibility and a safe recovery plan.
• Consider reversing flow. For pigs designed for bidirectional travel, a controlled flow reversal may move the pig back a short distance so forward movement can be attempted again. This approach may not be suitable for unidirectional designs, including certain cup pigs.

2. Pig stopped by an obstruction

A pig may also become lodged when it encounters an obstruction it cannot pass. Potential causes include heavy debris accumulation, partially closed valves, tools, or other foreign objects inside the pipeline.

• Evaluate available differential pressure. Additional differential pressure may provide more force to move the pig or debris, but pressure must remain within approved pipeline and equipment limits. Do not increase pressure without evaluating the obstruction and the risk of sudden movement or damage.
• Consider controlled pressure variation. The original procedure describes alternating pressure as a possible way to free a pig at tight fittings, valves or bends. Any such maneuver should be carefully engineered and controlled; rapid cycling can introduce surge and equipment risks.
• Relieve pressure and allow recovery. Where pig deformation may be contributing to the stoppage, safely relieving pressure and allowing time for the pig to regain its shape may help.
• Consider reversing flow. A controlled reversal may allow a compatible pig to retreat from an obstruction or be recovered toward the launcher. This approach is not appropriate for pigs that are not designed for reverse travel.

Operational and safety considerations

Before attempting recovery, verify the pig’s likely location and the cause of the stoppage as far as practicable. Review pipeline configuration, valve positions, pig design, flow medium, pressure ratings and receiving capacity. Monitor pressure and flow throughout any approved recovery attempt. If the obstruction cannot be characterized or operating conditions become abnormal, stop and consult experienced pigging and pipeline operations personnel.

No single recovery technique is suitable for every situation. A qualified team should determine whether controlled recovery, retrieval or another intervention is the safest course of action.

The following values are reproduced from the original Pigs Unlimited reference chart. They are general planning guidelines, not operating setpoints or guaranteed performance values.

Use and limitations

Actual pressure and flow requirements vary with pipe construction, conveyed fluid, deposits, temperature, pig type and system configuration. Confirm operating limits, equipment ratings and applicable procedures before selecting pressure, flow or pigging speed. Do not treat the table as a substitute for engineering review.

Recommended shelf life

Under appropriate storage conditions, the recommended maximum shelf life for foam pigs is one year. Storage conditions can influence the condition and performance of foam products over time.

Storage recommendations

• Protect from weather and temperature extremes. Store foam pigs indoors in a warehouse, away from rain, direct sunlight and extreme temperatures.
• Limit exposure to ultraviolet (UV) light. Package pigs individually to protect them from UV exposure, including sunlight and certain artificial lighting. Black plastic packaging and/or cardboard boxes are recommended.
• Keep products clean. Protect stored pigs from dust and other airborne contaminants.
• Control moisture. A climate-controlled storage area is recommended to reduce exposure to moisture-rich air.

Before use

Confirm the storage history and visually inspect each pig for signs of deterioration, deformation, contamination or damage before placing it into service. Contact Pigs Unlimited International, LLC if you have questions about the condition of stored foam pigs.

The one-year recommendation assumes proper storage and is not a guarantee of performance under every condition.