Pipeline Drying Pigs
Royal Poly Products — Manufactured in Western Australia
Dewatering removes bulk water from a pipeline. Drying removes what remains — the moisture clinging to the pipe wall surface, pooled in low-point dead-legs, and trapped in weld root imperfections and surface irregularities. For natural gas transmission pipelines and other moisture-sensitive systems, achieving and verifying the specified dryness level before product introduction is not optional. It is a commissioning prerequisite, a safety requirement, and a condition of regulatory approval.
Royal Poly Products manufactures pipeline drying pigs suited to the full range of Australian pipeline commissioning programs. Our drying pigs are used in conjunction with dry air, nitrogen, or methanol drying programs to progressively reduce moisture content to specification, supporting efficient and compliant pipeline handover.
What Is Pipeline Drying?
Pipeline drying is the process of removing residual moisture from the internal surfaces of a pipeline following hydrostatic pressure testing and dewatering. After bulk water has been displaced by dewatering pigs, a film of moisture remains on the pipe wall and at low points throughout the line. This residual moisture must be reduced to a level — typically expressed as a dew point temperature or a water content in mg/m³ — at which the pipeline can safely and reliably carry its intended product.
Drying is achieved by circulating a dry medium through the pipeline — typically instrument-quality dry air or nitrogen — while using drying pigs to assist the process by sweeping residual moisture forward and maintaining contact with the bore surface. In longer or more complex pipelines, methanol slugs may be used ahead of the pigs to assist water absorption from low points and dead-legs before the main drying circulation begins.
Why Drying Matters for Gas Pipeline Commissioning
For natural gas transmission and distribution pipelines, residual moisture is a serious operational hazard:
• Internal corrosion initiation: Even small amounts of residual moisture, combined with carbon dioxide or hydrogen sulphide present in some gas streams, create conditions for rapid internal corrosion. Corrosion initiated during commissioning can compromise the pipeline’s integrity from its first day of service.
• Regulatory compliance: Pipeline commissioning in Australia is governed by AS 2885 and associated project specifications, which typically mandate that the pipeline meets a defined dew point or water content specification before gas-up. Failure to meet this specification delays commissioning and may require additional drying cycles.
• Instrument and equipment protection: Residual moisture in the pipeline can damage downstream metering equipment, compressors, and process instruments, creating operational issues that persist well beyond commissioning.
Thorough pipeline drying is the final step between a tested pipeline and a commissioned one. The cost of a proper drying program is modest compared to the cost of managing hydrates, corrosion, or a failed gas-up.
Our Drying Pig Range
Royal Poly Products manufactures drying pigs across a range of foam densities and surface configurations, matched to the pipeline size, drying medium, and dryness specification to be achieved.
Low and Intermediate Density Foam Drying Pigs
For most pipeline drying applications, low and intermediate density foam pigs are the preferred choice. Their open-cell or semi-open-cell foam structure is highly effective at absorbing moisture from the pipe wall surface as the pig travels, physically removing water in addition to sweeping it forward. This absorption characteristic makes foam pigs significantly more effective at drying pipe surfaces than rigid pigs, which simply push water ahead without removing the film adhering to the bore.
Bare foam pigs are most commonly used for drying duty, as surface coatings are not required for this application. The foam pigs natural compliance with the bore geometry ensures consistent contact with the pipe wall throughout the run, including through bends and at weld root protrusions.
Medium and High Density Foam Drying Pigs
In larger diameter pipelines or where elevated drive pressures are available, medium or high density foam pigs may be preferred for drying applications. Their greater rigidity provides a more positive sweep of the bore and can be beneficial in pipelines with larger internal diameters where a lower-density pig may not maintain adequate wall contact throughout the run.
Multi-Pig Drying Trains
For long pipelines or those with tight dryness specifications, a train of drying pigs — often interspersed with methanol slugs or gel plugs — provides the most thorough moisture removal. A typical drying train might include:
• First foam drying pig — sweeps methanol and residual water forward, absorbs moisture from the bore surface
• Dry air or nitrogen — circulated behind the pig train to carry moisture out of the pipeline
• Second foam drying pig — second-pass moisture sweep as drying medium circulation continues
• Trailing pig — final bore wipe before dew point verification at the pipeline outlet
The number of pigs in the drying train, and the volume of methanol used (if any), is determined by the pipeline length, diameter, topographic profile, and the target dew point specification. Royal Poly Products can advise on pig train configuration as part of the pre-commissioning supply package.
Drying Program Sequencing
A complete pipeline drying program following hydrostatic testing typically follows this sequence:
• Swab runs — initial drying pigs remove the last free water and begin conditioning the bore surface.
• Methanol treatment (if specified) — methanol slugs are driven through the pipeline to absorb moisture from low points and wall surfaces.
• Dry air or nitrogen circulation — the drying medium is circulated continuously through the pipeline, carrying moisture to the outlet.
• Drying pig passes — further foam pig runs during circulation sweep residual moisture forward and maintain contact with the bore surface.
• Dew point monitoring — outlet dew point is measured continuously during drying. When the specified dew point or water content target is reached and sustained, drying is complete.
• Documentation and handover — drying results are recorded, including pig passage confirmations, dew point readings, volumes of drying medium used, and time to specification achievement.
Drying Pig Selection Criteria
Choosing the right drying pig for a given pipeline requires consideration of several factors:
• Drive medium and pressure: The available differential pressure from dry air or nitrogen determines the suitable pig weight and density. Lower drive pressures favour lighter, lower-density foam pigs.
• Pipeline geometry: Bends, tees, check valves, and diameter transitions all affect which pig designs can transit safely. Foam pigs offer the greatest geometric flexibility and are preferred for complex pipeline geometries.
• Dryness specification: Tight dew point specifications — particularly those required for high-pressure gas transmission pipelines — may call for more pig passes and a larger drying train than less demanding applications.
• Methanol compatibility: Where methanol slugs are used, pig foam material should be verified for compatibility with methanol to avoid degradation during the drying run.
Pig Signalers for Drying Runs
Confirming that drying pigs have completed their full transit through the pipeline is essential — a pig that has stalled in the line without detection would leave a section of bore undried, potentially invalidating the drying program and requiring repeat runs. Royal Poly Products supplies pig signalers that provide above-ground confirmation of pig passage at key locations along the pipeline route, giving operators real-time visibility of drying pig progress.
Drying Pigs for Operational Pipelines
Drying pigs are not used exclusively in commissioning programs. Operational gas pipelines that have been shut in for maintenance, repaired, or subjected to a re-test may require a drying program before being returned to service. Similarly, pipelines that have experienced internal flooding events — from groundwater ingress, valve failure, or inadvertent water injection — may require a structured dewatering and drying response before normal operations can resume.
Royal Poly Products supplies drying pigs for both pre-commissioning and operational pipeline programs, with the manufacturing flexibility to produce pigs at short notice for urgent project requirements.
Frequently Asked Questions
What dew point specification must a gas pipeline achieve before it can be commissioned?
The required dew point specification for a gas pipeline before commissioning varies depending on the pipeline design standard, the project specification, the product to be carried, and the operating pressure and temperature conditions. In Australian gas transmission pipelines commissioned under AS 2885, dew point specifications are typically in the range of -10°C to -40°C hydrocarbon dew point, though the specific requirement is set by the pipeline owner and the downstream facility operator. Ultra-dry applications — such as high-pressure transmission pipelines operating in cold climates or pipelines feeding gas processing facilities — may require dew points at the lower end of this range or below. Always refer to your project commissioning procedure for the applicable specification.
How long does pipeline drying typically take?
The duration of a pipeline drying program depends on the pipeline length and diameter, the volume of residual moisture to be removed, the flow rate and dew point of the drying medium, the topographic profile of the pipeline, and the target dew point specification. Short pipelines with modest dryness requirements may achieve specification within 24 to 48 hours of continuous drying medium circulation. Long-distance transmission pipelines with tight dew point specifications may require several days or more of continuous circulation, particularly if low points contain significant residual moisture. Methanol pre-treatment can significantly reduce drying time by absorbing moisture from low points before main drying circulation begins. Royal Poly Products can provide indicative drying duration estimates based on your pipeline parameters.
What is the role of drying pigs during nitrogen or dry air circulation?
During drying medium circulation, drying pigs perform two important functions. First, they provide a physical sweep of the bore surface, making direct contact with the pipe wall and carrying residual moisture forward toward the pig receiver rather than allowing it to evaporate slowly into the drying medium stream alone. Second, the foam pig material absorbs moisture from the bore surface on contact — a characteristic of open-cell and semi-open-cell polyurethane foam that makes foam pigs significantly more effective for drying duty than rigid pigs. Multiple drying pig passes during the circulation phase progressively reduce the surface moisture load, accelerating the rate at which the outlet dew point drops toward specification.
Is dry air or nitrogen better for pipeline drying?
Both dry air and nitrogen are effective drying media. Nitrogen is generally preferred for high-pressure gas transmission pipelines for several reasons: it eliminates the risk of introducing oxygen into the pipeline, which could create a flammable mixture during gas-up; it is inert and will not react with any residual product or corrosion deposits in the line; and it is the standard specified drying medium for most Australian gas transmission commissioning programs. Dry air is widely used for water pipelines and lower-pressure gas distribution systems where oxygen presence is not a concern. Instrument-quality dry air — with a dew point well below the target pipeline dew point — is essential for effective drying; using compressed air without adequate drying will not achieve the required result regardless of the volume circulated.
What is methanol treatment and when is it used in drying programs?
Methanol treatment involves injecting a calculated volume of methanol as a slug ahead of the drying pig train before dry air or nitrogen circulation begins. Methanol is hygroscopic — it absorbs water readily — and is used to address residual moisture in locations that are difficult for pigs to reach: low-point dead-legs, small-bore branches, instrument tappings, and crevices at weld roots. The methanol slug is driven along the pipeline by the drying pigs, collecting moisture as it travels. It is then collected at the pig receiver and disposed of appropriately. Methanol treatment significantly reduces the time required to achieve outlet dew point specification, particularly in pipelines with complex topography or multiple dead-legs. It is specified more routinely for longer pipelines and those with tight dew point requirements.
How are drying pig passes coordinated with dew point monitoring?
Drying pig passes and dew point monitoring are conducted concurrently during the drying circulation phase. A dew point analyser is connected at the pipeline outlet, and readings are taken continuously or at regular intervals throughout the drying program. Drying pig passes are typically scheduled at intervals during the circulation phase — for example, one pig pass per shift — to progressively reduce the surface moisture load and accelerate the dew point reduction. The pig signaller record from each pass confirms full transit. The drying program is complete when the outlet dew point has reached the specified target and remained there for the hold period defined in the commissioning procedure — typically a minimum of two to four hours at specification.
Can drying be done without pigs — just circulating dry gas?
Technically, yes — a pipeline can be dried by circulating dry gas alone without pigs. However, this approach is significantly less efficient than using drying pigs in combination with the drying medium. Without pigs, the drying medium must evaporate moisture from the bore surface through mass transfer alone, which is a slow process particularly at low points where water accumulates and where the drying medium flow may be less turbulent. Drying pigs accelerate the process by physically sweeping and absorbing moisture from the bore, dramatically reducing the circulation time and the volume of drying medium required. For any pipeline with a defined commissioning schedule, drying pigs are strongly recommended.
What happens if the outlet dew point does not reach specification?
If the outlet dew point fails to reach the specified target after the planned drying program, the commissioning team must investigate the cause before proceeding. Common reasons for failure to reach specification include: insufficient residual moisture removal during dewatering; low-point accumulations that were not adequately addressed by the dewatering train; inadequate drying medium flow rate or dew point; insufficient number of drying pig passes; or a leak in the drying system allowing atmospheric moisture to re-enter. The response typically involves additional drying pig passes, additional methanol treatment, and continued circulation. In some cases, a repeat dewatering sequence may be required if significant volumes of free water are still present.
Do drying pigs need to be a specific size relative to the pipe bore?
Yes — drying pig sizing is critical. A foam drying pig should be sized to provide interference fit with the bore: typically 2% to 5% oversize relative to the nominal pipe internal diameter, depending on the foam density. An undersized pig will not maintain adequate bore contact and will allow the drying medium to bypass the pig rather than being driven ahead of it. An oversized pig may stall, particularly in a pipeline with bore irregularities or tight bends. Royal Poly Products manufactures drying pigs to precise dimensional specifications for each pipeline diameter and can advise on appropriate sizing for your specific bore dimensions and foam density selection.
Can Royal Poly Products supply a complete drying package including pigs and gels?
Yes. Royal Poly Products can supply a complete pre-commissioning drying package including drying pigs in all required configurations, gel slugs for low-point moisture treatment, and pig signalers for run tracking. We work with commissioning contractors and project managers to specify the full package based on the pipeline dimensions, topographic profile, dryness specification, and project schedule. Contact our team early in the commissioning planning phase to ensure product availability and delivery in line with your program timeline.
About Royal Poly Products
Royal Poly Products is an Australian manufacturer of pipeline pigs and pigging solutions, based in Jandakot, Western Australia. The company designs and manufactures a full range of foam pigs (low, intermediate, medium, and high density), bi-directional disc pigs, cup pigs, solid cast pigs, dual diameter pipeline pigs, and pipeline gels, supporting pipeline operators across Australia and internationally.
ISO 9001:2015 certified and a recipient of the Western Australian Export Award (Emerging Exporter 2025) and the Australian Export Award, Royal Poly Products combines local manufacturing expertise with global quality standards to deliver reliable pigging products for gas, water, oil, and industrial pipeline applications.
Website: royalpolyproducts.com
Phone: +61 08 6117 9204
Address: Unit 5/41 Biscayne Way, Jandakot WA 6164, Australia
Email: sales@royalmechgroup.com