1 Introduction

After hydrotesting, water that remains inside of pipes will speed up corrosion and change the fluid’s purity. To guarantee their pipes operate as effectively as possible, all operators must endeavor to pipeline drying after hydrotesting.

Drying of pipes prevents water from breaking down normally, leaving space for ground pipework and fluctuations in flow rate. Even though pipeline drying is seen as a straightforward activity when done properly, it is really a complex process that requires great caution to complete. The nature of the workflow alters and gets harder to control if the pipelines are not managed appropriately.

A pipeline’s target dryness must be attained in order to properly commission the pipeline and monitor its integrity going forward. Achieving the right amount of dryness may help prevent problems like hydrates and microbiologically influenced corrosion (MIC).

Economic considerations and time constraints often influence the choice of drying techniques, leaving operability and corrosion problems unaddressed.

The requirements for dryness may be influenced by a variety of things. Considerations include the kind of hydrocarbon, hydrate formation circumstances, corrosion studies, pipeline topography, piggability, and environmental regulations. Pipes are loaded with hydrotesting fluids during commissioning; these fluids must be cleaned and drained from the pipelines before commissioning and start-up.

When deciding on dryness standards, the composition of these fluids must be taken into account. The typical moisture limit for natural gas pipelines is 40 lbs/mmscf, or a dew point of -4°F (-20°C). When assessing the pipeline dryness criterion, the dew point is a crucial factor. At anticipated operating temperatures and pressures, a dew point under the hydrate formation curve is preferred. For the majority of applications, a dew point of -4°F (-20°C) is often achievable and adequate. Tables that translate dew point to moisture content are easily accessible and might be helpful for establishing dryness standards.

A pipeline is typically dried out in phases using one or more of the following methods: Pipeline drying methods include using nitrogen gas, hot air, chilled air, vacuum, pigging/swabbing, and MEG drying.

2 Drying of Pipeline with Nitrogen

Gaseous nitrogen has the necessary chemical characteristics, notably its inert, non-reactive nature, which makes it highly stable, to be used for pipeline drying. Nitrogen gas priming makes pipelines less likely to have explosive mishaps. Oxygen, water vapor, and other contaminants that may degrade the pipes or change the quality of their contents will be replaced with nitrogen gas when it is released into the atmosphere.

For drying pipelines, nitrogen is a great option because of its low dew point (as low as -40F/C). Nitrogen has the extra benefit of being completely inert; it replaces oxygen, delays oxidation, and averts explosions. The 24/7 capacity of an onsite nitrogen generator provides the ideal option since a continual supply of dry nitrogen into the pipeline is required. With rates ranging from 2 SCFM to 1500 SCFM, an on-site nitrogen generating system can supply a consistent high-pressure feed of high-purity nitrogen. Turnkey N2 generators may be made to be mounted on skids or trucks. We may hire nitrogen generators for a short or extended period of time to meet urgent demands.

Typically, nitrogen gas is injected into a particular pipeline at one end to perform nitrogen purging. The remaining water and pollutants may be expelled via an exit port at the other end of the pipeline thanks to the gaseous nitrogen’s propulsion power and chemical makeup.

When the feed gas dewpoint and output gas dewpoint diverge by at least 10°F, the drying process is said to be finished.

3 Drying of Pipeline with Hot Air

The process for hot air drying is similar to nitrogen-based pipeline dewatering. The usage of hot air provided by an industrial air compressor is the primary distinction. When warm air is introduced into a damp pipeline, water vapor will quickly develop.

The exit port at the other end of the pipeline may then receive the evaporated moisture. Hygrometers that continuously identify variations between the inlet and output air dewpoints may be used to monitor hot air pipeline testing.

A supply of hot air from an air compressor feeds the pipeline at one end. The water that is present in the pipeline absorbs heat energy. As a result, the speed of evaporation of this water becomes very fast. Moreover, the rate of evaporation is directly proportional to the temperature of the hot air.  The hot air entrance port is at the other end of the pipe from the exit port. It is the place where the water vapor is transferred by the air flowing out of the port.

When the outlet air dewpoint considerably drops and in order to assess that the whole drying is completed, there is a need for a capacitive instrument. Easidew transmitter or Cermet II Hygrometer are the best choices in this regard. They can be located at the air outlet so that they can effectively indicate the finalization of the whole process of drying. At the current ambient temperature, the air will be almost saturated throughout the procedure. The dewpoint is likely to drop in relation to the feed air temperature by the compressor whenever the pipeline is dry.

Figure 1 Pipeline drying with hot air

The process for heated air drying is similar to nitrogen-based pipeline dewatering. The usage of hot air provided through an air compressor is the primary distinction. When warm air is introduced into a damp pipeline, water vapor will quickly develop.

The exit port at the other side of the pipeline may then receive the evaporated moisture. Hygrometers that constantly identify variations between the intake and output air dewpoints may be used to supervise hot air pipeline testing.

4 Pipeline drying with Chilled Air

Using very dry, cold air is another efficient method for removing moisture from pipes after hydrostatic testing. To remove any remaining water, extremely dry, cold air may be forced into a pipeline by very strong ventilators.

In order to remove moisture effectively, this approach uses chillers to cool down the air to below-freezing temperatures before usage. Dewpoint monitoring is done, much like other drying processes, to gauge how far along the drying cycle is.

Simple rules define the dry air approach of drying pipes. Air with minimum dew point has less moisture vapor pressure. Thus, by blowing it inside, the moisture would be absorbed. The main agent behind drying is the disparity in vapor pressure of the air’s moisture content of in the pipes and of the moisture content of dry air. The pipeline will dry up more quickly as the difference is larger. This is due to the reason that the pipes are underground and the ground temperature changes, this temperature becomes a deciding factor in how long the drying phase will last. As the temperature of the wall of the pipe will be similar to the temperature of the ground, therefore the dew point related to air within the pipeline will have a similar temperature as that of the pipe.  As a result, when there is a high ground temperature, the air within the pipeline may contain more moisture. When the earth is warmer, pipes might dry up more quickly.

Desiccant and refrigeration technology are both required to produce low dew point air. Dehumidification is the process of taking moisture out of the air. The real content of moisture related to the air might range from minimal in the winter to very large in the summer. Refrigeration is used in the initial step of the process of dehumidification to cool the air to less than its dew point. In Summer ambient air has great temperatures and a lot of moisture. Mechanical refrigeration dehumidification is particularly effective in these circumstances. Condensation then removes moisture by forcing air through the refrigerated cooling coil.

Usually, the air is chilled to only 2 degrees Celsius. Condensed moisture will freeze onto the cooling coil below this point, halting the dehumidification process. Prior to air entering the desiccant, a temperature of 20 degrees Celsius is crucial. The process of mechanical refrigeration involves the circulation of a refrigerant, such as R134a, via a condenser compressor, evaporator, and expansion valve in a closed, high-pressure loop.

A desiccant dehumidifier is used to complete the process of dehumidification. Just after the air stream has passed through the cooling coil, HPS, a desiccant, is utilized to remove moisture from it. The desiccant is highly effective at extracting moisture because the airflow is cool and saturated. As a result, the final dew point is reached at -40 degrees Celsius. Desiccant dehumidification can operate at any temperature regardless of freezing because moisture is separated from the flow of air during a vapor phase.  A Honeycomb wheel with HPS impregnated into it. The wheel spins at eight to ten revolutions every hour. The honeycomb wheel constantly rotates in all these processes. Electric heaters are used in the regeneration sector for drying the Honeycomb wheel.

With the help of low pressure, we tend to blow dry air via the pipeline. Resultantly, the air is dried at atmospheric pressure. Moreover, the roots blower is used to throw the dry air through the pipeline. This blower can withstand the pressure of one barg.

Because of the disparity in partial pressure of air and vapors, the dry air effectively absorbs the water within the pipeline. The absorbed quantity of water depends on the air temperature within the pipeline. Relative humidity of less than 50% prevents corrosion in the pipeline, therefore dry air offers good corrosion prevention.

Figure 2 Pipeline drying with chilled air

5 Vacuum drying of the pipeline

Water present in the pipeline is physically removed during the vacuum drying process. The vacuum-drying method depends on the observation that water’s boiling point changes depending on the applied pressure.  Therefore, we may make the water “boil” and extract it from inside using a vacuum pump by lowering the pressure of pipeline to the saturated vapor pressure for the ambient temperature.

There are three key stages of this process.

Figure 3 Vacuum Pipeline Drying


The saturated vapor pressure is reached in the initial phase by drawing lower pressure of the pipeline from atmospheric levels. The majority of the pipeline’s contents are being evacuated during this period. Typically, a leakage test is performed at this first stage to look for leaks that need to be fixed.


Water will begin to evaporate as the pressure rises toward the SVP, maintaining the pressure balance. As a result, more water evaporates as the pressure attempts to decrease. The vacuum pump draws this vapor out from the line, and additional water evaporation takes its place. Unless all of the free water has evaporated, this process keeps going.

Final drying

As soon as the free water has all evaporated, the pressure begins to drop since there won’t be any more water to maintain the balance. It is safe to presume that all of the air inside the pipeline has been completely expelled and that only the water’s vapor pressure is responsible for the pressure there. As a result, the dewpoint and pipeline pressure are closely connected. Once this pressure (1.032 mbara, equal to a dewpoint of -20 C) has been reached in the pipeline, it becomes obvious that the pipeline is dry. This phase may be changed by purging via a dry gas under vacuum on certain lengthy pipes where the force friction serves a significant role and delays the drying process. This may hasten the pace at which water is removed during the last drying stage.

A soak test may be used to conduct one more inspection. Here, the pressure is locked in and watched over for a while, usually for 24 hours. If any free water is available, it will evaporate, causing the pressure to increase once again to the temperature of the ambient pipework. After drying, the product may be put right into the vacuum, which is almost completely oxygen-free. This, however, requires that the product is readily accessible, and often, inert nitrogen gas is used to fill the vacuum in order to prevent any leakage. Before filling the pipeline, nitrogen may be purged under a vacuum to achieve very low dewpoints.

The air-drying technique, as well as the vacuum-drying process, both need the system to be heated for the water to evaporate. Latent vaporization is the term used to describe this. As the water must collect this heat from the environment around the pipeline, which is often dirt or ocean, vacuum drying may be a challenge. In air drying, the energy is continuously supplied to the system by the air moving into the pipeline.

6 Pigging and Swabbing

Pipeline drying may be accomplished mechanically by pigging and swabbing the pipeline. A pig, a device used for the swabbing procedure, is constructed of foam or another appropriate polymer material. A pig is put into the pipeline at one end and then driven all the way to the other end while being dried along the way. Pig devices are often made with a wider contact area between them and the inside walls of the pipes they dry to meet the internal diameter of the pipelines.

The most common materials are soft foam (also known as swabs), high and medium density polyurethane foam (also known as poly pigs), and solid polyurethane type pigs. The media already present in the pipeline, such as oil, water, food, paint, etc., may be used to push pipeline pigs as long as there is enough flow and pressure.

Using magnets, pig locators, or pig transmitters, pigs may be followed as they go through the pipeline. Even if the pipeline is underground, these tracking technologies may find a pig anywhere along the path. Pigging or swabbing is a highly efficient way to get rid of any particle contaminants and leftover moisture during hydrotesting.

Figure 4 Pigging and Swabbing

7 Drying with Mono Ethylene Glycol (MEG)

This technique of chemical pipeline drying removes moisture by taking use of the hygroscopic qualities of mono ethylene glycol. Dry air or natural gas is used as the driving force to move this chemical substance through the lumen of a pipeline from one end to the other. Any water left over during hydrostatic testing is removed by MEG drying, lowering the risk of quick corrosion damage and extending the usable lives of treated pipes.

MEG proving is especially helpful for operators with pipeline networks that pass through cold areas because of its entire anti-hydration qualities. The possibility of freezing, pollutant buildup, and channel obstruction are all removed since their pipes don’t contain any water.

For swabbing, mono-ethylene glycol (MEG) is often the preferred fluid. MEG is often selected because it can be recycled and is simpler to handle than methanol. After MEG treatment, it is typical practice to expose the equipment in the final park state to a minimum of 80% MEG concentration, where this number is to include the impact of residual hydrotest water on its dilution. This concentration is necessary because, whereas MEG requirements for hydration management typically call for a 50% rate in total fluid, bacterial growth tendency calls for a minimum concentration of 80%. Since MEG is manufactured using gas and is often processed by regeneration equipment, it must adhere to strict criteria wherever it is utilized.

Figure 5 Drying with Mono Ethylene Glycol (MEG)

8 References

  1. Y. Daib, Determining volume and concentration to dry gas pipelines. Oil and Gas Journal 81, 80±83 (1983)
  2. M. Gorislavets and A. A. Sverdlov, Numerical investigation of the process of ventilative drying of a pipeline. Journal of Engineering Physics 60, 615-623 (1991).
  3. A. LaCasse and T. Ingvordsen, Desiccant drying of gas pipelines. Material Performance 27, 848-851 (1988)
  4. Y. Daib, Techniques for drying pipelines by the three-spheres method. Oil and Gas Journal 81, 112-116 (1983).
  5. Battara and B. Selandari, Mathematical model predicts performance of pipeline drying with air. Oil and Gas Journal 82, 114-116 (1984)
  6. https://nigen.com/pipeline-drying-equipment-after-hydrotest/#nitrogen