Case Study: Pipeline Drying - Dry Gas Methods
Introduction
Following a successful hydrotest, or during maintenance outages, it is necessary to remove residual water from the pipeline before products can be introduced into the system. Residual moisture within a pipeline can present a unique set of costly challenges, with corrosion and hydrates posing a particularly serious problem. To prevent moisture-related issues, pipeline operators can choose from a range of pipeline drying methods, which typically includes using dry gas.
When a drying operation is executed properly, the results will protect the integrity and reduce maintenance costs throughout the lifecycle of the pipeline. The key to selecting the right drying strategy is a combination of project and client specifications, coupled with the expertise in designing the most economical and technically feasible solution. STEP Energy Services has recognized the opportunity to improve the predictability of drying operations through the development of modelling, which allows for effective planning of equipment and consumable requirements.
Drying Terminology
Pipeline Drying Process
Dewatering
Following a successful hydrostatic test, it is recommended that a series of bidirectional pigs are propelled through the line using either air or nitrogen gas to remove the bulk of the liquid water from the pipeline system. The bidirectional pigs provide an interface between the hydrotest water and the displacing medium so water can be swept out of all low points.
Cleaning and Swabbing
After a pipeline has been dewatered, it is recommended to clean the line of any rust or mill scale using a series of cleaning pigs. If rust and mill scale is not removed from the internal surface of the pipe, moisture will remain trapped and will bleed out over a long period of time, thus increasing the time and cost to dry the line.
Once the pipeline has been satisfactorily cleaned, a series of light density polyurethane foam pigs should be displaced through the pipeline to absorb residual water from the pipe walls. At this point in the drying operation, the physical absorption of water is the most efficient method for removing moisture from the line. It is recommended to continue displacing foam pigs through the pipeline until the water penetration into the bid body is minimal. At this point, displacing a dry gas through the line will be the most efficient way for moisture removal, however, foam pigs can still be displaced through the line intermittently to absorb any standing water that may have accumulated at the six o’clock position.
Drying with Dry Gas
Drying a pipeline with a gas is a mass transfer process that begins by injecting dry gas into a pipeline system. Dry gas flowing over a wet surface will pick up moisture and eventually reach saturation at the temperature and pressure within the system. The efficiency of drying with a gas is dependent on the initial dew point and flow rate of the drying medium, and the pressure and temperature of the system.
Convective mass transfer is required to obtain higher rates of drying. To achieve convective mass transfer, the bulk motion of the drying gas displaced through a pipeline system must be in the turbulent flow regime. If the drying gas passing over the wetted pipe walls is in the laminar flow regime, the mass transfer of liquid water to the gas phase will take place by molecular diffusion, which is a much slower mass transfer process than turbulent diffusion and will lead to longer drying times.
Saturated Humidity of Air at Various Temperature and Pressure
Figure 1 displays the saturated humidity of air with increasing temperature at various pressures. The back pressure on the pipeline should be kept to a minimum, as an increase in system pressure will cause the saturated humidity of the drying medium to decrease, which essentially means that the water-holding capacity of the drying gas is lower and will result in longer drying times and larger volumes of drying gas. The temperature of the system should be kept as high as reasonably possible to maximize drying efficiency, as an increase in water temperature increases the vapor pressure, thereby increasing the driving force for the mass transfer process.
Drying Mediums
Compressed Oil-Free Air
When drying using compressed air, it is recommended to use oil-free air compressors paired with a desiccant dryer before injecting air into the pipeline system. The desiccant dryer typically consists of two pressure vessels filled with desiccant that use a cyclic process to dry the air. As the atmospheric air passes through one of the desiccant beds, the moisture within the air is adsorbed onto the surface of the desiccant resulting in super dry air with a dew point as low as -60°C.
The desiccant contained in the pressure vessels is only capable of adsorbing a finite volume of moisture before it must be regenerated to dry out the desiccant. When the desiccant in the first pressure vessel becomes saturated with moisture, the valving on the dryer will switch and direct the atmospheric air to the second tower. At this point, the first pressure vessel will begin to regenerate. This cycle repeats itself throughout the entire drying operation.
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The main advantage of using air for pipeline drying is the endless supply of the drying medium (atmospheric air), however, if the quality of the desiccant is not managed accordingly, it could result in higher dew points and longer drying times. Air compressor systems also do not typically offer variable temperature and rate control to the operator and can be pressure-limited.
Nitrogen (Cryogenic)
Nitrogen is an inert gas that is typically supplied as a cryogenic fluid with a purity greater than 99%. Before the liquid nitrogen enters the pipeline system, it passes through a heat exchanger which vaporizes the liquid nitrogen into a gas with a consistently reliable dew point of -70°C. The nitrogen gas is then injected into the pipeline system using a positive displacement pump.
The main advantage of nitrogen gas is that it has a consistent and reliable dew point that does not rely on any drying processes. In addition, using nitrogen gas for drying activities provides an inert atmosphere inside the pipeline system for product feed into service. An inert atmosphere also provides internal corrosion protection if the line is to remain in hibernation for an extended period before it is placed into service. Nitrogen pumping equipment also gives the user full control over the flow rate and the temperature of the gas at the pump outlet which allows for a more versatile operation.
Case Study
A newly constructed 3.7 km NPS 12 pipeline in northeast BC was going to be temporarily hibernated following hydrotest and before commissioning. The prime contractor for the project would be responsible for dewatering and performing the preliminary cleaning and swabbing runs. Although cleaning and swabbing are effective in removing the bulk of the water from a pipeline system, residual water will remain that can contribute to corrosion and the formation of hydrates. Therefore, the client had requested an alternate solution to effectively dry the pipeline and preserve their assets.
Challenge
A detailed design is required for any pipeline drying operation to ensure the time and product requirements are identified to avoid major cost overruns. This is especially true for drying projects that use cryogenic nitrogen as the drying medium, since there is a finite volume of product available on site at any given time. The main driver of cost for nitrogen applications is the volume of nitrogen required and the proximity of the project to a nitrogen production facility. Therefore, a solid understanding of convective mass transfer is necessary when determining the volume of nitrogen required to dry a pipeline to a target dew point.
STEP provided an engineered solution to dry the pipeline with nitrogen to an absolute humidity of 65 mg/m₃ of water content, which translates to a dew point of -48ºC. or a relative humidity of 0.65% at ambient temperature and pressure.
Solution
STEP began by estimating the relative humidity and volume of water remaining in the pipeline based on the number of pigging runs performed following the initial dewatering run. STEP also considered the displacement medium used (i.e., atmospheric air) for pigging to consider any atmospheric moisture that may have been added into the system. With the estimated volume of water remaining in the pipeline, STEP applied convective mass transfer principles to determine the volume of nitrogen required to remove the remaining moisture and achieve the target relative humidity of 0.65% (i.e., dew point of -48°C).
Figure 2 contains the predicted and measured relative humidity based on the volume of nitrogen displaced through the system. The predicted and measured relative humidity data display identical decreasing exponential trends as more nitrogen is injected through the pipeline, as seen in Figure 2 below. STEP had predicted that a total nitrogen volume of 40,110 scm would be required to reach the target dew point. The final nitrogen volume used to achieve the target end point was 41,100 scm, representing a slight 2.5% increase over the anticipated requirements.
Predicted and Measured Relative Humidity
Summary
A drying campaign is a crucial step in the commissioning process and will reduce the lifetime maintenance costs of pipeline systems. Pipeline drying is performed to prevent the development of hydrates and corrosion, and to maintain the quality of the products to be delivered by the pipeline. In the above scope of work, STEP performed a thorough analysis using mass transfer theory to deliver a predictable pipeline drying operation with nitrogen. In performing a detailed analysis of the pipeline system, STEP successfully estimated the volume of nitrogen required to achieve the target dew point of -48°C and remained within 3.2% of the original budget.
Website: stepenergyservices.com