At the end of construction, and following commissioning tasks such as water flushing, water should always be present in petroleum processing equipment.
Whenever this water may subsequently lead to an operational problem, or if equipment may be damaged, it is necessary to perform a drying-out operation.
There are several different reasons for drying-out depending upon the type of equipment and its normal service: removal of moisture from the lining of heaters and vessels, prevention of hydrates formation in process equipment, prevention of corrosion in sealines, etc.
The dry-out operations are an integral part of the Commissioning, hence the Commissioning Team should:
- Establish the dry-out requirements, based on the Engineering recommendations and specifications, and on the Vendors requirements
- Prepare all specific procedures required to carry out dry-out operation
- Carry-out, or supervise if they are sub-contracted, the operations.
3.2 Dry-out of fired heaters
An internal lining is fitted to all fired heaters, the main purpose of which is to avoid heat loss through the heater casing. Before being brought into service, it is required to expel any moisture from the lining. The main types of heater lining are as follows:
- Refractory brickwork
- Refractory lining
- Ceramic fibers.
They all necessitate a drying-out operation achieved by progressively heating the lining with the heater burners, in accordance with dry-out temperature curves.
The dry-out procedure should always be provided by the heater Manufacturer, as should the dry-out temperature curves. As the drying is of primary importance to the quality of the performance and of the life time of the heater, the Manufacturer's procedure and Vendor representative's recommendations should be exactly followed.
The fuel-gas required to perform the operation should be supplied by the Operator, unless stated otherwise in contractual documents.
3.3 Dry-out of refractory-lined vessels
In some processing plants, certain vessels are lined with refractory. These should be reactors which should subsequently be filled with a patented catalyst.
As in fired heaters, it is necessary to remove absorbed moisture to prevent expansion damage. It may also be required to remove excess moisture to prevent adverse reactions with the catalyst which could cause future problems.
This type of dry-out is generally stipulated by the catalyst Supplier who is often the patent holder for the unit design; therefore, specific procedures should be supplied and are not further discussed herein.
3.4 Dry-out of petroleum processing equipment
3.4.1 Introduction: hydrates formation
Hydrates are solid chemical compounds which can form inside gas processing equipment when light hydrocarbons and free water are present.
Hydrate deposits can accumulate until equipment, such as control valves, off gas lines, gas trunklines, etc., become totally blocked causing expensive downtime. Chemically speaking, hydrates are generally defined as a gas molecular inclusion inside a dodecahedronic construction of water molecules. Hydrate formations visually resemble ice crystals but are quite different from the point of view of chemical construction.
For a gas, the dew point is, at a given pressure, the temperature at which the first drop of liquid forms when decreasing the temperature. If there is water vapor in the gas, the first drop may contain free water.
Hydrate formation requires the presence of free water. This shows the importance of the water dew point in hydrocarbon gases whenever there is a risk of hydrate formation.
Hydrate formation depends also on:
- The gas composition
- The temperature
- The pressure.
Whenever the operating conditions are within the hydrate formation area it is necessary to eliminate the risks of free water so as to avoid hydrate formation.
The real solution is to eliminate water from the gas until the water dew point is far below the operating conditions (this is the purpose of gas drying units such as ethylene glycol plants or molecular sieves).
Because of the hydrate formation risk during start-up operations, it may be necessary to perform a drying-out operation. This means that water should be removed until the water dew point is below the operating temperature.
Drying out operations tend to be expensive and therefore should be undertaken only if there is a high risk of hydrate formation and consequent operational problems. In low-risk applications a temporary injection of a suitable hydrate inhibitor, such as methanol, should normally be adequate.
Engineering's recommendation should therefore be followed.
The type of equipment most frequently requiring water removal is gas transmission pipelines (in some cases LPG pipelines are also considered), refrigeration loops, and low temperature systems.
The following procedures should describe the principles of the drying-out methods used.
- On pipelines and sealines
- On process equipment.
It should be a Commissioning responsibility to issue detailed procedures in line with these guidelines and perform them on site, or supervise the performance of a specialized Sub-Contractor.
3.4.2 Pipelines/Sealines dewatering and drying-out
Pipelines are generally handed over full of water following the hydrostatic test which takes place after construction. Normally pipeline draining is not feasible, with the possible exception of surface run lines. Certainly it is normally impossible where subsea lines are concerned.
The water in the pipeline is therefore expelled by pigging the line in the normal flow direction, pigs being propelled by either compressed air or inert gas. This stage may as well form a part of an overall procedure.
After the hydrostatic test water has been expelled, sufficient water should remain in the pipeline to cause hydrate formation problems. Water remains in low points, valve bodies, etc. and more importantly, in the interstices of the pipewall as a film of water, typically having a film thickness of 50 to 100 microns depending upon the pipewall roughness. Even in relatively short-length gas pipelines a considerable amount of free water should therefore remain.
Two main methods are commonly used for pipelines dewatering and drying-out:
- The "Vacuum dry-out" method. As this method is also used for other types of systems, its description should be found in paragraph 220.127.116.11
- The "methanol swabbing" method.
Briefly the process consists of charging methanol to the pipeline at the production end (i.e. offshore platform) pig launcher, the pipeline prior to charging methanol having been injected with nitrogen to prevent the formation of explosive mixtures. Methanol batches are contained between cupped pigs and slowly propelled through the pipeline, normally at not more than 10 km/hr and 7 bar pressure, thus absorbing residual water. At the pipeline reception point the methanol/water mixture arrives at the terminal pig receiver from which it is routed to temporary storage tanks. Samples of the mixture indicate the success of the operation.
Depending upon the situation the methanol swabbing slugs may be followed by process gas to complete a "gas in" operation. Otherwise the pipeline is generally left pressurized with nitrogen ready for a safe start-up when required.
As these operations require the availability of many equipment (tanks, pumps, pig trains, vacuum equipment) and of specialized personnel, they should be subcontracted to a specialized Company, who should be supervised by the Commissioning.
3.4.3 Process systems dry-out
The various methods used for drying-out process equipment are as follows:
- Dry air sweeping
- Nitrogen sweeping
- Loop circulation with dryers using air, nitrogen and even process gases
- Vacuum dry-out.
18.104.22.168 Dry air sweeping
The maximum quantity of gaseous water that can be contained in a defined volume of air is perfectly known, when the temperature and pressure of the air are known. When the quantity of water in the air reaches this maximum the air is said to be saturated with water. Non-saturated air should, in the presence of water, absorb more water until saturation.
Dry air does not mean that there is no water in the air, but generally that the ambient air has been dried by some means and is far below saturation.
Note: Any unfavorable changes in the conditions of saturated air (decrease of temperature or increase of pressure) should condense water. Therefore saturated air is at its water dew point limit.
Dry air is swept through the equipment involved.
The dry air becomes saturated with water and is exhausted to atmosphere. As more and more air is swept through the equipment, more water is "picked up" and removed, thus the system is progressively dried.
The quantity of water removed for a determined drying air volume depends on:
- The initial air water dew point. The lower the dew point, the greater the amount of water removed
- The air temperature. The higher the temperature, the higher the amount of water in saturated air
- The pressure. The lower the air pressure, the higher the amount of water in saturated air
- The flow. The quantity of water removed increases with the air flow.
The air water dew point that can be obtained depends on the initial dew point of the drying air, and on the time spent for drying. Therefore, for successful dry-out, the following conditions are required:
- The dew point of the air used for drying-out must be as low as possible
- The temperature of this air must be as high as possible
- The pressure must be as low as possible
- The air flow must be as high as possible.
Determine the dry air source. If the system to be dried has a high volume, the possibility of using a temporary heating system may be studied.
Carefully drain all free water from the system at all low point drains.
Line up the system so that all dead ends are included in the dry-out.
Commence dry air injection venting the exhaust air from drains, bleeds, etc., start by using vent points as close as possible to the air injection point, progressively moving to vents further along the system (there is little point in sweeping with saturated air). Do not forget dead ends, control valve bypass lines, etc.
The dry-out is considered complete when the air water dew point reaches its target at all sample points (dew point target to be decided upon depending on the type of installation).
The water dew point that can be obtained with this method should depend on the dried air water dew point. With normal instrument air, a water dew point of -15°C is possible. If air dryers are used, water dew points as low as -60°C can be achieved which should greatly speed up the operation.
Dew point measurement is normally carried out by laboratory personnel. However, simple portable dew point measuring apparatus is available which can be used by operators. This may be much more convenient in field situations. The Laboratory should nevertheless be called upon to conduct final dew point checks on critical equipment.
22.214.171.124 Nitrogen or inert gas sweeping
Drying-out operations carried out by sweeping with nitrogen or inert gas are identical to sweeping with air.
With a standard inert gas generator it is possible to obtain water dew points of -30°C. However, some inert gas generators can deliver inert gas with a water dew point < -60°C and therefore better performances can be obtained. Using pure nitrogen it is possible to reach -80°C.
126.96.36.199 Drying-out by loop circulation
The basic principle is to circulate the drying fluid with a compressor first through dryers, then through the system to be dried out, cooler exchangers, suction drum and back to the compressor suction.
The water is removed in the dryers that should be alternatively in service and in regeneration. Some water may be drained at the suction drum (depending on the after cooler temperature). This drying method whose principle is similar to air or nitrogen sweeping is very efficient and dew points as low as -100°C can be reached (depending on the dryer performance). It is used on cryogenic systems, and as it is part of every start-up operation it should be described in details in the operating manual. Commissioning teams having to perform this type of operation should therefore comply with the procedure given in the operating manual.
Note: A cooling exchanger can be inserted at the compressor discharge in some cases.
The method can also be used in other process systems provided the necessary equipment is available.
188.8.131.52 The vacuum dry-out method
This method is most frequently employed in processing units which comprise refrigeration or cryogenic processes, and for pipelines drying-out.
The system pressure is reduced to below atmospheric pressure and a vacuum is created. This reduction in pressure reduces the boiling point of water, hence even at ambient temperatures the water boils. The water vapor is then withdrawn to atmosphere.
Two methods are normally employed to obtain the required system vacuum:
- Vacuum Pumps (normally reciprocating but may be of other types)
- Vacuum Ejectors (normally operated by compressed air or steam).
Depending upon unit design the vacuum equipment may form a part of the system. If the system is not provided with the required equipment, vacuum dry-outs should most often be performed by specialist Contractors who provide all the necessary equipment, fittings and so on.
The major responsibility of the commissioning team is to first ensure the system in question is completely blinded or otherwise isolated from related systems. This should prevent equipment, not designed for vacuum operations, from being damaged. In all cases, ensure the equipment of the system involved is designed to withstand vacuum.
As a preliminary, a thorough leak test should take place before the drying-out.
The operation generally falls into three phases:
- 1st phase: A vacuum is pulled on the system, which removes the air contained in the system. When the vacuum is sufficient, any free water in the system should begin to boil.
- 2nd phase: The system vacuum is held constant until the water has completely evaporated. This is known as the "hold" phase.
- 3rd phase: The maximum possible vacuum, using the equipment to hand, is achieved thus ensuring complete removal of water vapor. When satisfied that this phase is complete, and depending upon operational requirements, a dry gas is then "backed-in" and the vacuum is broken. This gas may be process gas, dry nitrogen or other inert gas, or dry air. Air should obviously not be used where the risk of forming an explosive mixture is prevalent. In all cases a positive pressure is to be maintained in the system to prevent the ingress of humid air.
The detailed operating procedures should be issued by the Commissioning, or if the work has been subcontracted, by the Contractor who should be supervised by the Commissioning Team.
The drying-out procedures, with support drawings and sketches, all reports and results should be filed in the corresponding subsystem Commissioning Dossier.
Each drying-out operation should also be supported by a specific commissioning dry-out report form (see attachment).