Are humidifiers necessary?
During a recent consultation visit to inspect a couple of rigs for layup in Labuan, in the Bay of Brunei there was a lot of discussion on the importance of dehumidifiers and whether they are necessary at all.
The below information serves to highlight the 3 main class recommendations for layup and should speak for itself as whether in Asia you can afford not to use dehumidifiers. Some rig & vessel owners are tending not to use dehumidifiers due to perceived cost or instead use VCI (Vapour Corrosion Inhibitor) products as a substitute to dehumidifiers. The importance of following proper layup procedures cannot be underestimated. The humidity levels in Asia means that corrosion is accelerated if moisture is not controlled. As such dehumidifiers are of great importance for layup in Labuan and areas surrounding the Bay of Brunei.
Accommodation including navigating bridge and radio room should be protected against corrosion and other deterioration by means of a suitable system, e.g. by a dehumidified atmosphere having a relative humidity below 60%. It is advisable to maintain a dehumidified atmosphere on the navigation bridge, in control rooms and all other rooms containing computers and electronic equipment. The relative humidity should be controlled at regular intervals. Subject to manufacturer’s recommendations it may be advisable to keep equipment under constant voltage, or to put it into service at regular intervals both for additional moisture removal and to recondition components.
All provisions under section 9.3 apply, except that the temperature in the engine room and other spaces need not be kept above the ambient temperature, but not below 0°C. Spaces, equipment and machinery should be protected by the use of a dehumidified atmosphere with a relative humidity below 50%. A safe method for keeping this atmosphere under control with regard to humidity should be established, and arrangements should be made to maintain the relative humidity below the given limit. Cooling water should be drained from machinery. Dry lay-up of boilers and steam system to be applied.
Ballast tanks in use shall have efficient corrosion protection with coating in a GOOD condition throughout. For ballast tanks not in use, a dehumidified atmosphere should be secured, and the tanks sealed off.
Cargo tanks, pump rooms, etc.
For cargo tanks and holds not in use for ballast purposes, for pump rooms, cofferdams and pipe tunnels, a dehumidified atmosphere should be secured, and the tanks/rooms sealed off.
Cargo oil pipes, deck steam pipes, Butterworth lines, heating coils, exhaust, water and air pipes and ballast lines not in use should be cleaned, well drained, and kept protected by a dehumidified atmosphere.
Deck machinery which is not needed for instant operation should be protected by a dehumidified atmosphere.
Accommodation, including navigating bridge and radio room, should be protected by a dehumidified atmosphere. Parts of the accommodation in use may be held at a comfortable atmosphere with moderate relative humidity.
Engine and boiler rooms
Engine and boiler rooms should be protected by the use of a dehumidified atmosphere. Air supply to the engine room for possible working of combustion engines should be arranged in such a way that the dryness of the atmosphere is not influenced.
Crank case should be protected by the use of a dehumidified atmosphere.
Turbine machinery and reduction gears
Turbine and gear housings should be protected by the use of a dehumidified atmosphere.
Fresh water systems
Cooling water systems on engines not in use should be emptied and dried. Care should be taken to remove all water from cooling spaces, and thorough ventilation with dehumidified air is required.
Starting air system
Starting air receivers should be kept empty, clean, dry and open to the dehumidified atmosphere in the engine room. At least one auxiliary starting air bottle should be kept fully charged to maintain auxiliary engines ready for start. It should be confirmed prior to lay-up that drains are clear. Air piping should be drained and dried out.
Boilers and steam system
Arrangements should be made to circulate dehumidified air through the boiler and maintain a dehumidified atmosphere.
Dry lay-up condition should be used. After careful draining, drying should be executed by circulating dehumidified air through the boiler and the steam system.
The whole steam system should be preserved by means of dehumidified air. An arrangement combined with the boiler should be established.
Local arrangements with dehumidified atmosphere should be established for components such as switchboards, starter boxes, instrumentation units etc. Regular control of relative humidity on components as well as regular change/reactivation of possible drying agents should be carried out.
Instrumentation and automation control room
Control room in engine room and other rooms containing sensitive electronic equipment should be protected by the use of a dehumidified atmosphere
Personnel living on board should be accommodated in one area to allow all other areas to be dehumidified, or at least provided with heaters, to reduce humidity to an acceptable level. If the ship’s galley is being used by lay-up staff, the galley exhaust fans and grease trap should be regularly inspected and cleaned. For all unoccupied areas, the humidity level in accommodation areas should be reduced and maintained at 45-55% relative humidity (RH) by dehumidifiers.
This is particularly important for spaces such as the radio room, Navigation Bridge and other spaces housing electrical machinery or electronic control equipment.
Ships’ linen and napery should be stored in one single dry compartment with mattresses stowed on their edge to assist free air circulation. All provision room, cabin and cabinet doors should be secured in the open position. Water services in unoccupied areas should be shut off and drained and sanitary fittings and toilet bowls should be sealed.
The temperature in machinery spaces should be maintained above 0°C (32°F).
Dehumidifying equipment should be installed to protect machinery spaces from atmospheric corrosion, by maintaining relative humidity within the range of 30-50% RH. Power should be available for continuous operation of the dehumidifiers and the occasional turning of machinery. To achieve humidity control of the machinery space, funnel openings, grills, ventilator openings, doors, etc., should be closed and sealed. Access to the space should be restricted to two openings only and it is advantageous if these are double air lock doors.
Main engine crankcases should be supplied with dehumidified air which is suitably vented at the opposite end of the engine. Reference should be made to the engine maker’s recommendation for laying up. Water cooled systems for the main engine should be completely drained, washed with fresh water and left open to atmosphere. Air starting valves should be dismantled and lubricated. Fuel valves should be removed, overhauled and stored outside the engine. All bright work should be protected with a smear of grease or oil. Engines including all shafting should be turned weekly to circulate oil (e.g. one complete turn plus one quarter turn). Cylinder lubricants should be operated by hand before turning. Ideally diesel generators should be maintained in operational condition and operated (rather than simply turned) once per week for about two hours.
For lay-ups over three months, boilers should be drained and stored with the drums and header doors open to ensure maximum air circulation.
All SW and FW systems and pumps which are not in use should be washed with fresh water, drained and left open to the atmosphere when a dehumidifier is in use. (Otherwise they should be left full of suitably treated clean water.) Any pumps, for which power is not available, should be turned weekly by hand. Dehumidifiers should be suitably equipped with vent trunking to forcibly ventilate heat exchangers, condensers and steam piping, after the removal of appropriate inspections covers, crank doors, non-return valves, etc.
Electrical, electronic and software systems
Electronic and software systems
Of particular concern when laying-up modern ships is the large amount of computer processing equipment on board. It is very important that the following preventative measures are taken: Equipment containing printed circuit boards should be kept dry and free of moisture, and excessive temperatures should be avoided. The electric supply systems in modern ships consist of electro technical components, equipment and systems which could be susceptible to deterioration if the environmental conditions are inappropriate. These systems may require specialist protection and specialist testing during reactivation. Software back-ups should be made available for reactivation should computers fail to boot up or restart on their own. This may take the form of keeping all programs and databases (for planned maintenance, etc.) duplicated ashore.
Electrical equipment and machinery
Heating or dehumidification techniques should be employed to prevent condensation within electro technical systems where degradation could occur if the environmental conditions are inappropriate. These systems include:
a) main and emergency generators and switchboards
b) all motors and starters associated with propulsion machinery, pumping duties, steering gear, cargo handling, deck machinery and domestic services
c) converters, harmonic filters and transformers
d) all radio and navigational equipment, and
e) all engine, boiler and wheelhouse control consoles.
It is concluded that an inventory of these systems will be prepared by the shipowner before lay-up, as each vessel will have different requirements. Where required, anti-condensation heaters should be provided for those systems that are located outside any of the heated or dehumidified areas. Electrical equipment on deck should be covered and sealed, with a suitable method in place to ensure that any moisture within the sealed equipment is absorbed, such as the use of a desiccant material. Heaters should be distributed throughout the machinery spaces and those spaces that contain the electrical and control/software-based systems to maintain reasonable temperatures (above 0°C) and prevent condensation.
Whenever possible, accommodation spaces, navigation bridge, and other control rooms should be sealed and controlled by dehumidification. Complete dehumidification at 45% to 55% RH is normally required to prevent sweating or equipment damage.
Machinery Spaces and Machinery
The machinery and related engine room equipment should be protected against accelerated or localized corrosion, seizing and freezing. This normally involves the use of stabilized or preservative lubricants, prevention of acid concentrations, and regularly scheduled rotation or movement of machinery parts to shift contact surfaces. There should be a means or source of power for lighting and for turning over machinery. Continuous heating for the machinery spaces, including steering gear room, to maintain a temperature a few degrees above atmospheric; or alternatively, complete dehumidification at 35% to 45% RH is normally required to prevent sweating or humidity corrosion damage.
Equipment and engine manufacturer lay-up instructions should be referenced and adhered to.
All rotating machinery in the machinery spaces should be turned over several revolutions and stopped at a new position at least once a month. Where fitted, pressure lubrication systems should be actuated and cylinder lubricators manually operated prior to turning over the machinery.
The steering gear should be operated and moved full travel at the same intervals. Deck machinery should be turned over at least quarterly. Monthly engine turning may cause removal of preservative oil from some surfaces. The surfaces should be re-covered with preservative oil after the scheduled turning operations. Main and auxiliary engines which are not in operation shall be turned once a month with main lube oil pump (main engine) and pre-lube oil pump (auxiliary engines) running to ensure oil coverage of bearing journals. Cooling water circulation should also be considered.
Enclosed engine compartments should be dehumidified to avoid corrosion.
Lube oil in engines and turbines should be thoroughly centrifuged and water separated or dehumidified prior to shutting down, after which the oil should be periodically chemically analysed to confirm stability and the absence of harmful acidity. Any lube oil reservoir or sump vents to the exposed atmosphere should be closed off and opened only to a dry space. Where lube oil tanks are contiguous with the hull plating, accumulation of condensation should be provided for.
The electrical system should be protected against insulation deterioration, primarily from moisture absorption or water ingress, and the rotating elements protected against corrosion damage in the bearings.
Electronics are better protected with constant power applied in a warm and dry environment. Sealed and dehumidified ships will also preserve equipment longer. Desiccant should be placed inside all electrical units to avoid the risk of condensation. Desiccant should be changed at manufacturer’s recommended intervals. Open cable ways should be sealed to reduce desiccant degradation and other component damage.
Before securing, electrical motors and generators should be thoroughly cleaned of carbon or other hygroscopic foreign matter, heat dried so as to obtain acceptable insulation resistance readings, and the bearings lubricated with a stable grease or oil. Any carbon brushes should be lifted to prevent spot corrosion on the commutator or slip rings. Insulation readings should be taken and recorded at least monthly thereafter and where found abnormal, immediately corrected by heating, drying or cleaning.
All electrical apparatus should be maintained internally a few degrees above atmospheric by means of built-in heaters, if fitted, or by other means such as strip heaters or heat lamps. Alternatively, the humidity control may be maintained by opening the unit to the effects of a dehumidifier or by sealing the unit with desiccant inside. This latter approach would require replacement of the desiccant and re-sealing at least monthly or based on a moisture indicator.