عدد المساهمات : 3492
تاريخ التسجيل : 15/09/2009
العمر : 49
الموقع : مصر
|موضوع: Water Tank Design and Maintenance/تصميم وتنظيف وصيانة الخزانات السبت يونيو 23, 2012 10:42 am|| |
Water Tank Design and Maintenance
Purpose of Tank Maintenance
Every water tank must be periodically maintained in order to ensure long life of the tank and quality water within the tank.
Some tanks, such as the reinforced concrete tanks often used for ground level or standpipe reservoirs, have lower maintenance costs over the life of the structure when compared with steel tanks.
But all types of tanks require at least some maintenance.
Maintenance can be considered a cost-saving measure. Periodic maintenance is usually much cheaper than the large repairs which will be necessary in un-maintained tanks.
The cost of inspection is an insurance policy against premature failure of the tank.
The tank's paint coating requires the most maintenance since it must be replaced periodically.
When the coating is not well maintained, the tank will have to be repaired. Repair will usually cost two and a half times more than the cost of the original coating.
In addition to the financial cost associated with replacing badly maintained coats of paint, the coatings applied on such surfaces are usually lower in quality due to adverse application conditions.
Seventy five percent of all coating failures are attributable to deficient surface preparation or to improper application of the paint.
These failures lead to downtimes during which the tank cannot be used while it is being repaired. The downtime and loss of facility production can lead to potentially staggering costs.
Formal coating inspections and the associated maintenance will vastly increase the probability of achieving a successful coat application that will protect for the design life of the system.
They will prevent costly repairs and are financially responsible in the long run.
Tanks must be inspected at intervals as the first step of the maintenance procedure.
The purpose of the inspection is to determine if repairs are required and, if so, the exact nature and extent of the work required.
Inspection of water tanks is expensive, but the cost is insignificant compared to the cost of premature failure of the tank.
A thorough inspection must be performed every two or three years on the entire structure.
Some facilities choose to have annual inspections, as will be discussed later in this section.
In addition, inspections should be performed during the construction of new water tanks and during any repair, painting, and disinfection.
In many cases, inspection is considered to be a non-essential part of the maintenance process.
Inspectors are often hired only after there has been a costly premature failure in the tank coating.
In other cases, initial planning for coating jobs includes qualified coat inspections, but the inspection contract is dropped as a cost-saving measure.
The firm contracted to apply the coating then monitors itself or the tank owner's personnel schedule spot inspections.
This inspection technique usually results in improperly applied coatings which will result in tank failure.
Inspection should be considered a mandatory part of the maintenance procedure and should be conducted by an independent expert who will receive no benefit from any maintenance performed on the tank.
In other words, if a tank is inspected by a painting contractor or a paint manufacturer, then the inspector will probably schedule more repaintings since he will benefit financially from each paint job.
Such inspectors are not independent and should not be hired to perform inspections.
The AWWA Standard: Painting Steel Water Storage Tanks includes a brief section on inspection which should be followed but which is not adequate as a basis for the entire inspection.
The inspector should outline specific maintenance needed to restore the structure. This inspection should be the basis for all maintenance of the structure - only maintenance required by the inspector should be performed.
In addition, a complete record of inspections and maintenance should be kept.
Following the proper guidelines and selecting a qualified engineer to perform regular inspections will help avoid serious maintenance problems in the future.
Preparing for Maintenance
Inspections determine the need for maintenance. Then the tank must be dewatered and the surface prepared for maintenance.
The first step during many maintenance procedures is to drain all of the water out of the tank.
Before dewatering any potable water storage vessel, notice should be given to the state's Department of Environmental Management to allow time for any problems to be resolved by state and local officials.
Insurance carriers should also be notified before dewatering and inspection occurs.
Next the structure must be prepared for maintenance. This preparation is a very important part of the process.
If preparation is poor, the maintenance job will be poor.
The interior and exterior surfaces must be cleaned of all rust scale, paint scale, blisters, rust, dirt, and growths. This cleaning can be achieved through any of several methods - using wire scrapers, sand blasting, flame cleaning, and so on.
After cleaning the surface, loose rivets must be replaced and damaged seams must be welded.
To prevent rust, painting should begin as soon as possible after the structure is cleaned and repaired.
A coat of rust-inhibitive primer should be painted on all bare surfaces first. If the surface preparation resulted in an extensive removal of old paint, then the primer coat should cover the entire surface.
A protective coating should be applied on top of the primer coat. This protective coating is applied in a thickness ranging from five to fifteen thousandths of an inch and serves to protect the tank surface from the environment.
The protective coating is composed of a vehicle containing solvents, resins, pigment and inert ingredients.
The pigments add color, but may also perform a variety of other functions.
The pigments may provide resistance to the sun's ultraviolet light and may enhance the physical properties of the paint and the gloss.
The tank must be allowed to dry, or to cure, before being refilled with water and put back into service.
The paint on the inside of the tank may take longer to cure, especially during the winter.
Some types of paint, such as high build epoxies, will be problematic if applied during the winter.
Instead, high solid vinyl should be used during these months since they will cure more reliably.
If a tank has been emptied for inspection, cleaning, painting, or for any other purpose, the tank must be thoroughly sterilized before being put back into service.
Outside demands should never be allowed to force a tank back into service before it is properly cured and disinfected.
The structure should be thoroughly disinfected and chlorinated in accordance with Plumbing Codes before being used again.
Bacteriological samples must be taken and approved and a copy of the report must be sent to the Health Department.
A suggested method for sterilizing a tank is explained below:
Fill the tank 1/2 to 3/4 of the way full of water.
Mix the disinfectant into about ten gallons of water and pour into the tank.
The amount of disinfectant to be added will depend on the capacity of the tank.
You should add 12 pounds of HTH or Perchloron or 10 gallons of 10% sodium hypochlorite for each 20,000 gallons of water that the tank will hold.
So, if you are disinfecting a 75,000 gallon tank, you can either add 45 pounds of HTH, 45 pounds of Perchloron, or 37.5 gallons of sodium hypochlorite.
Completely fill the tank with water. This will evenly mix the disinfectant into the water.
Permit the tank to stand for 24 hours if possible, but for no less than 6 hours.
An alternative method may be used by experienced personnel:
Wash down all surfaces thoroughly.
Spray the bottom, side-walls, and top with a hypochlorite solution containing at least 200 ppm of chlorine.
The person in the tank must take special precautions. He or she must wear an effective gas mask and must be roped to an attendant on the outside of the tank.
Fill the tank and collect samples for bacteriological analysis.
Annual Maintenance Service
Annual maintenance service contracts are an alternative to hiring an inspector to periodically inspect a water tank and recommend maintenance.
Under this type of plan, a company is hired to clean the tank once a year, inspect the tank, and make all necessary repairs including painting, if necessary.
In most plants, the entire tank is thoroughly cleaned and given a coat of paint every fifth year.
Annual maintenance service contracts have become popular in municipalities with tanks and towers with capacities of less than 250,000 gallons.
The company hired will often require that their own company restores the tank before entering into a contract for annual maintenance service.
Then the company agrees to inspect and maintain the tank yearly for a set length of time, often for twenty years.
Due to municipal changes, some companies will allow a municipality the right to cancel or renew the contract on a yearly basis on the anniversary date.
Since the maintenance company will be given exclusive responsibility for the upkeep of the tank, great care should be taken in selecting such a company.
The company must conduct work with integrity, honesty, and professionalism. In addition, Comprehensive Liability and Workmen's Compensation Insurance should be carried on each contract to ensure performance of the maintenance work.
The water works should also demand a corporate bond, not an individual bond.
If an honest, reliable contractor is selected to perform the work, then annual maintenance service contracts can be a good choice for the maintenance of water storage facilities.
Water tanks can be divided into two types - ground tanks and elevated storage tanks. Each type of tank has advantages and disadvantages which should be considered when planning a new facility.
In addition, other factors such as structural stability, equipment design, tank location, elevation, and capacity should all be taken into account.
Corrosion is a serious problem in water tanks. The metal walls break down due to an electrochemical reaction.
Electrons flow from the walls of the tank (the anode) into the water (the cathode.) The third component of the corrosion battery is a closure circuit - a surface linking the anode and cathode through which electrons can flow.
A paint coating protects against corrosion by forming a physical barrier between the anode and the cathode.
The other type of protection, cathodic protection, provides a replacement anode so that the tank walls do not corrode.
Each type of corrosion protection has advantages and disadvantages when used singly, and they can also be used in combination.
Tank maintenance is essential to prevent costly tank failure. The tank should be inspected by an independent inspector at least every two or three years.
Some water works choose to hire an annual maintenance service to perform yearly inspections and maintenance.
In either case, the inspector will determine what type of maintenance should be performed.
The first step during painting, a common maintenance procedure, is to empty the tank and clean the tank surface.
Next, a primer coat should be applied and then a protective coat. Finally, the tank is cured and sterilized before being placed back into service.
Complete Assignment 10 on Water Tank Design. You may do the Assignment online to get credit or print it out and send it to the instructor.