About the authorThe Value of Coatings?
Much of the material used for ammonia refrigeration systems is carbon steel. Though various grades of this material are used for pressure vessel fabrication or piping there is one common problem associated with this.
Corrosion prevention or mitigation seems to be an on-going battle. One way to combat this is to use a sufficient coating to protect the base material. This is similar in concept to the use of hot-dipped galvanizing for air units and evaporative condensers. The galvanizing protects the base material from corrosion, thus extending the life expectancy of the equipment.
Like galvanizing, other coatings applied to carbon steel prevent the corrosion from occurring, if properly applied. For piping and pressure vessels it is important the correct surface preparation is performed. If the final coat of protection does not adhere moisture can find its way to the base material.
If oxidation is allowed to form, the continued growth of corrosion sites can decrease the material thickness. If this is allowed to continue, eventually the depth of corrosion can be greater than the minimum required thickness for pressure containment. At this time, replacement of the vessel or piping should be considered.
This leaves us with several points to consider; should we specify a corrosion allowance “thickness” to provide additional safety margin, or should this be considered in addition to protective coatings for the vessels and piping before insulation systems are applied?
What types of suitable coatings are available and with these coatings what type of surface preparation is required? Some requirements for surface preparation can extend to white-metal blasting with zinc primers and epoxy paints. Others may be a simple wire-brushing to remove surface rust and debris and then covered with a coal tar epoxy compound.
There are many different factors to consider; cost, application and drying time, dry film thickness, material compatibility, and coating repair procedures to name a few. At this point you might ask, why are we worried about all of this?
What happens if the vapor barrier is punctured and allows moisture into the insulation system? Oxidation can begin to develop on the metal. Is the first cost more important than the on-going cost of maintenance or unplanned downtime?
In a worse case scenario, what is the cost of an accidental release that resulted from corrosion?
Tags: cold storage warehouse, food processing, industrial refrigeration, operating issues
