Materials of Construction

One of the most important decisions that can be made to mitigate corrosion is selecting the right materials of construction. This includes the selection of the tube material, tube sheet material, and shell/support plate materials. The process flow make up will determine what shell and internal support materials will be selected. The cooling water source, fresh or salt water, will determine the materials selection of the waterboxes. A combination of both of these factors helps to determine the materials selection of the tubes and tube sheets. Failure to specify the correct materials of construction in a project can have long lasting implications on the system performance and reliability.

Common Fresh Water Tube Materials

               

Common Salt or Brackish Water Tube Materials

Warning: Carbon steel tubes should never be used in a vacuum service. The steel tubes will rust quickly impeding condensing and hurting the system performance. Problems with performance and tube failures are often seen when using steel tubes in a vacuum service.

Anodes

In order to help mitigate corrosion on the cooling water side of a vacuum condenser, sacrificial galvanic anodes are often utilized. Anodes are often installed in the water boxes of a vacuum condenser to protect the cooling water side of the condenser from corrosion. A metal that is more active per the galvanic table than the condenser material is selected for the anodes, so the anode material corrodes instead of the condenser components. Often the anode material is zinc, magnesium, or aluminum. Each material has its own advantages and is generally selected based on the conditions of each installation. Some considerations need to be taken into account when sizing the anodes such as the cooling water type, size of the condenser, and desired service life of the anodes between replacements, with the typical service life being 3-5 years of operation for most chemical and refining processes.

Coatings

The internals of the condenser’s waterboxes can also be coated to protect them from corrosion. At times the condenser tube sheets are also coated for this same reason. These components are often lined with a coating so that they can be made out of a cheaper material. For example, a salt water feed condenser might require titanium or brass water boxes, however steel water boxes coated with a protective layer on the inside are often used to lessen the capital cost of the equipment. Common coatings include Neoprene, Teflon, coal tar epoxy, and other various resins. All of these act as a barrier to corrosion by preventing the coated surfaces from coming into contact with the cooling water. Care needs to be taken with a coated surface to verify that velocity limits are not exceeded and that the cooling water does not have any suspended solids. Both of these can create an erosion concern and potentially compromise the coating. Often these coatings need to be reapplied during turnarounds to ensure that they provide adequate protection. Whenever waterbox coatings are used it is advisable to also consider installing sacrificial anodes.

Corrosion Inhibitors

One practice that is sometimes used to help mitigate corrosion is the introduction of a corrosion inhibitor into the process side of a vacuum condenser. While the type of inhibitor used is dependent on the material being protected and the process makeup, normally the inhibitor is an amine based agent for oil refining applications. Understanding how to properly introduce the corrosion inhibitor is important because if it’s not properly introduced into the vacuum system it can cause performance issues or reduce the system’s capacity.

Introducing a Corrosion Inhibitor

The best way to introduce and distribute a corrosion inhibitor is via spray connections designed into the condenser shell. By strategically placing spray nozzles along the length of the condenser shell, the corrosion control agent can then be sprayed into the condenser in a way that evenly distributes it throughout the condenser. An acceptable but less reliable method of introducing corrosion control inhibitors is by injecting it right into the process upstream of a condenser. This is done with the hope that the process flow entrains that inhibitor and distributes it throughout the condenser. This method can be made more effective if the corrosion inhibitor is introduced into the process flow via a spray nozzle. An atomized flow will be distributed more effectively.

Warning: Do not inject a corrosion control inhibitor into the suction of an ejector. The liquid inhibitor will negatively impact the suction pressure and stability of the ejector if injected in this manner.

Corrosion Inhibitors and Vacuum

The corrosion inhibitor is often introduced into the system mixed in a water solution. When that solution is introduced to the vacuum it has the potential to flash if the mixture is too hot. If this happens it creates an additional load to the vacuum system. While the corrosion inhibitor should not be injected upstream of an ejector, if it were to be injected there the flashing liquid would have a negative impact on the ejector’s suction pressure. To determine if the corrosion inhibitor will flash off under vacuum, the operating pressure of the system at the injection location should be taken and compared to a steam table. If the mixture will flash at that pressure and temperature, it could be cooled down prior to being injected. This analysis would ideally be done during the summer months when the supply water temperatures for the corrosion inhibitor will be at their hottest.

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