Tubesheet Cladding

Discussion

Tubesheet corrosion is generally caused by dissimilarities between higher alloy, more noble tube material, and the tubesheet metal.  This situation is becoming more and more prevalent as high-stainless steel and titanium tubes are being retrofitted into older condensers and heat exchangers.  Loss of tubesheet metal due to corrosion will, in time, lead to the compromise of tube-to-tubesheet joint integrity.

Generally the alternatives for tubesheet protection are cathodic protection (CP) or protective coatings.

 Cathodic protection of complex surface areas such as tubesheets with thousands of tube openings is difficult to achieve. Also, a CP system may embrittle titanium and ferritic stainless steel tube-ends if it exceeds – 750 mV. Additionally, CP systems may receive little attention from operation and maintenance personnel and may cease to function effectively over time. During outages with the waterboxes drained, galvanic corrosion can continue without the benefit of CP protection.

 While the passive protection of a coating system solves these problems, thin film coatings of 25 to 80 mils have failed due to improper application, and because a thin film thickness is insufficient to withstand the long-term abuse of cooling water systems such as impact during cleaning and inspection.  Counter productively, a partially failed coating may actually accelerate galvanic attack. (see photo below)

The Plastocor® Process entails the precise molded application of an ultra durable protective Cladding.  The tubesheet and tubes are completely encapsulated and locked in by a thick, 100% solids epoxy shield of 180 +/- 40  mils.  This polymeric shield is in addition to the thickness of material required to rebuild lost tubesheet metal.

The Process fashions a radially flared, high gloss, inlet and outlet tube opening, serving to dramatically reduce flow resistance and fouling.  For on-line tube cleaning systems, sponge ball wear is reduced.

The substantial nature of the System adds strength and integrity to the tube-to-tubesheet joint.  Tube push-out strength enhancement of 2000 lbs is routinely achieved.

Typically, the tube-to-tubesheet joint is a press-fit whereby the tube is expanded (by rollers) into the tubesheet.   Over time it is possible for this press-fit to be compromised.  Black-trips, water hammer, or partial blockage of a tubesheet are the most common sources of stress to the tube-to-tubesheet joint.  These stresses can cause the joint to “break” which allows the tube to slip in the tubesheet.  Small “joint leaks” usually result.  These leaks can be very difficult to locate as they often vary in intensity as condenser is subjected to transient forces during startup and other situations.   By epoxy cladding the tubesheets, all of the tube joints are permanently sealed and 2000 pounds of joint strength is added.

The Plastocor® Cladding withstands physical abuse, impact, and other stresses associated with the operation of a cooling water system.  The System is designed to endure over the very long-term.  Case histories date flawless performance back to 1963.

The Plastocor® System is engineered as an extremely long-term solution to tubesheet corrosion problems.