EN
Of all the views in an engine room, a weeping, dripping, or spraying PHE is perhaps the most unnerving. Not only do you lose fluid, but a leak represents an unacceptable breach between two fundamental systems - usually between the engine coolant and raw water systems, or between lubricating oil and engine coolant. An undetected or poorly managed pinhole leak can quickly turn into a cross-contamination event, generating emulsified lube oil, corroded engine block components, or even a complete failure of the jacket water system. Over the last 22 years SME Group has completed over 500 marine service and retrofit projects each year, and our technical experts have traced hundreds of PHE leaks in all types of vessels to roughly half a dozen common root causes. The article below discusses the most frequent causes and gives you tips on how to minimize the chances of your PHE leaking.
Gasket Degradation: Aging, Hardening, and Extrusion
Gaskets are the primary sealing elements of the PHE, and they are the weak link. As they age and endure high temperatures, corrosive chlorinated seawater, and cyclic compressions, the elastomer material loses its ability to recover from compression set. A nitrile (NBR) gasket used on oil coolers has been found to become brittle when exposed continuously to above 90°C and cannot conform to surface imperfections. An EPDM gasket is suitable for the seawater side but can swell or soften (leading to extrusion through the gaps between plates) by contamination by oil mist or grease. Extrusion is a particularly serious problem as the deformation is a thin lip of material which permanently deforms the gasket profile-once extruded an EPDM gasket will never seal again, even when re-tightened. At SME， we often find our service engineers removing gaskets that are up to five years old which have suffered more than 30% loss of compression set recovery (regardless of outward appearance). We favor a pro-active replacement schedule correlated to the total running hours and previous temperature exposure of the gasket rather than a reactive replacement schedule when the PHE is dripping and the performance seriously degraded. the installation date and thermal cycles (start-stop sequences) it has experienced will far better assess the useful life than a calendar reference alone.
Incorrect Assembly Torque and Uneven Compression
A PHE is precision mechanical equipment not simply piping requiring torque to the bitter end. One of the most common reasons we discover for a service leakage in an SME build relates to how tightly the tie bolts on reassembly following cleaning or a strip down for Inspection have been done. Each frame has a definitive compression dimension - often specified as overall gasket and plates overall thickness which needs to be achieved evenly across the plates applying torque diagonally. Over-tightness will result in gasket crushing and compression, damage to the incredibly thin corrugated (often only 0.5 to 0.6mm thick) plates and stressing of the bar across the bearings of the frame. Under-tightness insufficient gasket pressure, allowing in-service by pass across the individual channel and leaking out to the sides, far more critically a one-sided tightness will cause skewing of the plates which will induce micro gaps only expanded further when thermal expansion kicks in. When we recommend torque, we recommend calibrated use with a cross/star pattern on all assembly passes taking minimum of three increments up to the specified compression length and measuring the gasket stack length before and after the application of torque (if this value differs by more than 2mm from the nameplate specification we recommend stop and re- assess plate stack for contamination between plates or bent plates at point of contamination.)
Pitting Corrosion on the Plate Surface
Although modern marine PHEs (plate heat exchangers) made of titanium or 316L stainless steel are not fully resistant to localized corrosion. The pitting, characterized by tiny, deep holes in plate surfaces, tends to initiate at the gasket groove or the stagnation areas formed by the flow distribution zone. Aggressive concentrations of chlorides (prevalent in sea water of tropical climate or brackish water) and the low fluid velocity are the two main causes of pitting, since a settled deposit can form a concentration cell beneath itself when the flow is slow enough to allow sediments to deposit. Once pitting has drilled through the entire plate thickness the distinction between two fluid streams is eliminated and leaking will occur internally (mixing) as well as externally. We at SME found that pitting is usually diagnosed as a gasket failure leading to unnecessary routine gasket replacements without addressing the root cause. An early detection for pitting consists in carrying out dye penetrant test on plates when manually overhauling, particularly on the heat transfer surface around the inlet ports. Additionally, never let the velocity of seawater flow through PHE fall below 1.5 m/s to prevent settlement. If your vessel spends most of the operation at low load take into account a bypass or recirculation line to maintain the flow velocity above this threshold.
Plate Misalignment and Gasket Dislodgment During Reassembly
An even greater issue, however - and certainly the easier one to avoid - is leakage resulting from poor reassembly of a unit. Plates have a "right way round" orientation (with gaskets in the forward/reverse faces alternately) and they must be added to a stack in sequence. If a plate is either upside down or, far more commonly, added backward (with the gasket on the wrong face), the fluid channels on adjacent plates will not be aligned, and a gasket cannot make a true, metal-to-metal interface with the neighboring plate. In many cases, this not only results in immediate internal bypass, but if the adjacent plate comes sufficiently close, will produce leakage from the opposite-most gasket corner, to the outside air. Lastly, it is all-too-easy for the gasket, during the process of adding subsequent plates to a stack, to simply lift out and "unseat" its groove (especially if the latter still has old adhesive or debris within it). As little as 2-3 mm of upward displacement is enough to render the gasket ineffective, leading to both internal bypass and potential external leakage. This is why every plate in SME's workshop procedures is meticulously cleaned, the gasket groove inspected under magnification, and each gasket is lightly tapped into place (with a soft-faced mallet) prior to the stacking of the neighboring plate. A permanent arrow denoting the stack top and bottom allows our engineering team to visually inspect for "right way round" stacks.
Conclusion: Vigilance is the Best Sealant
It’s not a question of if or when will my plate heat exchanger leak, rather it’s about a when you fully control. Through the diligent application of a planned approach to manage aging gasket wear, applying the correct torquing procedures, vigilant inspection of material fatigue and evidence of mechanical stresses, and finally painstaking and careful reassembly can add decades to your PHE's operational life and help you sidestep costly contamination and costly emergency call outs. At the SME Group, with more than 100 engineers at their disposal and operating under an ISO9001, ISO14001, ISO45001 standards framework, we can service your vessels on board with comprehensive inspections, spare part provision and complete workshop overhauls in shipyards around the globe – not least in mainland China. A 12-month service guarantee is indicative of our standards and when dealing with our aim beyond your concern' philosophy we are satisfied only once every gasket is seen, every wear line recorded and every potential leak path identified.