Gasketed plate heat exchangers (PHEs) are used in some of the most common heat recovery and process cooling operations in all types of industries, including food, dairy, chemical and power generation. Like most types of heat transfer products, the operational service life of each PHE and its components will vary depending on the application, as well as on the operating temperature, pressure and corrosive nature of the process fluids.
Plate heat exchangers consist of a series of thin, individual corrugated metal plates fitted with gaskets and compressed in a frame assembly. The frame assembly comprises an upper carrying bar, lower guide bar, and stationary and moveable covers. The gasketed plates are compressed between the thick metal covers by tightening a series of threaded rods (figure 1).
With the recent volatility in energy prices, maintaining heat exchangers installed in critical applications has become more important than ever. But maintenance costs also must be kept to a minimum. Following the correct troubleshooting and maintenance procedures can help plants achieve both of these goals while also extending the service life of the unit.
Determining the Source of the ProblemEffective troubleshooting requires information from temperature and pressure gauges. Prior to opening the exchanger, it is crucial to determine whether the problem stems from decreased thermal performance, increased pressure drop, or a combination of these two factors. A gradual decline in thermal performance usually means that fouling deposits are adhering to the heat transfer surface areas - a situation commonly seen in process applications where the heat exchanger is used to cool products with a cooling tower or to heat products with steam. In these cases, the use of cleaning-in-place (CIP) chemicals often can be considered in lieu of opening the exchanger for manual cleaning. However, this method is only effective if it is used before the exchanger has been substantially fouled with deposits. An increase of more than 40 to 50 percent in the original design pressure drop indicates significant blockage in the flow channels, and CIP chemicals will not be able to make contact with enough of the deposits to achieve adequate cleaning.
For applications in which CIP is deemed suitable, sodium hydroxide (caustic) or alkaline cleaners normally are used to remove organic materials such as biological or hydrocarbon-based deposits. Sulfamic, citric or phosphoric acids can be used to remove the calcium carbonate scale that is typical in cooling tower applications. Hydrochloric acid (HCL) commonly is used in many industrial applications to remove calcium carbon scale from piping and heat exchangers; however, it is not recommended for use on 316 or 304 stainless steel material because it can cause chloride corrosion or surface pitting (figure 2). If HCL is used on plate heat exchangers, the surface of the stainless steel material will become etched and will appear to have dull finish (figure 3). However, etching will not affect the performance of the heat exchanger as long as the corrosion has not caused pin-hole leaks that allow contamination between the two fluids.
A significant increase in pressure drop can indicate a particulate obstruction rather than fouling. PHEs are available in a variety of plate pressing patterns and pressing depths. If the proper plate heat exchanger is selected, the process fluid should pass through the flow channels without being trapped in the port area or inside the flow channels. However, if the process fluid contains solid particulate that is too large to pass through the flow channels, the particulates will collect in the port area of the plate. Metal particulates that enter the port connection and do not pass through the flow channels can damage the blank end plate in the exchanger (figure 4). These particulates can be removed from the exchanger by backflushing, i.e., reversing the flow direction of the process fluid. If backflushing the exchanger is not an option, the installation of port filters or Y-strainers in the piping system should be considered.
Preparing to Service the UnitIf the PHE must be opened for manual cleaning or replacement of the elastomer gaskets, it is important that the maintenance technician follow the instructions in the operation and maintenance manual supplied by the original equipment manufacturer. Any external leaks should be identified on the outside of the plate pack prior to taking the unit out of service. Internal leaks that are causing cross-contamination between the two fluids can be identified after the PHE has been locked out and piping removed from the exchanger. First, the inlet isolation valves should be closed slowly, followed by the outlet valve of the exchanger. It is recommended that lockout tags be applied to all associated piping valves to prevent an accidental opening of process valves while the PHE is being serviced.
After the valves have been closed properly and the unit is out of service, the tightening bolts should be inspected for rust and corrosion. Any debris on the threads of the tightening bolts should be removed with a wire brush and a small coat of molybdenum (a high temperature, high pressure grease) should be applied to the bolts. Other anti-seize type of lubricants on the market tend to dry or harden on the bolts over time, causing galling between the running nut and tightening bolt.
Before opening the PHE unit, the maintenance technician should count the number of plates and measure the tightening dimension between the end covers. This information should be documented with a permanent marker on the frame to ensure that the proper tightening dimension is used when the unit is later closed. If possible, this measured dimension should be compared to the original certified drawing to determine whether anything has changed on the unit since it was shipped from the factory. The upper carrying bar should be wiped down with a lint-free cloth to remove any debris, and a light coat of lubricant should be applied to the upper carrying bar to ease the movement of the plates along the bar.
If the PHE shows signs of cross-contamination between the two operating fluids, the piping should be removed from the bottom port connection to enable a visual inspection inside the port connection. Any residual liquid in the bottom port connection should be vacuumed out of the exchanger so that any new liquid will be visible. The other side of the PHE should be pressurized to 75 to 100 psig. If the pressure gauge shows any decrease in pressure, the maintenance technician should use a flashlight to inspect the open port connection. Any internal leakage should appear as a rising level of liquid between the channel plates in the bottom of the port area and possibly lapping over from one channel to another. If the plates have experienced a chloride stress crack (typical with stainless steel plate material), it might take as long as 30 minutes for the liquid to flow to the bottom of the plate and gather in the lower port area.
If an internal leak is verified, the technician should count the number of plates from the inside of the stationary cover to identify the defective plate. If the leak is farther back in the plate pack, the leaking area can be identified with a tape measure. The measurement should be taken from the inside edge of the stationary cover and to the outside of the unit. The leak area then can be identified with a marker.
One other way to identify a leak is through a hydrostatic test, also called a hydro-test. In this test, the heat exchanger is filled with water and then pressurized to design pressure listed on the nameplate for about 15 to 20 minutes with the assistance of a positive-displacement pump. If the unit shows some signs of slow external leaks, placing cardboard underneath the unit during the hydro-test can make it easier to identify the approximate location of the leak. Any external leaks should be identified with a marker prior to opening the unit.
Following one of these simple procedures can allow maintenance technicians to focus their inspection time on repairing or replacing the defective plates instead of on visually inspecting a lot of plates that show no signs of leakage.
Performing an Internal InspectionMost plate manufacturers specify a proper bolt-loosening sequence in their operation and maintenance manuals. They normally call for the removal of all the bolts from the frames except for the four main tightening bolts, which are located on the corners near the inlet/outlet connections. Any piping attached to the moveable cover should be removed when the PHE is opened. The tightening bolts should be loosened in a diagonal pattern across the stationary cover. The moveable cover should be kept parallel to the stationary cover and should not exceed 0.375" out of square in the vertical dimension and 0.25" out of square in the horizontal dimension to prevent the cover from damaging the carrying and guide bars, which can be critical to proper plate alignment during the re-assembly process. Bearing boxes on the main tightening bolts on some of the 6"-port-size and larger PHEs reduce the friction load on the tightening bolts by more than 50 percent and can simplify the bolt removal process.
After the bolts have been removed from the heat exchanger, the moveable cover should be pushed against the rear support column, and individual plates should be removed to access the damaged plate. During the plate-removal process, each plate should be numbered with a permanent marker at the top hanger area to ensure that they will be reinstalled in proper order.
As the plates are moved down the upper carrying bar, the gaskets and plates should be visually inspected for any signs of swelling or deterioration. If substantial deterioration is evident, it may be wise to use a different elastomer material in that application. The lower ring and diagonal portion of the gasket grooves also should be inspected for any signs of deformation.
PHE gasket grooves are designed to provide a certain amount of compression on the gaskets to maintain the design pressure rating on the plates. If the PHE is tightened below these numbers, the lower ring area and diagonal portion of the gasket grooves can be damaged. Gasket grooves that have been deformed due to over-tightening or severe gasket swelling will have a reduced ability to compress the gaskets (figure 5). In these cases, the gasket groove must be repaired or the plates must be replaced before the PHE is returned to service to optimize the PHE’s service life.
Identifying the Cause of Plate FailurePlates that were identified during the initial inspection or hydro-test should be inspected for cracks or corrosion that could have caused interleakage between the two fluids. Once the source of the cross-contamination has been identified, it is crucial to determine the cause of the failure of the plate material. Most failures can be attributed to corrosion, erosion or fatigue.
If the plates are constructed of stainless steel and show signs of corrosion, the most common cause is chloride corrosion or stress cracking. Chloride becomes more aggressive as fouling deposits such as calcium carbonate and silica scale accumulate on the heat transfer surface area. If the corrosion is determined to be caused by chlorides, the source of the chlorides must be identified and eliminated. Alternatively, the plates can be upgraded to an alloy such as Avesta 254 SMO or titanium, which are more resistant to chloride attack.
Erosion can be identified by a shiny finish on the inlet port and neck area of the plate. If the plates show signs of erosion at contact points in the port or neck area, the cause often can be linked to high fluid velocity or process fluids that contain high concentrations of abrasive particles in contact with the heat transfer surface area. The port area has the highest velocity, and the fluid velocity decreases as it transitions from the port neck area of the plate through the distribution zone and into the heat transfer surface area. Reducing the fluid velocity through the channel by adding more plates (more flow channels) or by changing the plate pattern to a deeper pressing depth design can minimize the effects of erosion on plate material.
Process applications that have frequent process upsets or pressure fluctuations can cause fatigue cracks in the plates. Pressure fluctuations will cause some movement or flexing of the plates and, over time, fatigue cracks can occur in the center or distribution area of the plate. To minimize plate movement within the PHE, the exchanger should be tightened to the proper dimension. If the exchanger is only tightened to the limits of the tools being used and not to the proper dimension, the plates will not have the required metal-to-metal contact that prevents them from flexing with each pressure fluctuation in the system. Conversely, the exchanger should not be over-tightened. It is common industry practice to tighten bolts, nuts and valves to certain design torque specifications in an effort to prevent external leaks. However, this technique should not be applied to plate heat exchangers. Each plate manufacturer has published limits on the tightening dimension between the covers, and these limits should be followed to avoid damaging the gasket grooves (figure 6).