Diagnosing and Correcting Unusual Noises in PTFE Heat Exchangers

Last week the PTFE heat exchanger began rattling. It was faint at first but now you can hear it all across the place. Something is incorrect and ignoring it can result in the failure of the tube bundle.
Operator ears are a diagnostic tool – know what normal sounds like. The tubes in PTFE heat exchangers are purposefully thin and flexible to accommodate thermal expansion but this flexibility also makes the bundle very susceptible to vibration. If the flow velocity exceeds the design limit, usually greater than 2 m/s in the tubes, set by the manufacturer, the shedding of vortices generates periodic forces which excite the tubes at their natural frequency. The tubes then whip against baffles or other tubes creating the characteristic rattling or humming. Repeated micro-impacts over time induce fretting wear that eats away the PTFE surface and creates pinhole leaks. Water hammer is a sharp pressure wave caused by rapid valve closures or pump trips . It may cause a single loud bang or repeated hammering . Loose internal pieces – baffles that have come loose, bolts that have loosened, or supports that have shifted – cause erratic clanging or banging not related to flow. Any new noise should be attended to immediately as PTFE tubing cannot stand long term vibration.
Start diagnosis by monitoring flow velocity on both tube side and shell side. Compare the flow rate and the fluid density with the original design data sheet. If the velocity is only a little above the limit, the noise is virtually certain to be flow-induced vibration. Then listen carefully to the rhythm while the machine is running. If you hear a steady, continuous hum, or a clear tonal frequency, that is vortex shedding and tube resonance. Loose parts will frequently cause irregular, intermittent rattling or metallic clanging. Water hammer is traditionally indicated by a sudden, sharp bang or succession of bangs consistent with valve operation or pump start/stop.
Flow is reduced by throttling a downstream valve or by reducing pump speed for a short test period. If the noise stops or subsides promptly then the root cause is flow velocity. If the noise is consistent then it's a mechanical looseness not fluid dynamics. With the unit still isolated, check all external supports, hold-down bolts and nozzle connections for looseness. Check the shell for any observable movement or vibration transferred to the foundation. For water hammer, monitor how long it takes large valves to close. Anything faster than five seconds on lines greater than 150 mm can create devastating pressure waves.
Corrective steps are determined from the verified cause. The easiest way to correct flow-induced vibration is to permanently slow the velocity by inserting an orifice plate or flow restrictor on the afflicted side or by changing the pump speed with a variable-frequency drive. If reduced flow is not permitted by the process requirements, the bundle may require additional tube supports or redesigned baffles with tighter clearances to decrease vibration. In extreme circumstances the whole tube bundle should be replaced with one designed expressly with anti-vibration elements such as extended metal strips or additional mid-span supports.
To prevent water hammer connect a surge suppressor (bladder type accumulator) adjacent to the exchanger inlet or retrofit slow closing actuators to valves. Changing the valve closing time from seconds to ten seconds or more will usually stop the banging altogether. Loose internal parts necessitate controlled shutdown. Retorque all available bolts to the manufacturer's specification, replace any damaged baffles, and ensure tube supports are properly placed. Reassemble and rerun the flow-reduction test to verify that the noise has been reduced.
If the noise remains after these actions or if it reappears within days, it is likely that the bundle has suffered irreversible fretting damage. At this stage the exchanger should be taken out of service and the tube bundle sent for professional inspection and potential retubing. Rapid escalation to leaks and process pollution is still threatened by operation.
Water hammer is a pressure wave and a surge suppressor is its shock absorber. Every new rattle or hum is an emergency diagnostic warning, not plant background noise. Operators catch problems while they are still cheap to rectify. Early detection of flow caused vibration, water hammer or loose parts minimises fretting wear, tube collapse and costly bundle replacement. An effective and lasting exchanger is a silent one. PTFE heat exchangers are designed for years of safe and reliable operation with consistent attention to sound, velocity and support integrity.