How to Properly Mount and Connect a PTFE Heat Exchanger Without Damaging It

The PTFE heat exchanger is screwed down tight and the pipe is pushed into place with physical effort. The PTFE liner will crack at the nozzle within a few weeks. Why?' Thermal expansion stresses, pipe misalignments. You need some room to move for proper installation.
Polytetrafluoroethylene, or PTFE, provides unparalleled corrosion resistance in severe chemical service, but its mechanical behaviour must be considered. The linear thermal expansion coefficient of the material is about ten times that of carbon steel, or about 100 to 120 × 10−6/°C versus 10 to 12 × 10−6/°C for steel. The PTFE lining expands much more than the steel shell during process temperature transition from ambient to working conditions. If the nozzles are securely attached to the fixed exchanger, differential growth produces shear and tensile stresses which quickly surpass the elongation limit of the lining. This causes radial cracking of the nozzle neck, flange face distortion, or micro-tears which propagate under pressure cycles. A leak-free installation is a stress-free installation. The installer's goal, then, is to place the unit so that it can breathe axially and laterally, yet still retain exact nozzle alignment with the process piping.
Start with the foundation. Position the heat exchanger on supports for sliding rather than fixed restraint. Standard practice is to use PTFE sliding plates or low friction bearing pads between the exchanger feet and the steel support beams. This allows the shell to grow and shrink without fighting friction. Before lowering the exchanger, ensure that the support structure is exactly level using a precision machinist level or laser alignment device; deviation higher than 1 mm per metre will cause uneven loads. All support points equally distributed weight, never allow point loads under the shell or channel. Minor abnormalities in the foundation are taken up by shims or adjustable jack screws under the feet. Do not weld or bolt the shell rigidly to the framework once the exchanger is resting on its movable supports. Instead, put in axial guide rails or stop blocks that prevent sideways movement but allow unfettered movement along the vessel axis. PTFE expands when hot, make room to manoeuvre.
Once the level is set with the exchanger in place, the nozzle is aligned. Never force process piping to match exchanger nozzles. First of all, set the piping in a natural way. Place spool pieces on temporary adjustable pipe supports or with chain falls such that flange faces are parallel and concentric within 1.5 mm and 0.5 mm per 25 mm of bolt diameter respectively-industry regulations for lined equipment. Measure gaps with feeler gauges in several places around the circumference. Correct misalignment by shimming the piping supports or exchanger feet; do not hammer or winch the pipe. A misalignment of even 3 mm can cause bending moments that concentrate stress directly on the thin PTFE lining at the nozzle throat where the liner is glued or mechanically locked to the steel.
Install the right gaskets and align. The lining itself is matched for chemical resistance and creeping behaviour by the use of PTFE envelope gaskets or full face PTFE gaskets. Do not use rubber, compressed asbestos or spiral wrapped gaskets with metallic windings. They compress unevenly and can embed in the softer PTFE face, causing leak routes or localised crushing. Center the gasket on the flange, so the envelope lip does not fold or tear. If the manufacturer allows, put a thin film of appropriate lubricant (PTFE-based paste) over the faces of the gasket to minimise the installation torque without affecting the integrity of the seal.
Bolting is a slow, steady march. Tighten flange bolts in a star or crisscross pattern, starting from the center and working out in incremental passes--30%, 60% and 100% of the prescribed torque. Use a calibrated torque wrench. Typical values for PTFE lined flanges are from 40 to 80 Nm depending on size and pressure class. Always follow the equipment manufacturer data sheet. If the PTFE is over-tightened it will be compressed beyond elastic limit and cold flow or crack at the flange edge. Wait 15 minutes after final pass and recheck torque. PTFE gaskets settle in as they seat. Do not use impact wrenches or air tools on coated flanges.
Heavy process piping shall be independently supported on its own pipe racks or spring hangers, so that its dead weight is not hung from the exchanger nozzles. The nozzle is nothing more than an extension of the thin PTFE lining, and is not a structural anchor. Provide expansion loops or bellows-type expansion joints or flexible metal hoses on both the intake and exit lines. These elements absorb the differential growth (often 3-6 mm/m of pipe run) without delivering any axial thrust back to the exchanger. Keep flexible elements close to the nozzle but on the downstream side of the first isolation valve, so they are accessible for maintenance. A good rule of thumb is to leave a 5-10 mm gap between the exchanger shell and any fixed structural steel or concrete wall. This space allows for full thermal growth without binding.
Make all connections and check alignment in a cold condition prior to introducing process fluids. Scribe or otherwise mark reference locations on nozzles and on surrounding pipes. Measure the gap and parallelism with the system at ambient temperature. Then, re-measure when bolting is complete and supports are fastened. Any change larger than 1 mm shows residual strain. Loosen the flanges, re-set the supports and repeat until zero strain is indicated. The last check is the installer's insurance policy against hidden stress.
Proper mechanical installation and piping alignment prevents stress cracks and leaks." Thermal expansion is a consideration for PTFE heat exchanger life. Only in the case where the PTFE liner does best-in compression-will it do so when the exchanger is set on sliding supports, nozzles are aligned without force, gaskets are chosen and torqued properly, and pipework is allowed to expand. The outcome is years of leak-free service, not premature breakdown. The extra hour spent on careful levelling, sliding supports and flexible connections pays off in uptime and safety far beyond the initial effort in the end.