Dual Containment

PTFE LINED vs DUAL Contained

In this article we shall investigate the limitations of plastic and more specifically dual containment piping. We will also demonstrate why PTFE and PFA lined piping is a much better material choice for many of the more hazardous and volatile applications where plastic piping can sometimes be found. Plastic Dual containment piping is often found at customers that have not been aware of lined piping or where the piping for handling corrosive chemistries has been provided as part of a turn-key package. It is therefore worth considering this particularly and contrasting it with lined piping. Many of the comments also apply to single wall plastic piping systems.
PTFE and PFA have excellent, almost universal chemical compatibility. It is easier to list the materials that cannot be conveyed by PTFE and PFA than those that cannot. The plastics from which dual containment piping is manufactured have a much more restricted range of chemicals they can handle. Even if the chemical specified for a particular duty is compatible, any change to the plant requirements means that the use must be checked, whereas with PTFE/PFA their universal applicability means that most plant process changes can be accommodated with no change in materials. Chemical performance for PTFE/PFA does not decline with temperature as it does with other plastics.

PTFE/PFA lined systems have a higher pressure rating of 19.6 bar g at ambient temperatures as standard. Pressure drop with temperature is small – to 15 bar g at 200°C. This compares to a schedule 40 plastic system which in PVC-U is capable of 15 bar g at 20°C, dropping to 3.3 bar g at 60°C.  Moreover this pressure rating extends across the size range 1/2in to 12in in PTFE/PFA whereas at say 8in in schedule 40 PVC-U the performance is 9 bar g at ambient reducing to 2 bar g at 60°C. PTFE/PFA is also available as an option rated to 30 bar g at ambient.
PTFE/PFA is completely unaffected by ultraviolet light or other ageing effects of most plastics. Plastic piping systems are prone to “sagging” over time (because of UV exposure and weathering), which is both unsightly and leads to pooling of fluids in sections of pipe.
Because of the metal housing of lined pipework, it is significantly more resistant to mechanical damage than pure plastic pipework.  Moreover, many fewer pipe supports are required simplifying and cheapening the installation. Similarly in buried systems one must be aware of the limited strength of plastic pipework and the potential for damage from the static ground load or the dynamic load caused by traffic etc.
A lined piping system should never leak, but if for whatever reason one occurs, it is simple to re-torque the joint if it is a joint sealing problem, or if the liner has leaked the section of pipe or fitting can be readily replaced. Piping can also be dismantled for internal inspection if required. In contrast it is often difficult to find the leak in a dual containment system and it may be necessary to remove a significant part of the piping system to find and eliminate the leak.
There are so many examples of lined piping proving the solution for corrosive chemicals, we have decided to quote two where they have specifically involved the replacement of plastic piping. 

Case Study 1: Nickel Refining Plant, South West London

This plant uses hydrochloric and nitric acid and caustic for the refining of nickel for various applications. Historically corrosive fluids have been conveyed around the site using both single wall and dual contained CPVC pipework. After a serious incident involving an escape of nitric acid, it was decided to replace the pipework that conveys the acid from building to building with HiPerFlon® (paste extruded PTFE and PFA) lined pipe.
After this initial installation and having seen the excellent structural integrity of HiPerFlon® lined carbon steel pipe the site management decided that due to their ongoing serious safety concerns that they would extend the runs of pipe into the production buildings and ultimately up to the final connections on the reactor vessels themselves.
This plant is now running successfully with HiPerFlon® lined pipes and fittings for 4 years without any problem.

Case Study 2:  Commercial Aircraft Wing Manufacturing, North Wales
The plant in question has a series of large tanks for degreasing, acid pickling and plating aircraft wings. The tanks are GRP and the associated dosing, recirculating and waste pipework is in plastic.  Following incidents involving the health and safety executive it was decided to improve the integrity and robustness of the installation with lined piping and HiPerFlon® lined Atomac PFA valves.
This has proved extremely successful and they are pleased particularly with the mechanical strength provided by the system in an engineering environment.
These two examples show how HiPerFlon® lined piping consistently outperforms dual containment piping in most critical applications.



Contact Details

Corrosion Resistant Products Ltd
Todmorden Road
OL15 9EG

Phone: +44(0)1706 756400
Fax: +44(0)1706 379567

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