PVDF Hot conc sulfuric acid 3

[scorcher]

Hi,
I am trying to source information on the suitability of PVDF (piping) with hot (up to 100C) 90-98.5 sulphuric acid. Information I have found so far gives the full range of "suitable" to "not suitable". I know that the temperature itself is OK, but am looking for clarity on the sulphuric acid side at these temperatures. Some literature suggests it can go brittle with time. Practicle experience appreciated.
Thanks

 

[slojim]

are you sure the temp is ok if you don't know if the acid % is ok? I am not familiar with pvdf, but metal piping corrosion rates can very widely with temperature and concentration in the range you specify.

 

[moltenmetal]

The best reference I have is showing PVDF's suitability declines with increasing temperature and increasing % sulphuric acid (above 90%). 100 C is shown as poor to unsuitable at 100 C with conc acid.

As to "experience", it depends on what you're doing with it. Is this tubing, pipe, a tank, an exchanger, or something else? Is the 100 C temperature and conc acid exposure continuous or intermittent? And does anyone get injured or killed if the PVDF does embrittle over time?

PFA would be a better choice from an absolute compatibility standpoint. Or you could use the PVDF as a lining material, using something else (steel, FRP etc.) so that you don't worry so much about the embrittlement. But the correct solution depends on answers to the questions listed above.

 

[hacksaw]

pvdf is not suitable in an operating environment

 

[aspearin1]

Hacksaw,
Can you ellaborate on your statement?

aspearin1

 

[scorcher]

Moltenmetal,
To elaborate, the equipment is an instrument tee (sound velocity/acid conc). Exposure to acid is constant, conc constant to +/- 1%, operating temp in range 50 to 80C. Instrument is installed in a 1" branch off a process tank circulation line, pressure up to 4 barg, so potentially significant consequences on failure. The fitting supplied as a package with the instrument a few years ago.

We have previously seen possible "embrittlement" ie failure at one location (we have a number of these on site), however failure at a weld was put down to mechanical stress due to piping allingment or vibration at the time.

What reference are you using?

Thanks

 

[kenvlach]

scorcher,
See “Handling Sulfuric Acid” at

http://www.resistoflex.com/sulfuric.htm

Their chemical resistance table shows a maximum temperature of 65^oC for PVDF in 98% sulfuric acid. PTFE is recommended.

 

[BRT549]

I used PVDF (kynar) on a 93-98% sulfuric acid system in 1994. Worked very well, but temperature was ambient. It was red Sloan pipe, all kynar (not lined), socket fusion joining method. It was far better than the carbon steel system it replaced. It outlasted the plant, running until 2001 with absolutely no problems. Your application's problem is the temperature. My Chemtrol resistance guide lists PVDF good for 93 C up to 93%, then 94% and above only good to 65 C. Another chart by IPEX lists about the same conditions, basically that resistance falls off around 96%.

At high temperatures, PVDF pipe would need more support (unless it's lined pipe). Don't have any comment on the embrittlement issue - never observed it in my system, which never ran over 35 C. Try kicking it around with the Lurgi or Monsanto Enviro-Chem people - they have the most experience with hot sulfuric acid.

 

[moltenmetal]

The reference I was using was Chemline Plastics (

http://www.chemline.com),

which provides the best information I've found on plastic materials compatibility.

I personally would not use unreinforced PVDF for the application you're describing. With pressures potentially to 4 barg, and a hazardous fluid like conc sulphuric acid, PVDF at 50-80 C is not safe. The application would require extensive mechanical safeguarding at minimum, with total secondary containment being recommended. I would recommend a complete tear-out and replacement with either PTFE or PFA-lined pipe, or PVDF-lined pipe.

 

[mgp]

This site also gives chemical resistance of PVDF and other plastics.

http://www.piping.georgfischer.com/index.cfm?424D3C4A152F4697ABE3201FA7EC26D1

Regards
Mogens

 

[scorcher]

All,
Thanks for all the help. We have trialed modified tee (PVDF wiht steel support). Early indications seem encouraging. We will be upgrading the other strength meters following a longer successful trial.
Regards

 

[mjeglic]

The temperature stated is at the edge of the range for PVDF. PTFE lined steel would provide a greater margin of safety. These a readily available stock items at P&F distributors such as McJunkin