KROHNE OPTIMASS Sensors Corrosion guidelines EN - Manual (Page 5)

Corrosion Guide Rev 1.05 Jan 2006 5
Remember in all cases KROHNE cannot accept
responsibility for the final choice of material. The ulti-
mate responsibility lies with the end user. We can only
advise based on our experience!
Explanation of data tables
1. Key to symbols used
Symbol Meaning
Suitable, with a corrosion rate less than
0.05 mm (0.002”) per year
X Unsuitable, due to higher corrosion rate
nd No data is currently available
1.6 Problem Applications
Hydrochloric acid (HCl):
This acid normally contains fluoride and chloride impu-
rities that will promote stress corrosion cracking in all
tube materials. This effect will always cause a meter
failure even if there is no bulk removal of tube material.
For this reason we do not recommend Optimass
meters for any HCl application.
Instead look to alternative instrument technologies such
as capacitive MID (Capaflux) or variable area flowme-
ters (H250 PTFE).
Methanol:
“Pure” methanol (> 98% purity) tends to cause the
removal of the protective oxide layer on a titanium
measuring tube thus promoting corrosion. Titanium
therefore cannot be used. Stainless steel or Hastelloy
C-22 are suitable alternatives for these applications.
Methanol with a minimum 2%, or greater, water content
does not exhibit this tendency and can be freely used
with all tube materials.
Oxygen gas:
There is a risk of ignition where an oxygen rich (> 35%
O
2
) gas mixture is used with a titanium measuring
tube.Always offer stainless steel as an
alternative.Remember also that the “degreasing” option
for wetted parts should also be offered for use with oxy-
gen gas applications.
Passivation of meters
A “passivation fluid” is normally composed of a mixture
of nitric acid (HNO
3
) and hydrofluoric acid (HF) and is
used to ‘passivate’ (clean) stainless steel pipework in
high purity systems, typically found in the pharmaceuti-
cal industry. The process removes weld discoloration,
dirt, grease, metal particles, etc.
The nitric acid has negligible corrosive effect, but
hydrofluoric acid is very aggressive even at levels less
than 0.5% for all wetted tube materials. It is recom-
mended that any meter is removed and replaced with
spool pieces during the passivation process. This is
due to the relatively thin wall of mass flow meter tubes;
the customer’s pipework being relatively thick is toler-
ant to a small loss of material.
1.7 Using Material Compatibility Tables.
Identify the chemical to be used either by name or by
the Formula.
Check that you know the concentration of the chemical
and that it is within the concentrations listed in column
3.
Identify the best material for the application and then
check that this is acceptable for the customer.
2. Where a temperature is given, this signifies suit-
ability only up to this point (e.g. 212/100 denotes
up to 212
o F or 100 o C). Where no temperature is
given, this signifies suitability up to the lower of
either the boiling point of the product, or the
maximum operating temperature of the Optimass
flowmeter concerned.
The first temperature given is always in Fahrenheit
and the second in Centigrade/Celsius.
3. Always check the relevant technical data sheet
for the maximum operating temperature of the
meter series and wetted material.
4. For the purposes of corrosion resistance, stain-
less steel grades 316L and 318L can be consid-
ered as both being the same.