Purging Of Optical Process Interface - Siemens LDS 6 Operating Instructions Manual

Appendix
A.2 Purging of optical process interface
A.2

Purging of optical process interface

This appendix gives information on purging of optical process interfaces and gas flow
calculations for purging. There are four different types of optical process interface purging;
instrument air purging, elevated flow instrument air purging, purging with air blower and steam
purging.
Instrument air purging
This is the standard solution to keep the wedge windows free from contamination. It requires an
inlet flange with sintered filter for instrument air or gas, to create an air-flow in front of the
wedge windows and into the process, see the figure below.
The instrument air should have a pressure of 2-6 bar and be oil free. The optics housing is
supplied with a inlet connector for semi-rigid tube with an inner diameter of 4 mm (0.16") and
an outer diameter of 6 mm (0.24").
As a rule of thumb, a pressure of 2 bar generates an air flow of 40 l/min, 4 bar generates 80 l/min
and 6 bar generates 120 l/min (when the needle valve is fully opened).
Note
The dew point of the instrument air must be at least 10K below the lowest ambient
temperature.
Figure A-7
Elevated flow instrument air purging
The elevated flow instrument air purging is a type of instrument air purging but without the
needle valve (the needle valve limits the flow to maximum 120 l/min). This purging type allows
the flow to be up to 500 l/min, at an upstream pressure of 6 bar. Optics housings for elevated
flow instrument purging from Siemens are equipped with fan adapter, see the figure below.
50
Instrument air purging
LDS 6, Optics Housing CD 6
Operating Instructions, 01/2022, A5E01134377-05