K-Patents PR-21-S - Manual (Page 31)

4 Prism wash systems 25
4 Prism wash systems
Three alternatives of prism wash systems can be provided:
• Steam wash with integral nozzle, Section 4.2
• High pressure water with integral nozzle, Section 4.3
• Steam and water wash for flow cell mounting, Section 4.4
In most of the applications the prism wash is not necessary. However, installing a wash
nozzle should always be considered. A prism wash nozzle (Section 4.2, Section 4.3,
Section 4.4) can be useful in cases where normally no prism coating occurs, e.g. to
clean the prism after a process stop using a manual wash valve. Washing can also
be used for operational check (output signal decreases during wash) or as a part of
maintenance schedule. There is a special prism wash nozzle for Isolation and Retrac-
tor Valve HIMP-2 for LPH sensor. See Section 10.3
4.1 Prism coating
Deposit build up on the prism surface disturbs measurement. Look out for the follow-
ing indications of coating:
• Abnormally high concentration reading or upward CONC
• CCD value is getting higher and QF value drops down.
• High LED current, especially LED = 100 (max) (See Section 5.4)
• Prism wash (see Section 5.1.3) does not change optical image.
In most of the applications the prism will keep clean, but if coating occurs, check
the following:
• Sensor mounted correctly in respect to flow direction (arrow on sensor head).
• Sufficient flow velocity, Section 2.2.1
• A temperature difference between process fluid and sensor probe may cause
coating. This may happen for small flows if the thermal insulation is inade-
quate. In some cases it helps to insulate also the connection flange, Figure 2.4.
If there is a coating problem, it is recommended to try to increase the flow velocity,
e.g. by installing a pipe portion with smaller diameter. If this cannot be done,
the prism should be automatically washed at regular intervals, e.g. by steam or
hot water. A prism wash nozzle (Section 4.2) can be useful also in cases without
coating problems, e.g. to clean the prism after a process stop using a manual wash
valve.
4.2 Prism wash with integral steam nozzle
Prism wash with integral steam nozzle is for use in applications where steam can
remove coating from pipe lines. For other applications see Section 4.3 and Sec-
tion 4.4. The integral nozzle is mounted on the sensor head (Figure 4.1)
Integral steam nozzle (HPS) can be equipped for a standard length sensor or a