Water treatment of closed circuits

AKVO Ltd

Definitions

High temperature hot water Above 120 deg C

Medium temperature hot water Between 100 and 120 deg C

Low temperature hot water Below 100 deg C

Chilled water Less than ambient, for heat removal.

Water treatment selection

Consideration needs to be given to: Materials of construction

Dissimilar metals will always be prone to corrosion

See galvanic series Operating temperature and pressure Any pre-treatment of the water

Base exchange softening Demineralisation Reverse osmosis

Primary Problems Corrosion is the predominant problem

Galvanic due to the presence of mixed metal

Oxygen induced Chilled water systems are very

susceptible to biological fouling. Mal-odours

Deposition may be a problem in very high temperature systems or those with high levels of make up. A closed system should not make-up

significantly.

Galvanic series Graphite Most Noble Palladium Platinum Gold Silver Titanium Stainless steel (316) Stainless Steel (304) Silicon bronze Monel Phosphor bronze Admiralty brass Cupronickel Molybdenum Red brass Yellow brass Naval brass 464

Niobium Tungsten Tantalum Chromium Nickel Copper Cast iron Steel Lead Tin Indium Aluminum Uranium Cadmium Beryllium Zinc Magnesium Least Noble

Least Noble will corrode

0

2

4

6

8

10

12

14

16

0c 5c 10c 15c 20c 25c 30c 35c 40c 50c 60c 70c 80c 90c 100c

pp

m D

isso

lve

d o

xyg

en

Temperature/degrees celsius

Dissolved oxygen v Temperature/degrees celsius

Corrosion - Pitting by Oxygen

0

5

10

15

20

25

30

35

40

6 7 8 9 10 11 12

Ferrous

Copper

Aluminium

Free Corrosion Rates vs pH

mpy

pH

Secondary problems Blockages

Due to suspended solids settlement Corrosion debris Possibly precipitated hardness salts

Due to scabs on the walls of pipes being removed by the addition of treatment to an old fouled system

Erosion At bends due to abrasive action of

suspended solids Fouling of heat exchangers due to

biological slimes – chilled waters mainly. Poor thermal efficiency.

Treatment options – corrosion prevention For heating systems we can add oxygen

scavengers or blended corrosion inhibitors. Oxygen scavengers may be either tannin or

sodium sulphite based and would normally be added under alkaline conditions – depending upon metalurgy.

For very high temperature systems you may find hydrazine or a hydrazine donor in use.

Carbohydrazide Nalco Eliminox Accepta 2065

When copper or brass is present an azole type inhibitor would be present in any formulated material.

Benzo triazole Tolyltriazole

Treatment options – corrosion prevention For heating systems utilising aluminium

components extreme caution is required and usually an inhibitor based upon sodium molybdate. See where aluminium is in the galvanic

series Formulated inhibitors based upon

nitrites are frequently employed and may be blended with a range of ancillary components for pH control depending upon metallurgy. Silicates Borates Carbonates

Treatment options - biological fouling

Non oxidising biocides based upon slow breakdown materials are best AKVOB2 most frequently used AKVO125 is another option

Oxidising biocides are not recommended Halogen based Build up of breakdown products can

enhance corrosion

Treatment options – suspended solids

Addition of dispersants Problem is all this does is move the

solids arround the system. Continuation of blockages Erosion

Use of filtration systems Often with the use of dispersants Is a slow process Regular flushing/chnging of filters will

be required

Summary Chemical treatment is frequently

forgotten. Mixed metal systems will corrode. Design should consider minimisation of

oxygen entrainment. Mainly an issue with chilled water systems

Corrosion protection is readily available and effective if applied correctly. Consult your AKVO contact

Regular analysis is required to ensure treatment reserves are adequate at all times.