Closed Circuit Systems

Introduction

The problems encountered in closed circuit systems are basically the same as those for open systems, corrosion, scale and bacteria. Legionella is not usually a problem unless there is a possibility of sprays being formed during maintenance or process procedures (e.g. mould changes).

The obvious differences between the two types of system determine the way we treat them. The main differences between closed and open systems are: -

Closed

Fixed volume / low make-up

Low dissolved oxygen

Wide range of bulk water temperatures

No contact with process

Often multi-metal

Additives retained

Open

Continuous water loses and make-up

Water saturated with oxygen

Narrow water temperature range

Possible contact with process

Mainly steel and yellow metal

Additives have limited residence time

The low make-up to closed systems means that it is economical to dose water treatment products at higher levels than in open systems. This means that much better corrosion control should be expected in closed systems. Higher biocide levels and long contact times means that microbiological control in closed systems should be good.

At neutral and alkaline pHs the general corrosion rate is determined by the concentration of oxygen. In truly closed circuit systems the oxygen level should be low. Corrosion rates in closed systems should therefore be lower than for open systems. The corrosion rate increases with temperature.

Scaling should not be a problem unless there is substantial make-up.

Corrosion Inhibitors

Corrosion inhibitors control corrosion in one of three ways; Firstly, some corrosion inhibitors simply adsorb on to the metal surface forming a barrier between the metal and the system water; other inhibitors react with metal ions formed during corrosion to form an insoluble material that again forms a barrier between the metal and the system water; finally the third type promotes a state of passivity.

For more information on Closed Circuit Systems, please contact our offices on 0844 372 7344 or e-mail us on enquiries@bvwater.co.uk