Download
1 / 42
440 likes | 466 Views
Closed systems 101. What is a Closed System Treatment Techniques Chemical Protection The Guidance How to sample STSWH053 Issue 1. Section 1: What is a Closed system. A true closed system.
E N D
Closed systems 101 What is a Closed System Treatment Techniques Chemical Protection The Guidance How to sample STSWH053 Issue 1
A true closed system The definition of a truly closed system would be “ a system which is designed to be filled once with water and then run continuously for long periods of time without any significant amount of make-up”. All Closed Systems will contain the following apparatus: • A heating or cooling element • A re-circulating fluid • A re-circulating pump • A heat exchanger
What are the problems? • Listed below are the four main causes of problems within a closed system • Corrosion • Scale Formation • General Fouling • Microbiological Fouling • All of these problems can happen individually however if you encounter one you are more likely to suffer from a combination of the above.
What is the process of corrosion? Simply put corrosion is the “loss of metal from a component or system due to an electrochemical reaction with its environment”. This is not to be confused with erosion which is the “removal of material from a surface by means of a mechanical transport of solids”. Also very often system corrosion will lead to system erosion. For corrosion of a metal in contact with water to occur the following things are required: An Anode A CathodeAn ElectrolyteA Conducting Circuit
The Chemistry? The anode is the site at which the oxidation of the metal takes place, releasing metal ions into solution and generating electrons. The cathode is the site at which electrons are dissipated from the metal surface, usually by the reduction of dissolved oxygen in the water to hydroxide ions. The electrolyte is the water and the conducting path is provided by the underlying metal.
What can this lead to? • Reduced heat transfer due to deposits of corrosion products on heat transfer surfaces. • Reduced water flow resulting from blockages of control valves or strainers. • Excessive wear of moving parts such as pumps, shafts and chemical seals leading to failure.
The use of dissimilar metals Galvanic attack is of particular concern in closed re-circulating systems because joined dissimilar metals are frequently found in such systems. A common example would be the use of steel adjacent to copper or a copper alloy. In this case the steel being anodic to copper corrodes rapidly. Other couples to avoid are Copper-Aluminium and Steel- Aluminium. Copper-Aluminium is an especially bad combination as the Copper and Aluminium are widely separated in the galvanic series.
The consequences of scale Theoretically, under the conditions that exist in a true closed re-circulating system scale formation should not be a major problem as the common scale forming products in water (such as calcium carbonate, calcium sulphate, magnesium salts and silica) are present in small quantities and deposit on surfaces without very noticeable results. However where there is a high make-up to the system Scaling can become a problem as each time the system is topped up more scale forming products are added. This scale will then begin to block water passages and reduce heat transfer. Scale can also promote corrosion by creating an area ideal for oxidation to take place.
What are foulants and where do they come from? This is the term which describes deposits not occurring from scale formation, they are generally softer, less adherent and come from a variety of sources. Many of these fouling materials are the result of residue from the construction of new systems, contaminants introduced by make-up, leaks from process lines and inadequate corrosion control. These foulants reduce system efficiency by clogging valves, and re-circulating lines. They can also aid in system corrosion by producing conditions which promote cell attack.
The role of micro-organisms Bacteria and other micro-organisms can be very detrimental to closed systems. Firstly some species can produce a biofilm layer which prevents the Corrosion Inhibitor from making contact with the surface of the metal, this results in corrosion taking place beneath the biofilm layer.
How do micro-organisms affect the system? Secondly some micro-organisms feed on the corrosion inhibitor itself, a great example of this are nitrite reducing bacteria. These will feed on Nitrite based corrosion inhibitors producing nitrogenous compounds as waste. These need to be eradicated before the addition of more corrosion inhibitor. Another organism worth noting is Sulphate reducers as these cause very rapid pitting of metal surfaces.
How do micro-organisms create fouling? As already stated micro-organisms can play a huge role in the deterioration of a closed System. High make-up rates introduce oxygen rich water to the system along with nutrients, debris and even the organisms themselves. Process leaks can also provide rich food sources for any micro-organisms present. Where microbial growth has got out of control slime layers / films can develop and cause the system to lose efficiency, this is due to slime layers being one of the worst heat transfer media in existence.
What have you discovered? In groups discuss and write down 5 problems associated with poorly maintained Closed Systems Group Activity: 1
What can we do? Chemical Techniques None Chemical Techniques Anodic Inhibition Minimising water loss Cathodic Inhibition Selecting appropriate material of construction Film Forming Inhibition
How to minimise water loss The most important step involves reducing the loss of water from the system, this in turn reduces the amount of chemicals which need to be added to the system. This can be accomplished in the following ways… 1) Tightening up connections within a system. 2) Periodically checking all possible water take-off points. 3) Eliminating system draining or flushing.
The role of correct material selection The best non-chemical method for minimising system corrosion is by the selection of corrosion resistant metals. Ideally a single metal should be used throughout a system to avoid dissimilar metal corrosion.
How should I prepare my closed system? It is important to provide corrosion protection from the very start of a new system. If the system has been flushed with untreated water before use corrosion can begin immediately. Following cleaning it is not unusual to observe the formation of a thick film of corrosion products on metal surfaces, this is due to the reaction of the active surface of the metal with the oxygen in the air. These deposits will prevent a protective film from forming once the inhibitor has been added. It is most desirable in new and cleaned systems to remove deposits of every kind before starting up the system and minimise the time between system cleaning and the addition of corrosion inhibitors.
How to select an appropriate product? The choice of product will depend on: 1) Type of water system and its operation. 2) Water Quality. 3) Operating Temperature. 4) Materials of Construction. Success will depend upon: 1) Clean metal surfaces. 2) Maintenance of correct chemical reserve by regular monitoring and control.
Nitrite based corrosion inhibition One of the more common types of corrosion inhibitors are Nitrite based formulations. These offer excellent control of ferrous metal corrosion, although they must be checked regularly and an adequate reserve maintained. This is because low Nitrite reserves can induce pitting corrosion. Nitrite based inhibitors require specific Copper Inhibitors (Triazoles) within their formulation to promote adequate protection for Copper and alloys.
molybdate based corrosion inhibition Molybdate based corrosion inhibitors offer good protection for ferrous metals although the protection offered is slightly inferior to Nitrite based inhibitors. They tend to be more expensive than Nitrite inhibitors but are not susceptible to bacterial attack so are ideal for systems suffering from stubborn Nitrite reducing bacteria. They require specific Copper inhibitors to be included in their formulation and a good reserve must be maintained in the system.
Filming based inhibition Filming inhibitors offer effective control of ferrous metals but to a lesser degree than Nitrite or Molybdate and they are generally composed of a blend of ferrous metal and copper / copper alloy inhibitors. They are not so sensitive to a loss of reserve in the same way that Nitrite and Molybdate are.
Aluminium corrosion The prime concern in systems containing Aluminium is the protection of the Aluminium and the Copper, this can be to the expense of any ferrous metals which may also be present in the system. However the life expectancy of ferrous metals is only slightly reduced whereas if a system containing Aluminium is treated with an incorrect inhibitor the results can be disastrous and occur in a very short space of time. Aluminium heat exchangers if not treated with the correct inhibitor can fail within weeks of them coming on-line.
Aluminium protection Specific Aluminium inhibitors would include products such as Fernox Copal, Fernox CH3 and Fernox HVACF1. All of these chemicals are based on complex organic formulations and include a buffer to maintain pH control between 7.0 and 8.0. Again these products contain specific Copper / Copper alloy inhibitors but are not very effective against ferrous metal corrosion. Only these types of inhibitors can be used in systems containing Aluminium, this is particularly important when boilers contain direct fired Aluminium heat exchangers.
Where can I find guidance on closed systems? BSRIA is a test, instruments, research and consultancy organisation in construction and building services providing specialist support services for design, construction, facilities management, product testing and market intelligence. As a non-profit distributing member-based Association, they also publish best practice guides, hold an extensive library and run training and events.
Guidance for system preparation BG 29/2012 provides the industry with the latest guidelines and good practice cleaning techniques for commercial hot and cold water services systems in buildings. In particular, it aims to clarify the roles and responsibilities of the parties, improve the exchange of information between them and provide consistency between service offerings of the pre-commission cleaning contractors, includes:- • Design considerations • Inspection and witnessing • Installation considerations • System dynamic flushing • Chemical cleaning procedure • Connections between new and existing systems
Guidance for closed system MAINTENANCE BG50/2013 provides an introduction to current theory and practice of water treatment in closed building systems. It is intended for use by design engineers, installing contractors and the maintenance staff responsible for looking after the completed systems. In particular it will help facilities managers and others choose the most appropriate water treatment for their systems. The treatment of water in modern closed heating and cooling systems is essential for the avoidance of microbiological fouling (biofouling), corrosion and scale. These problems can result in energy wastage, poor system performance and the need for early replacement of plant and components. Many facilities managers have minimal understanding of how water treatment works, what it is intended to achieve and the consequences of ineffective water treatment can sometimes be disastrous.
Guidance on Sampling BS 8552:2012 addresses the particular issues of sampling water from closed-circuit heating and cooling systems in buildings and related infrastructure, from construction, pressure testing, pre-commission cleaning and commissioning to routine operation. The purpose of sampling a closed-circuit water system is to provide information about the current condition of that system and/or the water within it. That might include, but is not limited to, water treatment status, water quality, bacteriological contamination and corrosion activity. Confidence in the results obtained, which are crucially dependent on consistent sampling and analysis protocols, is extremely important to industry.
How to size a system Volume in Litres
How many samples should I take? In the context of this table “random” means locations chosen at random from those that have not previously been sampled since pre-commission cleaning. Additional samples may be taken from locations that have previously been sampled, for example to verify the effectiveness of remedial works.
Why is correct sampling important? • The method and point of sampling of a closed system can have a significant affect on the accuracy of the results obtained from the sample. This can lead to serious problems as the results will not be truly representative of the actual system conditions. We will now consider some important points when sampling from the following locations: • Drain Valves • Dosing Pots • Sample Coolers
Sampling from a drain valve The “first catch” sample from a drain valve will often contain suspended solids which have settled out in the small “deadleg” that exists above the drain valve. If this were to be submitted for analysis the results could be wildly inaccurate and lead to an incorrect impression of the systems condition. The drain valve should always be flushed with approximately one litre of water before taking the sample, this ensures the sample and results will be indicative of the systems true condition.
Sampling from a dosing pot Sampling from a dosing pot should be avoided wherever possible. However ,where it is necessary ensure that one of the following procedures is adhered to:- If the isolation valves on the dosing pot are open to the system and continuous flow is occurring flush the drain valve on the bottom of the pot with 2-3 litres of water before sampling. If the dosing pot has been isolated from the system, open the isolated valves and allow the system water to circulate through the dosing pot for 10 – 15 minutes before sampling. As before discard the first 2-3 litres before taking the sample.
Sampling from a sample cooler A sample cooler is by far the best place to obtain a sample from a closed system. It is important to ensure that the water isolation valve to the cooler is open before sampling. Next open the sampling valve to the system carefully and only allow low flow through the cooler to ensure adequate cooling. As with the dosing pots discard the first 2-3 litres of system water obtained before taking the sample. It is important to emphasize that failure to comply with any of these procedures will result in inaccurate results and incorrect system control!
