Pragmatic approach to better water quality

While the world is constantly changing, and many main plant items are becoming ever more technologically advanced, what do we (typically) do to the system water? The answer is that we often treat it just like we used to do when there were large water capacity sectional or packaged boilers.

All the aforementioned problems have one thing in common – they are the result of one simple inclusion in the system – air. Air, or more specifically the oxygen content in the air, corrodes the steel surfaces in heating and cooling systems almost instantaneously, creating the renowned “black sludge” or, to give it its more correct name, magnetite. This magnetite collects in still areas (i.e. the bottom headers of radiators), wears out pump seals, can block up heat exchangers, and fouls AAV valve seats. Additionally, any entrained air affects the pump’s ability to correctly circulate water, and the more air in the system, the more power is needed to drive the pump.

Control by deaeration

Air in systems
Air will be present in piped water systems as a result of incomplete purging after the system is filled, and due to the release of dissolved air contained in the water. In addition, no matter at what pressure the system is operating, air will leach in via microleaks, seals, glands, diffusion, etc.

The amount of air dissolved in the water will depend on the temperature and pressure – this may be determined and explained using Henry’s Law of Absorption of Gasses in Liquids (see Fig. 1). Henry’s Law states that, at a particular temperature, the amount of gas that will dissolve in a liquid is proportional to the pressure under which it exists. Hence the graph in Figure 1 (Henry’s Graph of Solubility of Air in Water), indicating the volume of air that can be dissolved by water at different temperatures and pressures. The graph clearly shows that the maximum air is dissolved into water at higher pressures and lower temperatures.

Cavitation
The principal places where cavitation occurs in piped water systems are pumps, or at system “restrictions” such as control valves, radiator TRVs (thermostatic radiator valves), fan coil metering station valves etc – all points in the system where relatively high pressure drops take place.

When the pump is at rest, the inlet and outlet pressures are equal, but as soon as the impeller rotates in the water, the water velocity increases but the pressure of the water on the suction side drops dramatically, allowing dissolved air to be released (the “champagne” effect). As the pressure reduces, there is a chance that the water will vaporise (if the “net positive suction head” is insufficient) and this is what is normally referred to as cavitation – i.e. the forming of cavities through the formation of vapour. The release of this air is in the form of microbubbles which, being very small, are also extremely “hard”, and consequently typically bombard the impeller and potentially cause pitting corrosion.

Cavitation in a pump can create a situation where, because of the high temperature created by not lubricating the seal, the water can “flash off” into steam, the result being that only the salts from the water are left on the seals, a factor that will then necessitate their early replacement.

When the bubbles later collapse on the discharge side of the pump (due to the increased pressure) they can cause very strong local shockwaves in the fluid, which may be audible and may even damage the blades. The change in pressure across obstructions such as valves and measuring orifices can create similar conditions.

Removing air from piped water systems
Initially AAVs (air admittance valves) were installed to remove air, but were found lacking. Air separators then emerged and appeared to improve matters, but eventually deaeration took over as the optimal solution.

Automatic air vents
Following a proper design process, and using a combination of manual and automatic air vents, the bulk of air can be removed from a piped water system. However, no matter how well the system is set up, there will always be air pockets trapped that may only be displaced when the system is in operation by venting, and some air pockets cannot be removed (e.g. in the top 0.5 in of radiators).

When water is being pumped around the system microbubbles (known to be around 2 to 3µm or even smaller in size) cannot be removed by AAVs (see Fig. 2), as the whole mass of water/air passing under the tee does not allow the air to rise. When the circuit is shut down, any air can then gravitate, in a still water environment, to the top of the pipe/system, which is why AAVs are normally located at the top of risers.

It is common knowledge that AAVs leak – a phenomenon typically attributable to one of a number of common problems, including the valve and float being mechanically connected, the water level being too close to the valve seat (any “scum” building up will stick to the valve seat causing seepage), or the float and body having too tight a fit, causing sticking of one to the other. However, there are leak-proof AAVs available designed to address all of the above issues.

Air separators
Air separators are normally cheap and rely on relatively low centrifugal forces. They can separate out the larger air bubbles suspended in the water, but will not be able to remove microbubbles because the environment in the vessel remains turbulent.

The deaeration process (temperature differential)

Before considering the Henry’s Graph (Fig. 1), the relationship between water and air needs to be appreciated: water is a natural element; therefore the amount of air in solution is controlled by the temperature and/or pressure under which it exists.

Consider an open tank of water at 10°C (containing 22.42 litres of air/m3 at atmospheric pressure – see Henry’s Graph). Heating up the water to 80°C (it will now contain 5.95 litres of air/m3 at the higher temperature) will result in 16.47 litres of air/m3 being released into the atmosphere. When the heat is turned off the water temperature will fall, eventually to the 10°C starting point. During the cooling process, the water’s natural characteristics will return to those it exhibits when its temperature is 10°C. Its air content will thus return to 22.42 litres/m3 simply because, when cooling, the water becomes absorptive. It thus requires more air content at the lower temperature and absorbs that air naturally from the atmosphere.

When introducing a deaerator it needs to be installed as close as possible within the system to the point where the bubbles are created to starve the water of gases – the system’s hottest point – on the common boiler flow header.

If one applies the above principles relating to air content in water, when the water leaves a boiler at 82°C it is quickly starved of gases at the deaerator. As the water circulates it can only cool, so that when it enters a radiator and passes to the outlet the temperature is reduced to 71°C. Most radiators have the bleed nipple 0.5 in down from the top, so there is always 0.5 in air pocket. As water is a natural element, it requires more air concentration at the lower temperature, so it absorbs some of the air pocket, which then returns back to the boiler, in solution. The boiler releases this air on the next pass and the deaerator removes a proportion of the released air until all the air pockets have been automatically removed by the deaeration process.

Such a process results in the water being conditioned to an extremely deep level of deaeration, to the extent that at no point in the circulating system can air be released; the absence of air means oxygen corrosion is so well controlled as to be almost non-existent.

As previously discussed, air cannot be efficiently removed in a turbulent zone; thus removing microbubbles from water requires specific laminar no-flow regimes (still water) so that the bubbles can be eliminated by buoyancy forces outside of the main water flow, in the upper portions of the deaerator body. Microbubbles do not readily coalesce; thus it is not sensible to rely on simply assuming that they will combine into larger bubbles.

Deaerators need either to be taller to create this still water environment, or of an increased bore, to create the laminar no-flow area outside the turbulent waterflow, and thus allow the microbubbles to gravitate upwards and be vented from the system. Otherwise they will pass through into the system and lead to the problems previously described.

The deaerator should always be installed at the hottest point in the system (on a boiler flow or a chiller return; chilled beams or ceilings require further consideration to locate the hottest point.) For the deaerator to operate properly it must be also be located where the static pressure is not excessive – manufacturers will advise on this.

Following the removal of air pockets, the system water is then conditioned to an extremely deep level of deaeration, so much so that at no point within the circulating system can air be released. Temperature differential deaeration (see Fig. 3) requires absolutely no input from operatives, and is fully automatic.

The deaeration process (pressure differential)

While temperature differential deaeration probably accounts for some 90% (100% in conventional domestic installations), of commercial installation deaeration, for the other 10% the solution is pressure differential deaeration. The pressure differential deaerator is the ultimate deaerator. In this process a small volume of water is removed from the system water, exposed to a vacuum of 0.05 bar absolute, deaerated, and returned to the system. This process is repeated until the entire system is fully deaerated equivalent to the 0.05 bar absolute. So, by installing this unit, which automatically starts up each day, a very deep level of deaeration is maintained throughout system life.

Pressure differential deaeration (see Fig. 4) is automatic, but does require a small amount of annual maintenance because of the components incorporated.

Control by dirt separation

Accumulation of dirt in system
Dirt (such as sand, fibres from cloths, swarf from pipe cutting, plastic, welding slag etc) will enter a piped water system while it is being fabricated. Systems should thus be properly flushed prior to use (see BSRIA AG 1/2001.1 (2004, AG 1/2001.1 – Pre-commission Cleaning of Pipework Systems, for details). 

Nevertheless, inefficient flushing will leave some of this debris in the pipes and, once in operation, scale and particles from corrosion – the latter a result of dissolved oxygen – will also accumulate. The reaction between iron, water and oxygen will form magnetite and, if oxygen is then present, the magnetite may, in some cases, be converted to the much more voluminous hematite.

A build-up of sludge and “dirt” in a system will reduce its effective operation.

Ensuing problems may include:

Removing dirt from the system
A common way of reducing particulates in piped water systems is to incorporate a filter or a “strainer”. There is always a compromise when using strainers – large mesh sizes allow larger particles to pass through, while a finer mesh will collect a large volume of particulates rapidly, potentially leading to obstruction of the waterway. To prevent problems, and ensure that system performance does not suffer, strainers require regular maintenance. Where there are large amounts of material circulating in the water stream (for example in open systems such as cooling tower circuits and swimming pools), sidestream filtration systems can be used. However, these filter only a proportion of the circulating water, and allow debris to circulate until it is removed on a subsequent circulation (if it has not already settled somewhere else within the system).

Dirt separators – (see Fig. 5) can remove particles down to 0.5 µm (compared to strainers that only remove down to 1,600 µm), in a completely different sphere (see Fig. 6). Tests have shown that, during the normal commissioning period, the separator will remove 90 per cent or more of all circulating material, which can then be “blown down” through the valve at the base of the separator at the end of the period. It then remains in situ to continue to remove the remaining dirt from the system. Since it is a full bore device with large waterways, its water pressure drop is relatively small, and remains constant from the first day because it will not become blocked as the dirt will only collect in the bottom of the vessel.

Maintenance is required only once every quarter to ensure that any possible dirt circulating around the system is removed. As mentioned previously, dirt separators can remove any dirt particle, not just magnetic dirt, provided that the particle is heavier than water. As with the deaerator, dirt separators require a still water zone to remove all dirt particles heavier than water.

Fitting dirt separators into existing systems has reportedly shown impressive reductions in solid matter. One particular independent test saw the dirt content reduced from 620 g/m3 (sized 5 to 10 µm) to less than 1 g/m3 of all particulates larger than 0.45 µm following the installation of a dirt separator, over a 7-week period.

Combined temperature differential deaerator & dirt separator
A combined deaerator and dirt separator (see Fig. 7) can be used to provide both air and dirt separation – reducing the cost and space requirements of separate devices. The largest units available in the UK have a 600 mm nominal bore.

Conclusion

Part 1 – Maintenance
By using properly installed and maintained air and dirt separators, (temperature differential deaerators need absolutely no maintenance, and dirt separators only require blowing down for the first two to three months – then just a quarterly blow down lasting around 5-10 seconds thereafter, while pressure differential deaerators require annual maintenance and solenoid valve diaphragm replacement) the problems arising from air and system sludge can be virtually eradicated.

Hence wear on equipment will be reduced, and maintenance costs on heat exchangers, pump seal replacement and radiators etc will be lower. Simultaneously, oxygen corrosion will now be controlled by deaeration.

Part 2 – Commissioning
Aside from the operational benefits, commissioning and setting up of systems will be far more consistent, while the labour costs of “chasing the air out of the system” will be eliminated. Meanwhile, should a system need, for any reason, to be partially drained down, the refilling process should be greatly simplified due to factors such as radiators subsequently only requiring venting once, with the deaeration process doing the rest.

This article is an adapted version of a feature article, “Deaeration and dirt separation to control water system quality”, that first appeared in the September 2007 edition of Building Services Journal.