Problems Associated With Off-Line Systems

In the current economic climate it is clear that significant plant may need to be deliberately left idle (shut down) for unknown periods due to commercial pressures. However, it needs to be understood that more damage can be done off-line if no management plan is put in place.

This article identifies areas for consideration. It is not intended as an action plan, but it clearly shows that there is a need to generate such detailed Action Plans with Plant Specific Details; timings; chemical quantities; responsibilities for both the initial set-up of storage regimes and their subsequent monitoring/control, in conjunction with appropriate staff.

Operators are relatively familiar with the need to use chemical regimes to minimise corrosion, scaling, fouling and microbiological problems under “on-line” conditions.

Corrosion

The dissolution of refined metals (i.e. plant) to the oxides (rust, for ferrous metals) resulting in early and unpredictable failures.

Scale

The deposition of sparingly soluble compounds, usually as hard material, on surfaces, which interfere with heat transfer and fluid flow.

Fouling

The deposition of solids, usually as soft, amorphous material, on surfaces, which interfere with heat transfer and fluid flow.

Microbiological

The growth of individually invisible micro-organisms into significant deposits which interfere with heat transfer and fluid flow. Some organisms can directly cause corrosion and others can cause health and hygiene issues.

What is, possibly, less well appreciated is that these mechanisms continue under “off-line” conditions with equal opportunities for equipment failure. Indeed, for well run boilers, most corrosion actually does occur off-line. For protracted off-line periods it becomes critical to manage a specific programme for storage and/or recommissioning.

General Points

There are physical considerations and chemical ones.

Freezing

With process heat removed, freezing becomes a major consideration when significant mechanical damage can be done to pipework and/or exchangers.

To avoid freezing, the options are to add heat to ensure the liquid does not freeze; add anti-freeze chemicals to the water or drain the equipment.

A small amount of heat is effectively added to the water by leaving the circulation pumps running or steam could be injected but dead-legs or stand-by equipment remain at risk. The cooling towers themselves need to be by-passed. Many boilers are natural circulation so they do not have pumps; flue ducts “suffer” natural draught and can rarely be successfully sealed. The costs are significant (electricity and/or steam).

Adding anti-freeze is theoretically feasible (eg glycols) but the quantities can be enormous; it becomes a microbiological nutrient, demanding biocide treatment and the material, ultimately, would need to be disposed of. It should not be put to general drain and therefore incurs “special waste” charges.

Draining is the cheapest and simplest option but great care needs to be exercised (in the detail) to ensure water is not trapped in sections - notably vertical U-tubes. It also leaves wetted metal surfaces with air access and insufficient corrosion inhibitor, resulting in attack of the metal unless some other procedures are practised.

Fire

Notably for cooling towers, which are natural chimneys, if water flow is isolated, the packing and/or support structures can dry. These can “twist”, leading to deformation and collapse but, more importantly they can become a bigger fire risk and periodic wetting is recommended (eg fire hoses).

It is not implied that “spontaneous combustion” will occur, BUT work involving cutting or burning, local to the tower(s) needs to be exercised with extreme care and extra precautions with respect to the risk of fire.

Geometry of Plant

If plant cannot be completely drained eg underground pipes; vertical U-tubes with top headers, it may be germane to treat different parts of the same plant by different techniques.
 

Open Evaporative Cooling Systems

The relatively low operational temperatures of open-evaporative cooling systems and low heat fluxes mean they may appear less critical than boilers but loss of cooling or plant shut down for repair can be just as damaging to a business.

The major saving graces are that, even if damage is done off-line, the rate of propagation to complete equipment failure and the operating pressures are much lower than in boiler systems such that they can often be more easily repaired, overcome or even “patched” on line.

On line, these systems contain aerated water, dissolved salts, micro-organisms and nutrients. The water is kept moving, by the circulation pumps and chemicals are dosed to minimise corrosion, scaling fouling and microbial growth. When the plant comes off line, the temperature reduces, which reduces the rate of (potential for) scaling and corrosion and, provided the pumps are left on and dosing is continued (needs adjustment/controlling), the equipment will be safeguarded. Freezing becomes a major problem, as discussed above – even if the cooling tower(s) are by-passed.

The options are often listed as:

  1) Use Vapour Phase Inhibitors (VPI’s)
  2) Use Wet Chemical storage regimes
  3) Use Dry Storage Regimes
  4) Drain

Vapour Phase Inhibitors (VPI’s)

These are water soluble, volatile chemicals which can be dosed into the water, prior to draining. They are more applicable to boiler systems (see relevant section) and are not recommended by this author for cooling systems where nominally high calcium levels exist. The pH needed to make the VPI’s work will result in precipitation of hardness salts and scaling unless softened or demineralised water is used. These detriments usually outweigh any potential benefits of VPI’s for cooling systems.

Wet Storage Regimes

There are a number of chemical regimes but most are more applicable to boilers (see relevant section) due to the volumes involved and the significant expense of the ultimate disposal of the solutions.

Dry Storage

This involves draining and then removing all traces of water via desiccants or passing dry gas through the system (~ -18°C Dew Point). This is often impractical on cooling systems which may have anti-syphon valves, “breathers” to allow draining and flooding of elevated pipework as well as major “open-ends” with no simple engineering method of sealing them.

Drain

Draining leaves wetted metal surfaces with air access and insufficient corrosion inhibitor, resulting in attack of the metal. Incomplete draining is actually “worse”. This leaves runnels of water which allows potentially significant amounts of corrosion to occur, in terms of pitting, leading to shortened equipment life on return to service. Dissolved and suspended solids in the water can settle and compact (exacerbated by evaporation of the water) resulting in increased differential aeration sites, increased corrosion and potentially increased resistance to fluid flow and heat transfer on return to normal duties.

This technique involves zero work and zero costs before and during the off-line period and it is probably the most pragmatic approach.

There is unlikely to be any indication of any problems during the off-line period (subject to the freezing and geometry limitations), but problems may be found upon recommissioning.

Observations and Actions On Return to Service:

a.    frost-damage in un-drained line sections or pumps is likely to be found through leaks when flow is re-established.
b.    water will be very dirty with high iron levels.
c.    the high iron will lead to fouling in low flow areas
d.    the high iron will interfere with the chemical performance of the inhibitors
e.    initially higher pressure drops will be seen around the system
f.     leaks may be seen in other sections of pipework, developing over the following 1-2 months
g.    chemical dosing should be increased to maintain three times normal residual levels for a minimum of one week
       (longer if increased demand for inhibitor continues to be detected) at the restart
h.    increase purge levels to reduce iron levels to < 3 ppm
i.    re-establish tower chemistry to “normal” parameters.
j.    check (by estimates on water losses) that there are no underground leaks where there are underground sections of  pipe.
 

Closed Cooling Systems

On line, these systems contain anoxic water, dissolved salts, anti-freeze and (possibly) micro-organisms and nutrients. The water is kept moving, by the circulation pumps and chemicals are dosed to minimise corrosion, fouling and microbial growth. (They are often charged with demineralised water, so there is no scaling risk).

When the plant comes off line, the temperature reduces, which reduces the rate of (potential for) corrosion. The normal chemistry is usually reasonable, even under low flow conditions, therefore (almost irrespective if the pumps are left on) the equipment will be safeguarded. Freezing is not normally a problem due to anti-freeze chemicals.

The options are:
  1) Drain
  2) Use Vapour Phase Inhibitors (VPI’s)
  3) Use Dry Storage Regimes
  4) Do nothing

Drain

Assuming the programme is typically a high nitrite/borate-based system the liquid can be drained and, despite wetted surfaces, it will be protected for a period of at least 6 weeks from corrosion issues.

The chemical inventory can be stored in IBC’s or temporary tanks and re-used. If the shut down extends beyond 6 weeks, the liquor can be pumped back into the system, recirculated until all parts have seen a high nitrite level and drained again, back into the storage facility.

VPI’s or Dry Storage

These are technically feasible but are relatively complicated to carry out so the ease of relying on the glycol for anti-freezing and the nitrite for corrosion protection rarely justifies examining any other technique for closed loop systems.

Do nothing

The systems contain inhibitors and anti-freeze which will overcome corrosion and freezing problems. Running the pumps would input some energy and further protect them from freezing. If the pumps are desired to be switched off (eg to save power costs) this would be acceptable but it would be recommended that the pumps were run for a period of, say, one hour per week, with monitoring to check inhibitor levels and microbiological activity. Dosing adjustments can be made, if necessary, but the main advantage is to move any settled sludge (keep it mobile) and ensure even distribution of inhibitors (including biocides) which can be come locally depleted in doing their work. In essence, this becomes a “Wet Storage” technique which is totally acceptable.
 

Boiler Systems

On line these systems contain deaerated water and chemicals to raise the pH.

The general comments, above, refer equally to boilers but it is usually considered more worthwhile to positively manage boiler systems since the operational temperatures and pressures are so much higher than ambient that even minor defects can rapidly escalate from small irregularities, on metal surfaces, to full equipment failures, necessitating complex repairs.

The principles of storage are identical, with the choice usually being dictated by the desired speed of return to service with options for:

a.    Keep Hot: Keep full with water and bleed steam back in from the steam header to keep at a positive pressure, eliminating air. (Circulation is not even critical in the absence of oxygen). Very simple but relatively expensive to maintain. Rapid return to service with no extra preparatory work. Only usually considered for short term storage of a few days or so.

b.    Nitrogen Blanket: Keep full of water and bleed nitrogen back in from a tapping on the steam header as the plant cools to maintain a +5psig pressure. This sounds simple but leaks and passing valves can make this difficult in practise and consume considerable volumes of bottled gas, becoming expensive to maintain. Rapid return to service with no extra preparatory work.

c.    Add VPI’s: Cool and drain. Good for small bore pipework (heat transfer sections and, say, up to 10” bore) but not usually effective in the steam drum unless “packets” of solid VPI are suspended in the steam drum. This means entering the drum to hang them and sealing it up again for storage. Before start up the drum has to be opened again and every (empty/used) sachet removed. Boiler needs to be refilled.

d.    Dry Store: Drain and dry by passing dry air (~-18°C Dew Point) through unit via adapted fittings. Check humidity of exhaust air and either seal up when dry or continue to blow dry air through, continuously. Needs lots of dry air (hire of dehumidifiers or special compressors). Dead legs or partially drained areas are difficult to protect or dry and, with the introduction of air, corrosion can occur quickly – even though (eventually) dryness may be achieved. If dryness is NOT achieved, corrosion is amplified in those wetted areas.

e.    Hot Air: The use of hot air, as in exhaust from, say, propane burners is most definitely NOT RECOMMENDED. It dries the equipment local to the hot gas but releases combustion products which provide acidic condensate resulting in enhanced corrosion in more remote areas of the equipment.

f.    Wet store – with superheaters – solid alkali treated, demineralised water boilers Where superheaters are fitted and the boiler water is caustic and/or phosphate dosed, the boiler water would need to be drained out, the boiler flushed (to drain) with 2-3 volumes of boiler feedwater (deaerated + oxygen scavenger + amine) chemistry and then filled with boiler feedwater until water is detected at a bleed point downstream of the superheaters. Extra dosing with oxygen scavenger and amine (or ammonia) will further enhance the storage. NO phosphate, caustic or polymer should be used whatsoever. A head tank arrangement can be set up (above the highest point of the steam drum) to ensure the system stays full of water, but any leaks or passing valves can make this more difficult than it sounds.

Before start up, the water levels need to be reduced to the normal start up level. Water left in any non-draining superheat passes will boil dry, without detriment to the tubes, during start up.

g.    Wet store – with superheaters – All Volatile treated As (f) but would not need to have the boiler drained and flushed before complete filling.

Before start up, the water levels need to be reduced to the normal start up level. Water left in any non-draining superheat passes will boil dry, without detriment to the tubes, during start up.

Alternatively, the drum can be filled to the top WITHOUT passing into the superheaters and without washing out the boiler water but, at start up, a degree of steam blowing would be required to strip corrosion products from the superheater surfaces, and vent them to atmosphere to avoid impingement on the HP turbine inlet blades. This presumes Cr/Mo superheater tubes which are less prone to corrosion than carbon steel. Steam blowing is time consuming, expensive and can be noisy, requiring significant pre-planning to carry out successfully.

h.    Wet store – with superheaters – softened water boilers.
Where softened water is used, the boilers can be filled to the top level with alkaline boiler water, free of oxygen. A head tank arrangement can be set up above the highest point of the steam drum to ensure the system stays full of water, but any leaks or passing valves can make this more difficult than it sounds. Precautions should be taken to ensure NO boiler water goes into any part of the superheaters or significant preparatory work will be needed before recommissioning to remove all traces of solids. This can be accomplished by thoroughly washing out with condensate before start up, although parallel pass superheaters can be notoriously difficult to completely flush. Residual softened water (eg vertical hairpins) or residual salts could lead to rapid failure of the equipment. Steam blowing is an alternative option but is time consuming, expensive and can be noisy, requiring significant pre-planning to carry out successfully.

i.    Wet store – without superheaters – softened water boilers. Where softened water is used, the boilers can be filled to the top level with alkaline boiler water, free of oxygen. A head tank arrangement can be set up above the highest point of the steam drum to ensure the system stays full of water, but any leaks or passing valves can make this more difficult than it sounds.

The wet storage, as above, exposes the plant to potential frost damage problems.

j.    Wet store – Borate/Nitrite: Where a solution of nitrite-borate in demineralised water (or condensate) is pumped into the boiler system to fill it completely, to wet all surfaces (≥200 ppm + ≥200 ppm at pH ≥8.5). The solution is then drained out (and retained in temporary tanks or IBC’s); the plant can be left open to the atmosphere with no further, sophisticated techniques. This will be good for up to 6 weeks when, if the plant is required to be off-line for longer, the solution can be pumped back into the boiler and drained out again. This gives a good passivation/ preservation without the frost damage concerns (subject to the exclusion of non-draining geometries).

Before the plant can be returned to service, all the residual chemical must be washed out (~3 washings with boiler feedwater). If the solution has penetrated the superheaters, these need to be flushed backwards with condensate quality water to remove all traces of solution. If the plant starts up with residual chemical in the superheaters there is significant danger of superheater failure and particle impingement on the HP turbine.

k.    Hot Blowdown: Involves shutting the plant down and blowing all the water out whilst the boiler is still hot, to use the residual heat of the metal to dry all the metal surfaces. This sounds effective but noise restrictions would probably restrict the pressure for final venting, to be below ~3 bar. This may not leave sufficient heat to dry-out poorly draining areas. Any wet areas will lead to localised corrosion. Weather induced, or simple day-night, temperature variations will lead to expansion and contraction of gas in the boiler with the introduction of water vapour followed by condensation resulting in wetting of many surfaces. The on-line passive surfaces may withstand this for a few days, but not much more.

Conclusion

Upon shutting water using plant, one could “Do Nothing” and allow corrosion to occur. At start up, extra purging and/or steam blowing (depending on the system) could be done to attempt to remove the corrosion debris. Failures may or may not occur but the probability of early failures of the boilers would be high (though not certain). The probability of early failures on cooling systems would be low (but not zero).

Chemical cleaning of the boilers, prior to their return to service, would produce clean, passive surfaces and could be used as a technique. This requires considerable planning, time and expense. If damage had occurred, to significantly weaken the plant, chemically cleaning would “find” those weaknesses, which could then be repaired, but they could not be accurately planned in terms of time or money.

Some form of pro-active storage/management is strongly recommended (and emphasised) that this needs significant planning and effort and is not zero cost.

This report has endeavoured to indicate the options available but the final choice of storage is a blend of technology, preference, practicality and balance of risk. It is an iterative process that would be finalised by “face-to-face” meetings to discuss all options and would likely vary by plant by individual restrictions.