Method of cleaning scale and stains in water systems and related equipment

A method for removing scale and stains from the interior surfaces of a water-containing system having a reservoir, auxiliary equipment and piping, without having to drain the water from the system. The use of inhibited mineral acid allows the water to be circulated throughout the entire system, cleaning all interior surfaces without damaging them. Once scale and stains are removed, the water is neutralized.

BACKGROUND OF THE INVENTION 
This invention relates to a method for cleaning scale and stains from the 
interior surfaces of a water-containing system having a reservoir, 
auxiliary equipment and piping without having to drain the water from the 
system. Scale and stains are objectionable because they can be 
unattractive, particularly in swimming pools and spas, and lead to the 
coating and plugging of lines, filters, pumps, heaters and heat exchangers 
in the system thereby reducing their efficiency and effectiveness. Scale 
can also lead to microbiological havens and to undesirable corrosion of 
surfaces which would eventually require replacement of the corroded part. 
In all cases it is desirable to maintain the water-contact surfaces of the 
system in a scale and stain free condition. 
Common practice has been to drain the water from the system and then 
physically scrape and/or wash the scale and stain from the surface with a 
strong mineral acid. This practice is dangerous to the owner or the 
service man who is required to wear protective clothing and also, most 
likely, a self contained air mask in order to comply with exposure limits 
for the mineral acids. The strong acid is also corrosive to the treated 
surfaces and does not clean the entire system. Additional cost would also 
always be incurred to fill the system with fresh water again. 
Several other methods have been suggested in the prior art but all have 
disadvantages. For example, Hamilton in U.S. Pat. No. 4,906,384 employs a 
strong mineral acid treatment of the water in the vessel to remove scale 
deposits, without draining the water, at a level at which the total 
alkalinity is zero or a pH of 4.6 (Col. 3, line 5). Because the relatively 
dilute acid reacts slowly with the scale and is thereby consumed, the 
total alkalinity must continually be monitored and additional acid added, 
in order to maintain the pH at 4.6 to achieve the descaling and stain 
removal. If, for example, a swimming pool is badly scaled and stained, 
Hamilton's method would require considerable manpower and chemical testing 
to maintain the pH at 4.6 since the mineral acid added would be consumed 
by the relatively large amounts of scale and stain present. 
Hamilton also suggests that the acidification treatment employ a metal 
sequestering and/or chelating agent such as ammonium salts or 
ethylenediamine tetraacetic acid (EDTA). By definition, sequestering 
agents react with certain metal ions to form coordination complexes. By 
definition, chelating agents react with metal ions to form a ring 
structure and strongly hold the metal ion through coordination bonding. 
Water scale ions, such as calcium, magnesium, iron and copper, may be 
changed in solution, by reacting with a chelating agent, from a positive 
ion to a complex negative ion. It is well known that this reaction is an 
equilibrium reaction and that chelation of the metal ion is less effective 
in acidic solution than in basic solution. Use of expensive chelating 
agents in acidic solution is not the most economical approach to scale and 
stain removal. In commercial literature, Hamilton also recommends that the 
circulating pump on the system be shut down during treatment. Therefore 
his method does not clean the entire system but only the main body of 
water. 
Similarly, Kisner in U.S. Pat. No. 5,108,514 teaches a method of removing 
stains, scale and calcium deposits from the interior surfaces of swimming 
pools without draining the water by treating only the pool surfaces with a 
concentrated inorganic or mineral acid in combination with a strong metal 
chelating agent while the pool pump and circulation system is not 
operating. Again, Kisner does not clean the entire system and also employs 
the expensive chelating agent in an inefficient manner by using a 
concentrated mineral acid mixture. Also, the Kisner method employs a 
tedious means of applying the treating solution only to the surface of the 
swimming pool and requires additional man hours and the prolonged exposure 
to and the additional handling of hazardous strong mineral acid. 
SUMMARY OF THE INVENTION 
This invention is directed to a method for removing scale and stains from 
the interior surfaces of a water-containing system having a reservoir, 
auxiliary equipment and piping, without draining the system. The method 
consists of first acidifying the water by adding mineral acid and mineral 
acid inhibitor to the water in the system in an amount sufficient to 
remove the scale and stains. Next, the water is circulated through the 
system and remains in contact with the surfaces of the system to dissolve 
the scale and stains without adversely affecting the surfaces. After scale 
and stains are removed, the water is neutralized by adding a basic 
compound in an amount sufficient to raise the pH. All of these steps are 
done without draining the water from the system. 
The acidifying mixture consists of an inhibitor and a mineral acid. The 
inhibitor is a nitrogen-containing organic compound, usually selected from 
the group consisting of an amine, quaternary ammonium compound, 
heterocyclic nitrogen compound, urea, thiourea, amide or mixtures thereof. 
In a preferred embodiment of the invention, the mineral acid inhibitor is 
a 1:1 soap of an amine or ammonia and an organic carboxylic acid such as a 
1:1 soap of alkanolamine and hydroxyacetic acid. The mineral acid 
composition preferably used to remove scale and stains includes 
hydrochloric acid, a quaternary ammonium compound and a wetting agent. 
This composition is added to the water at the rate of about 0.5 to 1.5 
gallons per 2500 gallons of reservoir capacity and in an amount sufficient 
to acidify the water to a pH of about 1.0 to 4.0, with the optimum range 
being a pH of from about 2.0 to 3.0. 
Once the inhibited mineral acid composition has been added, the acidified 
water circulates throughout the water-containing system for about one to 
seven days, or even up to twenty-one days, depending upon the amount of 
scale and stains on the surfaces. The acidified water circulates 
throughout the reservoir, as well as the auxiliary equipment and piping of 
the water-containing system. This is usually accomplished by activating 
the system's circulating pump during the cleaning process. 
If the scaling and staining are not excessive, the water containing system 
can also be cleaned effectively using the inhibited acid compositions at a 
pH of 1 to 4 by running the circulation pump on the normal periodic 
schedule. In some cases, the main body of water in the water containing 
system can be cleaned statically, without circulation, by employing the 
inhibited acids at a pH of 1 to 4 if desired. 
After the scale and stains have been removed from the interior surfaces, 
the water is neutralized by adding a basic compound. The compound is added 
in an amount sufficient to raise the alkalinity of the water to a pH of 
from about 7.0 to about 7.6. Typically, the basic compound will be sodium 
carbonate or sodium bicarbonate. 
This invention has several benefits and advantages over the prior art. The 
invention provides a safe, low cost and efficient method of cleaning scale 
and stain from the water contact surfaces of water containing systems such 
as swimming pools, spas, and associated equipment and piping systems and 
the like without having to drain such systems. Furthermore, the invention 
cleans the water transfer pipes, filters, pumps and heat exchangers in 
addition to the major water reservoir, without doing harm to the various 
surfaces being cleaned. 
In addition, the invention provides a simple method that can be used by an 
ordinary person and that does not require individuals trained and equipped 
in handling hazardous materials or in technical methods of analysis. 
Moreover, the mineral acid inhibitors used in this invention have several 
advantages. In addition to preventing the mineral acid from damaging the 
surfaces of the water-containing systems, the inhibitors are biodegradable 
and also act as a low cost metal carrying agent, maintaining scale and 
stain ions in solution after the cleaning process is complete. 
These objectives and advantages along with other aspects of the invention 
will be further understood with reference to the following drawing and 
description:

DETAILED DESCRIPTION OF THE INVENTION 
The present invention provides an improved method of removing water scale 
and stain from the interior surfaces of a water-containing system having a 
reservoir, auxiliary equipment and piping without draining the system. The 
cleaning is achieved by employing an inhibited mineral acid solution and 
circulating the water in the system until the scale and stains are 
removed. The cleaning solutions of this invention are most effective 
between a pH of 1.0 to 4.0 with the preferred range being between pH of 
2.0 to 3.0 or 0.01 to 0.001 moles of hydrogen ion per liter of water. At 
this hydrogen ion concentration, the inhibited acid solution readily 
dissolves the scale and stains without adversely corroding or attacking 
the water contact surfaces of the system. 
Various inhibitors for mineral acids have been well documented in the 
patent art. Typical, but not necessarily all inclusive, examples of acid 
inhibitors are disclosed in the following U.S. Pat. Nos. 2,758,970; 
2,807,585; 2,941,949; 3,077,454; 3,607,781; 3,668,137; 3,885,913; 
4,089,796; 4,199,469; 4,310,435; 4,541,945; 4,554,090; 4,587,030; 
4,614,600; 4,637,899; 4,670,186; 4,780,150 and 4,851,149 which are 
included in this application by reference. 
This invention in part depends upon the use of suitable mineral acid 
inhibitors in combination with mineral acid, for cleaning water-containing 
systems without draining. Generally, inhibitors for mineral acids are 
nitrogen containing organic compounds such as amines and amine derivatives 
like the ethylene oxide condensates; quaternary ammonium and quaternary 
amines and derivatives; various heterocyclic nitrogen compounds, urea and 
thiourea derivatives, amides, and the like. Surfactants may also be 
employed in the inhibited acid compositions. The surfactants reduce the 
surface tension of the solutions and enhance the activity of the 
inhibitors and acids employed. They may be anionic cationic, nonionic or 
amphoteric as defined in the art. 
As mentioned above, the preferred inhibitors of this invention are 1:1 
soaps, i.e., quaternary compounds of weak organic acids with amines as 
described in the co-pending U.S. patent application Ser. No. 07/700,780 
filed May 16, 1991 which is incorporated herein by reference. The 
preferred weak organic acids employed in the quaternary compounds for 
mineral acid inhibition are those that form relatively soluble derivatives 
of calcium and magnesium such as acetic acid, hydroxyacetic acid, citric 
acid, gluconic acid, tartaric acid, salicylic acid and benzoic acid. 
Oxalic acid is not preferred as it forms a very insoluble calcium 
derivative with solubility similar to calcium carbonate scale itself. The 
following Table 1 illustrates the solubility of various calcium and 
magnesium derivatives of organic acids in water solution. 
TABLE I 
______________________________________ 
SOLUBILITIES OF VARIOUS ORGANIC ACID 
DERIVATIVES OF CALCIUM AND MAGNESIUM 
______________________________________ 
Calcium Acetate 37.4 g./100 ml 
@ 0 C. 
Calcium Benzoate 8.3 g./100 ml @ 80 C. 
Calcium Hydroxyacetate 
1.2 g./100 ml @ 18 C. 
Calcium Citrate 1.0 g./100 ml @ 23 C. 
Calcium Oxylate 0.00067 g./100 ml 
@ 13 C. 
Calcium Carbonate 
0.00153 g./100 ml 
@ 25 C. 
Magnesium Hydroxyacetate 
7.9 g./100 ml @ 18 C. 
______________________________________ 
The preferred amines employed in the quaternary compounds for mineral acid 
inhibition are those with high water solubility such as ammonia; 
alkanolamines including monoethanolamine, diethanolamine, triethanolamine, 
triisopropanolamine, and isopropanolamine; and alkylamines such as 
diethylamine. 
The intrinsic character of the preferred quaternary compounds employed in 
the method of this invention makes them particularly suitable for their 
use as the mineral acid inhibitors in the compositions employed to remove 
the scale and stain. Not only are they mineral acid inhibitors, but they 
provide a means of carrying scale and stain causing ions, such as calcium 
and magnesium ions, in solution by forming the corresponding water soluble 
metal soaps of the organic acids upon neutralization of the mineral acid 
employed for the removal of the scale and stain. This is particularly 
desirable for applications in systems such as swimming pools and spas, 
where the mineral acid is neutralized prior to placing the system back 
into its intended use. 
Generally, the invention is practiced by the addition of the inhibited acid 
solution to the major water reservoir. The amount of inhibited acid 
solution required to remove the scale and stain from the water system will 
vary depending upon the degree of scale and stain present and the volume 
of the water in the system. We have found that a suitable inhibited acid 
solution can contain about 25% hydrochloric acid and about 20-25% of 
solution of a quaternary compound in combination with a wetting agent. 
This inhibited acid can generally be employed in cleaning applications at 
the rate of about 0.5 to 1.5 gallons/2500 gallons of reservoir capacity 
depending on the level of scale and stain to be removed and assuming that 
the system water is balanced to typical water management conditions with a 
pH of about 7. The pH of the system, upon treatment with this level of 
inhibited acid would be in the range of 2.8 to 2.3. 
The time required for cleaning depends upon the degree of scale and stain 
present, and is usually from one to twenty-one days. It is also 
recommended in the cleaning of swimming pools and spas that the reservoir 
be brushed periodically. This is normally required as most scale also 
contains a biomass, resulting from body oils, vegetation, and chlorination 
residuals, that tends to hold scale particles together and which will 
amass on the surface upon solution of the surface scale or stain. The 
inhibitors and wetting agents employed in the treatment solution also 
assist in breaking up the biomass bonding, thus further assisting in scale 
and stain removal. 
Because the cleaning process usually releases some silt and other acid 
insolubles from the scale deposits, a swimming pool or spa generally will 
require vacuuming to remove the remaining insoluble solids left after 
scale and stain removal. After scale and stain has been removed, the 
system then can be restored to normal operation by adjusting the pH to the 
normal use level of pH 7.0-7.6 with any of the usual water management 
chemicals such as sodium carbonate or sodium bicarbonate. When unusually 
heavily scaled and stained pools and spas are cleaned, it is sometimes 
best to increase the volume and frequency of filter backwashing or to 
drain the water from the system after the cleaning process to remove the 
high levels of scale and stain ions present. 
The invention provides several improvements over the prior art. The 
Hamilton method of U.S. Pat. No. 4,906,384 employs a mineral acid added to 
the system water to maintain a zero alkalinity or a pH of 4.6 equivalent 
to 0.000025 moles of hydrogen ion per liter of water to clean scale and 
stain from swimming pools. The inventive inhibited acid solutions are 
employed preferably at over 10 to 100 times more concentrated in hydrogen 
ion than the Hamilton method (pH of 3.6 is 10 times more and a pH of 2.6 
is 100 times more concentrated) which will inherently reduce the time 
required to remove the scale and stain. Hamilton requires additional 
manpower for the constant monitoring of the alkalinity and additional 
addition of acid during treatment as the low level of acid is rapidly 
consumed when dissolving the scale and stain. At the level of inhibited 
acid employed in our method, monitoring is not usually required as there 
is sufficient inhibited acid employed to dissolve all scale and stain 
present. Only in extreme cases of scaling would simple monitoring of pH 
with a broad range pH paper be required. Furthermore, the Kisner method of 
U.S. Pat. No. 5,108,514 employs a very strong mineral acid at high 
concentrations, i.e., 20-31.5% hydrochloric or 5.5 to 8.6 moles of 
acid/liter directly to the surface of the pool with the circulating pump 
off. This level of acid is over 500 times the preferred level of this 
invention and Kisner can be extremely corrosive to pool system surfaces. 
Kisner's treatment solution would destroy most metal parts if applied to 
their surfaces and that is why Kisner discloses treating only the surface 
of the pool in the procedure and not the auxiliary equipment and piping. 
By employing inhibited mineral acids of this invention, it has been found 
that scale and stain removal can be achieved efficiently and simply from 
all of the water contact surfaces in the entire system with no harmful 
corrosion of the surfaces. Additionally, by proper selection of the 
inhibitor, residual metal ions are held in solution upon completion of the 
cleaning. 
When inhibited acids are employed at a pH of 1 to 4 and the scaling and 
staining of the water containing system is not excessive, cleaning can be 
achieved by leaving the circulation pump running on the usual periodic 
circulation schedule. 
In some cases, it may be desirable to employ the inhibited acids at a pH of 
1 to 4 and leave the circulating pump off completely to achieve the 
cleaning of the main body of water in the water system, i.e., if the pump 
or filter is being replaced during renovation of the system. Using the 
inhibited acid at a low pH will dissolve the scale and stain rapidly 
particularly with periodic brushing of the water contact surfaces. 
The preferred method is to leave the circulating pump on during the 
cleaning process employing inhibited acids at a pH of 1 to 4. Cleaning of 
the entire system in the shortest possible time will then be achieved. 
EXAMPLE 
As an example of the method of this invention, a 20,000 gallon plastered 
pool was selected that was very badly scaled and stained over 80% of the 
visible surface (see DRAWING). The water was analyzed to have a pH of 8.2 
with total alkalinity of 180 and hardness of 800. Theoretically a pool of 
this capacity would require about 2.3 gallons of the preferred inhibited 
acid solution of 25% hydrochloric acid with 20-25% of a solution of a 
quaternary compound such as a 1:1 soap of ethanolamine and hydroxyacetic 
acid in combination with a wetting agent in order to reduce the pH to 3.0. 
It was concluded, because of the poor condition of the pool, 23 gallons of 
the inhibited acid solution to reduce the pH to 2 should be employed to 
assure scale and stain removal. The 23 gallons of cleaning solution were 
added to the 20,000 gallons of pool water and circulated throughout the 
entire system for a period of 27 hours. With reference to the FIGURE, the 
acidified water was circulated by opening the skimmer valve 3 and the 
bottom drain valve 4 and activating the circulation pump 5. The water then 
circulated through the pool 1, skimmer 2, and bottom drain 8 and through 
the pool filter 6. The pool filter 6 was a paper cartridge filter of 100 
sq. ft. surface area. After 27 hours the pH was at 2.5. The circulation 
was continued for an additional 12 hours at which time the pH was 3.5. The 
pool was then observed to be essentially clean of scale and stain. Sodium 
carbonate was then added to balance the pool to a pH of 7.2. The hardness 
had increased to 1800. The paper cartridge filter was intact and clean of 
scale and stain. 
Other modifications of this invention may be made without departing from 
its scope as will be understood to a person of ordinary skill in this art.