Industrial waters - quality assurance – sampling and testing

Chandran Udumbasseri, Technical consultant

cudumbasseri@yahoo.co.in.

Introduction

Industrial waters include boiler water, cooling tower water, condensate water,

make up water and waste water. The treatments differ in all these areas as their

purpose differ as industrial water. The treatments are monitored through regular

testing and analysis

Regular, representative sampling and accurate testing are essential parts of a

sound industrial water treatment program. This is to ensure that proper chemical

levels are maintained and the pH of the water is within the set range.

Quality assurance sample analysis

1. A boiler of capacity 100 HP or greater capacity needs quality assurance

each month.

2. All boilers treated with caustic, phosphate and sulfite should be checked

for the boiler water treatment level by collecting 1Lt of representative

sample.

3. For boilers of low capacity (les than 15psig), the caustic level must be

checked.

4. 1Lt sample should be taken for hot water boilers for testing purpose.

In plant testing

Test kits should be used for in plant testing (field testing)

Sampling and testing of cooling towers

Collect samples in clean plastic bottles and conduct the tests without delay.

In re-circulating cooling towers, sample may be taken by dipping in cold well or

from re-circulating pump discharge.

Frequency of samples

The table given below shows the sample frequency and the required tests.

pH M-

Alkalinity

Conductivity

(TDS)

Calcium

Hardness

Inhibitor

Make up - 1/w 1/w 1/w -

Small CT,

(<25 Ton)

1/w 1/w 1/w 1/w 1/w

Medium CT

(25-100 Ton)

2/w 2/w 2/w 2/w 2/w

Large CT

(>100 Ton)

1/D 1/D 1/D 1/D 1/D

Notes: w = week; D = Day The main tests for cooling towers are:

1. pH

2. conductivity

3. M-Alkalinity

4. Calcium Hardness

5. Inhibitor.

Sampling and testing of Boilers

Method of sampling

1. Cool the sample using sample cooler to get a representative sample.

2. Sample Cooler: A coil of 15-20 feet made out of copper or stain steel

tubing can be used as sample cooler. The coil can be immersed in a

permanent cooling jacket or it may be immersed in a bucket of cold

water. Keep a slow flow rate so that the sample gets cooled to ambient

temperature during its flow through the coil.

3. Sampling procedure:

3.1. Cool the cooling coil before taking boiler water sample.

3.2. Flesh the coil with boiler water for several times before collecting the

representative sample.

3.3. Extend the end of sample coil to the bottom of the sample bottle and

allow at least one volume of the bottle water to over flow from

container during the collection process. Plastic bottle are the

preferred containers (glass bottles should not be used if silica content

is limited)

Frequency of sampling and testing

The sample and frequency for boiler systems is given in the table

pH Total

Alkalinity

Hydroxyl

Alkalinity

Calcium

Hardness

TDS Copper

content

Ortho

phosphate

Sulfite

content Make up

water

- 1/w - 1/w 1/w - - -

Boiler

feed

water

(<25HP)

- 1/w - 1/w 1/w - - -

BFW

(25-

100HP)

- 2/w - 2/w 2/w - - -

BFW

(>100

HP)

- 1/D - 1/D 1/D - - -

Boiler

water

(25HP)

1/w - 1/w - 1/w - 1/w 1/w

BW (25-

100HP)

2/w - 2/w - 2/w - 2/w 2/w

BW

(.100HP)

1/D - 1/D - 1/D - 1/D 1/D

Condens-

ate

Return

Small

1/w - - -- 1/w 1/q - -

CR

Medium

2/w - - - 2/w 2/q - -

CR

Large

1/D - - - 1/D 1/q - -

Sampling location

1. Make up water: from the output of the softener

2. Boiler Feed water: from the discharge of boiler feed pump

3. Boiler water: from the continuous blow down line (between the boiler

and regulating valve)

4. Steam condensate: from the line that enter the deaerator

5. Steam samples: they are rarely taken; it can be taken from steam header

using sample cooler.

Feed water sample

Feed water may be checked for the following parameters:

1. M-Alkalinity

2. Conductivity

3. Calcium hardness.

Condensate sample

Condensate sample may be checked for pH and conductivity. If conductivity is

higher than 35 micromhos, then calcium hardness should be checked. If

applicable, Iron and copper should be checked.

Blow down sample

Blow down sample should be checked for the following parameters:

1. OH Alkalinity

2. Neutralized Conductivity

3. Ortho Phosphate

4. Sulfite

Sampling and testing other water systems

Hot and chilled circulating water

The testing frequency is given in the following table.

Additional points to be noted are:

1. Make up water to hot water boilers, chilled water systems and diesel

engine jackets usually comes from an ion exchange unit or dealkalizer

unit.

2. Re-circulating water from HTW (high temperature hot water boilers)

systems and systems using the sulfite caustic soda treatment program

should be tested once per day fro pH and sulfite.

3. Re-circulating chilled and hot water treated with nitrite-borax or

Molybdate-caustic program should be checked once per month for nitrite

or Molybdate content and pH.

Ion exchangers and dealkalizers

The testing frequency for these systems is given in the following table.

Additional points to be noted are:

1. The incoming water to ion exchanger should be checked for Total

Hardness once per week. The dealkalizer influent should be checked for

Total (M) alkalinity once per week.

2. The effluent from these exchangers should be checked for reduction in

hardness and reduction in total alkalinity once per day or once per shift.

3. The location of sample point is important to get proper representative

sample. If multi-port valves are used for automatic regeneration then

these are possibilities of sample to get mixed of incoming and outgoing

waters.

4. The brine used for regeneration should be tested by hydrometer to

measure its strength. It should be of 100% saturation ( 28% strength)

Frequency and testing requirements for other systems

System pH Sulfite Nitrite Molybdate Total

Hardness

Total

Alkalinity

HTW

SO3-caustic

1/day 1/day - - - -

Closed Hot water

NO2-Borax

1/mo - 1/mo - - -

Closed Hot water

Molybdate

1/mo 1/mo - -

Chilled water &

brine

NO2-Borax

1/mo - 1/mo - - -

Diesel jackets

Molybdate

1/mo - - 1/mo - -

Diesel jackets

NO2-Borax

1/mo - 1/mo - - -

Ion exchange

Feed water

- - - - 1/wek -

Ion exchange

Effluent

- - - - 3x/day -

Dealkaizer

Feed water

- - - - - 1/wek

Dealkalizer

Effluent

- - - - - 2x/day

Testing Procedures

Water testing is performed to provide the proper control of recommended

treatments. Any unexpected result should be acted upon immediately. The test

reagents should also be checked for conformance of the results. If results are

correct the treatment program should be evaluated.

Water sample testing procedures

Water test procedures will be provided with the test kits obtained from

commercial sources. Test kits will generally include tests for:

1. P-Alkalinity, M-Alkalinity or OH-Alkalinity

2. Conductivity

3. Organic polymer like Tannin or polyacrylate

4. pH

5. Hardness (Total, Calcium , Magnesium)

6. Phosphate (orthophosphate, total phosphate)

7. Sulfite

8. Nitrite

9. Chloride

10. Molybdate

Corrosion testing

A positive way to check corrosion is to place corrosion coupons (test specimens)

in the system

There are two corrosion test specimens commonly used

1. Corrosion test coupons for use in open and closed cooling systems,

closed hot water systems and domestic water systems.

2. Corrosion pipe inserts for use in steam condensate return systems

Corrosion test results are reported as a rate of corrosion penetration in to the

metal given in mils penetration per year (mpy)

A mills is one-thousandth (0.001) inch. Thus the corrosion rate of 10mpy means

that the thickness of a metal is reduced by 10x0.001 = 0.01 inches per year.

If the metal being studied is 1/16 or 0.0625 inches thick, this means that it will be

completely destroyed in just over 6 years.

Corrosion rate may also be expressed in millimeters per year (mmpy).

1 mpy = 0.0254 mmpy

1mmpy = 39.4 mpy.

Corrosion is also given a weight loss in milligram per square decimeter per

day (mdd)

For steel, the relation between penetration and weight loss is

1mdd = 0.2 mpy

1 mpy = 5 mdd

There are a variety of electrical instruments available which monitor and record

corrosion rates.

Corrosion coupons

The coupons are tags of various sizes. Generally low carbon steel specimens are

used as the most susceptible metal to corrosion. Other metals like copper,

stainless steel, brass and other metals are available

Weight loss:

The specimens are cleaned and then weighed. The general corrosion is

determined after exposure to the system. The coupons are again cleaned and

reweighed to find the weight loss.

Corrosion rate is calculated using weight loss, duration of exposure and area of

the coupon exposed.

Water velocity:

The coupons should be exposed to conditions that reflect those in the system as

whole. Water temperature and flow velocity will affect the results.

The corrosion rate will increase when velocity is too low or too high. For best

results the water velocity should be 3 to 5ft/sec.

Water temperature:

The corrosion increases with temperature. So in cooling water systems both the

cold water supply and hot return should be evaluated by coupon. The coupon

should be installed in the incoming hot water line and cold water going in to the

system.

Coupon exposure duration:

Usually the exposure time is for a minimum of 30 calendar days and maximum

of 90 calendar days.

Scheduling:

A regular schedule can determine the trends and recognize changes within the

system

While installing test specimens of different metals, the more resistant metals

(noble metals) should be installed down stream of less resistant metals.

Specimen shipment and storage:

The specimens are shipped in vapor inhibiting bags. The specimens should be

removed only at the time of installation. Use the same type of bag for returning

the exposed specimens.

Do not handle specimens with fingers since finger prints can initiate corrosion

sites.

Installing the specimens:

While installing the specimen into the system note the location and date of

installation.

After the exposure to the desired time carefully remove the specimen, air dry

without disturbing any deposits and return to the original vapor inhibited bags.

Record the removal date and send for evaluation.

Installation of coupons

Coupons are usually installed in pipe “Tees” by means of corrosion test racks

which include a 1 inch pips plug, drilled and tapped to accept a plastic or

phenolic rod with a nylon nut and bolt for attaching the coupon to the rod.

A by pass arrangement with 1 inch piping will facilitate installation and removal.

Poly vinyl chloride will eliminate electrical effects but cannot be used for hot

condensate.

For steam and condensate return line, installation is made where “Tees” are

present.

When using bypass arrangement, condensate from the outlet is piped back into

the system.

Bypass should be constructed of iron pipe and fittings and installed where there

is sufficient pressure differential to ensure a continuous flow through the bypass.

Important points are:

1. Flow through the corrosion test rack should be continuous and

measurable. (Flow meter).

2. The flow through the rack may be maintained at 3 to5 ft/sec.

3. The direction of flow should be as shown in the figure.

Interpretation of Tests

Many water treatment systems require that a specific level of a treatment

chemical be carried in the water. When a test indicates that a chemical is not

within the limits required for the treatment program confirm that the test was

performed properly and is valid.

When the test shows results at a lower level than required, evaluate the chemical

addition program and make the appropriate adjustment.

If adjustments do not correct the situation, then evaluate the system in total to

determine the nature of the problem.

When the results are too high then also check the chemical addition program.

Reduce the addition rate to bring the results the required level.

If test for hardness is found high then check the make up water treatment

program.

Alkalinity relationships

The three basic sources of alkalinity in water are:

1. Alkalinity resulting from bicarbonate (HCO3)

2. Alkalinity from carbonate (CO3)

3. Alkalinity from hydroxyl ion (OH)

The amount of each of these in water can be determined by titrating with an acid

to certain pH levels using phenolphthalein (P Alkalinity) and methyl orange (M

alkalinity) end points.

Relationship of P, M, and OH alkalinities

P and M alkalinities can be determined by titration and OH alkalinity can be

calculated. OH alkalinity is also known as caustic alkalinity.

Alkalinity relationship based on P and M tests

Conditions OH Level of alkalinity

contributed by

carbonate

bicarbonate

P = M M 0 0

P > M/2 2P-M 2(M-P) 0

P = M/2 0 M 0

P<M/2 0 2P M-2P

P = 0 0 0 M

Example 1

P=86ppm; M=118ppm

M/2 = 118/2 = 59ppm

So P>M/2

Then

OH alkalinity = 2x86-118 = 172 – 118 = 54

Causticity = OH alkalinity/3

So Causticity = 54/3 = 18ppm

Carbonate alkalinity = 2(M-P) = 2(118 – 86) = 2x32 = 64ppm

The table shows fro the condition P>M/2 bicarbonate alkalinity is Zero.

Total alkalinity = OH + Carbonate + bicarbonate

= 54 + 64 + 0 = 118ppm

Conditions

1. If P and M alkalinities give the same result, then all of the alkalinities are

due to hydroxyl ions (no carbonate and bicarbonate alkalinity)

2. If P alkalinity is greater than half of M alkalinity, then it indicates that

there is hydroxyl and carbonate alkalinity but no bicarbonate alkalinity

3. If P alkalinity is equal to half of M alkalinity, it indicates that all

alkalinity is due to carbonate. There is no bicarbonate alkalinity.

4. If P alkalinity is less than half of M alkalinity, it indicates that carbonate

and bicarbonate alkalinities are present

5. If P alkalinity is zero, then M alkalinity is due to bicarbonate only.

Hydrogen ion concentration

The pH is a measure of the acidity or alkalinity of the water. Neutral pH is 7.00.

The pH below 7 is considered acidic and above alkaline.

Causticity

When the pH of water exceeds about 9.6 to 9.8 a measurable concentration of

hydroxyl ions (OH) begins to appear. As the hydroxyl alkalinity increases, the

pH of the solutions also increases. Table shows relationship between pH and

causticity.

pH OH alkalinity pH OH alkalinity

As

CaCO3

As OH As

CaCO3

As OH

9.0 0.5 0.17 11.1 63 21.4

9.5 1.6 0.54 11.2 79 26.9

10.0 5 1.70 11.3 100 34.0

10.2 8 2.72 11.4 126 42.8

10.5 16 5.44 11.5 158 53.7

10.6 20 6.80 11.6 199 67.7

10.7 25 8.50 11.7 250 85.0

10.8 33 11.2 11.8 315 107

10.9 40 13.6 11.9 397 135

11.0 50 17.0 12.0 500 170

.

Conductivity and total dissolved solids

In neutral or alkaline waters there is no consistent relationship between

conductivity and TDS.

If alkaline water is acidified to the phenolphthalein end point with organic acid

such as Gallic acid (which neutralizes causticity but does not contribute to

conductivity), the TDS is approximately equal to two-thirds the neutralized

conductivity in micromhos.

If synthetic organic polymer is used as sludge dispersant a value of 0.7 may be

used as conversion factor.

If Tannin us used then the conversion factor is 0.7 to 1.0 depending upon the

amount of Tannin used.

In-plant Laboratory requirements

Select a suitable location inside the plant for Laboratory.

Select equipments to perform all tests routinely.

Provide sufficient quantities of standard materials (beakers, test tubes, graduated

cylinders and casseroles).

Provide appropriate test kit along with the equipment.

Keep stock solutions and reagents for each test.

Testing of sample from industrial water systems

The testing of industrial water is done to determine the amount of treatment

chemicals in the water. This gives the idea whether the dosage level are properly

regulated.

Portable test kits are to be used in the field from where the sample is collected.

There should be testing procedures that has to be followed during the testing.

Theoretically all water analysis should be made at 25oC (77

oF).

The shorter the time between the collection and analysis of the sample the more

reliable will be the results.

When water sample color interfere with analysis it may be necessary to filter the

sample through activated charcoal, except for the sulfite and nitrite tests.

Industrial water treatment chemicals

Commonly used chemicals in water treatment:

1. Antifoam: Polyamide or Polyhydric alcohol

2. Antifreeze: Ethylene glycol (minimum boiling point 300oF)

3. Biocide

3.1. Methylene bis (thiocyanate)

3.2. 2,2 Dibromo 3 nitrilo propionamide

3.3. Dodecylguanidine hydrochloride

3.4. n-Alkyldimethylbenzyl ammonium chloride

3.5. poly [oxyethylene (dimethyliminio)ethylene

(dimethyliminio)ethylene dichloride

3.6. Disodium cynodithioimidocarbonate

3.7. Potassium n-methyldithiocarbomate

3.8. 1-Bromo-3-chloro-5,5-dimethylhydantoin

4. Calcium hydroxide (slaked lime)

5. Calcium hypochlorite

6. Caustic soda

7. Cyclohexyl amine

8. Diethyl aminoethanol (DEAE)

9. Diphosphonic acid (HEDP) 1-hydroxylethylindine 1,1-diphosphonic acid

10. Hydrochloric acid

11. Morpholine

12. Octa decylamine

13. Disodium phosphate

14. Polyphosphate glass

15. Sodium hexa meta phosphate

16. Sodium tri polyphosphate

17. Tetra sodium pyro phosphate

18. Tri sodium phosphate

19. Poly acrylate

20. Poly acrylic acid

21. Poly methacrylate

22. Tannin extract

23. Soda ash

24. Sodium chloride

25. Sodium hydrosulfide

26. Sodium hydroxide

27. Sodium hypo chlorite

28. Sodium Molybdate

29. Sodium nitrite

30. Sodium silicate

31. Sodium sulfite

32. Sulfonated styrene copolymer

33. Sulfuric acid

34. Tolyl tri azole

35. Zinc sulfate

Note:

Technology of boiler water, cooling water and waste water treatment is

explained elsewhere