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U.S. Economy Grew 0.7% in the First Quarter of 2017

Friday, 28 April 2017  |  Chris Guy

Real GDP in the U.S. fell short of expectations, increasing at an annual rate of 0.7% in the first quarter of 2017; this according to the advance esti...

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Industry Headlines

Mueller Water Products Reports 2017 Second Quarter Results

1 DAY AGO

For its fiscal second quarter ended March 31, 2017, Mueller Water Products’ net sales were up 1.3% to $199.7 million and net income was $73.3 million, or $0.45 per diluted share. Operating income from continuing operations was $10.9 million. The quarter's results included $68.6 million of income...

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Weir Group Reports First Quarter 2017 Results

1 DAY AGO

The Weir Group's first quarter input was 15% higher than the prior year period with good sequential growth primarily driven by increased activity levels in North American Oil & Gas and strong aftermarket orders in Minerals. Group-wide aftermarket orders were 21% higher than the prior year period w...

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Permian Basin Oil Production Continues to Increase

2 DAYS AGO

Crude oil production in the Permian Basin is expected to increase to an estimated 2.4 million barrels per day in May, based on estimates from the U.S. Energy Information Administration . Between January 2016 and March 2017, oil production in the Permian Basin increased in all but three months, even as...

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East Coast Refiners Eye Texas as Alternative to North Dakota

2 DAYS AGO

“Major U.S. East Coast refiners profited from railing hundreds of thousands of barrels of discounted Bakken crude to their plants daily from 2013 until 2015. But as more and more pipelines were built in North Dakota, the discount began to disappear, and so did the rail cars,” Reuters repor...

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U.S. Economy Grew 0.7% in the First Quarter of 2017

1 DAY AGO

Real GDP in the U.S. fell short of expectations, increasing at an annual rate of 0.7% in the first quarter of 2017; this according to the advance estimate just released by the U.S. Department of Commerce. The 0.7% figure, down from 2.1% in the fourth quarter and 3.5% in the second half of 2016, and is...

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Survey Shows Small Business Confidence Increasing

2 DAYS AGO

The second annual Allstate/ Small Business Barometer finds increasing optimism and innovation among small business owners, despite the rising cost of doing business. Nine in 10 local entrepreneurs say the benefits of owning a business outweigh the challenges. This year’s Barometer found that, ...

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Indices that Measure Water Scaling

materials_q_and_a_graphicQ: Why do my carbon steel valves and piping corrode when the Langlier index says they shouldn’t?

 

A: There are a variety of indexes that measure the scaling ­tendency of water and the Langlier index is probably one of the more recognized and utilized. Notice I said that the index measures the scaling tendency of water, so there is really no direct correlation between the Langlier index and any of the other indices to the actual corrosion rate of steel in water. The belief that these indexes determine the corrosivity of water stems from the assumption that a scale-forming water will be protective to carbon steel whereas a non-scaling water will be corrosive.

 

Water’s Scaling Tendencies

Many people assume that water is not very corrosive but a water’s chemistry can vary considerably, and depending on its given chemistry it can be very corrosive to cast iron and carbon steel. In addition to corrosion another issue associated with a water’s tendency to cause scaling is its affect on a heat exchanger’s efficiency.

To determine a water’s scaling tendency, several indices or formulas have been developed over the years, with each considering different variables of a water’s chemistry. The more common indexes are the Langlier Saturation Index (LSI), the Ryznar Stability Index (RSI) and the Puckorious Scaling Index (PSI)... and, no, I am not making up these names! The following will explain how each of is determined.

LSI = pH – pHs

The LSI index, developed by Dr. Langlier in 1936, considers factors of a water’s tendency to be in equilibrium with calcium carbonate. Calcium carbonate is just one of many minerals or elements that can be found in water and is the one responsible for forming calcareous deposits. The LSI considers the effects of calcium, total alkalinity, dissolved solids and temperature to arrive at a computed pH shown as pHs in the formula. Determining pHs manually is mathematically onerous, but fortunately there are websites that let you plug in the water chemistry and then pHs is calculated for you. Once pHs is known, simply subtract it from the water’s actual pH and if the result is positive then the water will be scaling; conversely, if the number is negative the water will tend to dissolve calcium carbonate.

RSI = 2 pHs – pH

John Ryznar developed a modification of the Langlier index in 1944 upon realizing that it was possible for both low- and high-hardness waters to have the same LSI. By reversing the placement of pH and pHs in the formula the RSI always results in a positive number. The pHs for the RSI is determined by the actual pH and the concentration of the calcium and bicarbonate ions, total dissolved solids and temperature. This calculation for pHs can also be found on the web so you just have to plug in the appropriate variables to determine pHs. A RSI less than 5 should be scaling, whereas a RSI above 7 will produce little, if any, scale.

PSI = 2pHs – pHeq

The next improvement of the scaling index was developed by Paul Puckorius. The Puckorius Scaling Index is also called the Practical Scaling Index, which I suppose makes it sound a little more credible! The PSI accounts for two additional variables that the other indices do not: the buffering capacity of water and the maximum quantity of precipitate that brings water to equilibrium. Therefore, the PSI uses an equilibrium pH rather than the actual pH to account for the buffering effect. The equilibrium pH (pHeq) = 1.465 log (M alkalinity) + 4.54. The numbers resulting from this formula are the same as the RSI index so a value less than 5 will be scaling and a number greater than 7 will result in little, if any, scaling.

Another Method to Control Scaling

One method used to control scaling is to lower the pH as this lowers a water’s alkalinity. So it is to be expected that a scaling index should account for this change, which the LSI and RSI do, but not the PSI. However, be careful in lowering the pH much below 6, because this will result in significant general corrosion of carbon steel and cast iron.

So which index is best to use?

Tests done for a variety of waters found that the LSI seemed to be more reliable in predicting scaling tendency, but there is no guarantee that scaling will or will not occur. Keep in mind these indices were never intended to be predictors of corrosion for carbon steel or cast iron but rather a tendency for water to develop scale or not. As much as we would like to have a simple formula to tell us whether a given water chemistry will be corrosive to steel, there are other variables—such as dissolved oxygen, chlorides and sulfate ions—that influence the corrosion of steel.


Thomas Spence is director of materials engineering for Flowserve Corp. (www.flowserve.com), Dayton, OH. Reach him at This email address is being protected from spambots. You need JavaScript enabled to view it..

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