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Young Valve Professionals: Logan Moore

Young Valve Professionals: Logan Moore

In 2014, VMA's leadership created the Va...

VALVE Magazine: Readers' Choice 2017

VALVE Magazine: Readers' Choice 2017

Happy New Year to our VALVE Magazine rea...

Educational Opportunities Abound at New VMA Knowledge Forum

Educational Opportunities Abound at New VMA Knowledge Forum

From technical to manufacturing to human...

Offshore Production in a Low Oil-Price Environment

Offshore Production in a Low Oil-Price Environment

Three years ago, The Washington Post publi...

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Interior Coatings for Waterworks Valves

Interior Coatings for Waterworks Valves

Monday, 15 January 2018  |  John V. Ballun, P. E.

Since the 1990s, two types of epoxy coatings have been commonly specified and used for iron valves in the waterworks industry: fusion-bonded epoxy and...

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Industry Headlines

Velan Reports Third Quarter 2017/18 Financial Results

15 HOURS AGO

Velan Inc. announced its financial results for its third quarter ended November 30, 2017. Sales amounted to $87.7 million, an increase of $7.3 million or 9.1% from the prior year.

Net earnings amounted to $0.3 million or $0.02 per share compared to $1.5 million or $0.07 per share last year. After two...

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Emerson Named ‘Industrial IoT Company of the Year’

5 DAYS AGO

Emerson has been named the “Industrial IoT Company of the Year” by IoT Breakthrough. The IoT Breakthrough Awards, which received more than 3,000 nominations in 2017, recognize innovators, leaders and visionaries from around the globe in a range of IoT categories, including Industrial, Sma...

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Pipeline Capacity Expansions Accelerating in U.S. Northeast

13 HOURS AGO

With projects such as Columbia Gas Transmission’s Leach Xpress and Energy Transfer Partners’ Rover pipeline, “[t]he U.S. northeast should realize an increase of more than 3 Bcf/d of natural gas pipeline capacity by the end of this quarter, compared with an increase of 2.3 Bcf/d in ...

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EIA Forecasts Increasing Global Crude Production Through 2019

4 DAYS AGO

The U.S. Energy Information Administration’s (EIA) new Short-Term Energy Outlook forecasts Brent crude oil to average $60 per barrel (b) in 2018 and $61/b in 2019, slightly higher than the $54/b average in 2017. In both 2018 and 2019, EIA expects total global crude oil production to be slightly ...

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Texas Employment Forecast Calls for 3% Growth

5 DAYS AGO

Texas job growth is forecast to strengthen to 3% in 2018 from an estimated 2.5% in 2017, according to the Federal Reserve Bank of Dallas . The forecast means Texas should add about 366,000 new jobs in 2018. Texas added 305,900 jobs in 2017, ranking No. 3 in the nation for job growth after falling belo...

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U.S. Factory Orders Rose By 1.3% in November

7 DAYS AGO

New orders for manufactured goods in November, up five of the last six months, increased $6.5 billion or 1.3% to $488.1 billion, the U.S. Department of Commerce reported . This followed a 0.4% October increase. Shipments, up eleven of the last twelve months, increased $5.7 billion or 1.2% to $491.2 bi...

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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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