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Last updateMon, 11 Dec 2017 4pm

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Check Valves in LNG Cryogenic Service

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Industry Headlines

Construction Index Remains Strong in November

Monday, 11 December 2017  |  Chris Guy

The Dodge Momentum Index surged again in November, climbing 13.9% to 149.5 from the revised October reading of 131.3. The Momentum Index is a monthly ...

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Will Smart Machines Obsolete Human Resources?

Will Smart Machines Obsolete Human Resources?

Wednesday, 06 December 2017  |  Kate Kunkel

Is artificial intelligence (AI) going to take over manufacturing jobs and the jobs of human resources professionals? That is a question that concerns ...

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Industry Headlines

Classic Controls Certified by ITT Engineered Valves, REXA

4 DAYS AGO

Classic Controls is now certified by ITT Engineered Valves to repair Skotch brand FM approved and general purpose burner shut-off valves for oil and gas fired systems.

REXA, Inc. has also appointed Classic Controls as its exclusive authorized sales representative in the state of Florida. Classic Contr...

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Emerson Completes Paradigm Acquisition

7 DAYS AGO

Emerson has completed the purchase of Paradigm, a provider of software solutions to the oil and gas industry. Paradigm will be combined with Emerson’s Roxar software business to create a more comprehensive Exploration & Production (E&P) software portfolio offering.

Paradigm is headquarter...

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China to Become Second-Largest LNG Importer

4 DAYS AGO

The world’s no. 1 energy user, China, “is on the verge of passing South Korea to become the No. 2 buyer of LNG behind Japan, as environmental measures and winter heating needs boost demand,” Bloomberg reports .

“Tankers with total capacity of 33.6 million metric tons have visited...

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New Permian-Gulf Pipeline Proposed

6 DAYS AGO

Magellan Midstream Partners intends to develop an approximately 375-mile, 24-inch diameter pipeline from Crane to a location near Three Rivers, TX, providing shippers the option to ultimately deliver crude oil and condensate from the Three Rivers area to the Houston area via a new 200-mile pipeline or...

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Construction Index Remains Strong in November

11 HOURS AGO

The Dodge Momentum Index surged again in November, climbing 13.9% to 149.5 from the revised October reading of 131.3. The Momentum Index is a monthly measure of the first (or initial) report for nonresidential building projects in planning, which have been shown to lead construction spending for non...

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U.S. Adds 228,000 Jobs in November

3 DAYS AGO

Total nonfarm payroll employment increased by 228,000 in November, and the unemployment rate was unchanged at 4.1%, the U.S. Department of Labor reported today. Employment continued to trend up in professional and business services, manufacturing and health care.

In November, manufacturing added 31,000...

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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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