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Industry and Regulatory Changes in Offshore Operations

Industry and Regulatory Changes in Offshore Operations

On May 13, 2016, the offshore oil and ga...

Strategies for Successful SIS Valve Diagnostic Implementation

Strategies for Successful SIS Valve Diagnostic Implementation

End users and contractors alike strive t...

Stop Check Valves

Stop Check Valves

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Industry Headlines

Weir Opens New Safety Relief Valve Facility in Malaysia

-1 DAYS AGO

Weir has opened a new facility to increase the global reach and competitiveness of Weir valves by establishing a manufacturing and assembly plant close to Malaysia’s capital Kuala Lumpur.

The Malaysian operation has already successfully supplied several large and strategic projects, especially ...

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ValvTechnologies Names Garcia, Mermelstein to New Positions

5 DAYS AGO
ValvTechnologies Names Garcia, Mermelstein to New Positions

ValvTechnologies, Inc. has named David Garcia director of manufacturing and Steve Mermelstein as regional director, Americas and director, channel management.

Previously, West Point graduate Garcia served as director of operations for Merla Wellhead Solutions and held plant manager positions at Hallib...

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U.S. Still Top Producer of Petroleum, Natural Gas Hydrocarbons

3 DAYS AGO

The U.S. remained the world's top producer of petroleum and natural gas hydrocarbons in 2015, according to U.S. Energy Information Administration estimates . U.S. petroleum and natural gas production first surpassed Russia in 2012, and the U.S. has been the world's top producer of natural gas since 20...

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Why $50/Barrel Oil Means Stability to U.S. Producers

4 DAYS AGO

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Durable Goods Orders Surge 3.4%, But Demand Soft

4 DAYS AGO

New orders for manufactured durable goods in April increased $7.7 billion or 3.4% to $235.9 billion, the U.S. Census Bureau announced today. This increase, up three of the last four months, followed a 1.9% March increase. Excluding transportation, new orders increased 0.4%. Excluding defense, new or...

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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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