The Unique Requirements for Valves in SRO Applications

Global Water Intelligence reported that in 2011, new construction within the desalination industry was about five million cubic meters per hour (m3/hr), forecast to increase by 140% to 12 million m3/hr by 2016. To meet the forecasted increase in demand, individuals charged with valve applications in this industry know that, more than ever, paying attention to materials selection, equipment configuration and energy efficiency are critical for improving financial returns on the sizeable investments required.

This article briefly reviews the seawater reverse osmosis (SWRO) process and broadly addresses those valve applications. It discusses materials of construction and the all-important pitting resistance equivalent number (PREN), along with several new desalination valve configurations. It also touches upon energy recovery devices (ERDs) and the critical role they play in SWRO efficiency.

VALVE SELECTION

It’s important to note that valves used in the SWRO process are applied to low-pressure and high-pressure services, which affect both valve configurations and the materials of construction. The ultimate goals are to maximize process efficiency, minimize maintenance and downtime, and contain or reduce energy expenses. Proper selection can improve the chances of extending the life of valves and increasing system uptime, while improving energy efficiency.

The SWRO plant is typically divided into four units: intake, pretreatment, desalination and final treatment (Figure 1).

Intake

The goal of the intake process is to remove materials that could foul or damage the efficiency of the expensive, sensitive membranes. In the intake process, raw seawater, with its high content of dissolved chlorides, enters the plant and all solids, such as silt, sand and organic materials, are removed. This is accomplished through standard sand and cartridge filters and, more recently, by ultra-filtration techniques—along with chemical pretreatment. For the process, water intake valves are typically specified with rubber-lined cast iron or ductile cast iron bodies.

This low-pressure process also requires a large number of butterfly valves ranging in size from 3 inches (80 millimeters) to 110 inches (2,500 millimeters). These valves may be specified in a wide range of configurations and in a variety of seat and disc materials, including Viton, Halar, Hypalon and PTFE, as well as other plastics and elastomers. Valve bodies are typically offered in seawater-resistant materials such as aluminum bronze, duplex stainless steel and super-duplex stainless steel. Valve sizes to 24 inches (600 millimeters) will have lug-style bodies, while larger sizes are normally flanged.

Chemical Pretreatment

To clean the water even more, chemical pretreatment is employed. This process further protects the relatively delicate membrane equipment as well as related process equipment and increases the efficiency of the reverse osmosis process. Oxidizing biocides, pH adjustment, anti-scalants, etc., use valves typically found in any chemical dosing operation. Depending upon the corrosiveness of chemical agents and operator preference, the valves used here typically range from relatively inexpensive plastic diaphragm and check valves to more robust PTFE-lined plug, ball and butterfly valves.

Desalination

The most demanding valve application in the SWRO process is during desalination, which occurs in the high-pressure membrane area. Here, both the pressure (up to 1,015 psi or 70 bar) and the chloride content of the water are high (Figure 2).

The high-pressure system requirements dictate use of ASME Class 600-rated valves, typically plug valves, for membrane isolation service. Although the seawater is very clean at this stage, its high chloride content necessitates use of super-duplex stainless steel (grade 5A) for all wetted parts of the valve.

Typically, these valves are butt welded rather than flanged, as most users want to avoid the expensive ASME Class 600 super-duplex stainless steel valve and pipe-mating flanges. To protect valve investments against corrosion, these valves can be specified with an encapsulated PFA plug that rides directly on a machined body seat. These high-pressure membrane isolation valves are commonly available in sizes to 16 inches (400 millimeters).

To better protect the membrane from over-pressurization during startup, some SWRO plant operators use a bypass line. Industry standards dictate the reverse osmosis system pressure should gradually increase to reach full process pressure in about two minutes. Some installations achieve this by using a variable speed drive pump or control valves. A simpler, less expensive solution involves a bypass line on the main isolation valve. This line includes a smaller ball valve with a seat capable of withstanding high-pressure drops. At the time of startup, the main valve would be closed and the bypass valve opened. This bypass valve is sized to ensure the system pressure is increased at the recommended rate. Once the system has reached full operating pressure, the main valve would open.

Post-treatment

After the reverse osmosis process is completed, two streams emerge for post-treatment. The clean water is disinfected either by UV lamps or chlorination before further transmission. A brine discharge containing high salt levels, chemicals from the treatment processes and toxic metals is either discharged into the ocean or treated in a sewage plant with disposal to a landfill. Because of the high salt levels, chemicals and metals, rubber-seated butterfly valves with ductile iron bodies or full port ball valves are typically deployed in these services.

SELECTING THE RIGHT MATERIALS

In processing chloride-laden seawater, using the best materials prevents localized forms of corrosion such as pitting, crevice corrosion and stress corrosion cracking. The SWRO industry has experimented with various materials, such as aluminum bronze, 904L austenitic stainless steel, duplex stainless steels and super-duplex stainless steels.

A growing body of materials research and failure analyses within the industry has led to these general recommendations:

Conventional 300 series austenitic stainless steels—i.e., 316, 316L, 317 and 317L and their ASTM A744 cast equivalents: CF8M, CF3M, CG8M and CG3M —should not be used in either high-pressure membrane feed or in low-pressure pretreatment due to their susceptibility to pitting and crevice corrosion.
While the duplex alloys 2205 (ASTM A890, grade 4A) and CD4MCuN (ASTM A995 or A890, grade 1B) outperform the 300 series austenitic alloys, they are still at risk of failure because of localized attack; and, therefore, they should be used with caution. The same is true for the austenitic alloy 904L.
Only the stainless steels most highly alloyed with chromium (Cr), molybdenum (Mo) and nitrogen (N) offer sufficient resistance to localized corrosion to provide reliable service, especially on the high-pressure side. These are generally the super-duplex and superaustenitic materials, which include Sandvik SAF 2507 (grade 5A) and AvestaPolarit 254 SMO (ASTM A744, grade CK3MCuN).

Years of cumulative experience in designing and maintaining equipment in SWRO applications has shown that pitting resistance has the biggest impact on the overall lifespan of valves. This has driven manufacturers to use materials based on the pitting resistance equivalent rating number (PREN)1. The higher the PREN, the greater the pitting and corrosion resistance.

With high Cr, Mo, and N levels, the superaustenitic and duplex stainless steels were designed to resist chloride-induced corrosion. Not only do they have higher PREN than some of the other materials, their critical pitting temperatures (CPTs) are also much higher (Figure 3).

Figure 3. Pitting resistance equivalent
and critical pitting temperature for
selected stainless steels

Grade	PRE	CPI, C° (F°)
316L	25	16 (60)
317L	30	29 (84)
904L	35	60 (140)
2205	35	33 (91)
SAF 2507	42	84 (183)
254 SMO	43	85 (185)

To protect equipment investments, the use of super-duplex and superaustenitic stainless steels in high-pressure applications has been recommended, as they show the greatest resistance to the high corrosiveness of seawater. Duplex stainless steels are used to a limited degree in low-pressure applications.

A WORD ABOUT ENERGY

Energy is generally the most significant cost driver in any SWRO facility, making up on average 30% of the total cost of water, according to the National Water Research Institute. In terms of daily operating costs, energy represents about half of total expenses.

Reverse osmosis has grown in favor over thermal desalination processes (multistage flash and multi-effect distillation) because of relatively lower capital costs and significantly lower energy costs. Moreover, the SWRO process can readily incorporate ERDs into its system architecture.

Energy recovery turbines using impulse rotary technology, and
Specially engineered equipment based upon isobaric technology, such as the dual work exchange energy recovery system (Figure 4).

Both technologies recover substantial amounts of energy stored in the brine waste stream to drive the high-pressure membrane pumps. These ERDs can reduce the energy consumption/cost of the pumps by as much as 60%.

ERDs have limited valve applications, although there are some control valves and several highly engineered, special-purpose valves installed on these systems. Most importantly, because reducing overall energy costs in SWRO plants is of paramount importance, those charged with designing and specifying ERDs are seeking ways to incorporate valves, actuation and controls that enhance flow efficiency and reduce energy costs.

CONCLUSION

The SWRO industry is growing at a rapid pace. A substantial core of fundamental application experience and expertise is continuously cataloged to meet this need. Yet SWRO is still a young and developing industry that will require new and adaptive technologies to fully exploit its potential.

For those involved in system design and equipment selection for SWRO plants, creativity is needed to address the unique operating environment. Making the proper choices will minimize downtime, extend the life of valves and protect these investments.

Ben Lee is a product manager at Flowserve Corporation (www.flowserve.com). He has 32 years of experience working with Flowserve in various roles. Reach him at blee@flowserve.com.

Footnote

1. Pitting Resistance Equivalent Numbers (PREN) indicate the relative resistance of stainless steel- and nickel-based alloys to pitting corrosion. PREN are calculated as follows: PREN = % Cr + 3.3(%Mo) + 16(%N). The higher the value, the greater an alloy’s expected resistance.