Valve Magazine - Spring 2012

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 equiv-and its monopoly status, which has helped shield the water sector from certain economic factors.

Some analysts say that recovering more of the actual cost of water—if not moving toward full-cost pricing—could be an important avenue to increase the investment needed in our aging water infrastructure. They point out that water rates in the U.S. are typically less than in other developed countries. In fact, results of the International Water Report and Cost Survey, conducted by NUS Consulting Group in 2007, indicated that U.S. water rates were the least expensive at 66 cents per cubic meter average compared to the 13 other developed countries that were surveyed ( Denmark’s rates were the highest at $2.25 per cubic meter).

A significant source of funding for water and wastewater comes from the revenues generated by user rates; therefore, pricing water to accurately reflect the costs of providing quality water services is one strategy that utilities are using to maintain infrastructure and encourage conservation.

However, as rates increase, utilities will need to proactively educate consumers on the true value of water to absorb the shock of higher rates and ease consumer acceptance of those rates, according to Avoiding Rate Shock: Making the Case for Water Rates, a study sponsored by the AWWA [American Water Works Association] Water Utility Council. The study found that consumers get upset over rate increases because of misunderstandings about the true value of a safe, adequate supply of water, and that a consistent, structured communications strategy can build support for rate increases.

Another potential source of revenue for municipalities is increasing operational efficiencies. In AWWA’s State of the Industry Report 2010, respondents indicated they expect operational savings in 2011 to account for nearly 50% of utilities’ capital (in 2006, it accounted for 13%). Operational efficiencies that can result in significant cost-sav-ings can be enhanced through smart technologies such as automated meter reading (AMR) and advanced metering

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)

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

alent 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).

To protect equipment investments, the use of super-duplex and super-infrastructure (AMI), or by using non-invasive leak detection to improve water conservation and prioritize the replacement of deteriorating pipes.

Despite the estimated spending gap
for water infrastructure improvement,
everyone knows that a long-term solu-
tion to pay for much needed repairs and
replacement is necessary. (Respondents
to AWWA’s report indicated that inade-
quate attention toward the physical con-
dition of water infrastructure is the
austenitic stainless steels in high-pres-
sure applications has been recommend-
ed, as they show the greatest resistance
to the high corrosiveness of seawater.
Duplex stainless steels are used to a lim-
ited 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