Membrane-based reverse-osmosis (RO) filtration systems offer valuable service in a wide variety of industrial and commercial settings. They purify water, improve taste, and provide savings in food and beverage processing; increase energy efficiency in boilers; and supply a range of other benefits in applications from water jet cutting, vehicle washing, and humidification to restaurant and grocery use.
One important component of membrane-based RO systems, typically used at several critical points, is the solenoid valve. Design engineers working for original equipment manufacturers (OEMs) face multiple options — and issues — when selecting these complex, highly engineered devices for their systems.
Beyond the usual considerations of correct sizing and wattage, many current models may exhibit worrisome performance problems, as well as difficulties relating to certifications, availability, ease of assembly, and support, among others.
Fortunately, valve technologies are now available that avoid many or all of these problems, while providing significant benefits for the OEM and end-user alike. Here are seven key considerations for proper solenoid valve selection in RO systems.
1. Does it have the proper certifications?
OEMs must accommodate new regulations in many parts of the world, including those specifying the use of lead-free products and more. For instance, most current U.S. plumbing codes and state drinking water regulations demand that RO equipment for potable water applications meets NSF 372 leachate and lead-free requirements.
However, regulations and their means of adoption have been changing with relative rapidity. Caution is advisable. In specifying composite valves for these RO systems, pay close attention to how the certification status of each product is stated.
Some don’t mention NSF certification at all, which probably means the given product does not possess it. This should be a red flag. It could legally preclude use of OEM RO systems in certain North American localities, such as in California or Vermont.
Furthermore, S. 3874, the "Reduction of Lead in Drinking Water Act," became effective in January 2014 throughout the United States. This act modified the Safe Drinking Water Act (amended in 1986), specifically regarding lead-free standards for pipes, pipe fittings, plumbing fittings and fixtures.
Some statements will indicate that a product complies with a certain standard. However, untested and uncertified compliance does the OEM little good in jurisdictions that may require the certification itself. Look for valves that possess all the certifications necessary for the intended application. Valves suitable for applications such as water jet cutting, vehicle washing, boilers, and humidification systems are tested and certified to NSF 372 (weighted average lead content for lead-free applications using industrial purified water). For potable water applications, such as food and beverage processing and restaurant steamers, relevant models are tested and certified to NSF 61 Annex G (for drinking water leachate requirements, and low lead content) and NSF 169 (for special-purpose food equipment and devices). They’re also tested and certified to NSF 42 for drinking water treatment units — aesthetic effects.
2. Will it be available when needed?
More and more often, today’s lean manufacturing methodologies demand the ready availability of composite valves used in RO systems — as do the lower work-in-process inventory requirements now common in OEM operations.
When evaluating valve makers, consider availability issues such as quick-shipment programs, local coverage, and the specific inventory of required composite valves. What are the lead times and delivery schedules? Request the suppliers’ record of meeting shipment dates for its quick-ship program.
3. Is it reliable?
Certain other characteristics must also be carefully considered when selecting composite valves for RO applications. They can have a real impact on product reliability, maintenance costs, and service life.
Unfortunately, it’s not uncommon for some composite valves used for RO applications to leak — even after only a few thousand operating cycles. This issue may be caused simply by poor connection system design. However, it can also be due to out-of-spec pressure or thermal stress on the connection or other parts of the valve.
Not all composite valves are designed or rated for the higher pressures and temperatures inherent in some RO system applications. Ultimately, it behooves the designer to make sure the valve’s ratings are adequate for the job. As a protective measure, many designers select valves with ratings that slightly exceed levels expected when installed on a given piece of equipment.
4. Can it connect easily and securely?
Connections can be a major concern when specifying composite valves for RO applications.
For example, valves utilizing traditional NPT and solvent bond connections are often a source of frustration. These may require considerable trouble and expense in equipment design, installation, and assembly. The logical alternative would be quick-connect valves. But while there have been concerns in the past with such solutions, designers today now have a better choice — new, advanced types of quick-connect technology that can maximize ease and speed, while also preventing leaks.
5. Is it easy and economical to design and install?
Several situations can make integration of valve components into purified water and reverse-osmosis filtration systems especially difficult and time-consuming.
Often, desired performance characteristics are maximized by using, for example, PEX tubing in one part of an RO system, and PVC or copper (CTS) tubing in another part. However, this requires the design, specification, purchase, assembly and installation of custom fittings or adapters at the place or places where components of differing types or materials must come together. When multiple such situations exist in a single piece of equipment, or in cases where OEMs must turn out high volumes of such equipment, added costs and lost time can be substantial.
Another issue may arise where composite valves threaded for NPT connection must be assembled or installed. The concern here is cracking. When a piping assembly is put together, the valve is screwed onto the pipe with a given torque or pressure, to the tightness required for a good seal. However, this procedure usually will not leave the valve aligned correctly, for instance in an upright position, for proper mounting. So the valve must then be further rotated into its final position.
Problem: this further tightens the valve. With brass or stainless steel valve bodies, such over-tightening does no harm. But depending on the degree of rotation involved, the thermoplastic threads of composite models may easily split or crack under the increased strain — ruining the entire valve. This cracking may only be evident when the system is pressurized (worst case: in the field), after which the entire component must be disassembled and replaced.
(Note that some models include stops to prevent overtightening. However, this presents assemblers with almost equally aggravating problems: they must then search for some other point in the system that can be tightened or rotated to compensate.)
In critical assemblies using composite valves, then, cautious engineers had traditionally gone through the trouble and expense of specifying two extra parts per valve: adding unions to each end. With the introduction of new technology models, however, both these problems are resolved — by providing as many as three possible varieties of union coupling on each end — even on NPT-threaded models. Thus, piping and valves of disparate types and materials may be connected quickly and easily, with fewer piping accommodations and no custom fittings or adapters. Additionally, each NPT end coupling can be screwed onto the pipe with the required torque for a proper seal. The valve is then rotated freely to the proper position, and the union joint quickly and easily secured.
6. Is it available in both normally closed and normally open versions?
Many composite valves are offered only in normally closed versions. This can force OEM designers to make unwelcomed equipment compromises. For example, they may have to purchase expensive diverter valves to compensate for this lack of normally open models.
Identify sources that offer both normally open and normally closed composite solenoid valve versions. This can furnish needed design speed and flexibility, while simplifying sourcing with fewer part numbers to keep in stock.
It may even save money. For instance, purchasing a normally open plus a normally closed valve and wiring them in series delivers the functionality of a diverter valve for less cost.
7. Does it come with the right support?
When selecting valves for an RO system, don’t stop at hardware characteristics. Evaluate potential buys also on the level of support available from the supplier.
Consider the quality of the support as well as basic geographical reach. With North American customers, for example, products made by Asian or European manufacturers naturally face greater logistical challenges in terms of time to get answers, technical fixes, and onsite service or training.
Additionally, satisfactory support should now extend beyond phone calls and site visits. The Web has greatly expanded support possibilities for providing technical product information, specification assistance with sizing and application issues, and even 3-D engineering drawings and similar enriched content.
New composite valve technologies can offer notable benefits when improved valves are specified for membrane-based filtration functionality in reverse-osmosis systems. To maximize the benefits of the selection of a specific solution, compare widely and evaluate carefully. Consider up-to-date certifications; ready availability; high reliability even in challenging applications; quick, secure connection; ease of use and ensured savings in design, assembly and installation; a choice of open and closed versions; and comprehensive, resource-rich support. For the OEM, they all add up to greater cost savings, longer ensured equipment life, and shorter time to market.
Roy Bogert has served as Principal Engineer with ASCO Valve since 2006 and has been a practicing engineer since 1984. He holds multiple solenoid valve patents including the FasN™ pipe connection system for ASCO’s 212 valve. Mr. Bogert can be reached at 973 966-2530 or Roy.Bogert@emerson.com.
David Park is Senior Product Development Engineer for ASCO Valve, working on NSF certified composite valves since 2010. Mr. Park has experience working with water industry approval agencies, such as NSF. He can be reached at 973 966-2255 or David.Park@emerson.com.
For questions and inquiries about the technology covered in this article, please contact Rob Lindquist, ASCO product marketing manager, at Robert.Lindquist@emerson.com or 973-966-2266.