Important Considerations to Performance of
|An Adobe PDF version of this article is available by clicking here.|
Click to get
Over the past 20 years, disinfection technology and chemical treatment for wastewater has been consistent with the use of Chlorine as the standard chemical to safely and effectively treat wastewater media. The delivery of the chlorine to the flow stream ranges from gaseous, liquid chemical, or carrier chemical agents. Today, through the development and realization towards improving safety and reducing risk, Sodium Hypochlorite (NaOCl) has commonly been used in various applications where disinfection or sterilization of water is needed. NaOCl is becoming more and more popular based on its inherent chemical characteristics of being a safer, less costly, lower risk chemical, and still effective means to treat waste water media. The problems and practical solutions for NaOCl and related piping systems are the discussion topics in this article.
Sodium Hypochlorite Characteristics
Sodium Hypochlorite can be considered a solution of dissolved Chlorine gas in Sodium Hydroxide. Simply put, its character is that of common household bleach or "swimming pool" chlorine; however, sodium hypochlorite for wastewater treatment usually is found in 12.5% concentration. Chlorine is easily released from the Sodium Hypochlorite due to the breaking of weak ionic bonds with its base molecule, Sodium Hydroxide. Piping material, valve selection, seal materials, pressure relief, and stagnation control, are a few design, safety, and operational measures to consider when designing NaOCl systems.
Sodium Hypochlorite carries a relatively high pH (about 12.8) and a concentrated chlorine gas element that again is easily released into solution. As the NaOCl is added, the solution becomes diluted in pH scale, but not by much. The solution of diluted water with sodium hypochlorite (12.5%) still has a pH of about 9 or higher. The reason behind this is due to the release of the chlorine and high concentrations of the caustic byproduct, Sodium Hydroxide. In some plants, various chemicals, such as lime or calcium oxide, may be added to help stabilize alkalinity of the media. This may cause problems with scaling inside piping systems thereby clogging up piping, interfering with valve operation, or making flowmeter and rotameter type equipment nearly impossible to read (see picture at right.)
Furthermore, outgassing occurs with sodium hypochlorite due to common decomposition of the chemical. Chlorine and oxygen are the most prevalent gases to consider when designing safety relief of piping systems or when making valve selection. Equipment used in piping systems must be carefully selected to withstand the high and fluctuating pH levels, outgassing concerns, and scaling effects. Chemical resistance, design safety, and system performance become major issues with respect to above.
PVC is the most widely used piping system material for sodium hypochlorite applications due to its good chemical resistance, availability and economic value.
Piping System Material Selection
For past years, PVC and CPVC piping materials have been used successfully for Sodium Hypochlorite applications. PVC is the most widely used due to its good chemical resistance, availability and economic value.
The biggest concern for PVC piping is the joint quality during installation. A few points to watch out for are:
Designing for valve selection over the past years has been discovered in some cases by trial and error. Today, we know to use caution with ball valves in Sodium Hypochlorite applications due to possible stem fractures caused by crystallization problems. Ball valves can explode due to excess gas buildup in the interstitial space of the ball valve while in the closed position.
In many cases, the downstream side of the ball has been machined with a small purge hole. This allows the interstitial space to "vent and wet" in order to avoid gas buildup or crystallization. In any case, ball valves should be cycled periodically to keep crystallization from occurring and to avoid stem fracture due to the ball "freezing up."
For Sodium Hypochlorite applications, true-union and flanged end diaphragm valves are recommended over ball valves due to their control capabilities, seal materials, durability, less maintenance, and longer life. PVC bodies are sufficient for choice of design with CSM (chlorosulfonated polyethylene - Hypalon - see picture at right) diaphragms and EPDM (ethylene propylene polymer) o-rings.
For sodium hypochlorite applications, spigot, true union and flanged end diaphragm valves, like the George Fischer Diaphragm Valve, are recommended over ball valves, due to their control capabilities, seal materials, durability, low maintenance and long life. This valve offers additional advantages with its snap-on handwheel and optional lock.
CSM diaphragms and EPDM O-rings
The benefits of using CSM diaphragm material is based on its good chemical resistance to Sodium Hypochlorite and Sodium Hydroxide (high pH) concentrations and its excellent mechanical properties like long flex life, tear and abrasion resistance, and toughness.
Other materials have been used in the past, such as FPM (Viton®) or PTFE (Teflon®) backed with EPDM or FPM. The lack of success of FPM to withstand concentrated caustic solutions and the expense of PTFE backed materials cannot compete with the effectiveness and added value considerations of CSM.
It may appear that EPDM diaphragm material would be adequate for Hypochlorite and Hydroxide applications from a chemical resistance standpoint; however, it does not stand up to the combination of properties CSM offers (mechanical, flex life, and chemical resistance). EPDM is more effective as an o-ring material due to its good chemical resistance and compression set characteristics for this particular application. CSM exhibits poor compression set resistance, thereby not making it a good material for static o-ring selection.
Measurement and Instrumentation
Instrumentation for flow and pH are critical for Sodium Hypochlorite applications. Measuring flow can be accomplished by use of a rotameter or by electronic means such as a vortex style flowmeters.
Rotameters can give both electronic and visual indication pending the need of the user. PVC connections, PSU (polysulphone) tube material, and EPDM o-rings are recommended. Again, consideration to scaling should be taken into consideration when selecting where to install the rotameter. A rotameter site tube that is "scaled" is nearly useless to visually measure the float position. Rotameters are effective instruments to measure flow in a simple and cost effective way, but can be problematic with respect to system scaling.
Another way to detect flow is to utilize a vortex style flowmeter. A vortex style flowmeter provides extremely accurate and reliable flow measurement with no moving parts. Essentially, it uses vortices created by the flow stream to measure the flow rate. It is relatively safe from becoming scaled and can be installed with remote and direct visual indication. Vortex flowmeters are highly accurate, reliable, but more expensive versus the rotameter type.
System pH instrumentation is also very crucial in order to monitor the concentration levels of Sodium Hypochlorite. The reaction rate of Sodium Hypochlorite increases by:
Outgassing of chlorine and oxygen have been mentioned as concerns for system design. Gas buildup in the piping system Tank Transfer line, valves, and pumps is a topic for "design discussion". The consideration to design and system operation can be approached in the following:
George Fischer V85 Pressure Relief Valve
Pressure relief valves and high point venting should be practically designed into strategic parts of the system with consideration to shutdown, service maintenance, and standard safety functions.
Investigate the elements of Sodium Hypochlorite and, surprisingly, dont "over-engineer" your system. Be practical. Piping materials and components, pressure relief design, and standard operating procedures are of significant importance for optimum performance of Sodium Hypochlorite wastewater treatment systems. Explore the options closely when choosing the materials as what is presented as Sodium Hypochlorite may not be what to specifically design around. In retrospect with this examination of Sodium Hypochlorite, designing for the actual chemical "Sodium Hypochlorite" has been of short discussion. Sodium Hydroxide, Chlorine, and operational issues are the engineering considerations caustic, corrosive, "neat and smart".
Kurt Lind is a Production Manager / Mechanical Engineer for George Fischer, Inc.,
2882 Dow Avenue, Tustin, California 92780-7285, (714) 731-8800, Toll Free (800) 854-4090
Fax: (714) 731-6201, e-mail: firstname.lastname@example.org,
Viton® and Teflon® are trademarks of E.I. DuPont, Inc.
All trademarks are properties of their respective companies.