Important Considerations to Performance of
Sodium Hypochlorite Wastewater Treatment Piping Systems

by Kurt Lind, George Fischer, Inc.

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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:

  1. Strictly adhere to the material manufacturer’s joining procedures and also ASTM D2855 "Standard Practice for Making Solvent-Cemented Joints with PVC Pipe and Fittings." This is a very important factor as solvent-cement joining is sometimes taken for granted. Even leakage that isn’t seen by the naked eye will show up eventually if a sound joint is not made. Threaded joints should also be kept to a minimum and MIL spec P-27730A-rated Teflon tape should be used when making joints. Particular attention must be made when making the joint...each one.
  2. Good ventilation and Chlorine chemical resistant materials should be selected for NaOCl systems, especially in confined spaces like sheds, tank farms, pump houses, and the like. Over the course of time, chlorine gas will escape from the system causing corrosion problems for equipment and safety concerns for operating personnel in and around confined spaces. Anticipate leakage paths from joints and connections, granted you may not see them, but chlorine gas will find its way out. Larger leakage paths will be apparent with the formation of white crystallized salts forming around the leak port.

Valve Selection

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:

  1. the reduction of pH (releases more chlorine with reduced pH)
  2. higher Hypochlorite concentration
  3. higher temperature
  4. the presence of metal due to its volatile ionic chemical structure

Outgassing Considerations

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:

  1. Stagnation of flow or system isolation has been one of the most noted operational problems with Sodium Hypochlorite systems. Why? When the system is stagnant or "not allowing gas to be circulated through the system," gas becomes trapped in the isolated system. The Hypochlorite will continue to outgas and will buildup extreme high-pressure if not allowed to vent or recirculate. Diaphragm pump ruptures, short run piping system and ball valve explosions, pump malfunction (airlocking), and gas pressure relief valves are a few sensitive issues in which to take precautionary measures. Generally, if a system is left stagnant for long periods, the system should be drained, flushed, or allowed to be bypassed through a recirculation line.
  2. 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. Pumps, system isolation piping, tank piping, injection piping, and quality plumbing installation should be scrutinized. (See picture at right).

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.

Conclusions

Investigate the elements of Sodium Hypochlorite and, surprisingly, don’t "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".

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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: info@us.piping.georgefischer.com,

Internet: www.us.piping.georgefischer.com

 

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