Mechanically actuated diaphragm metering pumps in chemical applications.
by Tom O’Donnell
September 24, 2018

The use of sodium hypochlorite (liquid bleach) is common in many industrial applications. Its uses include bleaching, cleaning, disinfecting, deodorizing and water treatment, all of which can be daily applications in an industrial or manufacturing setting.

The list of industries in which sodium hypochlorite is often used is also a long and diverse one: agriculture, chemical, paint and coatings, lime, food and beverage, glass, pulp and paper, pharmaceutical, synthetics, and heating, ventilation, air conditioning and refrigeration (HVAC/R).

The companies that manufacture sodium hypochlorite and use it for industrial purposes are well aware of the dangers that can be associated with its production and handling.

Confronting Off-Gassing Challenges

Sodium hypochlorite is an oxidizer with potentially corrosive oxidation reactions. When used in concentrated forms, the solution—if handled improperly—can burn exposed skin and cause severe eye damage. Also, when sodium hypochlorite solutions come in contact with some metals, flammable hydrogen gas may be created. If these containers are heated, they may explode due to the buildup and release of chlorine gas.

From a production standpoint, the main challenge in handling sodium hypochlorite is overcoming its tendency to off-gas when it is being pumped or metered. Off-gassing occurs when the sodium hypochlorite begins to decompose in a storage tank or transfer pipe. When sodium hypochlorite decomposition occurs—which happens at an accelerated rate in higher pH concentrations and at elevated ambient temperatures—it transforms into a salt solution that generates oxygen bubbles. As the oxygen bubbles accumulate in the pumping or metering system’s suction lines, they can cause a loss of prime, which will inhibit the effectiveness of the pumping or metering process.

Therefore, any industrial manufacturing operation that uses large amounts of highly concentrated sodium hypochlorite must identify and use pumps that will not be susceptible to off-gassing, which can result in negative effects that can hamper the overall production process.

In general, facility managers who must optimize the metering of sodium hypochlorite in their production processes should consider six things before choosing a metering pump.

6 Considerations when Selecting a Sodium Hypochlorite Pump

  1. Use a high-stroking pump model with a short stroke length. This operational configuration is less likely to permit the accumulation of gas in the pump head, which will lead to a lower risk that off-gassing will occur and result in optimized performance.
  2. If the operation requires the pumps to be idle for an extended period of time, gas can build up in the suction lines and pump head. In these conditions, it is recommended that the pump be outfitted with a bypass line that features an automated valve so that the sodium hypochlorite can be recirculated when it is not being actively fed by the pump.
  3. The production area should be constructed in such a way that the tank that is used to store the sodium hypochlorite is not located in direct sunlight. The higher ambient temperatures that can be created by direct sunlight will increase the sodium hypochlorite’s off-gassing rate.
  4. Much like it is not wise to oversize pumps, any application that requires sodium hypochlorite should use it in a concentration that is just necessary to complete the process. Higher-than-needed concentrations will increase the rate of off-gassing. The proper concentration should also be combined with a metering pump featuring a straight-through head design so that no gas can nest and bind the pump. This will also reduce the rate of off-gassing.
  5. When configuring the pumping system, a short, flooded suction-pipe arrangement with the suction piping traveling down from the storage tank to the metering pump is most effective. This design allows any gas bubbles that are produced to flow back to the storage tank, rather than into the pump. This also means that the metering pump should not be mounted on top of the storage tank, unless the pH level of the sodium hypochlorite is very low.
  6. If the setup allows, an air vent should be used to purge the valve in the head of the metering pump. Purging the pump head will enable any trapped sodium hypochlorite to be evacuated, which will prevent it from reaching the pump head.

Identifying the Perfect Pump

One of the best types of pump to use in sodium hypochlorite service—especially when limiting the harmful effects that off-gassing can have while providing exceedingly accurate metering rates—is the mechanically actuated diaphragm pump (electronic or motor-driven).

This pump technology excels in sodium hypochlorite applications because the piston in the pump is attached to the diaphragm with the forward (positive) movement of the piston ensuring positive diaphragm return, which optimizes the pump’s suction capabilities. More specifically, the best choices for sodium hypochlorite handling are metering pumps that have a high stroking speed (300 strokes per minute) and short stroke length, which makes it less likely that gas will accumulate in the pump head. In addition, these pumps can be outfitted with an automatic vent valve that allows any trapped gas to be purged from the pump head. Most models of this nature are also offered with manual control, automatic control or fully programmable control features that allow them to produce flow rates of up to 20 gallons per hour (gph).

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