Gas Chlorine Dosing
Gas chlorine is safely dosed from cylinders or drums using remote vacuum gas chlorination equipment. The vacuum regulator (the heart of the chlorinator) is mounted directly on the cylinder isolation valve using a lead gasket. Chlorine gas under pressure is fed to the inlet pressure reducing valve, the needle of which is seated on a diaphragm. At the water end of the system, pressure differential is applied across a venturi by a boost flow which causes the venturi to suck. This vacuum is applied to the diaphragm, causing the needle to be pushed into the spring loaded valve, thereby allowing chlorine gas to flow into the chlorinator. Gas chlorine under vacuum then proceeds through a flow regulating device (needle valve V notch) and through the vacuum tube to the venturi where it is dissolved in water.
The key feature of the gas chlorination system for water disinfection is that it operates completely under vacuum and is therefore inherently safe. If the booster pump stops or the venturi becomes blocked or the vacuum tube is broken, vacuum is lost at the chlorinator. The inlet pressure reducing valve immediately closes and the cylinder or drum is completely isolated. Even if the cylinder was to fall over and the body of the chlorinator was to smash, the inlet pressure reducing valve would still stay closed and no chlorine gas would escape.
Gas chlorination can occur from 920kg drums. Chlorinators are equipped with a drop leg and heater where any liquid chlorine is vaporised. Normally, this is only required for connection of the drum immediately after transport. After a short operation time, the heater is not required but is normally left on.
Gas chlorination systems are usually supplied as duty-standby units so that chlorine gas is continuously available for water disinfection. Change-over from the duty cylinder or drum to the standby unit is via a vacuum shuttle valve. When the cylinder is empty, vacuum increases to such an extent that the valve shuttle is pulled over to the other side, allowing the standby cylinder to come on line.
Sodium Hypochlorite Dosing
Sodium Hypochlorite is supplied as a 12.5% solution and is readily available in 20L drums, 200L drums and tanker loads for larger installations. Chemical storage tanks are usually manufactured from polyethylene. Sodium hypochlorite is dosed with solenoid operation dosing pumps or motor driven metering pumps.
12.5% sodium hypochlorite undergoes gasification as it decomposes to give off oxygen gas. This gasification can cause problems with chemical metering pumps as the gas will accumulate in the metering pump suction line, eventually making its way into the liquid end where loss of prime is the result. The ProMinent Gamma/L and Beta dosing pumps have solved this problem with the specially designed auto-degassing liquid end. Gas is removed from the suction line through a vent valve and directed back to the storage tank with a small amount of liquid. The clear chemical is then dosed through the discharge valve.
Larger metering pumps (>20 L/hr) don’t usually suffer from loss of prime as the valves are much larger and the volume of chemical being pumped is also larger.
Suction connections between the bulk sodium hypochlorite storage tank and the metering pump should be designed to minimise formation of gas pockets. A calibration cylinder immediately prior to the metering pump will function as a gas accumulator and allow gas to rise and vent off. If the suction line is run downwards from the tank to the pump, gas will rise back into the tank.
One way to prevent gasification is to dilute the 12.5% solution to 1% using potable water. Although the metering pump will have to be 12.5 times larger, the 1% solution will last decay approximately 144 times slower than the concentrated solution.
Definition of Strength
Five common definitions of sodium hypochlorite solution strength are as follows:
- Grams per litre of available chlorine = The weight of available chlorine in grams in 1L of sodium hypochlorite solution. This is determined by analysis.
- Grams per litre of sodium hypochlorite = 1.05 x grams per litre available chlorine.
- Trade percent available chlorine = grams per litre available chlorine/10
- Weight percent available chlorine = grams per litre available chlorine/ (10 x sg)
- Weight percent sodium hypochlorite = weight percent available chlorine x 1.05