Chemical Dosing

[tipp79]

I'm currently working as a process engineer on the design of a new chemical dosing sytem at a water treatment works . The chemicals to be dosed are MSP, Sodium Hypochlorite and Sodium Carbonate.

The water authority's specifications stipulate that the velocities in dosing lines should be in the range of 0.8 - 1.5 m/s and if the diluted chemical's velocity is less than this, carrier water should be utilised. Other guidance, however, recommends that carrier water is not used for sodium hypochlorite and MSP dosing. The authority have agreed in principle that carrier water need not be used for dosing hypochlorite and MSP so long as there is no negative consequence from dosing below the minimum velocity they recommend / without carrier water. The table below shows the velocites achieved using a 6mm diameter dosing line (smallest practical bore) and the diluted chemical dosing rate:

Chemical Dosing Concentration Velocity w/o Carrier Water
Min Max
(%) m/s m/s
Sodium Hypochlorite 3 0.02 0.04
Sodium Carbonate 5 0.26 0.52
MSP 25 0.01 0.03

Can anyone foresee a problem with using these rates?

Many Thanks,

James

 

[bimr]

You should be concerned about the hypochlorite.

It is very common problem for hypochlorite to offgas. The offgas will collect in pockets in the piping, especially since the velcoity is low.

Of course, the extent of the problem will depend greatly on the length of your discharge piping. The length of the discharge piping should be minimized.

 

[aquaseaulutions]

A peristaltic dosing pumps system will eliminate all off-gas concerns. I am using peristaltic pumps since 3 years and the only problem is changing the tube once a year(3-4 minutes) but it is cheap comparing to all the problems you may have with diaphragm pumps.

 

[mxmaciek]

aquaseaulutions: it depends mainly from the pump construction and accesories. For the diaphraghm pumps you can have wide range of solutions, like the head vents or automated (timed) gas release systems.
Tipp79, your solution will depend as well from the injection point backpressure, dosin line length, suction line length etc.
In principle, if it's below 3 bars: use the peristaltic pump. If it's in range from 3 to 6 bars, you can use the flexible dosing line (special hoses). Benefits will be: lower nominal diameter (3-4mm) will increase your linear speed, and flexi hose will act as the pulsation dampener.
Above the 6 bars: membrane pump with rigid pipe and gas releasing device.

 

[ch81pc]

You will primarily have problems on the suction side of your pump. Ideally provide your dosing pump suctions with positive head by elavating your storage tank slightly. What are the summer tempratures like where you are? The hotter it is the more problems you'll get with gasing off. You'll probably have stand by pumps, again dependant on temprature, you may need to swap duties over after a matter of hours rather than days as hypo will gas if left standing in your pipework. Lower pressures in the dosing lines may cause problems as without the pressure, the scaling at the actual point of injection may lead to blockage as the pressure won't be there to clear it, the knock on is that you'll have to clean your dosing lances more regularly.