We have concentrated bulk HCl storage on site.  We also have a supply and return HCl header that is always pressurized.  What is the best way to setup acid injection on the RO feedwater line?

I thought about using a daytank that operators could fill once per week and would draw down and get injected into the 16" feedwater line using a chemical pump.  However, the thought of an acid 'daytank' was not well received.

I'm hesitant to just run a line from the acid header to the 16" feedwater line for fear that the regulating valve could fail, causing large amounts of acid to get dumped into the line.

Looking for alternative ideas.

Day tank would be a better and cheaper option. Why can't you use Venturi type dosing units (like Dosmatic) with a automatic shutoff valve upstream, controlled by an online pH sensor? 

Regards,

Believe it or not : Eratosthenes, a 3rd century BC true philologist, calculated circumference of earth with the help of a stick and it's shadow. The error was just 4% to the present day calcuated value.

The regulating valve that you mentioned would probably work fine. You may have issues with materials of construction since you have to be concerned with heat of dilution.

You should install a 3 valve block and bleed to prevent the water pressure from backing into you acid system in the event of failure of the acid pump.

If you are that concerned with control valve failure, you could add a second pH instrument to shut off the acid in the event of a failure.

The pumping scheme that you are describing appears to be unsafe. You should also consider installing a 3 valve block and bleed to prevent the water pressure from backing into your acid system in the event of failure of the acid pump, valve, check valve or power. A 3 valve block and bleed is a commonly used technique to be ensure that an water into an acid tank accident will never happen.

A 3 valve block and bleed uses two normally closed valves and one normally open. In the event of a process failure, excess pressure is relieved to drain rather than to an upstream acid tank.

pipeline<---------X---------X-------<acid pump & tank
                       |
                       |
                       X (normally open)
                       to
                      drain

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