Acid injection system for RO pretreatment 1

[ProjectEng]

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. Thanks!

ProjectEng

 

[quark]

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.

 

[bimr]

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.

 

[rodesign]

If this is for a municipal water treatment plant, then the Recommended Standards for Water Works (10 States Standards) will support your reasoning for a day tank. It states that ‘Day tanks should hold no more than a 30 hours supply’, and ‘bulk chemicals require at least 30 days of supply or a minimum storage volume of 1 ½ truck loads.’

The acid injection system this is typically configured with a bulk tank, day tank, metering pump, pressure relief loop back to the day tank, pressure sustaining valve at the injection point, an injection quill, a static mixer, a pH sensor, and a control loop that controls the dosing rate of the acid for the specified pH valve.

Acid injection and scale inhibitor injection are two of the most important systems to be engineered in a RO process, along with pre-filtration (typically 5 micron) of the raw water stream.

Good luck...

 

[WaterWizard]

If you go with taking acid directly from the pressurised acid supply, I would suggest using a mixing tee lined with teflon or other protective substance. I would also strongly suggest a very good non-return valve on the acid line. I have seen a pressurised acid line spring a leak, twice in a matter of months, due to water being forced into the acid line during operational disruptions, passing an inadequate NRV. The plant was built by a sub-contracting company and had to be redesigned twice to correct the problem. It seems to work well now but I will admit that every time we have a disruption to flows; I go feel the acid pipe upstream of the mixing tee to see if I detect any heat from water mixing with the concentrated acid in the line.
We have a dedicated pump supplying our acid which cuts out in event of a plant trip. Perhaps you should have a supply valve at the point you connect to the pressurised acid line that cuts the supply in event of a plant trip. This would negate the possibility of continuing to feed acid in case of a plant trip, and also give you an isolation point if you need to do maintenance downstream.

Edd

 

[bimr]

Waterwizard,

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

 

[semo]

If you are looking for a reason to recommend a day tank, Bimr gave a good study for onsite generation of hypochlorite in another FAQ. It discusses the use of bulk storage with day tanks using pumps and vacuum injection. See:

http://www.consulthai.com/2002/news/papers/On-site_versus .pdf

 

[ProjectEng]

I have decided to try a spring-to-close Collins control valve tied in to 2 pH meters. The 2 pH meters must agree within reason or else the control valve will close.

Off the pressurized header, I already have a manual valve. I plan to add a strainer right after the manual, then the control valve. At the point of injection, I plan to add a ball check valve, then a manual valve, then an injection quill with its own little &quot;cheapo&quot; built-in ball check.

Wanted to bounce this off you guys to make sure I wasn't overlooking anything. Your thoughts and suggestions?

Thanks,

ProjectEng

 

[semo]

Are you going to include some method of flow measuring (tube or meter) so that you can see what quantity you are feeding?

 

[bimr]

As I said in my previous posts, you need a double block and bleed arrangement. Check valves are not a substitute for a double block and bleed. Check valves are notorious for leaking. Generally, in process safety evaluations, when you are trying to develop potential accident scenarios, it is assumed that the check valves will leak.

 

[FLwater]

You are working with dangerous chemicals and high pressures and dont appear to have addressed all the important issues.

I highly recommend that you hire a consultant with experience in RO plant design & construction to design the pump and injection system for you, otherwise OSHA may be writing about your proposed system in the near future!

 

[25362]

As an aside and speaking of pH levels, has anyone heard of negative pH values?

 

[bimr]

In a solution, the hydrogen ion activity (H+) can never be reduced to zero, no matter how basic a solution may be, so it is not possible to have a negative pH.

pH = - log {H+}

reference: Chemistry for Environmental Engineers

 

[kenvlach]

pH is < 0 for strong inorganic acids having [H⁺] > 1 mole/Liter.
Corning and Orion have pH meters capable of measurement range -1.99 to 19.99. Electrode probes are usually rated for pH 0-12 or 0-14 since materials of construction usually deteriorate at the limits. It is more accurate to titrate for concentrations of strong acids, anyway.