Erratic pH results in product stream from PD chemical injection pump?
Erratic pH results in product stream from PD chemical injection pump?
(OP)
I have a very low flow PD pump that puts an basic chemical into a product stream to raise pH. The flow of the chemical is typically 0.3 GPH and doesn't vary much. The PD pump flow is controlled by the stroke of the piston on a 1200 RPM motor. The flow is measured with a small coriolis meter. The chemical pipe is 1" diameter and the distance to the product stream is approximately 300' away. There is a 50' horizontal run before it 90's down into the product piping.
The product stream that the chemical is pumped into is 4" and the flow is approximately 10000 GPH. After the chemical ties into the product stream, it flows through a static mixer and then travels approximately 100' away to a product storage tank.
The results for pH are taken just before the combined stream enters the storage tank.
The problem we're having is that the pH results vary between 0.5 pH to 3 pH from sample to sample even though the flows have remained the same.
We sampled the tank and the pH is relatively constant at 2.5 pH across samples.
I'm wondering if we have a situation where we have a partially full pipe for the chemical injection and even though the chemical flow is reading fairly constant, the material could be building up and we get waves of the chemical in to the product stream causing the variation in pH.
Am I correct in my thinking?
The product stream that the chemical is pumped into is 4" and the flow is approximately 10000 GPH. After the chemical ties into the product stream, it flows through a static mixer and then travels approximately 100' away to a product storage tank.
The results for pH are taken just before the combined stream enters the storage tank.
The problem we're having is that the pH results vary between 0.5 pH to 3 pH from sample to sample even though the flows have remained the same.
We sampled the tank and the pH is relatively constant at 2.5 pH across samples.
I'm wondering if we have a situation where we have a partially full pipe for the chemical injection and even though the chemical flow is reading fairly constant, the material could be building up and we get waves of the chemical in to the product stream causing the variation in pH.
Am I correct in my thinking?
-Mike





RE: Erratic pH results in product stream from PD chemical injection pump?
A pulsation dampener with an injection valve may improve the operation.
Review figure 1 in the link below:
http://www.miltonroy.com/corp/details/0,10294,CLI1...
The flow from the pump is not constant so you should expect different pH readings over time if you are sampling the pipeline. The pumped volume averages out over time in the Storage Tank so that the pH readings in the Storage Tanks should be constant.
If you can not live with the pulsed flow from the PD pump in the piping system, then you will have to install a different type of pump.
RE: Erratic pH results in product stream from PD chemical injection pump?
-Mike
RE: Erratic pH results in product stream from PD chemical injection pump?
In addition, the static mixer is not going to help that much since there is no back mixing.
RE: Erratic pH results in product stream from PD chemical injection pump?
A partially full 1" line? Maybe. Is there a good check valve where the 1" line tees into the 4" line. If not the fluid in the 4" line may backflow up the 1" line.
At 0.002 ft/sec there is no "injection" into the main stream. It "drools" in and clings near the walls and gets smeared down the line against the wall.
Do you have a detail drawing of the union of these two pipes? I hate to go on and not know what is really there.
Good luck,
Latexman
RE: Erratic pH results in product stream from PD chemical injection pump?
Bimr, looking at your post closer, I have to check if the drive from the motor to the pump has a gear ratio on it. It does seem like an awfully low flow/stroke if its 20/second.
-Mike
RE: Erratic pH results in product stream from PD chemical injection pump?
Good luck,
Latexman
RE: Erratic pH results in product stream from PD chemical injection pump?
RE: Erratic pH results in product stream from PD chemical injection pump?
RE: Erratic pH results in product stream from PD chemical injection pump?
It is generally not practical to pump 300 feet with such a small pump. The discharge line should be short and direct, typically 10-20 feet.
If piping contains pockets, it is possible that periodic siphoning or other odd flow conditions may occur. Avoid pockets or high point where air can accumulate in the discharge piping. An ideal pipe run is one where the piping gradually slopes up from the pump to the outlet. This will ensure that any air in the discharge side of the pump can be evacuated to the outlet.
Conveying fluids are sometimes desirable to meter a chemical dose into a stream, Conveying fluids help when pumping over long distances and/or with low chemical doses. Not sure if conveying fluids are practical for your application.
RE: Erratic pH results in product stream from PD chemical injection pump?
Going back to it, the majority of the piping is actually 1/2" and expands to 3/4" right before it injects into the 4" main. There is a check valve just upstream of the injection point. There is a 50' horizontal run that contains the check valve, and then expands to 3/4". The 3/4" piping goes another 5' and then 90's down into the 4" main. The connection into the main piping is just a 3/4" coupling at the top of the pipe.
We have a parallel unit that is essential a mirror process, fed by a buddy pump right next to the one in question. This unit runs very similar conditions, however the pH results only swing between 2.5-2.7 between samples. The biggest difference is the length of piping to the unit is about half of that of the other.
Walking out the chemical injection line to the problem unit, I noticed there is definite bowing of the line as excepted with such a small pipe diameter.
Both units have been in operation for at least 20 years. This unit has always been a problem. The comments above seem to point to the problem being in the piping design. I'll have to go back and see what we can do to eliminate the excessive length and pockets in the piping.
-Mike
RE: Erratic pH results in product stream from PD chemical injection pump?
Maintaining higher back pressure in the chemical line also reduces the chance of vapor pockets. It prevents any siphoning.
Johnny Pellin
RE: Erratic pH results in product stream from PD chemical injection pump?
Good luck,
Latexman
RE: Erratic pH results in product stream from PD chemical injection pump?
The injector is definitely something we're already looking into.
Awesome ideas from everyone here. Thank you again for all the input!
-Mike