pH monitoring loop piping layout

[jari001]

Hi everyone,

I'm looking at a 100 gallon tank wastewater tank (acidic but no solids) that is continuously re-circ'd through a 1" OD PVC pipe and pH adjusted before discharging to the sewer. The pH adjustment is controlled by operators via the DCS (dose in NaOH as needed) and there is an interlock that prevents discharge unless both pH sensors are reading the same pH (within some tolerance). The acceptable range is 6 to 8. My question is regarding the shown layout for how the sensors are installed; does the branching afford any advantage in measurement performance or some other advantage? My only thought is that you can isolate one line at a time so you don't have to lose re-circ if you have to replace a sensor.

 

[bimr]

Don't believe that your question can be answered without some background on the application. Are you neutralizing strong acids or weak acids? Continuous flow or batch operation? Are there organics? What type of business? Why the complicated and expensive arrangement? In addition, note that the fluid that you are processing will have an effect on the probe maintenance. Are you trying to resolve an existing problem?

Why don't you use a retractable assembly that is suitable for "Hot-Tap" service, allowing for removal from a pressurized process line or vessel without interrupting the flow?

Retractable

 

[ashtree]

I agree with bimr comment about the retractable type unit being used to avoid a process shut down if that is important to you.

However you also asked the question about whether the branching has any advantages or impacts on the system.

Theoretically if the fluid you are treating is already well mixed and completely homogeneous then what you get down each branch should be the same and the pH probes should read the same within the limits of the accuracy of the system.

However if the liquid is not well mixed and or it changes pH rapidly then unless the flow down each leg is the same or the flow path very short then there is a chance that the two units will read differently. The units mounted in the same line could also suffer the same problem unless mounted very close together.

How close do they need to be? That will depend on the velocity through the pipe, the sensitivity of the pH monitoring and control system and the likely rate of change of the solution you are dealing with. pH sensors are relatively slow to pick up changes(seconds rather than milliseconds) so unless the velocity is very slow in the pipe mounting the two sensors within a few feet of one another should be okay.
The same would apply to the branch line. Keep the flow path relatively short, make the two paths the same length and same design and put the pH sensors at the source end of the system so that the separation distance between the two is short and it should be okay.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[georgeverghese]

Can only see a reduction in the number of isolation and bypass valves for the parallel arrangement as compared to the series arrangement. 4 valves in total for parallel, 6 for the series. Whether this operates as an inline probe or through a fast loop sampling line would have no effect on performance or response time.

 

[jari001]

Waste water is acidic because the process uses the water to wash sulfuric acid from the product, with only trace organics from the environment being present. Industry is semiconductor materials but the water does not have elevated levels of metals.

The production process runs as a batch process and the washing that produces the waste water is the last step, so the waste water collection tank receives shots of water as one batch is finished. This pump is used to recirc. the waste water tank contents, the tank doesn't have an agitator or baffles. The pH adjustment takes on average about 2 hours to complete, and the operators have instruction to always confirm stable pH readings (+/- 0.1 for 5 seconds on each probe's transmitter is considered stable) before adding the next dose of NaOH; the Ops team has a long track record of doing this well so I believe the mixing from the recirc. and the time used to complete the pH adjustment allows for proper bulk pH adjustment. Currently looking for an interlock matrix to know what the interlock tolerance is but historically the pH reading from each instrument is roughly within 0.3 of each other and that satisfies the interlock.

Given the current uptime requirements for the process that creates the wastewater, the Ops and Maintenance team have a 1 week window to troubleshoot and repair if the pH probes ever became suspect. The tank is large enough to collect the water for 2 weeks work runs, so even if the recirc. was stopped for repairs, it wouldn't impact production or our ability to safely collect the water. Given that, I don't think the retractable style of probe mounting is required.

Thanks for the comments,
~J

 

[jari001]

Just another anecdote to add to the list about the field matching what you're being told: I'm in California working on a project and helping on addressing some PHA items that this post was related to in New Jersey. I thought it was going to be a paperwork exercise to complete an MOC to tie these pH probes into the DCS and I finally get some pictures of these components and I was shocked at how horrible it all looked. Also, water that looks white and GRITTY somehow qualified as no solids to the people that I was working with

I think a sequel to this thread is in the works.