PISONICS
PS7020 Series
Ultrasonic Concentration Meter
Sound velocity · inline spool
Nitrogen Trichloride Discharge Treatment — Caustic Strength Monitoring
—— The second mandated caustic measuring point in the same standard, and almost nobody writes about it ——
【Key measurement: sodium hydroxide wt% (5–20 %)】
Where This Point Comes From
Nitrogen trichloride (NCl₃) concentrates during liquid chlorine production and vaporisation, and decomposes explosively when heated or shocked. GB 11984—2024 clause 5.1.1 addresses it:
Liquid chlorine systems shall be designed without dead legs and shall have nitrogen trichloride discharge and treatment facilities meeting the following:
a) the discharge shall be analysed periodically, with discharge frequency set by keeping NCl₃ in the discharge below 0.5 % by mass;
b) sodium hydroxide in the treatment facility shall be 5–20 %;
c) the discharge facility shall connect directly to the treatment facility, with no chlorine recovery upstream of it.
Clause b) gives a concentration range without naming “online” the way 6.1.3 d) does, so strictly it is not a mandated online point. Read it alongside a), though, and the logic is clear: discharge frequency is set by NCl₃ content, and the decomposition risk depends directly on whether there is still caustic in the treatment facility. Once the caustic is spent, the NCl₃ coming out is no longer being destroyed. The safety of clause a) rests on clause b) holding.
Why This One Gets Forgotten
- It is not the main unit. The treatment facility is small, makes neither chlorine nor caustic in normal operation, and appears on the P&ID as one tank in a corner. Rounds tend to pass it by.
- Its caustic is spent a little at a time. Every blowdown consumes some, and the blowdown frequency itself follows NCl₃ content rather than a fixed schedule — so there is no cycle from which to work out how much life the last charge has left.
- The span is wider than on the emergency absorber. The standard gives 5–20 %, a full fifteen points, with 5 % as the floor and no margin below it.
Taken together those are the textbook conditions for an online instrument: irregular consumption, no attendance, a concentration-based criterion, and manual analysis costing more than the meter.
The Measurement Is Simpler Here
The good news is that the medium is clean: a binary system of caustic soda in water. What NCl₃ decomposition leaves behind — ammonium salts and chlorides — is small relative to the bulk liquor, so this tank does not turn ternary the way an emergency absorber does after a single release. This is ordinary binary caustic measurement — the same problem as the caustic storage tank page, over a wider span and in a less visited corner of the plant.
| This point | For comparison: emergency absorber | |
|---|---|---|
| System | Binary (NaOH + water) | Binary in standby; one-dimensional ternary after absorbing chlorine |
| Span | 5–20 %, given in the clause | 5–20 % (hold the 15 % floor, see down to the 5 % change-out point) |
| What the standard asks | Gives a range; does not name online monitoring | Names online monitoring facilities |
| How it is consumed | By blowdown events, irregularly | Carbonation and dilution in standby; eaten by chlorine on an event |
| Temperature | No limit set for this facility | Not more than 45 °C leaving the tower |
Where It Goes
- If the treatment tank has a circulation pump, in a full-bore run on the pump discharge, as on the emergency absorber;
- if it has no circulation pump — common on small facilities — this needs assessing. There is no flow in a static tank, so a single point represents it poorly. The practical answer may be a small circulation loop, or to keep managing the tank by laboratory analysis. We are not going to tell you a static tank measures well just to sell a meter;
- sample valve at the same location, temperature element close to the sound path.
Plainly: whether this point deserves an online instrument depends on your blowdown frequency and the tank's arrangement. A few blowdowns a year and no circulation — laboratory analysis is entirely adequate, and the money is better spent on the emergency absorber's instrument. Frequent blowdown, a circulated tank, and nobody willing to sample often — that is where this meter earns its place. Separating those two cases matters more than making the sale.
Before We Quote
- Roughly how often is NCl₃ discharged, and on what criterion?
- Treatment tank volume, whether there is a circulation pump, and the bore.
- How caustic strength is managed today — a fixed change interval, or analysis-driven?
- Is the tank open to atmosphere (which sets the carbonation rate)?
- Hazardous-area classification and rating.
- Any historical laboratory results.
What this page is: a method and selection note, not a case study. There are no unit counts, saving percentages or payback periods here — we do not invent those. Where the page says what the industry does, it cites something you can check for yourself (published vendor literature, granted patents) rather than implying we have installed base there. Send us the process conditions and you get a selection opinion you can check.
