PISONICS
PS7020 Series
Ultrasonic Concentration Meter
Sound velocity · inline spool · no moving parts
Emergency Chlorine Absorption — Online Monitoring of the Circulating Caustic
—— GB 11984—2024 clause 6.1.3 d) makes this instrument mandatory ——
【Key measurements: sodium hydroxide wt% / tower outlet temperature °C】
The Clause
GB 11984—2024, the technical code for chlorine safety in chemical enterprises, was issued 2024-11-28 and came into force 2025-08-01, replacing GB 11984—2008. It is mandatory and applies to chemical enterprises that produce or use chlorine (the standard notes that chlorine includes liquid chlorine). Clause 6.1.3 d), on the emergency chlorine absorption system serving liquid chlorine storage, reads:
Sodium hydroxide in the circulating absorption liquor shall be 15–20 %, and its temperature leaving the tower not more than 45 °C. Facilities for online monitoring of the circulating liquor's sodium hydroxide concentration and temperature shall be provided; periodic laboratory analysis shall be carried out; and the liquor shall be replaced or renewed before the sodium hydroxide falls below 5 %.
Two related clauses sit in the same standard:
- 5.2.3: the electrolysis unit's emergency chlorine absorption system shall absorb for not less than 30 min, sodium hydroxide concentration shall be monitored online, and a chlorine detector shall be fitted at the vent; that system shall not be combined with the sodium hypochlorite production unit.
- 6.1.3 a) c) g): independently installed, two-stage absorption; capable of 24 h continuous operation, with the caustic circulation tanks able to be switched, held in standby and made up; circulation pumps and emergency chlorine fans provided with spares, on the most critical class of primary electrical load.
This is not a softened recommendation. The previous edition, GB 11984—2008, contained none of it. On the industry side, the China Chlor-Alkali Industry Association's T/CCASC 1005—2023 was the first to put online monitoring of caustic concentration into an inspection checklist; the national standard took it up in 2024 and attached numbers.
The Fact That Decides the Selection
An emergency absorber looks much like a process tail-gas scrubber, but most of the time the two are doing entirely different things:
| Emergency chlorine absorber | Process tail-gas scrubber | |
|---|---|---|
| Normal state | Standby. Fresh caustic circulates; no chlorine is entering | Continuously loaded. Always absorbing |
| Liquid phase | NaOH + water, binary (99 % of the time) | NaOH + NaOCl + NaCl, a one-dimensional ternary along the reaction path |
| How strength is lost | CO₂ picked up from air, dilution, evaporation — slowly, and invisibly | Eaten by chlorine — quickly, and predictably |
| What the standard says | GB 11984—2024 6.1.3 d) / 5.2.3, online monitoring mandatory | A purely process tower does not fall directly under either |
| Change-out criterion | Replace before caustic falls below 5 % | Switch out on residual alkali or available chlorine |
So the quantity the standard asks you to measure online is, most of the time, a binary system. Sound velocity in caustic soda varies monotonically with concentration over a wide span, which makes this the most standard duty sound velocity has. No multi-component method is needed, no “dual parameter” is needed: one PS7020 covers it. Only after the tower has actually absorbed a release, with hypochlorite now in the liquor, does the medium become the one-dimensional ternary described on the tail-gas scrubbing page, and that page's scaling method takes over.
Where the Meter Goes
The Shandong provincial standard DB37/T 1933—2022 sets out what one of these units consists of, which is a convenient way to locate the measuring point: suction ducting, induced-draught fan, emergency chlorine absorption tower, caustic circulation pump, caustic circulation tank, caustic cooler.
- In a full-bore vertical run on the circulation pump discharge. It is the one place in the system with steady flow in a pipe that is genuinely full.
- Not in the circulation tank. A tank has a free surface, a gas-liquid interface, stratification and vortexing; aerated duty is on the PS7020's own exclusion list.
- Temperature element close to the sound path. The standard asks for concentration and temperature, and the 45 °C applies leaving the tower — the element has to represent that section, not just any convenient pipe.
- Sample valve on the same spool. The clause also requires periodic laboratory analysis; the further the sample point from the probe, the less the two data sets agree.
- The standby tower must be able to circulate without starting the main fan. For the meter to read, the pump has to turn. Most units already have a circulation mode; confirm it.
Most Existing Units Need Their Caustic Raised
One retrofit item that is easy to miss: the Shandong standard DB37/T 1933—2022 gives a caustic strength of 7.0–15.0 %, while GB 11984—2024 requires 15–20 %. Liquor made up to the older figure sits below the new national lower limit, or right against it.
That has two direct consequences for selection. The calibrated span has to be built for the new window rather than carried over from an older instrument; and once the strength is raised, viscosity and density change with it, so the “looks about right” judgement that used to serve becomes less reliable — which is precisely why the standard now asks for an instrument.
How Standby Caustic Goes Bad Quietly
In a continuously loaded tower the caustic is eaten by chlorine: fast and predictable. A standby tower is different. Its caustic degrades two other ways, and no online method reads free alkali directly:
| Path | What happens | Does the instrument see it? |
|---|---|---|
| Carbonation | The ducting and circulation tank are open to air all year; 2 NaOH + CO₂ → Na₂CO₃ + H₂O. Free alkali is consumed while total salt content barely changes | Not clearly. On a first-order estimate, each point of free alkali eaten moves sound velocity by only about a fifth of a point's worth — the reading sits higher than the truth. Density hardly moves; conductivity falls more, but conductivity is poorly behaved in this window anyway (see the companion page) |
| Dilution | Make-up water, rainwater or wash water entering the tank; a leaking cooler | Yes, and this is what an online meter is best at — a step change in concentration, obvious at a glance |
| Concentration | Long-term evaporation, or caustic topped up without water | Yes, in the opposite direction |
This is why the clause says “monitored online” and then also says “periodic laboratory analysis”. The two are not redundant: the instrument holds the trend, the step changes and the alarm; the titration holds the absolute free-alkali figure. Carbonation is the path only titration catches, and it is the main way a standby tower's caustic degrades. Any supplier telling you that an online meter removes the need for laboratory analysis has either not read this clause or has read it and not said so.
Materials
Materials for this unit are written into two mandatory standards, so there is nothing to guess:
- AQ 3062—2025 A.2.11: equipment in direct contact with wet chlorine or chlorine water shall use titanium-palladium alloy, duplex stainless, Hastelloy or similar chloride-stress-corrosion-resistant metals, or CPVC / PVDF verified against chloride permeation; austenitic stainless steel shall not be used.
- AQ 3062—2025 A.2.10: equipment in direct contact with dry chlorine shall not use titanium or titanium alloys.
- GB 11984—2024 5.3.4: titanium equipment and pipework shall have temperature monitoring to prevent wet chlorine turning dry and reacting with the titanium; where a titanium fan boosts wet chlorine, the inlet shall be humidified.
- GB 11984—2024 6.1.2 e): local level gauges on liquid chlorine storage shall not be titanium.
The standby liquor is plain caustic and 316L is usually adequate for it — but once the tower has absorbed a release there is hypochlorite and chloride in the liquor and A.2.11's logic applies. Specify materials on the basis that one day it will absorb chlorine, not on the standby state. That one day is the entire reason the unit exists.
Before We Quote
- Does this unit serve liquid chlorine storage (6.1.3), the electrolysis unit (5.2.3), or process tail gas as well?
- What caustic strength is currently specified — is it still on the older 7–15 % basis?
- Is there a usable vertical full-bore run on the circulation pump discharge, and at what bore?
- In standby, does the circulation pump run continuously, on a timer, or only on an event?
- Is the circulation tank open to atmosphere? Is there a breather or a nitrogen blanket?
- Is there a caustic cooler, and how is tower outlet temperature measured today?
- Hazardous-area classification and rating; whether the supply is already on the most critical class of primary load.
- How often the liquor is sampled and analysed today, and by whom — that continues after the retrofit.
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.
