How to determine over-throttling of concentrated water?

To determine if the concentrated water is over-throttled, the key is to look at four indicators: flow rate, pressure, recovery rate, and scaling risk. Combined with on-site observations, this can quickly lead to a diagnosis.

1. Check the parameters: Typical abnormal performance (most intuitive)
（1）Brine flow is too low, permeate flow is abnormally high
· Brine flow is significantly lower than the design value, even approaching a 'stalled' state
· Permeate flow is temporarily high, but the pressure drop and conductivity quickly deteriorate
· Calculation formula: Recovery rate = Permeate flow ÷ Feed flow × 100%
If the recovery rate is consistently 5%–10% higher than the design value, it can generally be judged as over-throttling.

（2）Abnormal overall system pressure increase
· High-pressure pump outlet pressure, interstage pressure, and concentrate pressure are all high throughout the system
· Total pressure difference (feed water pressure − concentrate pressure) increases rapidly
· Reducing the concentrate valve and further throttling will cause the pressure to continue to surge

（3）The pressure difference in the second stage rises rapidly, with a clear tendency for scaling
· The second-stage pressure difference (second-stage feedwater − second-stage concentrate) is significantly larger than the first stage
· The concentrate side LSI/Langelier Saturation Index and S&DSI are positive, indicating a high risk of scaling
· Product water conductivity gradually increases, and the desalination rate slowly decreases

2. Observe the Phenomena: Typical On-Site Symptoms
Adjustment of Concentrated Water Valve Too Low
- The on-site concentrated water control valve is set very low (for example, long-term <20%–30%)
- If the bypass or return valves are not fully closed, it can also create a false appearance of "over-throttling"
Concentrated water pipeline/valve vibration and loud noise
- Flow velocity at the throttling point is too high, causing whistling and vibration
- Obvious wear and erosion at valves, elbows, check valves, etc.
Membrane Elements Scaling/Fouling Rapidly
- Cleaning cycles are significantly shortened (normally 1–3 months → need cleaning in a few weeks)
- Upon disassembly, white/yellow scaling can be seen on the surface of the two-stage membranes (calcium, magnesium, silicon, barium, strontium scale)

3. Rapid Verification Method (On-site One-step Confirmation)
Gradually open the concentrated water valve and observe changes:
o Open the concentrated water valve slightly each time (5%–10%)
o If the following occur:
§ Significant increase in concentrate flow
§ Overall pressure drop
§ Total pressure differential decreases
§ Recovery rate returns to the design range → basically confirms that the concentrate was previously throttled excessively.
Compare with the design conditions using the design/operation manual and check:
o Design recovery rate
o Design concentrate flow
o If actual long-term operation deviates from the design pressure range, it indicates excessive throttling. Perform a "standard condition reset"
o Set the recovery rate according to the design value
o Re-adjust the concentrate valve and operate stably for 1–2 hours
o If pressure, differential pressure, and conductivity all stabilize, it indicates that the previous issue was due to human operation or valve control.

4. Common Consequences of Excessive Brine Throttling (Helps You Verify in Reverse)
· Accelerated membrane scaling and fouling, frequent cleaning
· Decreased salt rejection, increased product water conductivity
· Increased energy consumption (high-pressure pumps running at high pressure for long periods)
· Shortened membrane lifespan, premature replacement
· Erosion and damage to pipes, valves, and instruments

5. Simple Judgment Process (for on-site use)
（1）Check the recovery rate: Is it consistently higher than the design value?
（2）Check the pressure: Are the overall pressure and total pressure difference relatively high and rising quickly?
（3）Check the concentrate valve: Is the opening noticeably too small?
（4）Check the second stage: Is the pressure difference in the second stage much greater than the first stage, indicating a high risk of scaling?
（5）Perform verification: Open the concentrate valve wider, does the pressure/pressure difference drop significantly?
If 2–3 of the first 4 items are met, and then verified by item 5, it can basically confirm excessive concentrate throttling.