Pump flow verification
Before you spend forty thousand dollars replacing an impeller, it is worth knowing whether the pump is actually off its curve or whether the system is. A clamp-on meter on the discharge answers that in an afternoon.
The three-number test
Pump performance is suction pressure, discharge pressure and flow. Two of those you already have on permanent gauges. Flow is the one that usually is not instrumented, which is why pumps get condemned on differential pressure alone.
With a clamp-on meter you can plot the actual operating point against the manufacturer curve, and the answer is usually one of three things:
- On the curve, wrong place. The pump is fine. The system curve moved, normally because a valve is throttled or a line has fouled.
- Below the curve at all points. Genuine mechanical wear. Impeller, wear rings or a failing coupling.
- Nowhere near the curve. Check the entered pipe data before you condemn anything. This is nearly always a measurement setup error.
Where to put the sensors
Discharge side, ten diameters downstream of the pump and five upstream of the next fitting. Never immediately at the pump nozzle — the flow leaving a volute is swirling badly and will read high or low depending on which side of the pipe the sensors happen to sit.
If the only accessible straight run is on the suction side, that works too, but watch for entrained air on a lift application. Air in the line will drop the signal strength before it corrupts the reading, so watch signal quality as your warning.
Verifying the pump against a permanent meter
A common use for these rentals is checking a permanently installed mag or turbine meter that the plant no longer trusts. Clamp the ultrasonic meter on the same line, at a different location with good straight run, and compare. If the two disagree by more than a few percent, the permanent meter is the usual culprit — they drift, ultrasonic clamp-ons do not, because there is nothing in the flow to wear.