Flow Meter

A fault.

Go on:

• Tell the forum what type of flowmeter it is!
• Tell the forum its size, its age, its manufacturer!
• Tell the forum about the nature and name of the <...material...>, its temperature and pressure!
• Tell the forum about the ambient conditions!
• Tell the forum about the pump, the key dimensions and features of the piping installation and the difference in elevation between the <...material...>'s start and end points!
• Tell the forum about the way this equipment is operated!

Pretty-please?

Sounds like it's measuring the differential flow (relative to previous reading) rather than actual flow.

In a way, a bit like trying to use a DC meter to measure AC current/voltage.

I suspect that the meter also indicates maximum negative flow rate when turn pump off.

Have fun resolving this.

Without knowing any more about it, my guess is that on startup, there is little resistance to the flow and maybe the steady state flow rate is low but not zero when back pressure builds up.

So, that smiley face is to indicate you aren't actually serious, right?

<...Without knowing any more about it...smiley face...> What good would crying do, though (rhetorical question - NNTR)?

No, actually I was envisioning some sort of liquid filling empty pipes on startup ahead of some constriction downstream that limits the flow when the "product" reaches it. There was not much information provided in the original question, leaving a lot to the imagination.

I'm with you.

Ah, okay. I can see where you are coming from. The meter would have to be poorly chosen or functioning improperly to read zero flow with the tank being filled at a rate not deviating enough from normal to be noteworthy.

If it wasn't filling the tank, any obstruction that acted like an overfill prevention valve with the pipe as the tank, could allow air to be pushed out initially and then stop flow. Simply have a fair amount of air caught in a long closed line would likely yield oscillation.

If this is a dp flow meter, initial maximum indication may be due to sudden increase in pressure reaching the first leg before the second and the inertia within the device.

The subsequent reduction to zero is because the flow meter is broken/in need of maintenance, or severely in need of calibration, or of insufficient resolution, or improperly installed, or not actually a flow meter after all.

Maybe this flow meter isn't even attached to the system you think it monitors. Perhaps the jump and return to zero is just coincidental with operation of a nearby system and has nothing to do with the fluid flow into the tank you reference.

We can't tell from this end. You have left us fairly blind.

It's real. You forgot about the accumulator that's between the flow meter and the orifice (nozzle) that feeds the tank.

...and the closed valve upstream of the pump...

Doesn't account for zero flow shortly after start.

It would be indicative that you have a valve closed past the flowmeter location....

Except the OP notes the tank is filling.

The he's probably using the wrong type of flow meter for the application....

https://www.keyence.com/ss/products/process/flowknowledge/sources/trouble_02.jsp

It broke.

My guess is the feeder pipe is a different one than the one where the flowmeter is on.

Both are empty at start and then most of the flow goes through the other pipe.

That doesn't make even the tiniest shred of sense.

Now, that is your problem!

This is my guess ( it's probably wrong ) my guess is about as good as anyone's so what the fudge.

The pump is a two stage diaphragm, there is a faulty check valve before the pump. When the pump starts up, the pipe is full of fluid and the check valve is open, the gauge reads high. Then the check valve sticks closed, the viscosity of the fluid drops after the pump and flow still comes out of the pipe because the pump is still pumping on it's exit diaphragm, but the gauge drops to zero because this has been going on for so long the gauge is worn out and will only indicate full or nothing at all. When inlet side pressure finally builds up enough, the check valve opens, the gauge reads full, the viscosity in the line is high and flow continues out of the pipe. This process repeates itself over and over.

Remember this is just a guess and I'm probably wrong, again, as usual.

We do not know what is pumped and how and not even why, so any guess is good until we have more details!

Quite.

pair o' ducks.

Magnetic flow meter electrodes (inside the pipe) are going dry after start, pipe not full

Incorrectly operating pump contact, used to clamp magnetic flow meter output to zero when pump not running, usually used when air is likely in the meter body. If contact closes while running, meter output goes to zero.

Bad magnetic flow meter transmitter (the box with all of the electronics), or less likely open / un-grounded circuit on one of the excitation coils (wrapping the pipe).

I'd suggest some scenario which may be resulted in the problem you've described:

Most flow meters (or at least those I've come across) work according to a certain working pressure in a process, and when this pressure goes below that value, then the meter would become zero... That's why you need to check if there is a pressure relief valve(s) in your process being damaged or leaked out a severely ..

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It remains to be seen what type of flowmeter we are talking about. It could even be in the set-up of the meters electronic parameters (if electronic), where a low threshold was set too high, for example, with a magmeter.

Agree with you. Until we know the flowmeter operating principals we are all shooting blind.

Initial thought was floating earth with capacitive effect. Initial (change in) flow shows as reading but then steady state flow indicates zero/null. That's why I suggested OP to provide meter observations at pump stop, which I suspect would be a negative flow indication.

Anyway, it's their problem to solve. We can only hope that the collective responses here have been of some assistance to them.

Yes,.. I suppose it as a variable area flowmeter which has a constant pressure drop,... and nevertheless,.. without more information from the OP, all we do is a kind of mere guessing!!

What I can say here that I experienced something similar to the OP's issue, and the the flowmeter was Krohne-made,.. We've found out that relief valve -directly past the feeding pump- is responsible for that problem..

Perhaps the line is empty, when pump starts it's against zero head so the fluid travels rapidly to fill the pipe then the pump runs out of head or it meets a closed valve and the velocity drops to zero.

That's my guess for what it's worth

Is there a prize for getting the right answer?