Water Pump Tripping Overload. (are you sitting comfortably)?

[Grizzly]

Following on from the previous brilliant post "breaker type".

I started to wonder about a recent event where I on a Friday was called to a specialised "Controlled Environment Cold Store"
This very neat install was supplied, Installed and is remotely monitored by a Dutch Company "Reftech".
Anyone heard of them?

My brief was to arrive at the Lab's collect a replacement 3ph chilled water pump from the stores and fit it as over the previous week it had been tripping out frequently.

On arrival and prior to collection of the pump I decided to check out the original.
The pump is a small 3ph with a overload scale of 1 to 1.6amps.
Which was pulling a stable 0.8amps with the overload set to 1amp.
Luckily the installation's electrical drawings were available and the designed pump amperage was 1.23amps.

I then advised the customer that as the trip was set to 1a and the motor was pulling 0.8a. I felt these were to close. Recommending that the overload was reset to 1.3a.
I then spoke to the Dutch companies technician who agreed with my assessment. Thanking me for not just changing the pump. (which had yet to arrive in the stores).
I left a happy man!!
On the Monday the site phoned in saying that the pump had yet again been tripping over the weekend.

I was asked to return and change the pump. After a 1.5hrs in a 3ft roof void above the cold store I changed the pump.
On turning the pump on the site engineer with me remembered that there was a remote valve yet to be turned on.
When he turned this on there was no noticeable pitch change in the pump which was pulling a steady 0.7a across each phase!
When I checked the system water pressure (gauge within the chiller upon roof). It was at 1bar and the design pressure should be 2.25 bar.
When re pressurised the pump amperage was still at 0.7a.
Therefore I made the assumption that neither pump was faulty and the real problem had been low system pressure.
When I expressed this to the Dutch Guys they disagreed!
So shut up but did explain to the end user the importance of monitoring the system pressure.
Adding that if it continued to trip then logically the overload would need to be looked at!
What does anyone else think?
Also what the guys discuss in the faulty breaker post could it be relevant here?
Grizzly.
Thanks for your patience!!!

[Dilip sahasrabu]

Is overload thermal type, is it getting heated up, it can cause false tripping.

[casstrig]

Recently I had a similar problem with a pump,although the starter was star delta.It was pulling 6.7 amps and the overload set at 8,it would run ok for hours and then trip.I dismantled a contactor and found the contacts carbonized and splashed I changed the contactors and all ok.

[nike123]

In situations like this, I always change contactor (or maybe both, contactor and overload) and wait if problem is repeated. Also, I would check for loose wires at contactor or motor terminals. And last thing to check is if in that circuit is need for non-return valve, and its proper function.

[1torr]

Yes Steve, I've had this many times on process chillers, not enough restriction on head of pump. Regards, Kev.

[Peter_1]

Same comment as Nike 123, replace both, pump and relay.

But, if there's not enough water in the system, then the pump has nothing to pump and AMP's will be lower than normal. So, in my opinion, the Dutch guy was right.

[Grizzly]

Same comment as Nike 123, replace both, pump and relay.

But, if there's not enough water in the system, then the pump has nothing to pump and Amp's will be lower than normal. So, in my opinion, the Dutch guy was right.

Peter thanks for your valued opinion.
Normally I would totally agree but if you read my post again.
My dilemma was that the running amperage with an under pressurised system was 0.8amps.
Whereas when the pump was changed it's running amperage was 0.7amps Pre and post re pressurisation.

So my interpretation of events is that if the what appears to of been a poorly pressurised system also had air pockets in it.
Surely the spinning impeller would behave rather like a car wheel spinning on a wet surface!
Meaning when a pocket of air (carried around in the water) hit the impeller. Resistance would drop and it's rpm would increase and when the water behind it met the impeller. Resistance would return and the motor amps would shoot up!
I suspect this what 1torr and Nike are both refering to in their posts, although I have to admit this is an area where I have a lot to learn.
So no offence meant in my reply.
I actually think the Dutch Guys are very clever and have done a beautiful install.
Being in their shoes having payed for the pump to of been changed under warranty.
I would also dismiss my thoughts!
I just wanted to sound out the "collective"
What do you think?
Grizzly

[chemi-cool]

Hi Grizzly,

Pumps behave funny and not always make sense to the normal way we are thinking.

If you block the discharge, amps will decrease and rpm will increase.

The more liquid the pump will deliver [regardless of the pressure[ the amps will increase and rpm will decrease.

[DEVITG]

When using centrifuge pumps, head pressure shall be controlled as if it is so low the flow will increase at a rate it will overload the motor.
It have to be check the pumps performance curve.

A low head pressure will overload motor , same considerations have to be take with centrifuge fans with out self limiting flow blades

[nike123]

For centrifugal water pump in closed circuit it doesn't matter what pressure system is until that pressure (static pressure) on pump height is higher than pump suction pressure drop (to avoid cavitation) and not lower than pump suction pressure drop plus height of water column of instalation above pump converted to static pressure.
In that case and higher static pressures, pump only see height difference as some resistance against its rotation plus all resistance for water flow of that instalation (pipes, fitings, valves, elbows etc...).

So, if you have pump with pressure drop of 0,3bar and instalation height of 10 m with 1,5 bar pressure in system and same instalation with pressure in system 4 bar, for that pump load is same.

If, in other hand, you have lower pressure than above minimum you have cavitation.

[slingblade]

sounds to me like the pump is off it's curve, throttle back a valve on the discharge pipework to attain 2.25 bar. the flow should reduce and with it the current draw and possible overheating of the windings.
running at 1 bar it seems that it is trying to pump more than it's capacity in ltr/min.

[frank]

As Sling says, you need to take a flow measurement and plot it onto the pump curve.

Does the system have any Binder points fitted?

Also, consider why the pump appears to run OK while you are on site but trip the breaker intermittently when you are not there - what happens to cause the trip?

Is it a type 3 mcb?

[Grizzly]

As Sling says, you need to take a flow measurement and plot it onto the pump curve.

Does the system have any Binder points fitted?

Also, consider why the pump appears to run OK while you are on site but trip the breaker intermittently when you are not there - what happens to cause the trip?

Is it a type 3 mcb?

Good points Frank.
And many thanks to all you guys for your helpful advise.
I have learnt a lot that I did not know.
Simply put I will look at all of the points raised IF and it's a big IF.
We are asked back because the problems still exist.
Somehow I don't think we will hear anymore for a while.
Remember we don't maintain the systems they belong to Reftech and are told what and when to do things via a facilities company (as required by Reftech).
If you get my drift but some very good advise guys non the less!
many thanks again one and all!!!!!!
Grizzly

[slingblade]

As Sling says, you need to take a flow measurement and plot it onto the pump curve

Woohoo. i make that another one i got right. cheers frank. congrats.to all who pointed this out before me as well.

[frank]

Woohoo. i make that another one i got right. cheers frank.

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