Pump Run Problems

[ar9]

All,

We have a vertical pump with discharge after pump, with CenterLine(CL) at El. +8'. We then have a 6' straight pipe (23" ID) with a 6" ARV on top. After the 6' of straight pipe, the discharge pipe has two (2) 90D Long Radius bend (vertical CL change), which brings the pipe CL to El. +2. (see attached schematic)

How do the 2 bends going down right after the pump affect the TDH? Does this create a suction effect, negative TDH? When we run the pumps, the AMPS and HP climb until the pumps trip. Can this be caused by low head?

Any help is appreciated.

-ar9

 

[ar9]

Thanks everyone for the replies.

The reason why an orifice was being discussed was because of the issue of running out of the curve because of low head. I have to correct myself on a previous post. I mentioned that the impeller poured was between C&D, however, that is incorrect. The impeller was poured between A&B. We are fluctuating between 8-10 TDH (solely due to intake elevation drops). The motor size is a 75 HP motor, one speed.

Would a change of impeller to a full B, or halfway between B&C improve? Like I said, my concern and initial theory was that the system did not have enough head and was operating at a point beyond the curve.

Once again, thanks for all the input.

 

[Artisi]

Correction to my previous post- it should read as head reduces -- NOT FLOW - the power reduces. Running at the end of curve gives the minimum power input requirement.
Sorry for any confusion

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[Compositepro]

You could just remove the air release valve and replace it with an open stand pipe. That will break the siphon and put a constant 8 ft of head on the pump.

 

[LittleInch]

Ar9,

Can you describe what your problem is better. Is it on start up or after running for some time?

The curves don't really describe what happens at start up but your motor looks quite low powered to deal with the start up.

Do you have any handle on what flow rate you're running at?

If it just a transient starting up then you might just need a short term overload overide, but your motor does look quite close to the.limit.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[DubMac]

From your performance curve, propeller blade angle between A-B gets you near 60HP when you are pushing 10'. If you consider that the efficiency may be lower than published, the blade angle may be closer to A than B, and top it off with extra bearing friction due to sand intrusion......that may explain tripping.

Another consideration on propeller pumps is how far off of the liner the blade is set. Similar to setting the clearance on a vertical turbine pump by raising or lowering the shaft, you perform the same operation with a prop pump to gauge the proper clearance between the bowl liner and the blade edges. The manufacturer should be able to tell you this setting. It will affect efficiency if it is way off.

If your problems are mechanical, the tail, or sand bearing could be the culprit. In very high sand content, many users like a separate pressurized lube line to it, either grease or water.

 

[itsmoked]

orifice plates are simple, cheap and easy to instal between existing flanges

Thanks for the info LittleInch.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[georgeverghese]

Based on revised information, with impeller performance between A and B curves:

The 8-10 ft TDH matches up with info provided : head = 5ft for submergence + 4 ft for the overweir = 9ft, for which power demand is 45HP for the B curve and 60HP for the A curve.

At startup, we have a total head of approx 13-14ft (8+5+1), given that all water in the pumping column would have previously dribbled back in to the river. That indicates a startup power demand of between 60HP for the B curve and 75HP for the A curve. There seems to be a pencil mark between the A and B curves at 13ft head which I presume may be the actual pump operating point at startup? - this indicates 70HP at startup.

Where is this pump curve actually between A and B - on the pencil mark? I can see your reasoning for going to a full B curve.

What amps are we tripping at now? Is the setting correct? Else, would the electrical engineer allow for some minor short term exceedence at startup? That will avoid having to change out impellers. Adding a foot valve at the intake may help but could get blocked up.

 

[Artisi]

One more check to be considered, is full voltage being supplied to the motor and is the motor / starter wired correctly, for instance is it wired star/ delta ?? is it switching to delta.
At this point based on the information given I can see no reason what so ever that there is a pump hydraulic problem.

A pump of that flow, with that length of column would would go from start to full operating condition in the bat of an eye.
You really need to prepare a detailed written description as to that is happening once the start button is pushed.
For me, if not a major mechsnical problem with the unit i would be looking at electrics.


It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[ar9]

Thanks everyone for the responses. I will try to explain more in depth what is going on with the station.

All conditions as previously set, we run the pumps. Startup is not an issue, pumps start flowing and within seconds (2 at most), the weir is full up to +4 Elevation. The pump then runs for periods between 3-7 minutes until the amps reach approximately 105 and it trips. The pencil mark on the curve is where the existing tilt would fall, in between A&B. As well, this pump is not in-taking from a river, this is a drainage pump with a concrete sump and it has a trash screen before the intake. I don't see sand/debris being an issue here as it was mentioned.

As far as wiring, I will have to check with the Electrical Engineer of the project. I dont have a flow reading from the time's we have run it.

If any other information can be helpful, please let me know and I will try to post it ASAP. THANKS EVERYONE!!

 

[LittleInch]

What voltage and phase?

Do you have a pressure guage on the discharge? Can you fit one? Will only measure a few psi, but would tell you what is going on. I calculated you're flowing at 4m/sec. With only one our two feet head difference making such a big power impact small impacts might have an effect.

Was there ever a performance test on your particular pump? The vendor should have something rather than guess work of between a and b lines. More details on your motor would be good such as the data on the plate.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

Is the motor tripping on high amps (overcurrent) or on motor fault at 105amps?

We have a 13-14ft long pump shaft here - would we say "guilty till proven innocent" on this shaft intermediate bearing?

 

[Artisi]

What is full amps of the motor?
And:
Quote "The pump then runs for periods between 3-7 minutes until the amps reach approximately 105 and it trips." Unquote
This doesn't make sense, -- until the amps reach approximately 105 and it trips -- why is it taking 3-7 minutes to reach 105 amps - the pump should come up to duty load within a few seconds.

I still think the pump is ok - electrics - electrics - electrics.


It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[georgeverghese]

Sounds like there's heat building up somewhere till it causes the overload :

a) The shaft intermediate support bearing is heating up for some reason - lube oil cooling?
b) Motor windings or motor bearings heating up?

 

[LittleInch]

Given we don't know what the start water level of the sump is or what happens to it, are you sure the incoming water level stays the same? If it starts to drop off after the pump starts then the effective head required will climb along with the power required?? With that pump adding a couple of feet head makes a lot of difference to power absorbed.

Is your 75hp motor a 75 hp shaft output motor?

Either that or your trash screen gets blocked, amps increase, pump trips, flow stops, a bit of back wash, the "trash" falls off so when you inspect there is no "trash" until you start up, the flow swirls around again and ....

This gives you enough things to investigate - let us know what you find.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Artisi]

Any insight to the problem as yet?

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[ar9]

Everyon thanks for the responses. As of now, the pump manufacturer is set to check the tilt of the prop and the bearing. I will keep you posted as soon as this happens. Thanks

 

[ar9]

Everyone, sorry for the lack of response. There has been some troubleshooting going on between pump manufacturer and contractor but no results yet. Can anyone quickly back-check my TDH calculations? Using the schematic attached in the first post and having a total equivalent length of pipe (with fittings) of 180' for the 24" pipe and a total length (again, with fittings) of 65' for the 30" discharge pipe. Both pipes are 1/2" thick. Bear in mind that there is a weir installed at the discharge end that brings the water elevation to +4.

Just want to make sure from an unbiased mind.

THANKS!

 

[ar9]

In reference to the post above, the design flow is 35 cfs (approx. 15709 gpm).

 

[LittleInch]

At that flow and pipe lengths (which are a bit different from the first post...) I get a TDH of about 7.5 ft (4ft for the weir and 3.5 for losses in the pipe). This is a bit off your pump curve and implies a higher flow, but if your water level drops below the zero point on your schematic, then this directly adds to the TDH.

the question is how far between A and B is your pump.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.