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Pump Run Problems

Pump Run Problems

Pump Run Problems

(OP)
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

RE: Pump Run Problems

The missing information here is what happens after your 30 " pipe?

If this fills a tank and the water level is higher than the pump discharge elevation, the two bends have no significant impact ( a small amount of extra friction loss, but only equivalent to a few feet of pipe)

If the water level is less than the pump discharge elevation then yes it will act as a syphon and essentially lower the head at the discharge point to virtually zero.

The same applies to if the discharge just flows away.

In both instances it is doubtful that the pump can cope with this and what you describe is a pump running beyond it's duty point and off the end of its curve to the right hand side, resulting in increased power demand and eventual tripping on high amps and possibly high vibration.

To avoid this and reduce flow rate and climb the head you need to add pressure drop, either by adding a valve or possibly an orifice plate or some other sort of flow restriction.

First though complete the hydraulic description and state where is the end water level or pressure this pump is pumping into

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

RE: Pump Run Problems

To fully understand your application, you should post a pump performance curve.

This appears to be runout of the pump. Your pump is pumping at too great of a capacity because there is low resistance (differential pressure) to flow.

For a given centrifugal pump, the flow rate through the pump is dependent upon the differential pressure or head developed by the pump. The lower the pump head, the higher the flow rate. A specific pump has a curve of pump flow rate versus pump head called a pump performance curve. After a pump is installed in a system, it is usually tested to ensure that the flow rate and head of the pump are within the required pump curve specification. A typical centrifugal pump characteristic curve is shown below.

There are several terms associated with the pump performance curve that must be defined. Shutoff head is the maximum head that can be developed by a centrifugal pump operating at a set speed. Pump runout is the maximum flow that can be developed by a centrifugal pump without damaging the pump. Centrifugal pumps must be designed and operated to be protected from the conditions of pump runout or operating at shutoff head.



The pump may not have enough HP to pump at the rate that you are pumping.

Post the pump performance curve to confirm.

RE: Pump Run Problems

(OP)
LittleInch - Thanks for the reply.

The discharge is open to atmospheric pressure. Therefore, we installed a weir so that it would create a tank operation . The weir brings the water to an elevation of +4 when the pumps are running (with this, the discharge pipe is fully submerged). You are right, the amps are climbing up and making the pump trip.

I thought about blocking the discharge end pipe in some sort in order to add even more pressure, but I wanted to confirm that the bends are not working against the system essentially. Would it be better to have a sort of pipe reducer right before the bends?

bimr - Thanks for the reply.

I do have the pump performance curve and it is not near the shut-off head, instead as LittleInch mentioned, it is going off the curve to the right side (getting into low efficiency)Pump Runout. We need to add head into our system and wanted to make sure that the two bends were not working against the system. We need more head in our system, do you think by adding an orifice block at the discharge end (after the bends) would drive us back to healthy pump operating points? It is currently running ~1' above the pump runout. (I will try to post the performance curve ASAP.)

RE: Pump Run Problems

The bends are not likely to have any significant affect.

The installation and use of an orifice plate or a valve to restrict the flow is recommended.

You can see from the following equation, that as the flow increases, the required HP increases as well.

How do I calculate Brake Horsepower Required for a centrifugal pump?

Brake Horsepower Required = GPM Required X (Total Dynamic Head) / 3940 / Efficiency

If you slow the flow down with a restriction, the HP consumed decreases.


RE: Pump Run Problems

Adding a suitably sized orifice plate anywhere in the system from pump discharge onward would help get the pump into a better place.

As said the elbows add a little bit more friction than straight pipe but the effect is very small.

Your alternatives are maybe reduce pump speed or alter the pump by taking out a stage or reducing the size of the impellor. All depends on cost and how many hours and days this runs for.

Would love to see the pump curve....

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

RE: Pump Run Problems

That's an interesting pump. I haven't seen many where power goes down at flow goes up. Given the high efficiency shown you really must be operating at virtually zero head. Try an orifice and see what happens.

Let us know what happens.

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

RE: Pump Run Problems

The orifice plate should have an orifice of 18 Inches diameter in the 24-Inch pipe. The orifice equation shows that would generate a headloss of 14 feet at 14,000 gallons per minute.

You might have to experiment a little with the orifice size.

RE: Pump Run Problems

(OP)
I appreciate all your answers bimr and LittleInch. I will keep troubleshooting and post once resolved. I was thinking of an 18" opening for the orifice. I will let you know how that goes.

RE: Pump Run Problems

It is obvious that the pump is axial flow design, high flow, low head, very steep H,Q curve with increasing power with increasing head. Running at end of curve is normally no problem within NPSH constraints.
What is causing the overload - good question, is the motor correctly sized, is the impeller hitting the casing.
Can see no obvious reason for overload if the motor is correctly sized.

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.)

RE: Pump Run Problems

What are the impeller curves A-D, diameter or impeller tilt.
Is thus a new or existing installation?

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.)

RE: Pump Run Problems

(OP)
Artisi - Thanks for the reply.

It is indeed an axial flow pump as you have described. The impeller A-D curves are the impeller tilt. At this point, an impeller between C & D was poured. This is a new installation and when tested, had the issue of the pumps tripping. My concern was having low TDH and running off the curve, therefore, lower efficiency, and was wondering if this was the cause for the AMP's climbing.

RE: Pump Run Problems

Is this pump a single stage vertical pump with the pumping element down below el 0ft? You say pump discharge nozzle centreline is at +8ft? If so, is there adequate submergence to prevent vortexing? Do we need a vortex breaker here?

Shaft support bearing issues?

RE: Pump Run Problems

(OP)
georgeverghese- Thanks for the reply.

It is a single stage pump. Minimum submergence is 5' per manufacturer and that is covered. As well, a splitter plate was installed in the intake bell to avoid vortexing.

RE: Pump Run Problems

Could the intake cowl be blocked or the pump impeller be seized up with mud or silt ? is this in river water service or something similar?

Any way of inspecting this shaft support bearing? These are now mechanical issues and getting beyond my depth..

RE: Pump Run Problems

Time for stupidity check,is the motor speed / power correct, is the pump rotation correct, has impeller blade angle been confirmed?
Don't assume rotation is correct unless confirmed by yourself - secondhand confirmation is always
suspect.
Has flowrate been checked and any pressure readings been taken?


Agree with George, it could be a mechanical problem.


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.)

RE: Pump Run Problems

Mechanical issues aside why would one reach for an orifice plate instead of a valve that can be quasi-infinitely adjusted?

Keith Cress
kcress - http://www.flaminsystems.com

RE: Pump Run Problems

For simple one flow rate systems like this, orifice plates are simple, cheap and easy to instal between existing flanges.

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

RE: Pump Run Problems

Why are we discussing orifice plates, this is an axial flow impeller, as flow reduces so does required power input.
On this pump assuming half C-D vane tilt and a head across the pump of 4ft - flow of 12000 gpm and efficiency of 70% - power required is approx 17hp which hopefully is a lot less than the installed motor.
We need to be looking elsewhere for the problem.

NOTE: see my next posting correcting an error in this post.

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.)

RE: Pump Run Problems

(OP)
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.

RE: Pump Run Problems

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.)

RE: Pump Run Problems

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.

RE: Pump Run Problems

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.

RE: Pump Run Problems

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.

RE: Pump Run Problems

Quote:

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

Thanks for the info LittleInch.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Pump Run Problems

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.

RE: Pump Run Problems

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.)

RE: Pump Run Problems

(OP)
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!!

RE: Pump Run Problems

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.

RE: Pump Run Problems

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?

RE: Pump Run Problems

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.)

RE: Pump Run Problems

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?

RE: Pump Run Problems

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.

RE: Pump Run Problems

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.)

RE: Pump Run Problems

(OP)
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

RE: Pump Run Problems

(OP)
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!

RE: Pump Run Problems

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

RE: Pump Run Problems

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.

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