Adding stages to a VTP 1

[jamaican]

Can anyone tell me why after increase the stage of a 10inch vertical turbine pump from 3 to 4 stages the flow fell from 288 m3/hr to 240m3/hr (1500RPM)instead of increasing. Please note the pump is operating in the same system. The system of operation is pumping directly inline.

 

[bimr]

The gallons per minute should stay the same no matter how many stages are added.

 

[TD2K]

I'd expect the flow to go up since the addition of another stage should increase the head from the pump. By "pumping directly inline" do you mean the downstream system curve sets the operational point for the pump on its head/flow curve?

Something left in the pump during the addition of the stage that is obstructing flow?
Some change in the downstream piping such as a valve partially closed?
Suction restriction?

 

[jamaican]

I will agree with you TD2K I was expecting an increase in flow with the additional stage, that's the reason the stage was added. I am sure that nothing was left in the pump during fitting up. With the additional head do you thing the NPSHR might have changed and is now insufficient? Please note that the pump operates in an open thank. Of note is that the new pumping pressure is slightly lower than before the modification was made, its confusing.

 

[Pumpsonly]

Both flow and pressure should increased if there is no change in the down stream condition.

1) check pump rotation is correct or not.

2) Check you installed the correct impeller.

Was the original pump with dummy stage casing before?
if not, the addition of another stage could push the suction closer to the bottom of the tank and cause inlet restriction and resulting in cavitation.

 

[bimr]

If you did not put a larger motor on the pump, does your existing pump motor have 33% excess HP for the new flow and head conditions?

The discharge flow may only increase if the system curve that you are pumping against will allow more flow. If you have a pumping against a system curve consisting of mostly static head, then you should not expect more flow, just higher discharge head.

Vertical turbine pump stages (bowls) can be added in series. By doing this, the head capability is increased. The head-capacity curves and horsepower capacity curves are additives
at a given discharge. Head and horsepower are doubled if a second bowl is added to a first bowl; three stages would triple the head produced and horsepower required.

Adding stages usually means furnishing the pump initially with sufficiently large shafting and motor base to accommodate the increased future horsepower.

Does the existing motor have adequate HP for the new condition of 288 m3/hr @ 33% more HP or did you change the motor?

 

[TD2K]

BIMR, if the pumping system is mostly static head that means as you said the discharge pressure is fixed. Adding another stage should in effect push the pump curve to the right shouldn't it? That would to my thinking mean a higher flow.

If the motor is undersized for the new stage, wouldn't the motor likely be tripping due to overcurrent? I can't see the slip increasing enough to reduce the flow if this is a standard AC motor.

Jamaican, what is the fluid you are pumping? You said you are pumping from an open tank so the vapor pressure of the fluid is the main factor in the available NPSH. You said that the discharge pressure had also dropped along with the flow, can you describe the system that this pump is installed in?

 

[JJPellin]

A number of possibilities occur to me. Pumps of this type have a number of different impeller arrangements. We have some with open face impellers that require a certain lift setting. If the lift is excessive, the impeller will allow flow to slip back along the face. Pressure and capacity will be lost. We have others with closed impellers that use a lateral wear ring. These also require a certain lift setting because the axial clearance to the wear rings provides the seal to prevent internal recirculation. We have others that have closed impellers with radial wear rings. These have a more generous lift setting and are thus, not as sensitive to the final lift. Which type is your pump?

If lift is not the problem, I would be concerned with wear ring clearance, impeller pattern (cast for correct rotation), and impeller mounting. If the impellers are mounted to the shaft with tapered collets, an impeller could have come loose and not be spinning.

Some pumps of this type have internal porting within the head so that the seal chamber can be maintained at discharge pressure or suction pressure depending on the installation and removal of certain internal plugs. If plugs were not correctly installed, this could also provide a path for internal recirculation.

As Pumpsonly noted, you need to verify that the clearance between the pump and the sump floor did not change, creating a suction restriction.

Johnny Pellin

 

[bimr]

TD2K, in reviewing this again, jamaican is actually stating that the pump is not operating on the pump performance curve, ie lower flow and pressure.

I saw this occur once when the Contractor incorrectly installed a pump too close to the floor, (as Pumpsonly suggested). The increased suction headloss caused the pump to operate off the pump performance curve.

 

[Artisi]

As per Pumpsonly, is direction correct - wouldn't be the first time a pump has been removed then re-installed and reconnected to the power incorrectly.

Something else to think about, are the new impellers fitted backwards??

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.)

 

[jamaican]

In response to all the comments made so far, the impeller is of the closed type, the rotation is correct (CCW), the pumping fluid is water, the impeller was place in a dummy bowl which was already on the turbine, so the pump length did not change, the current is well within the full load capacity of the motor. The impeller can be installed only one way, the pumping main is a 18 inch, so there is no worry about flow over restriction.

 

[Artisi]

If the flow has dropped and if nothing else has changed then something is wrong - which of course you already know - you need to check all "stupid" reasons,ie, correct direction (you check - don't rely on someone else telling you it's correct), loose impellers - check all impellers locked correctly to the shaft, check flow meter, check any valves in the system etc. as it doesn't make any sense that adding a stage will reduce flow / head.



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.)

 

[bimr]

Regarding "the pump operates in an open tank"

what type of tank and what are the dimensions?

 

[Artisi]

Following on from bimr, it is possible that the pump is trying to achieve additional flow which pushes the inlet flow to the pump into pre-rotation or is causing a free-surface or sub-surface vortex which impacts on the pump performance.

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.)

 

[Pumpsonly]

If you are sure the impellers are fitted correctly, I would still "speculate" the high chance of the problem is wrong rotation. You did not post any info on the construction of the pump to exclude this posibility. As Artisi said,.. check it your self.
You did not post any head figures before and after or any performance curve. So we will just continue to speculate.
A centrifugal pump will produce typically 50 -60% of the performance when in reversed rotation depending on the Ns. Assuming each stage produces 50 Meter at 288 m3/hr. A 3 stage pump in correct rotation will give 150M and 4 stage at 200 M. 60% of 200M = 120M. Bear in mind that now you only get 240M3/hr, the corresponding head on the curve would be higher. Therefore it would be 60% of >200 m. which would be just below the 150M head of 3 stage pump in correct rotation as you mentioned in your second post.

Any way please let us know what was the final finding.

 

[jamaican]

Gentlemen thanks for the assistance, I have found the problem, the limiting factor is the NPSHA. The NPSH requirement above 300m3/h is 53ft. The pump is set at 9ft below the water surface, adding the 33ft available plus 9ft (42ft). So it obvious that there is a short fall in in the NPSHR.

 

[1gibson]

What do your power readings tell you? Is there a major loss in efficiency, or is power lower than expected?

High power (matches expected 4 stage requirements) - forgot to install wear rings, new impeller different hydraulic design (wrong part), leak in system somewhere before location you are measuring

Low power - suction issue (cavitation, vortexing, restriction), loose impeller


Note - it is uncommon to have a "dummy bowl" that is fully functional just minus an impeller. There will be losses in the dummy configuration, without an impeller to guide the flow through the bowl diffusers. It is more common to include a short column/spool piece that matches a bowl length, bearing location, and bolt circles/mounting details. This spool is then pulled and replaced with an actual bowl at the same time an additional impeller is installed.

Are these collet mount or ring/key? In theory you could throw an impeller wherever you wanted on the shaft if it is collet mount. If it was installed above a "top case" and discharging into an open column pipe, I can imagine that flow disturbance to have the symptoms you are describing. I have trouble thinking that this could actually happen, but you never know.

 

[Pumpsonly]

The NPSH requirement above 300m3/h is 53ft. What is the NPSHR at 288 m3/hr and 300M3/hr?
You were doing alright at 288m3/Hr which is not far from 300m3/hr. The NPSHr can not shoot up so much with small increase in flow. Unless you are operating very near to the end of the curve.