hello ,
please advice !
The problem is related with the pump and motor mismatch. the whole
scenario is as follows:
Water is required for a power plant of more than 1000 MW. to fullfil
this requirement a pump has been installed at the nearest river. the
pump is a vertical turbine pump with a 180KW induction motor attached
to it. the pump is rated at 1550 meter cube/ hr of discharge. the rated
speed of the pump is 1450 rpm. now the induction motor selected was of
1500rpm of 180KW.
After installation , during commissioning it was found that the motor
is overloaded. the motor has a rated current of 18.7A but during
testing it was found to be 22A.
Also the discharge is 2000 meter cube/hr.
this is due to the obvious miss match between the motor and the pump.
The pump and motor details are as follows:
induction motor:
output : 180KW
speed : 1492
type of duty: s1 (continous)
rated voltage: 6600V
no of phase: 3
frequency: 50Hz
full load current: 19A
starting % full load current: 6.5 times full load current
guaranted efficiency at pump duty point without any tolerance: 93.4%
Pump:
type of pump: vertical turbine pump
type of drive offered direct
suction bowl: 530mm
discharge pipe 500mm
rated capacity: 1550 M3/Hr
total effective head at rated capacity: 33.4m
pump losses:
suction losses: 0.6m
column losses:
discharge head losses:
bowl head at rated capacity : 34.0m
shut off head: 53.m
pump speed: 1450rpm
bowl input required at rated capacity without energy improvement
coating : 163.09KW
transmission losses: 1.2KW
i think the above data is sufficient. Please advice on what can be done
to reduce the motor loading.
few suggestions as per our technical team
1) use a oriface to increase the system resistance.
2) trim the pump
also the pump/motor is started with valve closed. the is a Butterfly
valve at the end of the discharge.
By keeping the valve only 30% open the motor rated current reaches to
19A, which is appropriate.
Also if you could tell me why exactly the motor is overloaded. ?
Irrespective of ratings, the fact is that the pump is overloading the motor.
If you throttle the discharge to reduce the flow this should reduce the
power consumption as well as the delivery flow. The most useful document for
working through this situation is the pump curve, which I assume either you
have or is obtainable. A quick way to test this out may be to throttle back
on an isolation valve, but you need to be careful of possible mechanical
problems in the line such as vibration or cavitation, isolation valves
aren't designed for throttling..
As you mention, you can also reduce the size of the impeller, either by
fitting a smaller one or machining the existing one. This is likely to
provide more improvement than throttling the output, which involves needless
wasting of energy. Pump curves often provide information on pump
performance with a few impeller size options, otherwise the vendor should
certainly be able to advise. In fact, if the vendor is any good, they should
be able to do some of the work in engineering a solution for you.
What you have isn't so much a pump-motor mismatch as a pump-system mismatch.
The motor, being an induction motor will run pretty much constant speed,
regardless of the load (up to a point obviously). Thus, the pump is running
at a fixed speed. But you have too much flow. One of the basic pump
affinity laws is that power required to drive a pump is proportional to the
flow rate cubed. So a slight excess in flow causes a significant increase
in power demand.
My guess is, if you look at the discharge head of the pump, you'll find that
it is a bit lower than rated as well. This confirms that the pump is
operating beyond it's design operating point.
It is not unusual for a new system to operate beyond it's design. Keep in
mind, the engineer that specified the pump probably wanted to get rated flow
after the piping and components had been in operation for a while and the
system had some amount of fouling (either silt, scale, or bio). So when
brand new, flow is higher than the final design point.
Unfortunately, it seems the engineer didn't consider this higher flow rate
when the system is new, when he sized the drive motor. So the load when the
system is clean/new is more than the drive motor is designed for. You must
reduce the load on the motor by reducing the system flow.
There are several ways to reduce the flow in the system to within the power
capacity of the pump. Throttling the discharge is only a temporary fix as
it will eventually destroy the butterfly valve internals. If the final
outlet/discharge from the system has a globe valve designed for throttling,
it would be best to adjust that instead. Probably the best fix, as
suggested by your technical team is to increase the system resistance curve
by installing an orifice plate. If there is a concern about keeping the
components supplied by the pump fully flooded, it probably is best to put
the orifice at the extreme outlet so that the system piping remains
pressurized. The down side of using a fixed orifice plate is that as the
system ages and fouling increases, flow will then drop *below* the desired
operating point and you'll either have to periodically clean the system, or
re-size the orifice.
daestrom
Please excuse a dumb blonde question but this isn't my field and I hope
that you can help me understand what is going on.
The motor is running at constant speed. It would, presumably, run at the
same speed if no water was present in the pump at all - only with much
reduced motor current. If water was actually pushed through the pump (by
another pump), then I assume the same would apply and the current taken
would also be low.
Now, putting a restriction in the pipe is going to make the pump's job
harder, surely? And thus the motor current greater?
I had expected solutions that would make the pump's job easier - for
example:
1) Putting an adjustable bypass pipe across the pump - so that outlet
water was forced into the inlet. This, I imagined, would make the pump's
job easier and reduce the load on the motor.
2) Putting an adjustable air vent into the inlet pipe - so that air
could be bled into the inlet. Obviously, open too far and the pump would
only be "pumping" air - which presumably it can do without a lot of
effort. I dread to think of what an aerated mixture going through a
turbine does to it - it doesn't sound good practice. But I thought that
it would reduce the load on the motor.
As I started saying..dumb blonde..
I didn't understand the point about "if you look at the discharge head
of the pump, you'll find that it is a bit lower than rated as well."
Does that mean that the output pressure is lower than it should be? I
didn't understand the original post either, when it talked about "bowl
head".
If you do have the time and patience to explain, I would be grateful..
Not necessarily. You see the actual work being done is the product of the
work done on each unit mass of water and the number of units of mass
flowing.
If you have a positive displacement pump (not what we have here), then the
mass flow rate is always the same (if you keep the speed constant). In that
case, throttling the discharge makes it harder to push each kg of water
through the system, and yes, the power required goes up.
But centrifugal pumps are not so simple. When you throttle the discharge,
the amount of energy needed to push each kg of water through the system does
go up. But the number of kg's flowing through the system goes down. So now
it becomes a question of 'Does the work/kg go up faster than the flow in
kg/s goes down?" For just about any centrifugal pump, the actual result
is that the pump discharge pressure goes up only a little bit, while the
flow through the pump drops markedly. So the overall power needed goes
down. The so-called 'pump laws' discuss this as well as how the flow/head
change with pump speed.
There are some types of pumps whose discharge characteristic (the 'pump
curve') shows the head remains pretty much constant with changes in flow
(these have a deep suction and are sort of a cross between an axial
propeller type and the traditional radial centrifugal type). With these,
the power is directly proportional to flow.
I screwed up in my last post, the pumping power doesn't follow flow cubed,
it follows speed cubed when you have a variable speed setup (not the case
here). For a constant speed centrifugal pump, the power is almost linear
with flow.
But if you look at *just* the pump, inlet to outlet, you'll see that in that
case we have *more* flow through the pump. And the discharge pressure may
drop some depending on the exact pump curve, it will not drop as much as the
flow has increased. So a bypass will end up loading the pump/motor down
even more.
No, bleeding air into the inlet would be a bad idea. Yes, it is much less
energy to move the same volume of air, but there are some problems with the
pump. Most pumps cannot move air very well at all. When a pump has a lot
of air in the casing, it becomes 'air bound'. With air around the impellor,
it cannot pressurize it to the same discharge pressure as with water. So it
can't 'push' the air out of the impellor to make room for more incoming
water. The result is that flow stops completely. So the air (and perhaps a
trace of water) just spins around in the pump casing, gradually warming up
due to friction. The motor doesn't have much load on it, since spinning air
is pretty easy. But left for a long time, the casing can overheat. When
the metal expands, the impellor can come into contact with the casing and
friction will quickly heat the metal (remember, we have no lubricant now)
and the whole thing seizes up in a bad way.
A little bit of air will form bubbles in the inlet (duh...). But the
bubbles will collapse as they travel through the pump and to the higher
pressure region of the discharge (higher pressure squeezes the size of the
bubbles down). This is a bit like cavitation, and the collapsing bubbles
mean that water rushes into the 'imploding' space and impacts the
casing/impellor. This causes accelerated corrosion/erosion.
Best to keep the suction completely flooded with water.
If you were to set up a centrifugal pump on a test stand, you could operate
it with a lot of different flow rates. For each flow rate, measure the
discharge pressure. Now, plot the points with flow rate along the
horizontal axis of a graph, and the height above the horizontal line the
measured discharge pressure.
For most rotory pumps, the resulting 'pump-curve' will start out almost
horizontal near the zero flow point, and curve downward as the flow
increases. (sort of a cutaway view of 1/2 of an upside down bowl).
Remember that as you change the flow through the system, you simply move
left/right on this curve. So as you move left (less flow), the discharge
pressure rises slightly, and as you move right (more flow), the discharge
pressure lowers (more and more as you move further to the right, it gets
steeper and steeper). The overall slope is closer to horizontal than
vertical (less than a 45 degree down angle or if you prefer, a very shallow
upside-down bowl). So moving left means the 'rise' in pump head is less
than the reduction ('run') in pump flow. So total power drops.
Since this pump is running with more flow than design, it stands to reason
it is operating further to the right on the curve, at a lower discharge
pressure.
(if you've read this far, you can skip to the end if you want. This is just
some more 'stuff')
Now, pushing water through a fixed set of pipes/fixtures has a certain flow
versus pressure relationship as well. For almost every case, the pressure
needed to push water through a fixed system is proportional to the flow rate
squared. So if you run some experiments on the system, keeping track of the
pressure needed to develop a given flow, you can plot those points on the
same graph. Of course, zero pressure develops zero flow, and as you
increase flow to the right on the graph, the pressure needed rises. This
plots one side of a perfect, concave up, parabola.
If you drew the 'system curve' on the same set of axis as your 'pump curve'
you'll find the two curves intersect at some point. And this is the
operating point. It will operate there with stable flow. If some random
pertabation caused flow to drop slightly, the pump curve shows that the pump
discharge pressure would rise slightly, and the system curve shows that less
pressure is needed to develop that lessor flow. So, the excess pressure
will accelerate the fluid and flow rate is restored. Similarly if flow
should rise slightly, the forces in the system will act to slow the fluid
and restore the original flow rate. We have a nice stable flow rate.
Closing a throttle valve slightly, or adding an orifice plate changes the
system curve. More restriction makes the parabola 'steeper' as the pressure
needed to develop each flow point on the curve becomes higher. So it
intersects the pump curve at a point that is somewhat to the left of the
original operating point.
Another fix as 'bruce varley' mentioned, is to change the shape of the pump
curve by modifying the pump internals. This lowers and steepens the pump
curve's drop so that again, the intersection point between pump curve and
system curve has moved to the left.
Hope this helps...
daestrom
P.S. Although I've met a few 'dumb' people that happened to be blonde, I've
also met some smart ones. Never assume....
Many thanks. Yes it did and yes I did read every word. I hadn't ever
thought, or had to think, about how water pumps perform before. I shall
read up some more now.
I am wondering what would be the effect of adding a pump *before* a
centrifugal pump fitted with a restricted outlet. This would,
presumably, increase the flow through the pump? And hence *increase* its
power consumption? Could a much lower power "pre-pump" thus be used to
greatly vary the output of a biggie?
Many thanks again, I always find your posts interesting,
Whilst the discussion on pump mechanics is very interesting and
enligthening to some.
A 1000MW powerstation is to put it mildly QUITE LARGE and this pump must
have cost a lot of money presumably from a large engineering company.
The specification was wrong if it called for a 1500 rpm induction motor
as that would be the synchronous speed and induction motors always have
slip and as you stated the nameplate speed is 1492RPM.
The real and only practical solution is to get the designer and the
supplier to fix the problem.
When you put two pumps in series like that, it can get 'interesting'. If
they have identical pump curves, then it behaves much like one *big* pump
with a pump curve that is twice as 'tall' (double the head) but the save
'width' (same flow scale). Now, when you put this on the same graph as the
system curve, obviously it intersects the system curve at a different place
(further to the right, implying much more flow).
If the first pump 'pushes' the water into the second, the flow in the system
(including the second pump) rises. If you increase flow *far* enough, you
can reach the point on the second pump's curve where the head drops faster
than the flow increases. Just think of increasing the flow through the pump
to the point where the pump curve comes down and actually touches the
horizontal axis. Now there is a lot of flow, but no work (because we're
multiplying by zero head). Sounds great doesn't it? But how much work did
we have to supply to the other pump to increase the flow that much? (care
to wager if it's more than the original pump in the original flow setup??)
Since there is more flow through the system, the system curve tells us that
it takes more pressure input to develop that extra flow. It's coming from
your booster pump. So the booster pump has to raise the pressure higher,
and it has more flow. So we've eliminated the power requirement for the
original pump, but had to supply even more power to the new one. A losing
situation.
You're welcome.
Oooh, a redhead. Do you have the stereotypical temperment to go with it?
;-)
daestrom
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