The pump you start with the valve open
Every pump you were taught flings water outward and draws its lowest power at zero flow, which is why you were told to start it against a closed valve. An axial pump is a propeller in a pipe, and its power curve is upside down. Start that one the way you were taught and you trip the motor.
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A propeller in a pipe
A centrifugal pump throws water outward. Liquid arrives at the eye of the impeller, the vanes fling it to the rim, and the volute, whose cross section grows all the way round to the discharge, turns that speed into pressure. Spin becomes head, and you trade flow for head.
An axial pump does not throw anything outward. It is a propeller on a shaft inside a column, a ship's screw in a pipe, and it pushes water straight along the axis it sits on. Behind the propeller sit stationary guide vanes whose only job is to take back out the swirl the propeller put in, so the water leaves straight.
That difference in geometry is the whole story. A radial impeller can develop a lot of head and not much flow. A propeller can develop an enormous amount of flow and almost no head.
flow 1,000 to 500,000 m³/h
head up to about 15 m per stage
efficiency 70 to 90% Cooling water intakes, storm drains, irrigation and flood control: services that move a river and lift it a couple of storeys.
For scale on what that means in the field, the West Closure Complex outside New Orleans is rated at 19,140 cubic feet per second, about 540 m³/s, and its job is to lift stormwater a few metres over a closed floodgate. At that rate an Olympic pool, 50 by 25 by 2 metres, which is 2,500 m³, goes past in 4.6 seconds.
Both curves, and a valve you can close
Here is the part nobody draws for you. On a radial pump the power curve rises with flow, so shut off is where the motor works least. On a propeller pump it does the opposite: the head climbs steeply toward shut off, the blades stall, and the power peaks at zero flow.
Close the valve below and watch the operating point walk left. It is a real solve, not an
animation: the system curve is H = Hstatic + kQ², closing the valve
raises k, and the point shown is where that curve meets the pump curve.
Close the discharge valve
Pick the pump, then close the valve.
Motor is fine Shaft power is inside the 1.15x service factor the motor was sized on.
Run the axial pump to zero and the power reads about 1.9x its best point value while the head reads about 3x. A motor bought for the duty point, with the usual service factor, has nowhere to go. Do the same on the radial pump and the power falls to under half. That is the whole of the rule you were taught, and the whole of why it does not travel.
The specific speed ladder
Which behaviour you get is not a brand or a model number. It is one dimensionless group, specific speed, which is what fixes the shape of the impeller and therefore the shape of every curve that comes off it.
| Specific speed | Impeller | Head at shut off | Power at shut off | Start against |
|---|---|---|---|---|
| 500 to 2,000 | Radial | 1.1 to 1.2x | 0.4 to 0.6x | a closed valve |
| 2,000 to 5,000 | Francis, mixed | 1.5 to 2x | about 1x | either, check it |
| 5,000 to 9,000 | Mixed flow | about 2x | 1.2 to 1.5x | an open valve |
| 9,000 and up | Axial, propeller | 2.5 to 3x | 1.8 to 2.2x | an open valve |
Read the bottom row as the sentence it is: at high specific speed the shut off head reaches about three times the best efficiency head, and the power input reaches its maximum at zero flow. Everything about start up follows from that one column.
The start up rule that follows
The reason you were taught to start against a closed valve is that on a radial pump it is the gentlest thing you can do to the motor: minimum power, minimum torque, and the discharge line fills in a controlled way. That rule is correct, and it is correct for that pump.
On a propeller pump the same action is the worst thing you can do. Shut the discharge and you drive the blades into stall, the head runs up toward three times duty, and the shaft power runs up with it. The motor sees its highest load at the instant it has the least help from the load's own inertia.
axial start against an open discharge Vendor start up instructions for propeller pumps say the same thing in the same words: the discharge valve is open, or there is no discharge valve at all and the column simply runs to a siphon or a flap. Check the curve you were sent rather than the habit you were taught.
The practical tell, if you have nothing but the datasheet: look at the power column of the performance table. If the kW figure at minimum flow is larger than the kW figure at duty, you are holding an axial or mixed flow machine and the valve is open on start.
What a fixed efficiency block never sees
In a steady state flowsheet the pump block usually carries one number: an efficiency. Give it a rise in pressure and a flow and it returns a duty, and it will do that happily at any flow you ask for, including zero. It has no curve, so it has no shut off, so it cannot tell you the thing this whole page is about.
The check that matters is not the duty point. It is the power curve laid over the system curve, evaluated at zero flow as well as at the best efficiency point, against the motor's rating and its service factor. That is a five minute check and it is the difference between a commissioning morning and a tripped breaker.
That is the layer being built at Reflux: the model that knows which curve it is on, checks the end of it, and tells you what it verified.
Run it on your own pump
Reflux Student drives your Aspen Plus V14 from plain English: open the case, change the pump curve, re-run, and read back what moved. Three free runs, Windows, your own licence.
Sources: flow and head ranges and the cooling water and sewer duties from the CHE 480 pumps notes; the power input maximum at zero flow, the open discharge start and the 15 m per stage figure from the KSB centrifugal pump lexicon, propeller pump; the shut off head of about three times best efficiency head at high specific speed from Pumps and Systems on specific speed; the 19,140 cfs rating of the New Orleans West Closure Complex from ENR. The pump type at that station is not stated in the source, so it is quoted here as the duty and not as an axial installation. The curves plotted above are representative shapes for each specific speed class, scaled to the best efficiency point, and are not a fitted curve for any one machine.
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