Pro/Forums - View Single Post - Pump rated wattage vs actual power draw vs pumping power

myv65 · 11-14-2003, 09:19 PM

Hey Bill,

I'm still here, though not too often anymore. I skimmed this thread purely because JD linked it over at AMDMB. I thought I had beaten this subject to death a long time ago. Much of what I'll say next (like ALL of it) I've said before, so I'll try to abbreviate. Oh, and I gotta say that you guys are really off into the Rube Goldberg region with your block hold-down devices. It's about time for someone to draw a free-body diagram of the situation and figure out that it really isn't that tough a nut to crack. If time allows I'll get back with you on that topic, just don't hold your breath.

Power to the impeller always increases with increasing flowrate.

Head from the pump always decreases with increasing flowrate.

The impact of flow is greater on shaft power than the impact of head.

Cathar didn't see max power because he is not truly running wide open. Running wide open is a sort of utopian point that doesn't exist. As folks here know, the suction conditions can easily push you over into cavitation. Well, even when there is no cavitation there are still suction (and discharge) losses. An idealized suction and discharge would generally correspond to zero-TDH and peak shaft power.

You also need to have a basic understanding of electric motors. Motors are generally rated for a specific base speed and corresponding torque. At lower loads, the speed increases ever so slightly and torque drops. Vice versa for higher loads. Provided you don't stall the motor, that is.

With no water in the pump, the rpm probably went up a dozen or so and torque dropped dramatically. Neglecting inefficiency, power is nothing more than torque * rpm, so power drops off quickly when output torque is basically zero. ie, all energy put in is essentially lost as motor inefficiency.

There's an easy way to hear this with fans, which follow the same basic laws as pumps. LEGAL WARNING TO BONEHEADS: Do not stick your fingers in a fan doing this! Hold a 60 or 80mm fan in your hand and plug it into a molex. It'll have a distinct pitch. Cover the inlet with your other hand. The pitch will increase, yet no air gets pumped. The speed goes up because the shaft torque goes down. Delta_speed is a heckuva lot smaller than delta_torque, so net power goes down a lot. Cover the outlet and the result is the same. Wanna be really *cool*? Do this in your BIOS screen with a three plug fan or with some fan monitoring software running for a (somewhat inaccurate) measure of rpm.

Phaestus,

I haven't done what you suggest, but it's pretty routine to keep pumps running 24/7 in the systems we used in the paper industry. The pump energy is enough to keep the oil warm during machine outages. A fun one was our hot oil systems. At room temperature the oil is pretty thick and our flow control valves would be wide open yet system flow well below setpoint. Pump energy would get you up to ~140°F and almost bring the valve off its wide-open position. By the time the burners took it to 550°F and the oil's viscosity dropped below water the flow control valve would darn near close to keep from overshooting the desired loop flow.

11-14-2003, 09:19 PM	#22
myv65 Cooling Savant Join Date: May 2002 Location: home Posts: 365	Hey Bill, I'm still here, though not too often anymore. I skimmed this thread purely because JD linked it over at AMDMB. I thought I had beaten this subject to death a long time ago. Much of what I'll say next (like ALL of it) I've said before, so I'll try to abbreviate. Oh, and I gotta say that you guys are really off into the Rube Goldberg region with your block hold-down devices. It's about time for someone to draw a free-body diagram of the situation and figure out that it really isn't that tough a nut to crack. If time allows I'll get back with you on that topic, just don't hold your breath. Power to the impeller always increases with increasing flowrate. Head from the pump always decreases with increasing flowrate. The impact of flow is greater on shaft power than the impact of head. Cathar didn't see max power because he is not truly running wide open. Running wide open is a sort of utopian point that doesn't exist. As folks here know, the suction conditions can easily push you over into cavitation. Well, even when there is no cavitation there are still suction (and discharge) losses. An idealized suction and discharge would generally correspond to zero-TDH and peak shaft power. You also need to have a basic understanding of electric motors. Motors are generally rated for a specific base speed and corresponding torque. At lower loads, the speed increases ever so slightly and torque drops. Vice versa for higher loads. Provided you don't stall the motor, that is. With no water in the pump, the rpm probably went up a dozen or so and torque dropped dramatically. Neglecting inefficiency, power is nothing more than torque * rpm, so power drops off quickly when output torque is basically zero. ie, all energy put in is essentially lost as motor inefficiency. There's an easy way to hear this with fans, which follow the same basic laws as pumps. LEGAL WARNING TO BONEHEADS: Do not stick your fingers in a fan doing this! Hold a 60 or 80mm fan in your hand and plug it into a molex. It'll have a distinct pitch. Cover the inlet with your other hand. The pitch will increase, yet no air gets pumped. The speed goes up because the shaft torque goes down. Delta_speed is a heckuva lot smaller than delta_torque, so net power goes down a lot. Cover the outlet and the result is the same. Wanna be really cool? Do this in your BIOS screen with a three plug fan or with some fan monitoring software running for a (somewhat inaccurate) measure of rpm. Phaestus, I haven't done what you suggest, but it's pretty routine to keep pumps running 24/7 in the systems we used in the paper industry. The pump energy is enough to keep the oil warm during machine outages. A fun one was our hot oil systems. At room temperature the oil is pretty thick and our flow control valves would be wide open yet system flow well below setpoint. Pump energy would get you up to ~140°F and almost bring the valve off its wide-open position. By the time the burners took it to 550°F and the oil's viscosity dropped below water the flow control valve would darn near close to keep from overshooting the desired loop flow.