PUMP HORSE POWER QUESTION 1

[dkm0038]

I've seen the correlation of

(hydrualic horsepower) = (total head)x(flow rate)/(constant)

I want to set up a system in which it is important to maintain a constant flow rate but with a varying total head. but does the above equation mean that if I have a certain hp pump then the flow rate will vary with the total head, or does it mean that I will have a certain flow rate and the horse power being drawn from the pump will vary?? or something totally different??

THANK YOU

 

[BigInch]

Power will be consumed by the pump according to what is needed to make the flow going through it. The motor will attempt to draw the power it needs until it overheats and shuts down on hi temperature indicator, or goes into meltdown, burns, trips a fuse, melts the power cables if they arn't designed for the excessive amps, or otherwise burns or breaks up from excessive loads on the shaft and bearings when trying to do so. If there is a way for the motor or pump to exceed its ratings, you must either limit the flow to power levels within both the motor's and the pump's ratings, or shut it down before it gets to that point. A flow control valve limited to flows under that maximum power rating, FCV failing closed, would normally do the trick, in addition to the normally required fuse or breaker in the pump power supply, if that happens to be electric.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[raronen]

dkm0038,

(hydrualic horsepower) = (total head)x(flow rate)/(constant)

does the above equation mean that if I have a certain hp pump then the flow rate will vary with the total head, or does it mean that I will have a certain flow rate and the horse power being drawn from the pump will vary??
__________________________________________

As BigInch noted, it is the motor where HP is fixed.

The pump HP equation doesn't really tell you anything about the pump itself. Pump HP is not a fixed design criteria for the pump. Change the fluid pumped from propane to sulfuric acid (4X the specific gravity), the pump will consume nearly 4X more HP, and the pump doesn't care. The motor, however, will care a lot.

So, in the early design phase, consider using the pump HP equation to determine the max HP needed for the operating condition at maximum head, and then apply efficiencies and factor of safety to determine the size of the motor you need.

Once your pump is in place, what the equation says is that as head increases, the HP draw on the MOTOR will increase.

 

[lcarter157]

dkm0038,

You stated: "I want to set up a system in which it is important to maintain a constant flow rate but with a varying total head."

To meet the above conditions, you need a positive displacement pump - not centrifugal. Calculate required horsepower based on stated flow at maximum head, don't forget to calculate head at maximum viscosity at lowest pumping temperature.

 

[dkm0038]

thanks lcarter157, So is it right to say that as long as you have a powerful enough pump then it will produce a constant flow rate within a range of total head?

 

[lcarter157]

Essentially yes, however nothing is 100% efficient. Positive Displacement (PD) pumps have slip. At higher viscosity, less slip but more head loss in the piping system. At low viscosity, less slippage, less head loss. Specify maximum capacity at minimum viscosity and specify required maximum horsepower at maximum viscosity. Especially consider cold start up, will you start up a pumps will a pipeline full of cold fluid?

 

[dkm0038]

I've been looking at gear pumps and notice that you'll need to purchase a motor seperately. What are the general considerations when selecting a motor for a given gear pump? And would a gear pump be advisable for pumping hot oil?

 

[BigInch]

kcarter,

I think that what you're saying is a considerable bit of misinformation. When you say you "need a PD pump - not centrifugal" for a constant flow application, don't you agree you are implying that centrifugal pumps will not work with flow control. We all know that it is simply not true.

PDs do any head at a given speed and a given flowrate within their power capacity and robustness, irregardless of the system curve, while fixed speed centrifugals do any flow and any head at a fixed speed within their "curve" and with regard to the system curve. You should recognize that the performance of both can be equalized simply by adding an equivalent VFD if electric, or a VSD, ie. driving the centrifugal with a diesel engine.

This is a PD pump curve on fixed speed, yet a centrifugal can reach the same H-Q point, simply by changing its speed, ie, reving the diesel up or down to get to it. So, I ask you, does that look like I "need a PD"? I'd say, NOT.

http://www.driedger.ca/ce2_pdp/CE2_PDP.html

However, in some cases a PD may be more advantageous, such as in viscous product, high pressure and low speed - low flow range situations, or a combination of those, in which case I would probably select it over a centrifugal. If I needed wide flow range at low to medium pressure ratios, for suitable products, centrifugals have many advantages over recips.


**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[lcarter157]

BigInch,

I stand corrected, as a matter of fact, that hit me last night. I'm deeply involved in a project involving marine loading of base oils at a considerable range of viscosities and temperature. Apparently I have developed tunnel vision. Of course a centrifugal will work. I regret any misinformation, it was not intentional.

 

[BigInch]

I get the same way with centrifugals sometimes too.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/