Can I Calculate Thrust for Different Pump Sizes?

In summary, Jordan is building an ROV and wants to use bilge pumps for thrusters. They want to calculate the thrust for a potential pump and consider using a 2000 gph pump for more thrust. Jordan is a Chemist and is seeking help with calculating thrust. A general formula is provided but it is also suggested to experiment with nozzle modifications and variable flow rates for optimal performance. The pumps were not designed for propulsion but rather for removing water from boats.
  • #1
coffejor
2
0
Hello All,

I am building an ROV and would like to use bilge pumps for the thrusters. I would like to calculate the thrust that a prospective pump is going to create so as to decide which pump to buy. I know that the pump can output 1250 gallons per hour and has a nozzle with a 2" diameter opening. Is there anyway that I can calculate how much thrust this pump will produce? How much more thrust would I gain by going to a 2000 gph pump?

As you may be able to tell, I'm not a Physicist but rather a Chemist, so please don't hold that against me :wink:

Thanks for any help.

Jordan
 
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  • #2
as a general formula I worked out that the thrust in Newtons is going to be

(rho) (F)^2/A

where rho is the mass density of the liquid being pumped in kg/m^3

F is the volume flow rate in m^3/sec

and A is the area of the nozzle measured in m^2
 
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  • #3
Welcome, coffejor. You might want to experiment with various nozzle modifications as well as just tailoring the pump and motor to your needs. Might I suggest even using variable ones, such as on a garden hose, with remote-control actuators so you can vary the flow to suit your purposes at any given time? It would be sort of like having a transmission.
 
  • #4
note that the flow rate will be the same no matter what the size of your nozzle is, it will just become far less energy efficient as a propulsion system as the nozzle gets smaller and the exit velocity increases
 
  • #5
True. My thinking was based upon the fact that he might at some point want more speed, as in moving from one interesting location to another one some distance away. Once the thing is underway at high volume flow, it can obtain more top-end with a faster discharge.
 
  • #6
Thanks for all the help, I got through all the calculations and got the force I was looking for.

I think that I'll experiment with various nozzles on the ROV to see which gives me the best performance, my thought is that the manufactorer of the pumps probably already determined what the optimum flow rate was and that's why they chose the hose diameter that they did, but a little on site test won't hurt either.

Thanks for all your help, couldn't have done it without u guys.

Jordan
 
  • #7
You're welcome. Glad it worked out for you. One thing to bear in mind, which is why I suggested the nozzle variation, is that these things were not designed for propulsion. They're optimized to get the water out of your boat fast.
 
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1. How do I calculate the thrust for a pump?

To calculate the thrust for a pump, you need to know the pump flow rate, total head, and specific gravity of the fluid being pumped. The formula for calculating thrust is: Thrust = (Flow Rate x Total Head x Specific Gravity) / 550. This will give you the thrust in pounds (lbs).

2. What is the flow rate for a pump?

The flow rate for a pump is the volume of fluid that is pumped per unit time. It is usually measured in gallons per minute (GPM) or liters per second (L/s). The flow rate can be calculated by dividing the total volume of fluid pumped by the total time it takes to pump the fluid.

3. What is total head for a pump?

Total head is the sum of the static head (the vertical distance between the pump and the surface of the fluid) and the friction head (the energy lost due to friction between the fluid and the pump). It is measured in feet or meters and is used to determine the amount of energy needed to pump the fluid.

4. What is specific gravity?

Specific gravity is the ratio of the density of a substance to the density of a reference substance. In the case of pump calculations, the reference substance is usually water. It is a dimensionless quantity and is used to determine the weight of a fluid relative to the weight of water. The specific gravity of water is 1, so fluids with a specific gravity greater than 1 will have a higher weight and require more thrust to pump.

5. How does thrust affect pump performance?

Thrust is an important factor in determining the performance of a pump. A higher thrust means the pump is exerting more force and can pump fluids at a higher flow rate and head. It is also important to consider the maximum thrust a pump can handle, as exceeding this limit can cause damage to the pump and decrease its efficiency.

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