Pressure change in manual foot powered pump

[sherryliang]

Hi,

I'm working on manufacturing a foot powered irrigation pump, a pressure treadle pump, that lifts water from up to 7m depth and can deliver water vertically 7m above ground or 100m horizontal distance. It is operated by stepping on a set of treadles, like a stair stepper machine, which moves 2 pistons up and down. In each piston, there is an inlet and outlet valve, which is connected to a chamber that the water passes through before going up to the piston. When a treadle is lifted, it creates suction which draws water into the inlet channel and up through the piston, and when the treadle is pushed down, the water is expelled out of the piston and through the outlet channel. This happens with both pistons so it's a "continuous" flow of water.

My question is, I'm currently reducing the diameter of my pistons to try to accommodate local pipe sizes. I don't want to lose volume of water, so I calculated the increase in length the piston has to travel. However, I know this will increase the pressure by around ~15.8% due to the decrease in area. Is my logic correct? If so, is it safe to say this increase is somewhat negligible as a person is exerting force on the treadles by standing, or would it be recommended to increase opening in valves to decrease the pressure inside the valvebox/piston chambers?

Thanks!

 

[micalbrch]

The pressure will not change. What changes is the load to the piston at a given pressure but that should not bother you.

 

[LittleInch]

Micalbrch, I beg to differ. You're thinking about the far end of the system not the"pump" end. Pressure = force / area. Assuming the force, which is essentially sometimes weight, stays constant between the two piston sizes, then reducing the area will increase the pressure.

Sherryliang, what you need to do is some tests to see what the optimum stoke height is for your human motor and how much power is available from a person and then optimise. There's no point having a strike length longer than someone finds comfortable, ditto making them do more work than they are comfortable with. Making your valves as big a possible is a good idea as any losses need to be minimised.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[sherryliang]

Thank you both your responses! I really appreciate your help. The stroke length is somewhat optimized, as we have already tested and happy with the flow rate of pump with required exerted power. Human power is approximated to be around 70W with a short stroke length of 60mm. But now we want to decrease the piston diameter from 121mm to 110mm. In order to have the same volume of water, I will have to increase the stroke length up to 73mm, which I believe is still a comfortable stroke length (not tested), only a 13mm height difference.

My concern now is whether the 15.8% pressure increase will affect the ease of use. If I want to keep pressure the same, I can increase the valve opening but it's already somewhat maxed out with current geometry of the valvebox. Do you think there is an easy way to predict pressure/optimize the system without making a physical model, such as a simplified CFD model? Thanks so much!

 

[Artisi]

sherryliang: Why are you trying to re-invent the wheel, there must be dozens of designs already out there tried and proven in the field in nearly all third world countries.
Further, although a human output may well be 70W, I wouldn't account for this in your modelling as it is unlikely that can be sustained for long periods and, if this is for a third world country, in many cases it could well be children using the pumping system.

If you would like to let me know what you are doing, I could put you in contact with a group of (now ex)university students in the US whom I mentored in the development of a pump unit for a project in Africa.


It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[sherryliang]

Hi Artisi,

Yes, great point. I am actually working off a current design, but because that design required pistons that didn't fit any metal pipes currently available in local markets, it is really time consuming for the manufacturer to shape the pipes to fit the pistons. That is why we are reducing the size of the cylinders in the tested design so that they can fit locally available metal pipes. However, I also don't want to compromise pump performance and wanted to somehow confirm we can make changes without doing so.

Thank you so much for the offer. In short, I worked on developing and manufacturing treadle irrigation pumps in Cambodia a few months ago. I'm in the states now to finish school but remotely helping the local team with making another batch of pumps, but I want to make the new batch easier to manufacturer. I would love to contact the students you have mentored and would greatly appreciate the introduction.

Thanks for your help!

 

[Artisi]

Sherryliang. It is against good housekeeping to post email addresses but if you use my title at Hotmail - and send me yours I can get back to you.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[sherryliang]

Thank you Artisi, I sent you an e-mail. I really appreciate you offering to connect me.