Calculating Force required to move a piston

[carla00]

Hi Again, I thought I'd better start a new thread.

I need to find the force required to push a piston along a tank.

I know the force atcing on the opposite side (the other side is pressurised at around 6 bar) and will find out the friction coefficient of the piston rings on the tank surface.

Could anyone tell me how to calculate the force needed to ovrecome the initial friction and then the force required to move it at a given velcity ?

Thanks

Carla

 

[kcj]

I think you have answered your own question: the force on the back side is 6 bar x area, plus the friction to slide the rings. There is a breakaway static friction, then a a sliding friction/

The difficulty is in determining the seal friction as the piston moves. It is highly variable depending on surface finishes, pressure energizing effects on the inside of the seals pushing outward, lubricity of the fluid, velocity of sliding, etc. I suspect you waill have to find it by experiment.

Overall though, it may be small percentage considering total force of the pressure on the back side. In hydraulic cylinders, breakaway can often be maybe 5 to 25% of rated loads.

k

 

[carla00]

Thanks for that,

I've checked wiht the piston ring manufacturer and can discount the friction, so thats one less thing to think about.

Obviously the balancing force is now 6 bar, however, I'm not sure how to find the force needed to move it at a given speed. Is there a calculation I can use using overall desired speed required ? All I can find are equations using acceleration, which would make things a bit more complicated.

 

[IRstuff]

A piston usually implies a sealed volume (6 bar?). If so, then you're performing compression, and the pressure will increase as you compress the gas on the other side

TTFN

 

[carla00]

The pressure is a constant 6 bar. Basically, it's gas on one side of the piston and water on the other. The gas is used to push the water out of the system.

Not a prob any more, I'm just going to have to assume an acceleration using the speed I require.

I guess it's just a simple F=ma + opposing force.

 

[IRstuff]

I'm confused now. Is the gas at a constant 6 bar?

If so, then the orifice through which the water escapes is the dominant factor. Since water is essentially incomressible, the piston cannot move any faster than the amount of water it displaces. So the volume rate of water leaving divided by the area of the piston is the maximum rate at which the piston can move.

TTFN

 

[carla00]

You're right, but I've already worked out the speed of the piston using the desired flow rate of water at a given pressure. The pressure is in the water side of the system so basically, I'm working out the required gas pressure.

I've posted another thread previously that explains what the project is. The thread is called fluid flow in pipes, pressures and flow if you want to have a look.

 

[carla00]

Done (I think) ! - it hasn't taken me this lomg to work it out by the way, I've only just started looking at it again !

I've worked out the pressure required in the gas side of the tank using F=PxA + pressure on other side of piston. it turned out to be a tiny increase on that of the other side. i.e if I need the sprinkler system to be pressurised to 6 bar (to obtain the desired flow rate given in supplier's brochure), the gas side of the tank needs to be at a pressure of very slightly over 6 bar.

Does this sound about right ?, here's a description of the process:

Initial conditions:
gas pressure = 0 (gas comes from a pyrotechnic device once actuated), water is not initially pressurised and is held in tank above piston.

Actuation: gas is produced to give a P of 6 bar and moves piston to achieve desired flow rate. (gas is produced at a rate that maintains a constant pressure underneath the piston). From supplier info (tables of nozzle flow rates at given pressures) the flow rate results in pressurisation of the water side to 6 bar.

Friction and head losses are negligible so I have discounted them.

Is there anything else I need to consider or is it as simple as I've described above ?