Valve at inlet side of pump

[JBartlett]

Hi everyone, I'm thinking about building a piston type valve and placing it near the inlet side of a pump but my concern is that the piston may rise up due to suction at the inlet which I don't want to happen.

I'd really appreciate it if anyone could assess this drawing and determine if this would be the case.
I know suction causes the poppets in foot valves to rise up while the pump is running but does that occur because the poppet is facing parallel to the flow of water?
Could the piston rise up if it's facing perpendicular to the water flow or will this not happen?

 

[pointsman]

Is the pressure inside the suction pipe less than atmospheric pressure? If so you will have a pressure difference across the cylinder, creating a force pushing the cylinder into the pipe. This assumes that the exterior face of the piston is exposed to atmospheric pressure.

Conversely, if the pressure in the tube is greater than atmospheric, the piston would be forced out. You could solve all this by having an actuator of some kind which controls the position of the cylinder. The actuator (like your seals) needs to be beefy enough to resist the pressure differentials in play.

 

[LittleInch]

Why not just connect the moving valve bit to your actuator stem?

But agree - not enough is known about whether you can get a higher pressure on the other side of the moving piston. If it exposed to atmospheric pressure and not connected tot he actuator stem then yes, it could move.

 

[Doug Hunter]

There is definitely a risk of that, but it depends on the numbers. An alternative design would be a piston bore and piston that run across the opening, with a flow window in the piston. That would remove the axial force imbalance, but you might still have a lateral imbalance in the flow direction to resolve.

 

[JBartlett]

Is the pressure inside the suction pipe less than atmospheric pressure? If so you will have a pressure difference across the cylinder, creating a force pushing the cylinder into the pipe. This assumes that the exterior face of the piston is exposed to atmospheric pressure.

Conversely, if the pressure in the tube is greater than atmospheric, the piston would be forced out. You could solve all this by having an actuator of some kind which controls the position of the cylinder. The actuator (like your seals) needs to be beefy enough to resist the pressure differentials in play.

Yes, the pressure inside the valve will be lower than atmospheric pressure.
Could the weight of a piston prevent itself from lifting up in a lower pressure area? The weight of the piston is about 240 grams.

 

[pointsman]

Yes, the pressure inside the valve will be lower than atmospheric pressure.
Could the weight of a piston prevent itself from lifting up in a lower pressure area? The weight of the piston is about 240 grams.

In theory it could, but won't your valve have a control device (like a handle, or threaded stem) that controls the position of the valve? I think this would be a better solution that relying on the valve's weight to keep it in place.

Have you ever seen a valve that uses its weight to stay in position? I haven't. Very often there is a need to install valves in positions other than vertical, and then your solution to rely on the valve's weight would not work. Also, if the valves weight keeps the valve open, how do you close it?

 

[LittleInch]

whats the diameter of the piston?

You seem to know its weight, so can you provide all the information?

 

[JBartlett]

whats the diameter of the piston?

You seem to know its weight, so can you provide all the information?

Sorry, the diameter is 16.00mm.
Is there any other information you need?

 

[JBartlett]

whats the diameter of the piston?

You seem to know its weight, so can you provide all the information?

Length is 70.00mm

 

[Snickster]

Why not just do a force balance on the piston.

Forces acting down is pressure of suction in absolute times area of piston, plus weight of piston, plus any frictional force of piston against sides of cylinder.

Force acting up is the pressure on bottom of piston times area of piston which would be 14.7 psia if bottom side of piston is vented to atmosphere.

 

[JBartlett]

Why not just do a force balance on the piston.

Forces acting down is pressure of suction in absolute times area of piston, plus weight of piston, plus any frictional force of piston against sides of cylinder.

Force acting up is the pressure on bottom of piston times area of piston which would be 14.7 psia if bottom side of piston is vented to atmosphere.

Hello, apologies for the late reply, are you saying that the bottom of the piston would be pushed upwards with a force of 14.7 PSI if the bottom side is vented to the atmosphere?

If the bottom side wasn't vented, would the force be greater or less? Sorry if this is a silly question.

 

[LittleInch]

Force is not pressure, it's pressure times area, but yes that's what we're saying. This is of course counterbalanced by the pressure in absolute pressure times area on the other end. It's 14.7 psia at sea level. So if the areas are the same then they cancel out and you're left with the difference in pressure between the two times the area as the force moving the piston one way or the other.

There's also friction (static and kinetic) which is not easy to calculate which prevents or restricts movement. Or the force from the water pushing horizontally increasing friction, maybe.

 

[Snickster]

Hello, apologies for the late reply, are you saying that the bottom of the piston would be pushed upwards with a force of 14.7 PSI if the bottom side is vented to the atmosphere?

If the bottom side wasn't vented, would the force be greater or less? Sorry if this is a silly question.

Little Inch basically explained it, but yes if the bottom of the piston is vented to the atmosphere then you will have 14.7 pounds per square inch absolute pressure acting up on the bottom side of the piston so force acting up is 14.7 times area of piston in square inches. On the top side of the piston you have the pressure of the process fluid pushing down multiplied by area of piston plus the weight of the piston to give the force acting down. The process pressure is 14.7 psia plus or minus gage pressure in the line. If line gage pressure is positive the net force is down = gage pressure plus 14.7 psia times piston area plus weight of piston. If process gage pressure is negative the net force is up if 14.7 times area of piston bottom acting up is greater than acting down = 14.7 psia minus gage pressure times piston area on top plus the weight of piston. In other words since the 14.7 psia absolute atmospheric pressure cancels out, the piston will move up if the negative gage pressure times area of piston is greater than the weight of the piston. However there is also friction force of piston on wall that acts opposite to direction of motion that needs to be considered.

If piston were not vented to atmosphere then it gets complicated. If there is enough clearance between piston and walls the process fluid will enter the cylinder area and be felt underneath the piston so any pressure force will be balanced. Therefore only weight of piston itself will keep it from rising. However if the flow/pressure would suddenly decrease and not enough clearance exists so the pressure instantaneously equalizes, the piston may rise.

If the sides of the piston with wall were liquid tight then with negative topside process pressure, if piston would tend to rise a vacuum would form on the bottom side further tending to hold it down.

 

[LittleInch]

I forgot the weight of the piston....

Doing the calculation for you (I must be in a good mood), in metric.

atmospheric pressure is 101kPa or kN/m2. you have said this is 16mm diam so area is 0.008*0.008*3.14 = 0.0002 m2 * 101000 = 20.3N Max force assuming the upper side is at 0Pa.

You've said this has a mass of 240g which on earth is 2.35N force.

So basically any pressure less than about 0.86 bara on the upper side has the potential to move your piston, depending on friction etc IF the bottom end is exposed to the air AND the cylinder is vertical