Positive displacement pump for spray nozzles

[TheRedDevil18]

Hi all, I have a positive displacement (diaphragm) pump that I am looking to use to supply spray nozzles. The pump has a max pressure rating of 69 bar at 30.6 L/min (1450 rpm). I however want my nozzles to spray at 15 bar pressure and from the nozzle performance data sheet that equates to 0.68 L/min per nozzle. If I have a total of 4 nozzles that gives a total flowrate of 4*0.68 = 2.73 L/min.

I know that to vary the flow in a PD pump, you vary the rpm of the motor, however my rpm is fixed at 1430 rpm and from the pump curve that's about 30 L/min.

My questions: (Pump datasheet is attached below)

So I need to shunt the excess 27.27 L/min back to the tank through a relief valve ?

I'm a bit confused on the pressure output. From my understanding, a PD pump is like a plunger or syringe. If I put my thumb on the output, the force I need to push down increases the more I close the output. Which means the pressure seen by the pump is dependent on the load or in my case nozzle resistance.

So would the pressure output on the pump vary depending on the nozzle ?, or would it still push 69 bar regardless ?

 

[Baluncore]

The flow would be that of the positive displacement pump.
The pressure will rise until the outlet flow matches the pump flow, or a relief valve opens, or something breaks.

 

[Lnewqban]

No resistance ⇒ No discharge pressure.

From that catalog, which one is your pump?
Would you use one of the pressure regulating valves shown?

 

[TheRedDevil18]

No resistance ⇒ No discharge pressure.

From that catalog, which one is your pump?
Would you use one of the pressure regulating valves shown?

From that catalog, which one is your pump?​

It's the D10-X.

Would you use one of the pressure regulating valves shown?​

Are you referring to the one in the catalogue ?

I believe I will need a flow control valve with 3 ports. Inlet, outlet and discharge excess to tank. I need to take the flow down from 30 L/min to 1.5 L/min. I think they call it a back pressure regulator ?

 

[gmax137]

So, your pump capacity is 20 times what you need? Can you get a smaller capacity pump? Can you put some kind of speed reduction between the motor and the pump to slow it down?

Pumping 30 l/min and sending 28.5 l/min back via recirc seems like a waste of effort.

 

[TheRedDevil18]

So, your pump capacity is 20 times what you need? Can you get a smaller capacity pump? Can you put some kind of speed reduction between the motor and the pump to slow it down?

Pumping 30 l/min and sending 28.5 l/min back via recirc seems like a waste of effort.

Hi gmax, unfortunately that's the pump I was given to work with. I could probably slow it down with pulleys else yeah its just a waste of power.

Coming back to the datasheet, to calculate the motor horsepower, the equation they gave is,

I know my motor rpm and the flowrate the pump can provide at that rpm. So basically I need to calculate all the pressure drops of the system (nozzles, fittings, piping, flow control valve) to get the input pressure of the pump ?

 

[Tom.G]

Another possibility is drive the motor with a Variable Frequency Drive (VFD). That way you can slow down the motor to a crawl to get the flow rate you want.

A possible problem running at 9% speed is the bearings. Sleeve bearings would probably need a much lighter oil to work at that low a speed, but may not be compatible with the rest of the internal mechanics. Ball bearings may be needed.

Worth looking into anyhow.

Cheers,
Tom

 

[Baluncore]

I could probably slow it down with pulleys else yeah its just a waste of power.

If you stick with the same pump ...
You could simply set an adjustable bypass valve to 15 bar.
But with high bypass volumes it is good to avoid the pressure drop across the bypass valve, since it will continuously heat the circulating fluid and cost fuel.
Power = pressure * flow rate. In SI units; watts = pascals * cubic metres per second.
15 bar = 1500 kPa; 30 litre / min = 0.5 litre / sec;
Power = 1500 * 0.5 = 750 watt ≈ 1 HP; That is a significant problem.

Now, consider a 15 bar hydraulic accumulator / reservoir, with an outlet going to the spray nozzles.
That reservoir is supplied from the pump through a non-return valve = check valve.
The pump also feeds directly through a pilot operated bypass valve, back to the main tank.

When the pressure in the reservoir falls below a set low point, the pilot closes the bypass valve, pressure then rises and the reservoir is re-filled through the check valve. When reservoir pressure reaches the high set point, the pilot opens the bypass valve, pressure falls, and fluid circulates freely through the pump and tank. Yes, just like an energy efficient switching power supply.

All you now have to do is find a reservoir, and a pilot operated valve with hysteresis.