Throtelling Pump systems

[fcuneo]

Hi all,

I will be using a 10-15kpsi (20 US gpm) pump to perform water blasting operations. Due to the existence of different type of paintings and substrate materials to be stripped, I will need the capability of working at different water pressures.

For an electrically driven pump I was wondering if somebody could advise me on energy efficient methods for controlling the ouput pressure of a pump.

Is the use of a VFD a reasonable option? or would a bypass system connected in the output be enought?

The pump is to be used for periods upto 20 minutes, and the total operation per day would probably not exceed 5 hours.

Thanks
F.

 

[bulkhandling]

I believe you are using a positive displacement pump such as a gear pump or piston pump. For a PD pump at a fixed speed, the pressure is determined by the back pressure in the piping system, in your case, mostly the blasting nozzle. If you have a good method to adjust the diameter of the nozzle, you'll get a whatever pressure you want within the adjustable range.

VFD is very commonly used for a PD pump. Although a VFD is normally used to control the flowrate, it can also control your discharge pressure. When the speed of the pump is reduced, the flowrate is reduced. Since the nozzle size is the same, pressure drop through the nozzle will be reduced due to reduced velocity.

A bypass system is possible in theory. The problem is how to adjust the huge differential pressure without creating cavitation problem. I would not try this if I have other choice.

 

[dcasto]

thats a 200 HP application! A VFD would go a long way as would a variable opening in the nozzle.

 

[itdepends]

Presumably if the unit (as indicated above) is a PD pump it will also include a pressure safety cutoff and a pressure safety valve.

I have no idea how one of these units is plumbed up- but seeing them operate- the flow is not constant- it turns on and off as the water jetter starts/stops flow. As such a VFD probably won't help a great deal (if the flow stops- the pump would also have to stop). I can only presume (guess) that the units are using some sort of internal recycle to control the system pressure.

 

[fcuneo]

Thanks for the feedback. Yes, we are talking about a piston pump (3 cylinders) and from what I've been seeing so far it would take a 150-200 HP electrical motor to drive this system.

I'm trying to figure out how to control the output pressure electronically (via a PLC), so far I have come across two options offered by the manufacturers of this pumps:

1)a VFD that would control the rpm of the driver, changing the flow rate; and by keeping the nozzle diameter constant thus varying the output pressure.

2)a pneumatic regulator which will bypass the water at the exit.


Unfortunatelly varying the nozzle diameter is not possible. I'm not too sure about the second option, as it would call for the need of a control loop system say a PID with a pressure transducer, and I am a bit hesitant if it would end up reducing the pump's life by not effectively controlling the output pressure and putting the pump under an oscilating load.

Also this would mean dumping a portion of the liquid that has been risen to a high pressure to meet the output pressure requirements, not the most energy efficient method.

 

[dickon17]

Use different jet nozzles (orifice diameters)for each application. Most high pressure water jet lances have the option of rotating nozzles specifically for this purpose.