precharge pump job on a high pressure water jetting machine

[oscyes]

greetings engineers,

I am a mechanical engineer from a hidroblasting enterprise, I am new in this field so there are some things that I dont completely understand.

we are doing maintenance to a high pressure hydroblasting (20000 psi) and I had to buy a new precharge pump for connecting it with the main plunger high pressure pump.

the question is why is it necessary to have a precharge pump (centrifugal) which pumps from a big enough tank full of water to the high pressure plunger pump.

I mean I think it could work without the precharge pump and connect the high pressure pump directly to the tank, having the same speed at the nozzle end because it would depend only of the gpm delivered by the high pressure pump passing trough the nozzle hole, increasing the pressure at the high pressure pump discharge.

maybe I am Wrong thats why a need some illumination.

does the precharge pump gpm affects the final pressure at the discharge of the high pressure pump?

thanks in advance, I hope you all write your comments and expiriences because I am not as experienced as you

 

[btrueblood]

Boosting the inlet pressure to the piston/plunger pump reduces the possibility of cavitation in the entering fluid stream. Without this, some type of pulsation dampener, or oversized inlet piping, would be necessary to reduce the low-pressure minimum of the pulsating inlet flow.

 

[oscyes]

thank you so much I knew about the cavitation problem but the oversized inlet piping makes too much sense, it all depends of the GPM delivered by the high pressure pump.

so what I undertood is that if you dont use precharge pump it is possible to have cavitation because of the losses in the inlet piping or housing, and if you want to minimize the losses you would have an oversized inlet piping to achive that.

leaving aside the cavitation problem and the oversized piping. Could I Raise my outlet pressure to 20000 psi without precharge pump?, I mean just pumping from the reservoir through the nozzle.

thanks in advance all of your comments

 

[LittleInch]

oscyes,

In all honesty, how is anyone supposed to be able to answer that question when you've provided no technical information on the most important thing here - your high pressure pump - other than it's delivery pressure.

Start by looking at the data sheet and technical requirements of the pump to see what pressure the guy who designed the pump says he needs. I guess this is a piston pump of some sort and hence there is an acceleration head to take into account when calculating the NPSHA value.

btrueblood is probably correct in that a pre-charge pump will keep the inlet piping or tubing to a minimum size.

could you raise the pressure without it? - possibly, but also possibly not. Too many unknowns here to make a judgement call.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[btrueblood]

What LittleInch said. Among other things, it would depend on the rpm of the pump (which determines the acceleration of the piston), or in other terms, how many gpm you want. Operate a plunger pump slow enough, and it won't cavitate. Whether you can get 20000 psi at that rpm will depend on how well the seals and packing and check valve seats hold, i.e. does the leak-by (also called slip) exceed the pump flow rate.

 

[oscyes]

thanks for all your interesting comments.

let me describe what happened while testing the machine.

at first we had our precharge pump with a 5 inch pulley but while starting the diesel engine at minimum rpm the pressure at the high pressure plunger pump discharge was like 10000 psi.

then we changed the pulley to a 3 inches one, the pressure at minimum rpm in the same point was like 30000 psi.

then we changed the pulley to a 4 inches one, the pressure at minimum rpm in te same point is now 20000 psi.

my real question is why is the pressure behavior like this, I though the pressure at the discharge of the high pressure plunger pump was only determined by the flow rate of the pump and the nozzle hole restriction by a formula like this Q=K*(deltaP)^1/2

I can´t understand that behavior, if any of you could clarify that to me I will apreciate it a lot

thanks in advance

 

[EdStainless]

How much does the pressure at the outlet of your charge pump/inlet of your HP pump change with pump speed?
Changing the speed of a centrifugal pump that much is dangerous, as you are almost certain to running well outside of its designed conditions. If its design conditions and run that way or you will eat this pump up and maybe damage your HP pump in the process.
Why is your HP changing? It must be because you are starving the inlet at the lower charge pressures.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[Compositepro]

the purpose of a precharge pump is to make sure that the HP pump fills completely on every suction stroke. So your measurements make perfect sense. With insufficient precharge pressure the HP pump does not fill, and the flow rate goes down. Therefore the pressure output goes down.

 

[oscyes]

thank you so much compositepro

 

[cannondale]

It's all about NPSHa as already stated for the change pump, differential pressure, cavitation, and most importantly the nozzle size on your jet nozzle, oh and the line associated that gives you the back pressure. Changing the belt drive pulley isn't the answer, as the speed/torque availability will be different. Did you measure the consumed amps of the engine power?
However with a PD pump, the torque is almost always constant, because the pressure should be what your trying to keep constant. Therefore pressure=torque=amps (for electric). Change any one and the others will be affected.
What is it your trying to achieve? Higher flow or pressure, or just understand the relationships?

 

[cannondale]

Having mulled over this for some time, I'd recommend not experimenting. These pressures are extremely hazardous. Caution must be exercised when changing any equipment from its original build, especially for this application. Have you checked the pressure rating of the whole system (all its components)? Is the PD (positive displacement) pump rated for that pressure/rod load? What's the PSV/burst disc set at?

There's a very good chance that the engine is reacting to the different load conditions. By changing the charge pumps drive pulley diameter by such a margin, you'll be altering its performance a great deal. The PD pump will be attempting to pump using the power available and back pressure exerted on it by the discharge. It should be an important factor to monitor the engine speed and the PD pump flow rate. Also the drive ratio of the charge pump. A charge pump for a triplex PD pump should be approx. 150% of the flow of the PD pump, at between 1 and 3 barg, this is to ensure the centrif is slightly cavitating but to protect the PD pump, as the consequences can be catastrophic for the PD pump.
A PD pump is a flow producing device, and a centrif is a pressure producing device.
I would just recommend caution, as all examples are at engine idle, and nowhere near the optimum power of the engine (torque or power). I've spent years with this type of equipment, and would be happy to help, as this type of equipment can be very dangerous.
If your at all in doubt don't just take my word for it, consult an expert, like Calder, Jetstream or one of the other smaller companies for advise.
If you have the plunger diameter/stroke and a few details on the engine, I can help/show you how to calculate the absorbed power very easily, then look up the engine performance and read off the data, and then maybe we'll understand a little more about what's really going on.
The centrif will increase in flow when the engine gets up to speed (not directly proportionally with pressure), but the PD pump could well increase in discharge pressure at a faster rate than the flow increases (again not directly proportionally). So any data at idle isn't representative of the performance at optimum speed.
Unless the impeller of the centrif has been trimmed for a particulate speed, it would have a relatively normal performance of 60-70% eff at a given speed, which I'd hope was the optimum for the PD pump design flow rate. You didn't state how the PD pump is driven? is that belt driven, or via a flywheel flexible coupling?

I hope you understand I'm not preaching, only trying to assist, with some potentially dangerous equipment.

 

[oscyes]

thank you all for your comments.

I wiil have caution, I was trying to understand the relationship between all these variables, because i did not have the hydraulic schematic of this machine.