Nozzle flowrate at the discharge of the centrifugal pump (washwater)

[Andrew88]

Hi all,

I have created a system curve and overlaid it on the centrifugal pump curve provided by the manufacturer - see attached file.

I would like to install on nozzle at the end of my pipework so that I can achieve about 2 l/s flow at 30m of head but not sure how to include the nozzle in my hydraulic calculation. In the system curve I allowed for 1.5" pipework with the same outlet size.

Could you please advise on how to deal with this issue?

Would 2 l/s at 3 bars of pressure behind the nozzle provide sufficient stream of water for manual concrete tank cleaning (let's say 5m x 10m x 2m WxLxH)?

Thank you,
Jed

 

[1503-44]

If you want 2 l/s flow, you need to raise the system curve up so that it intersects the pump curve at a flow of 2 l/s, so the nozzle must provide that additional 38-8m = 30m of resistance at a flow of 2 l/s. Discharge will be at 3.8 Barg.

If you want a discharge pressure of 3barg, 30m head, then your flow rate will be around 2.6 l/s. Your nozzle will have to provide 20m 2bar resistance at 2.5 l /s


A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[Andrew88]

Thank you for response.

If I do that, then it means the pump will deliver 2 l/s at 30m head on its discharge port. The hydraulic gradient will be close to zero at the pipework outlet where nozzle will be connected. I struggle to understand how the nozzle be exposed to 3 bar of pressure. Could you please elaborate?

 

[LittleInch]

You need to work out what size and type of nozzle will give you 2l/sec with s 3 bar pressure drop.

Nozzle flow is well known but there are various options.

I don't know what jet velocity you need. How dirty is your tank?

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

 

[1503-44]

At 2.0 l/s your pipe inlet will see a pressure of 3.8 barg 38m.
After your pipe pressure drop of 0.8 bars 8m, the pipe's discharge pressure will be 3 barg 30m. That pressure is also the inlet pressure to your nozzle. A nozzle exit pressure is atmospheric pressure, 0 barg. The function of the nozzle is to convert all its inlet pressure (potential energy) into kinetic energy (velocity). That 3 bars pushes all the fluid through the 30m resistance provided by the nozzle and no pressure is leftover when the fluid makes exit. Its all velocity at that point, at least until it hits the tank. Then it's converted back to pressure again as trhe fluid impacts the tank wall. The velocity of the jet will be sqrt(2gh) = sqrt(2*9.81*30m) = 24.26 m/s that's going to rock and roll.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[LittleInch]

What's that velocity?

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

 

[1503-44]

Did I do the SI units correctly?

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[Andrew88]

Thank you both.

Does it mean that I can treat 2 l/s 3 bar nozzle ( if I can find it on the market...)as a headloss coming from the fitting? That would allow me to incorporate it into pump curve with some conversion factos depending on velocity/flow.

Yes, looks good in SI units but shouldn't in be 30m instead of 8m in the jet velocity calculation?

Edit/ What would be the recommended jet velocity for cleaning rag in a storm tank application with a bit of solids and rag? If there is any rule of thumb like this...Should I be looking more at velocity or flow or both?

 

[1503-44]

My 22:35 post tells you the pressure at each point in the system. Inlet nozzle pressure is 3 barg 30m head. Nozzle Outlet pressure is 0. Velocity is not minimal. Velocity is 24.26 m/s. The skin on your thumb will be pealed off in a few seconds.

I think Q/V = 20something diameter Outlet of the nozzle, but please check that SI math.


A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[Andrew88]

I misunderstood and talked about low velocity in regards to flow through the 1.5 " pipe not jet velocity

 

[1503-44]

Right. The smaller nozzle outlet diameter will speed that up a bit.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[1503-44]

Yes, you're right. 30m! Stand back. 24.26m/s. Diameter 20 something. I corrected the math above. Thanks. Too late at night to do math.
Yes. The 30m loss is a loss at a "fitting", across the nozzle itself.
A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[Andrew88]

Yes, for me it is also quite late so need to check the diameter in my spreadsheet !

It should be about 10mm so I guess reasonable? However the nozzle reaction force is about 0.5 kN (112 lbf) so probably a bit too high and would pick up larger nozzle.

I asked before but probably it was lost in conversation: what would be the recommended jet velocity for cleaning rag in a storm tank application with a bit of solids and rag? Is there is any rule of thumb like this...Can't decide for optimum flow vs velocity. Do you have any experience in application like this?

Really appreciate your help.

 

[1503-44]

Not me. I just know that the little portable spray washer machines say 50 to 100 bars on the bottle and I'd guess they use a small volume high pressure recip pump. They're easy to handle because they don't put out anywhere near 2 l/s. I think you'd really want higher pressure & velocity and less flow, but I really can't say.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[bimr]

"For washing walls, a 9-mm (3/8-in.) nozzle at a nozzle (pitot) pressure of 620 kPa (901b/in.2) that accelerates 2.5 L/s (40 gal/min) of water to a velocity of 35 m/s (115 ft/s) is adequate. The commercial high-pressure washers used for cleaning sewers would also serve. High-pressure, low-flow-rate washers of the kind used by painters are useless."

Pumping Station Design by Garr Jones

 

[1503-44]

90 psi?
Yes, they'll clean the loose paint and dust, but nothing more.

Do you think the 24m/s will work? Being that's a theoretical max.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[LittleInch]

Yes but a larger nozzle may not give you the pressure drop you need to make your pump happy (end of curve operation).

Like all things there is a trade off between a lot of water (2l/sec is quite good) at high velocity versus needing two men to hold it and stop it whipping around like an out of control fire hose.

Do you just have a pump you're trying to use or something? Centrifugal pump may not be the best idea when you have a very large pressure drop at one end and not much pressure drop in your hose.



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

 

[bimr]

My understanding is that the OP is interested in spraying down a tank after accumulating debris after stormwater events, not necessarily removing materials that have been accumulated on the surface for years.

 

[1503-44]

Good point. 24m/s should do.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.