Fluid Flow in Pipes, pressures and flow rates help needed 4

[carla00]

Hi

Could anyone point me in the right direction ?

I am trying to work out an inlet flow rate (or pressure, whichever is easiest)required for a water sprinkler system.

All the info I have is the outlet flow rate and pressure of the nozzles, and pipe specifications and layout.

I can't use Q1 = Q2 (sqr p1/p2) because I dont have P1 or Q1. I tried guessing P1 as a pressure higher than that of the nozzle (assuming that the pressure in the pipework is higher than that of the nozzle) but the resulting flow rate was greater than that of the nozzle. Surely it should be less ?

I'm probably going about this completely wrongly so would appreciate any assistance.

Cheers

Carla

 

[katmar]

Carla,

I think you are making a mountain out of a molehill here. Re-read what BRIS has said. S/he has laid out the problem as simply as it can be done.

Just a couple of practical tips:

With liquid systems and normal velocities you can often disregard the Bernoulli effect. i.e. the velocity head is usually negligible. This means all you have to take into account is the friction loss using Darcy (or Hazen Williams) and any change in static head due to elevation changes.

With gases it is different. The velocities are much higher and velocity head is often important. But with gases you can generally disregard static head, so you still only have two factors to deal with.

If you are designing a sprinkler system and you want equal flows out of each nozzle the easiest way is to slightly oversize the distribution piping so that the friction losses in the pipe can be ignored. This gives each nozzle virtually the same pressure.

You say the pipe losses in your system are very small, so it looks like the original designer took this route. Remember that the friction loss varies with the fifth power of the pipe diameter. This makes the pressure drop in a 3" line about an eighth of that in a 2" line. Going up one pipe size can dramatically change the picture.

A similar approach is taken with spargers. If you want an equal distribution along the length of a sparger you could get into all sorts of fancy calculations for varying the hole sizes and spacings, but this would make it very difficult to fabricate. As long as the pressure drop down the sparger pipe is small relative to the pressure drop through the holes you will get a nice evenly distributed flow with equally sized holes equally spaced.

Your comment on working out the pressures and forces on the pump piston makes me think you are over-complicating that aspect as well. Once you have the total flowrate (= sum of nozzle flows) and the required pressure (from catalog to give required flow through nozzle - neglecting pipe losses) you simply select the pump from a catalog. Call in a pump vendor to help you. In the early stages of your career you can learn a lot from these reps and their catalogs. Don't be afraid to get them to do the work. They need to tell their bosses about all the sales calls they have made so they will welcome the invitation.

From your comments here I can see that you are not afraid to ask questions. That is always the sign of a good engineer. The guy who knows it all is the guy to be wary of (unless he is old like me!).

 

[carla00]

Hi there,

You're right, I am over complicating things, I always do !!

It's not actually a pump that I require. I'm designing a piston, pushing a liquid out of a tank using an increase in gas pressure in a chamber behind the piston. I've just realised it is very easy to do !

The only thing I've got to think about is the fact that the system needs to be able to deal with different flow rates. Anyway, I'm going to look into it so will let you know how it goes.