pump pressures 1

[panelman]

Guys

we have a pump that pushes 16 l/s through 5m of 80mm pipe with 5 elbows and an open end...can someone tell me what pressure we will see at the pump discharge please

and...how would it change if the pipe was 10m long (everything else the same)

Cheers

 

[TD2K]

16 l/sec of what?

To know the pressure at the pumpp discharge, you need to know the downstream pressure also as you can only calculate the pressure drop through the system you described. Where is this liquid going to?

 

[25362]

Even, when discharging to an open end, besides the full characteristics of the pipe and elevations involved, one needs to know the physical characteristics of the fluid being pumped. My advice: pay attention to TD2K first question.

 

[katmar]

Hi Panelman,

It is difficult to define a problem outside your own area of expertise - I have regularly frustrated our electrical people with my "dumb" questions so I understand where you are coming from.

The first thing to determine is how accurately you need the answer i.e. what are you going to use the information for? Unless you are pumping something very different from water the friction pressure drops are going to be very low - about 23 kPa for the 5m line and 30 kPa for the 10m line.

This may be all the info you need. But if you want to get it as accurately as possible you need a lot more info :

You need the viscosity and density of the liquid. If it is a common liquid you can give the piping engineer the composition and temperature and he will have tables to give him the properties.

You have given the length and number of elbows - that's a good start, but are there any valves or other fittings? What class (schedule) of pipe is it - this affects the actual diameter. The pressure drop varies with the 5th power of the diameter, so if you want accurate pressure drops this is important. Are the elbows screwed or flanged or welded? Long, standard or short radius?

You need to know the pipe roughness. Usually this is done from look-up tables from the pipe material, finish and age or condition.

Finally, you need to specify the change in elevation from the pump discharge to the open end.

But before you go hunting down all this info, determine the accuracy needed. If you just wanted to know if the discharge pressure would be less than (say) 5 bar then it would be a waste of time to go to these lengths.

You owe me an answer to my next dumb electrical question ;-)

regards
katmar

 

[panelman]

Catmar, well understood about “dumb” questions

Fluid is water with some grit, the pipe comes out of the pump, goes up 600mm then horizontal for 5 (or 11) metres via the elbows. At the end of the pipe there is a final elbow which is used to turn the flow through 90 degrees in plan and 45 down in elevation. Pipe length between final elbow and open end discharge is 600mm

Elbows are short radius (maybe medium) and flanged, pipe is new 80mm nb DI. Height change from pump to discharge point is about 300mm.

I need to fit a pressure switch to the pump discharge to sense when it is pumping….my thoughts were that when the pump is not pumping the pressure at the pump discharge will be 600mm of water ie 1 psi but I need to know what this will change to when the pump is pumping….gut feel is about 4-5 psi but my gut is firmly electrical.

 

[TBP]

What about a paddle switch?

 

[panelman]

I did consider that but rejected it as

a. the water is sewage and does contain some “rag” which would catch on the paddle

and

b. the grit content would be erode the paddle

Thanks for the thought though

 

[25362]

Assuming the grit doesn't plug the pipe, at the elbows or otherwise, the flow rate as indicated would be ~254 GPM. The old Hydraulic Institute Pipe Friction Manual tells us that the friction drop for water flow on a 3 in. line, for such a flow rate, would be about 14 ft of water head/100 ft of pipe.

If the equivalent length of your pipe, including entrance and outlet losses, comes to, say, 50 ft, the friction drop would be around 7 ft of water, or 3 psi, to which you must add any the elevation difference.

In short, these findings show your estimate is in the ballpark.

 

[Scipio]

Out of curiousity, instead of a pressure switch on the discharge to sense when the pump is running, couldn't you just monitor the motor run status at the starter?

 

[panelman]

Scipio...It's an alarm system...I need to know when the motor is running but the pump is not pumping ie lost prime, blocked or belt broken

 

[dicksewerrat]

Then use a waterlevel switch in the holding tank. do you have pump on, pump off switches? If water level doesn't drop when the pump is on, it will raise put your alarm level above the pump on level. You may have this capability in the control panel.

 

[panelman]

Nice thought....but, the pump is used to remove grit from the incoming flow to a sewage works, the incoming flow runs into a wide channel which slows it down so the grit settles out.

The pumps are mounted on a travelling bridge which spans the channel and runs up and down once a day. The pumps suck a mixture of settled grit and water up and discharge into a classifier whish separates the grit from the water..grit goes to a skip and water is returned for treatment