6 replies to this topic

[guoyinyanliner]

The centrifugal pump has a head of 50 m. The pump suction is at atmospheric pressure, and water is pumped into an atmospheric storage tank with a liquid level 3 m above the pump. What is the pressure in the pump discharge piping? Is it 150 kPa, or around 500 kPa?

[breizh]

Hi,

my understanding from your query: Head of 50 meters means to me:

Head discharge -Head suction equal 50 meters

Head discharge means head at the discharge of the pump

Head suction means at the suction of the pump.

The intersection between the pump curve and system curve is your operating point.

 

I've difficulty to understand where 150 Kpa or 500 Kpa are coming from.

To answer to your question, the pressure at the discharge of the pipe is equal to the back pressure, most probably about 130 Kpa  ( 100+3*9.81*1000/100) .

Submit a sketch to avoid confusion. 

Share with us the pump curve and operating point.

Based on your query, you need to review basics about centrifugal pumps.

Attached a few documents to review.

Breizh

Attached Files

•  centpump basics.pdf   74.45KB   6 downloads
•  Centrifugal pump and fluid flow - practical calculations.pdf   1.44MB   17 downloads
•  understanding centrifugal pump.pdf   434.73KB   5 downloads
•  Understanding NPSH.pdf   823.84KB   4 downloads
•  Pumps hydraulics.pdf   1.37MB   2 downloads
•  KSB Centrifugal-Pumps-data.pdf   3.47MB   3 downloads
•  Suction piping design.pdf   397.47KB   4 downloads
•  Pump sketches.docx   141.91KB   5 downloads

[guoyinyanliner]

thanks

[Fletch]

It sounds like you have an existing pump that is getting retrofitted into a new arrangement.

While your pump is capable of producing 50m of head at some flow conditions (i am assuming this is the shut-in conditions), the head the pump produces is a function of your discharge piping and destination pressure.

Based on the information you provided you only need 3m of head or ~30 kPag of discharge pressure, towards the lower end of the pressures you provided (noting that the discharge pressure is a function of the piping frictional losses, destination pressure and elevation and any control valves or restrictions in the system).

Also note, the pump curve provides you with differential head, so if you have suction pressure of x kPag, then the pump only needs the difference to meet the differential pressure you need. So account for the suction pressure too.

 

Sounds like you have an existing pump, and therefore a pump curve?

Where this required pressure intercepts the pump curve, is the flow rate that your pump will be operating.

Given the low discharge pressure requirements, this sounds like it will be well down the pump curve at a very high flow rate. (which has high power and high NPSH demands)

 

If this does result in a very high flowrate, then the piping frictional losses will be much higher, so ensure you factor in the discharge piping effects (size and length etc)
 

What you will be determining is the system curve and the new 'duty' point of your new arrangement.

 

To push you back up the pump curve you need additional resistance in the system e.g. control valve or restriction orifice.

All these will increase the required discharge pressure and have you up the pump curve.

 

If you don't have anything, the pump will operate down the curve and you may experience pump issues as a result.

[guoyinyanliner]

It sounds like you have an existing pump that is getting retrofitted into a new arrangement.

While your pump is capable of producing 50m of head at some flow conditions (i am assuming this is the shut-in conditions), the head the pump produces is a function of your discharge piping and destination pressure.
Based on the information you provided you only need 3m of head or ~30 kPag of discharge pressure, towards the lower end of the pressures you provided (noting that the discharge pressure is a function of the piping frictional losses, destination pressure and elevation and any control valves or restrictions in the system).
Also note, the pump curve provides you with differential head, so if you have suction pressure of x kPag, then the pump only needs the difference to meet the differential pressure you need. So account for the suction pressure too.

Sounds like you have an existing pump, and therefore a pump curve?
Where this required pressure intercepts the pump curve, is the flow rate that your pump will be operating.
Given the low discharge pressure requirements, this sounds like it will be well down the pump curve at a very high flow rate. (which has high power and high NPSH demands)

If this does result in a very high flowrate, then the piping frictional losses will be much higher, so ensure you factor in the discharge piping effects (size and length etc)

What you will be determining is the system curve and the new 'duty' point of your new arrangement.

To push you back up the pump curve you need additional resistance in the system e.g. control valve or restriction orifice.
All these will increase the required discharge pressure and have you up the pump curve.

If you don't have anything, the pump will operate down the curve and you may experience pump issues as a result.

[guoyinyanliner]

So what is the pump discharge pressure

[breizh]

Hi,

Only you can perform the calculation!

We need the pump curve, you know the TDH (50 m), from there your get access to the flow rate (intersection of the pump curve and system curve).

We need to get access to your system to calculate the head loss on your discharge line (pipe head loss, minor head loss, instrument head loss and equipment head loss) then it's a back calculation, Bernoulli application between discharge pipe and discharge pump.

 

Good luck

Breizh