Pipe Pressure Drop Calculations from Pipe Flow Software
View basket of Pipe Flow Software for fluid flow and pressure drop calculations (0 items)
Search for  
Wednesday 18th October 2017   
Pipe Flow Software » Pipe Flow 3D » Pumping the Fluid
   Pipe Flow Software
   Pipe Flow Expert Software
   Pipe Flow Wizard Software
   Pipe Flow Advisor Software
      Pressure Drop Theory
      Fluid Flow Factors
      Pumping the Fluid
      Fluid Database (PF3D)
      Pipe Flow 3D FAQs
   Convert 123 Software
   News & Press Releases
   Software Downloads
   Who Uses Our Software
   Customer Testimonials
   How to Buy Online
   Get Your License Code
   Software Release Dates
   Pipe Flow FAQs
   Technical Articles
   Europump Guides
   Contact PipeFlow
   About PipeFlow
   Piping Related Links
   Site Map


Secure Online Payment

How to buy Pipe Flow Software online today


Like Pipe Flow Software?



   

   


Pipe Flow 3D - Checking if you can pump the fluid?

Will the pressure on the fluid surface & the effect of any positive or negative fluid head be sufficient to ensure that the full flow rate required will reach the pump inlet?

Will the lowest fluid pressure, which occurs in the pump inlet, fall below the fluid vapour pressure, and hence cause cavitation to occur?

You will needs to establish the inlet condition to the pump
and establish the discharge pressure that the pump must overcome.

Getting Fluid to the Pump?
Pipe Flow 3D, pressure drop and pressure loss calculations on a network of pipes.

Buy Pipe Flow 3D to calculate pressure drops for known flow rates

[ Now superseded by
          Pipe Flow Expert
]
What about pump suction? Pumps do not suck!

It is a common belief that pumps provide the energy to lift fluid to the pump inlet. This is not true.

The pump simply moves fluid from the immediate inlet pipework and discharges this fluid against the outlet pressure in the discharge system. This action creates a local suction effect, which allows the external forces acting on the fluid intake system to push the remaining fluid in the intake system towards the pump inlet. This alternative (actual) view of what is happening within the pipework system leading to the pump inlet will help in understanding the limitations introduced by bad pipework system design.

If the inlet system arrangement does not provide enough energy to move the required flow rate to the pump inlet, the pump will be starved of fluid and the required flow rate will not be delivered.


Getting fluid to the pump and checking NPSHGetting fluid to the pump

The air pressure on the fluid surface is the usual energy source used to push the fluid into the pump.

A supply container positioned above the pump inlet will increase the available energy.

A supply container positioned below the pump inlet will reduce the available energy.




Resistance to fluid flow

Fluids in motion are subjected to various resistances, which are due to friction. Friction may occur between the fluid & the pipe work, but friction also occurs within the fluid as sliding between adjacent layers of fluid takes place. The friction within the fluid is due to the fluid’s viscosity.

When fluids have a high viscosity, the speed of flow tends to be low, and resistance to flow becomes almost totally dependant on the viscosity of the fluid. This condition is known as ‘Laminar flow’.


Checking the flow rate at the pump Will the required flow rate actually reach the pump inlet?

The energy losses in the pipework system must be calculated. This energy loss must be subtracted from the available energy to obtain the condition at the entrance to a pump.

The inlet condition is commonly referred to as the ‘suction condition’ - Leading to the idea that pumps suck.

If the theoretical pump inlet pressure is too low the system will operate at some lower flow rate or the pump may not operate at all.


Boiling fluid (Cavitation)

Many fluids will boil at ambient temperature if the pressure is reduced below a particular level. This pressure is referred to as the ‘Vapour pressure’ of the fluid.

If the pump inlet pressure falls below the vapour pressure of the fluid, gas bubbles will form in the fluid. These bubbles will be moved through the pump. The bubbles will collapse when the fluid pressure is raised on the discharge side of the pump. The effect of this is to reduce the flow of delivered fluid. In some systems the effect can cause dramatic vibrations, and may result in damage to the system and the pump.


Increasing the pressure at the pump inlet

Small pipe sizes will result in high pipework energy losses. Increasing the pipework size will help to reduce this energy loss.

In the case of high viscosity fluids, increasing the pipework size may not have the desired result. Also, commercial considerations may limit the size of the pipe that can be used.

Increasing pressure at the pump inletUnder these circumstances, the easiest solution is to raise the position of the supply container, to increase the positive head available, thus more force will be available to push the fluid through the pipework.


If is not practical to raise the supply container, it may be necessary to enclose the supply and introduce some positive pressure above atmospheric onto the fluid surface.

Look out for sealed supply containers where the force moving the fluid will reduce as the container is emptied.


Suction units

Suction conditionsSuction conditions can be described in many different ways.

Normal atmospheric pressure (about 1000 mBar, 14.5 psi.g) will support a water column of 10.2 metres (33.45 ft) high.

If the fluid column was Mercury the column height would be 750 mm (29.52 inches)



Net Positive Suction Head Check


Pump inlet loss N.P.S.H.r

An energy loss occurs during fluid entry into most pumps.
This loss is described as N.P.S.H.r (Net Positive Suction Head requirement).

The N.P.S.H.r is determined by the pump manufacturer.
The N.P.S.H.r is usually plotted on pump performance curves.
The N.P.S.H.r is expressed in metres head ( or ft head) of fluid.

The value of N.P.S.H.r will be dependent on many factors including flow rate, Impellor design, inlet type, pump speed etc.


Boiling fluid (Cavitation) (repeated from last section)

Many fluids will boil at ambient temperature if the pressure is reduced below a particular level. This pressure is referred to as the ‘Vapour pressure’ of the fluid.

If the pump inlet pressure falls below the vapour pressure of the fluid, gas bubbles will form in the fluid. These bubbles will be moved through the pump. The bubbles will collapse when the fluid pressure is raised on the discharge side of the pump. The effect of this is to reduce the flow of delivered fluid. In some systems the effect can cause dramatic vibrations, and may result in damage to the system and the pump.


Minimum pressure at the pump inlet and N.P.S.H.a

The minimum pressure at the pump inlet minus the ‘Vapour pressure’ of the fluid is usually known as the Net Positive Suction Head available (N.P.S.H.a)

This must not be confused with the N.P.S.H.r published by the pump manufacturer.


Net Positive Suction Head Check

Net Positive Suction Head CheckThe NPSH calculations featured in Pipe Flow 3D will display the N.P.S.H.a

The example graphic here shows how the calculation is performed.

The N.P.S.H.a must be greater than the pump manufacturer’s N.P.S.H.r to avoid the fluid pressure falling below the point at which ‘Boiling of the fluid’ will occur.





Use Pipe Flow 3D to carry out your calculations

Download PipeFlow 3D for a FREE trial!




Pipe Flow Software: Piping design & pressure drop calculations from www.pipeflow.co.uk    Copyright © Daxesoft Ltd. 1994 - 2017, All rights reserved.