3 replies to this topic

[jack33]

The head vs volumetric curve for a PD pump is a almost vertical. I know the volumetric flow rate is constant.
But can you tell me how can there be different heads at same flow rate?

[thorium90]

Backpressure valve?

[Art Montemayor]

Jack:

 

The head of a pump is the difference between the suction and discharge pressures, expressed in feet of liquid.

 

A positive displacement pump does not, of itself, create pressure.  The pressure on the discharge side of the pump is built up (or increased) when there is a resistance to flow introduced in the discharge side - such as a process pressure downstream, a throttle valve on the discharge, a reducing orifice, pipe and fittings resistances, height of final discharge, etc., etc.

 

Because of the nature of the pump's basic design, it will continue to pump essentially the same quantity of liquid regardless of the discharge conditions.  Since liquids are basically non-compressible, the hydraulic discharge pressure increases rapidly when the discharge resistances increase - and during this time the pump's disharge flow remains the same.  This is a potentially hazardous condition typical of positive discharge machines and a pressure relief device is required on the discharge side of the pump to avoid a sudden, unexpected increase in discharge pressure (or an increased "head" while the suction pressure remains the same).

 

Note: I keep repeating over and over again: Please do not post duplicate topics in our Forums.  Read the Forum Guidelines before you post and follow the instructions.  I have removed the duplicated postings you have made.  Please follow Forum rules and guidelines.

[fallah]

how can there be different heads at same flow rate?

 

jack33,

 

This is exactly the consequence of positive displacement phenomena...repeated displacement of a specified liquid volume in the unit of the time leading to a fixed flow rate regardless of how much is the destination pressure...