Reading some of the claims on this board about positive displacement pumps and how they affect the oil flow and pressure made me do some digging to try and clarify few misconceptions.
I'm not going to go off with a lengthy post as I'm by no means and expert on the subject, but instead people can read the below link and I'm going to quote several points from that source.
Quote:
Flow Rate and Pressure Head
The two types of pumps behave very differently regarding pressure head and flow rate:
The Centrifugal Pump has varying flow depending on the system pressure or head.
The Positive Displacement Pump has more or less a constant flow regardless of the system pressure or head. Positive Displacement pumps generally gives more pressure than Centrifugal Pump's.
Quote:
Capacity and Viscosity
Another major difference between the pump types is the effect of viscosity on the capacity:
In the Centrifugal Pump the flow is reduced when the viscosity is increased
In the Positive Displacement Pump the flow is increased when viscosity is increased
Liquids with high viscosity fills the clearances of a Positive Displacement Pump causing a higher volumetric efficiency and a Positive Displacement Pump is better suited for high viscosity applications. A Centrifugal Pump becomes very inefficient at even modest viscosity.
http://www.engineeringtoolbox.com/classification-pumps-d_55.html
In other words the two pump types serve different design characteristics and needs. In systems where the flow is not critical and can vary greatly or even be stopped, a centrifugal pump is ideal because it will maintain a certain pressure regardless if the fluid in the system is flowing or not, such as in the cooling system or house water pipes. In these pumps the pressure is constant and the flow varies.
A positive displacement pump is used when a certain amount of fluid has to be delivered, or constant amount of flow maintained. In these pumps the flow is constant and the pressure varies. That is why these pumps have pressure relief valves, to protect the system and the pump from over pressurization. Because these pumps will keep on moving the fluid until something gives way.
I'm not going to go off with a lengthy post as I'm by no means and expert on the subject, but instead people can read the below link and I'm going to quote several points from that source.
Quote:
Flow Rate and Pressure Head
The two types of pumps behave very differently regarding pressure head and flow rate:
The Centrifugal Pump has varying flow depending on the system pressure or head.
The Positive Displacement Pump has more or less a constant flow regardless of the system pressure or head. Positive Displacement pumps generally gives more pressure than Centrifugal Pump's.
Quote:
Capacity and Viscosity
Another major difference between the pump types is the effect of viscosity on the capacity:
In the Centrifugal Pump the flow is reduced when the viscosity is increased
In the Positive Displacement Pump the flow is increased when viscosity is increased
Liquids with high viscosity fills the clearances of a Positive Displacement Pump causing a higher volumetric efficiency and a Positive Displacement Pump is better suited for high viscosity applications. A Centrifugal Pump becomes very inefficient at even modest viscosity.
http://www.engineeringtoolbox.com/classification-pumps-d_55.html
In other words the two pump types serve different design characteristics and needs. In systems where the flow is not critical and can vary greatly or even be stopped, a centrifugal pump is ideal because it will maintain a certain pressure regardless if the fluid in the system is flowing or not, such as in the cooling system or house water pipes. In these pumps the pressure is constant and the flow varies.
A positive displacement pump is used when a certain amount of fluid has to be delivered, or constant amount of flow maintained. In these pumps the flow is constant and the pressure varies. That is why these pumps have pressure relief valves, to protect the system and the pump from over pressurization. Because these pumps will keep on moving the fluid until something gives way.