Is an outlet check valve necessary for positive displacement pumps?
Thread Content
The pump selection manual states that \"positive displacement pumps generally do not have check valves.\" Why is that? In principle, it seems that metering pumps or reciprocating pumps are easier to understand, as these pumps already come equipped with check valves. But why can a rotary pump also be omitted? Such as gear pumps, vane pumps. Won’t the high pressure at the outlet push the impeller to rotate in the reverse direction? Seeking advice!
A check valve prevents backflow. What if there’s a power outage? What about unexpected situations? It’s better to be prepared for the worst. Your profile picture is too distracting
So why can positive displacement pumps usually be omitted? . . ? Or is this statement completely wrong?
There shouldn’t be any mistakes in this sentence; after all, the discharge pressure depends on the back pressure. But gear pumps, piston pumps, screw pumps, etc. must be equipped with filters, and metering pumps definitely need check valves. Take a look at what others think
Which selection manual states this? That’s not scientific – your profile picture is amazing!
Positive displacement pumps rely on the reciprocating or rotating motion of working elements within the pump cylinder, which causes the working volume to alternate between expansion and contraction, thereby enabling the suction and discharge of liquid. A positive-displacement pump in which the working element moves back and forth is called a reciprocating pump, while one in which it moves in a rotational motion is called a rotary pump. In the former, the suction and discharge processes take place alternately within the same pump cylinder, and are controlled by a suction valve and a discharge valve ； The latter forces the liquid to move from the suction side to the discharge side through the rotational action of working elements such as gears, screws, vane rotors, or slides. An outlet check valve is installed to prevent backflow of the medium from causing the rotor to rotate in the reverse direction. It is usually necessary to install this at the outlet of a centrifugal pump, as reversing the impeller can cause the threaded parts of the impeller or its nut to reverse and come loose! In contrast, backflow of the medium in positive displacement pumps does not have such a severe impact.
“The Manual for Selecting Industrial Pumps” was compiled by the Pump Technology Committee of the National Chemical Equipment Design and Technology Center; it was published by the Chemical Industry Press. I have been reading this book, and it’s written very well. I don’t quite understand this point. You too, Sharp Guy {:3_57:}
If the pump stops but is not turned off, why doesn’t the high pressure at the outlet cause the impeller to spin in the reverse direction and fall off?
First of all, when the pump stops, there is no pressure! Since the impeller does work, once it stops doing work, it can no longer increase kinetic and potential energy, and thus the high pressure also disappears. Secondly, even in the absence of a high-pressure medium, gravity is still present, and gravity can also cause the impeller to rotate in the reverse direction. As for why it doesn’t fall off, some manufacturers of centrifugal pumps have incorporated mechanisms to prevent reverse rotation, or there is backflow of the medium, which fortunately prevents the impeller from falling off!
I added a check valve at the outlet of the DN25 small metering pump, and our chief engineer asked me to remove it