Best pump for closed valve conditions?

[RobD7]

I am designing a control system where there will often be a closed valve downstream of a pump. What type of pump will work best under this condition for long periods of time? I know positive displacement pumps are out of the question, but I'm not sure centrifugal would be the best either.

I'm not sure if it will change anything, but the pipe is only 1/2".

Thanks in advance.

 

[micalbrch]

An air driven diaphragm pump will work well against a closed valve. But if it is suitable for your applications will depend upon the operating conditions: flow, pressure, fluid properties.

 

[hydtools]

Use a pressure compensated variable volume piston pump. It will stoke to near zero flow when the valve is closed.
Or use a fixed displacement pump with an unloading valve which unloads the pump when the valve is closed.
What is the application? Fluid?

Ted

 

[BigInch]

One with an off switch.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[RobD7]

Thanks, BigInch.

Yes, HydTools, it is for fluids. I like the fixed displacement pump idea. Thanks.

 

[bimr]

Relief valve?

One would think that any pump would overheat during a situation where the pump is deadheading for a prolonged time period.

 

[Oldhydroman]

The sort of variable volume pump that Ted suggested reduces its displacement to near zero whenever the outlet pressure rises to a particular value (set on the pump's compensator). The pump has a deliberate non-zero internal leakage so there is a small case drain flow coming from the uppermost port on the pump body. This flow carries away the heat that would otherwise continue to build up and eventually damage the pump. So the pump displacement is not actually zero - hence the reference to the displacement being "near zero".

In some cases a deliberate flushing flow is fed into the pump case, taken either from the pump's own outlet (I don't particularly like that) or from a separate cooling circuit (much better). Some pumps have no separate case drain port and leak instead into their own suction port - this type cannot stand running for long periods in a deadhead condition because they will overheat (usually a matter of minutes).

If there is enough "capacitance" in the downstream circuit then you can use a fixed displacement pump and an unloading valve. The valve can sense the downstream pressure and divert the pump outlet flow back to tank/bypass at low pressure. A check valve prevents the downstream circuit from de-pressurising when the unloading valve does its thing. Insufficienct "capacitance" will mean that the downstream circuit depressurises too quickly when the pump goes off load and the unloading valve switches it back in again too quickly (hunting).

If the time periods are long enough then the unloading valve can be replaced with a pressure switch, a timer and the on/off control. Sensing the system pressure being up at relief valve setting and remaining there for a certain period of time will cause the pump to turn off. When the pressure drops the pump can start again. You don't want the motor to suffer from too many starts per hour so you can only do this trick for systems with infrequent cycling.

An alternative scheme could employ a very small pump as well as the larger (unloadable/turn-off-able) pump. The small pump would keep topping up the downstream circuit and just blow the excess flow across a relief valve. Yes there is energy wastage and heat generation but this might be within tolerable limits.

DOL

 

[bimr]

Most of the zero discharge mowers use piston pumps. These are axial piston, variable displacement, bi-directional pumps.

http://www.hydro-gear.com/Main/Products/PistonPumps