Gerotor pump, will it work in a closed system without air?

[hxtasy]

just hypothetical question,

First off I am not very knowledgeable in pumps. Let's say you have a pump to pump fluid, i don't think the type matters, self priming as I understand it creates a vacuum with the air in the system to pull the fluid towards the gear pumps.

I'm wondering what happens when you have a cavity with oil, you are pumping it, and there is a valve in between the tank and the pump, this valve suddenly closes. what will happen? since the valve is closed, there is no vacuum on the other side. will the oil just sit there and spin? or will it push the oil out creating a vacuum in the oil line from the valve to the pump?

 

[Lnewqban]

A gerotor pump is of positive displacement type.
Those can’t work against a closed valve.

The non-compressible fluid trapped between that pump and the valve will act like a solid.
Something will have to yield, normally the thermal protection of the pump motor will stop it.

Please, see:
https://en.m.wikipedia.org/wiki/Pump#Positive-displacement_pumps

https://en.m.wikipedia.org/wiki/Gerotor

 

[jrmichler]

or will it push the oil out creating a vacuum in the oil line from the valve to the pump?

If the valve is on the suction side of the pump, this will happen. When the oil is pumped out, the pump will just spin. The vacuum on the suction side will pull a small amount of oil backwards into the pump, which might be enough to lubricate it. Or it might not, in which case the pump will be damaged or destroyed.

Systems with positive displacement pumps normally should not have valves. If you need to control the flow, change the speed of the pump or open a diversion valve.

 

[anorlunda]

Systems with positive displacement pumps normally should not have valves.

I agree, but sometimes valves on both sides allow removing a pump for maintenance while the remainder of the system continues in service. But of course, those valves should never be closed when the pump is in service. Even major valves sometimes have shutoff valves immediately before and after to allow the valve to be removed for maintenance.

The electrical analogy is to put switches on both sides of a circuit breaker.

 

[hxtasy]

I agree, but sometimes valves on both sides allow removing a pump for maintenance while the remainder of the system continues in service. But of course, those valves should never be closed when the pump is in service. Even major valves sometimes have shutoff valves immediately before and after to allow the valve to be removed for maintenance.

The electrical analogy is to put switches on both sides of a circuit breaker.

Thanks for all the comments guys. in this case, I am talking a type of fuel, and the shut off is there to stop the flow before ignition, however the pump is mechanically operated on the engine. So this is a real condition that would happen, during an emergency shut off. I kind of want to build a small setup and see what actually fails in the system.

 

[hxtasy]

A gerotor pump is of positive displacement type.
Those can’t work against a closed valve.

The non-compressible fluid trapped between that pump and the valve will act like a solid.
Something will have to yield, normally the thermal protection of the pump motor will stop it.

Please, see:
https://en.m.wikipedia.org/wiki/Pump#Positive-displacement_pumps

https://en.m.wikipedia.org/wiki/Gerotor

I was reading up on those but it did not describe in detail how a Gerotor acts as an air pump, and what would happen in non ideal scenarios.

 

[anorlunda]

So this is a real condition that would happen, during an emergency shut off.

OK, but wouldn't the emergency shut off stop the pump at the same time as it closed the suction valve? If not, the emergency shut off design is faulty.

 

[Baluncore]

I'm wondering what happens when you have a cavity with oil, you are pumping it, and there is a valve in between the tank and the pump, this valve suddenly closes. what will happen?

There should be a filter before the pump. The scenario described occurs when the filter becomes blocked and the pump inlet is starved of fluid.
In that situation:
1. A bypass valve should allow fluid to bypass the filter.
2. The filter may be damaged by the vacuum, allowing contaminated fluid to enter and damage the pump and following valves.
3. The fluid may boil or foam, then flow back to fill the tank through a return path, overflowing the vent. You need to specify the fluid, then you can predict the partial pressure at the pump inlet, and work out how the fluid will react or be denatured.

Testing part of the system to destruction is not wise, it is dangerous.

Describing only a small part of a system makes it difficult to know what protective components should be included in the system. There should be a pressure relief valve after the pump that allows fluid to return to the tank for cooling.

 

[Lnewqban]

I was reading up on those but it did not describe in detail how a Gerotor acts as an air pump, and what would happen in non ideal scenarios.

Copied from:
https://www.michael-smith-engineers.co.uk/resources/useful-info/gear-pumps

“Gear pumps are self-priming and can dry-lift although their priming characteristics improve if the gears are wetted. The gears need to be lubricated by the pumped fluid and should not be run dry for prolonged periods. Some gear pump designs can be run in either direction so the same pump can be used to load and unload a vessel, for example.”

Copied from:
http://www.liquiflo.com/v2/files/pdf/Gear_Pump_Basics.pdf

“Another important advantage of the gear pump is its self-priming capability. Gear pumps are capable of self-priming because the rotating gears evacuate air in the suction line. This produces a partial vacuum that allows the atmos- pheric pressure to force the liquid into the inlet side of the pump. This ability of the gear pump makes it an ideal choice when the application requires that the pump be located above the liquid level, and the liquid must be lifted to the pump. Because a gear pump cannot create a perfect vacuum, the total lift (including pipe friction losses) should not exceed about 7.5 PSI, or about one-half of the atmospheric pressure.”

Liquids can’t be compressed or stretched; therefore, no vacuum will form.
Only vapor will form for ever if the pressure reaches enough low value.

It is the atmospheric pressure above the free surface in the tank what would push the liquid towards the inlet of the pump once the pump evacuates the trapped air, if any.