I suppose it is possible as it would also tend to reduce the suction pressure given that a pump supplies a fixed differential pressure. If discharge pressure decreases for any reason (other than a reduction in flowrate) while the pump maintains a given differential pressure, suction pressure would drop, potentially to levels below NPSHR.
What exactly do you mean by a"grinder" pump?. Is this a sewage unit?
Given that pumps exist to create a difference in pressure, if you apply a vacuum to the discharge whilst it is running, then yes the inlet pressure will drop and you will cause cavitation, possible excess flow and overload of the motor.
Give us a few more figures and data and you might get a more considered response.
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Also: There's usually a good reason why everyone does it that way
Applying a vacuum to the discharge will in effectively reduce discharge head to zero or below causing the pump to run way right on the curve probably resulting in cavitation as well as other likely problems.
More detail of your operation wouldn't hurt the discussion.
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
Then the natural tendency would be for a vacuum to cause cavitation due to BOTH effects, the pump's tendency to run to the right and increase flowrate thereby increasing friction and reducing head throughout the system, plus the previously mentioned effect of a suction head reduction if the flow happened to remain constant.
Thinking a bit more on the case of a vacuum on the discharge side of a pump, to be able to have a vacuum condition on the discharge side means that the inlet side would have to be blocked / closed / off -- or, the pump is installed with a "suction" lift condition.
Something that could cause a problem, assuming it is a submersible grinder pump and depending on the pump configuration is that the mechanical seal faces could be opened as a result of the lower pressure on the impeller side of the unit compared to the pressure on the motor / drive side of the unit.
More detail is required.
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
The pump specified is a Hydromatic Pump Model S3HRC750M52 —Recessed Impeller Sewage Pump. There will be a vacuum truck connected to the end of the pump discharge pipe. the vacuum truck will be creating a negative pressure on the pipe pulling the liquid while the pump pushes the liquid.
Ok. Data to work with. This is a pretty simple robust looking unit pumping around 60 ft @ about 300 gpm. What is the vertical head, what size pipe and how far away is the truck from the pump?
If it's a 150 ft or more then it probably doesn't make a lot of difference, but will increase flow a bit. Do you need the vacuum truck to apply sub atmospheric pressure or does the pump manage to pump anyway?
If it's right next to the pump it won't do it a lot of good, but even then this is a recessed impellor pump so will probably survive without much impact. If it starts making an odd noise, or the motor trips, turn it off. You will still get flow with this sort of pump if you apply lower pressure on the far side.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
Total height from pump to truck is 12600mm, 12 x 45 deg bends, total length of pipe is 69100mm. the pipe will be 3" diameter. the goal was to size the pump so that it will need the vac truck running at the same time to get the liquid to the surface and into the truck. If the pump was running without the truck connected it would not be able to pump the liquid to the street making a mess.
I couldn't quite work out which your pump curve was, but looked like you're generating about 18m head, so once you take off the 12.6m head, you haven't got a lot left for your bendy 70m of pipe. The vacuum truck will add about 5m to that as "vacuum" is a relative term for these sort of trucks.
Your pump should be able to pump without the truck, but not at a very high rate.
With this vertical lift and the length of pipe from truck to pump, this will not cause the pump to cavitate or be harmful to the pump.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
