Entrained Air Issue?
Entrained Air Issue?
(OP)
Hello. I have a diesel operated 6" trash pump. I'm pumping from a undergound chamber which is 17' deep, 23' long and 8' wide. A 36" line dumps into the chamber @ 6' from the top. My pump suction is located at the opposite end. Vacuum gauge reading when operating is 15". On the discharge side, I have a constant reading of 35psi. The pump is operated via a float switch so the suction never runs dry, never creates a vortex. Whenever the pump turns off via the float switch, and goes to come back on, we are getting air trapped at the top of the volute and the unit won't prime for some time. The unit is equipped with a venturi type priming system which incorporates a float in the priming chamber. We put a small ball valve in the top of the volute and cracked the valve to allow for air release. We also ran a small tube from this back into the water source to ensure the priming system still works when it needs to. This seems to work most of the time, but isn't failsafe. Without it, we have torn engine couplings. We even tried an electric drive unit with the same results. I guess my question is am I dealing with entrained air and if so what are my options. Any guidence would be appreciated.





RE: Entrained Air Issue?
If this is the case I would suspect small air leaks through the shaft seals or various gaskets.
RE: Entrained Air Issue?
RE: Entrained Air Issue?
Are you certain that the installation vents the stuffing box completely and the seal is designed for the service? Once fluid begins to move, any air that is back in that area will become trapped by the heavier water being slung to the outside. The seal then may leak.
Is the ambient temperature higher than the water in the sump? If the water warms up in the suction line while idle, less gas is soluble causing air to come out of solution, even if it weren't air entrained from the plunging inflow. You might be able to look at the Flowserve pump manual, which I recall as having a diagram for the proper relationship between in and outflows in a sump.
RE: Entrained Air Issue?
I assume you are talking about an air-driven venturi system. Is this a commercially built pumpset or put together for the job
2. The problem of self-priming after shut down sounds as if it connected to your priming system not the aerated water, float switches etc.
2.1 I would check that the venturi system is not partially blocked and that air supply line is not blocked.
2.2 Check that the nozzle and venturi are not worn and that the nozzle / venturi are positioned correctly in relation to each other
2.3 Check to make sure that the float system is operating properly and not stuck in the "shut" position ie, negating the air-handling capability of the venturi.
2.4 is the unit fitted with a non-return valve on the discharge?
What is a "long time" for repriming - 30 seconds - 10 minutes?
I do not understand why you are tearing couplings. What is the mode of failure as I fail to see why repriming or not repriming has anything to do with coupling failures.
RE: Entrained Air Issue?
RE: Entrained Air Issue?
RE: Entrained Air Issue?
RE: Entrained Air Issue?
RE: Entrained Air Issue?
As Gorman Rupp pumps are designed to run all day "snoring" (pumping then not pumping then repriming- pumping not pumping repriming)the driver and coupling should also be selected to accommodate this mode of operation.
I assume you are shutting the diesel engine down at each cycle, for interest, what is the frequency and timing (run time - stop time) of start / stop cycles.
RE: Entrained Air Issue?
The unit runs for about 15 minutes and is down for @ 40 minutes. The unit is designed for snoring applications however we have found that whenever we are pumping into pressure, ie forcemain, etc., the volute tends to get airlocked at the beginning but once its released, the air doesnt come back. Unless we vortex.
RE: Entrained Air Issue?
On some occasions the pumps would "half-prime" on a re-prime sequence - to overcome the problem they invented and patented the "Univac" pipe - this consisted of a tapping point into the pump case behind the impeller and close to the stuffing box - from there it lead to the priming tank via the "Univac pipe" in which the float level control apparatus was positioned - the point of entry into the tank was below the nominal water level maintained in the tank when the pump was operating- a later refinement was fitting a non-return valve in the univac pipe with the direction of flow from the back of the pump housing to the priming tank.
This was to overcame the problem of air entraped in the volute, this is a catch 22 problem -- the air can't be pushed from the casing until the pump is fully primed and pumping and it can't be fully primed and pumping until the air is removed.
I did have a copy of the patent - but think it has vanished and I can not find it on the web so hope this description is clear. It is probably worth considering as I can see now other explanation.
As for the coupling failures - still in the dark.
RE: Entrained Air Issue?
RE: Entrained Air Issue?
Other alternatives are to fit a check valve on the suction or a foot valve at the inlet pipe (not recommended for slurry)
RE: Entrained Air Issue?
RE: Entrained Air Issue?