Pumping a fluid that is under vacuum?
Pumping a fluid that is under vacuum?
(OP)
Hello group
Im looking a repeat seal problem where the shaft sleeve has contacted with the bore of the seal plate.
The contact area is at about "2 oclock"
So it seems the shaft has move up and over a touch.
I have been told that the pumps operate at vacuum.
This is where Im getting a bit lost as the pump itself
if a Flowserve End Suction Centrifugal. THere doesnt appear to be anything 'special' about it.
What could this mean?
Does this mean that vessel/tank from which the fluid is pumped is under a vacuum and the pump is moving the said fluid? So suction is vac yet discharge is 1-2 Bar say?
Any advice is welcomed as this has gone over the edge
of my basic pump knowledge
M
Im looking a repeat seal problem where the shaft sleeve has contacted with the bore of the seal plate.
The contact area is at about "2 oclock"
So it seems the shaft has move up and over a touch.
I have been told that the pumps operate at vacuum.
This is where Im getting a bit lost as the pump itself
if a Flowserve End Suction Centrifugal. THere doesnt appear to be anything 'special' about it.
What could this mean?
Does this mean that vessel/tank from which the fluid is pumped is under a vacuum and the pump is moving the said fluid? So suction is vac yet discharge is 1-2 Bar say?
Any advice is welcomed as this has gone over the edge
of my basic pump knowledge
M
-
Milkboy





RE: Pumping a fluid that is under vacuum?
Second, you need to be sure that the pump is operating at a good point on the curve. If the flow is too low (below minimum stable continuous) the pump will cavitate even if there is an adequate NPSH margin. If the flow is too high (>120% of Best Efficiency Point), then the NPSH required may have increased so much that it exceeds the NPSH available. If you want greater reliability, this range should be tightened further. We would attempt to keep this pump in a range between about 70% and 110% of BEP flow.
Third, you need to have positive pressure on the mechanical seals. If the seal runs under vacuum, it could "suck air" if it fails. In my business, a mixture of air and product inside a pump or pipe is a very dangerous thing. But, even if the seal does not fail, the negative pressure across the seal can affect the lubrication across the faces. We normally have our vacuum service pumps designed with restricted balance holes through the impeller to keep the seal chamber pressure positive. This has to be done carefully, by a qualified pump designer since it will also have a large affect on thrust loads. Positive pressure at the seal can also be accomplished with a seal flush plan such as API plan 54 using a separate pressurized stream between two seals.
The damage you describe does not seem to be to be tied to the fact that the pump runs at vacuum. It could be a simple mechanical problem. You should check the concentricity of the shaft within the bearing housing, the fit from the bearing housing to the head and the fit from the head to the case. If you mount the bearing housing on the head (with no seal) and sweep the concentricity of the seal chamber and the head wear ring, this will give you the result. Even if each of the fits is within specification, the tolerance stack-up could result in a situation where the shaft is not running down the center of the seal chamber or the head wear ring.
If the pump has two volutes, then you need to check that the condition of the volutes is good. The volute lips (cut waters) should be symmetrical and even. The volutes should be unrestricted (no foreign material obstruction). If the volutes were uneven, this would result in a side load on the impeller when running away from BEP flow.
The seal pilot to the head should be checked. The seal needs to be located positively to be concentric to the seal chamber. The seal installation needs to be performed correctly. The seal needs to be positioned on the shaft, but not tightened to the head until after the bearing housing is bolted solidly to the head. Then the seal should be bolted to the head and the drive collar locked to the shaft. This is especially important if this is a non-API seal that does not have a pilot fit to the head.
Johnny Pellin
RE: Pumping a fluid that is under vacuum?
Much appreciated
M
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Milkboy
RE: Pumping a fluid that is under vacuum?
have a good week end guys, Johnny I would live to see your plant one day!