Illustrated Case Studies in the Maintenance Reliability Engineering World of Failure Analysis, Predictive Maintenance, and Non Destructive Evaluation
|Pump packing is about as unglamorous as a mechanical device can get. There's just no "star power" in it to draw any enthusiasm. Needless to say, it's a never ending battle to keep it running properly. Who wants to spend their time on something so boring (even though you ARE being paid to make that time). This is one of those "nickel and dime" problems that adds up.
The picture to the left illustrates the cross section of a typical "packed pump." In this particular case, the sole function of the packing is to keep the process from leaking out of the pump. This is accomplished by using square braided packing rings (usually five) that are pushed into a "stuffing box" and then compressed further by a "gland plate." The more the gland plate is compressed, the more the packing rings are deformed and close any gaps between the shaft and stuffing box. It's really quite simple.
[Note: Less there be any confusion, the cross sectional drawing shows two types of sealing arrangements. The top of the shaft shows a packed pump with a gland plate and five sets of packing rings and a lantern ring. The bottom of the shaft shows a mechanical seal with a seal plate and the stationary ring.]
|Of course now, if you've been paying attention, you're wondering about the friction between the rotating shaft and the packing. Rub your hands together fast and they get warm. Rub a shaft at 1,400 to 2,800 feet per minute and watch out! That packing and the shaft are going to develop enough heat to destroy the packing and gall the shaft. In short order, the pump will be leaking. Designers have overcome this problem with some basics. First, lubrication is provided to the packing in the form of water (usually). Second, it is evenly distributed by means of a "lantern ring". The lantern ring is a hollow perforated ring that is physically located between the 2nd and 3rd rings (from the impeller, see drawing). Looking at the cross section illustration, there is a port drilled at this specific location. This port allows a flushing medium, usually water, to enter the stuffing box at the lantern ring. From there, the flush is evenly distributed between the rotating shaft and the packing. This simple action lubricates the shaft and keeps both the shaft and the packing cool. The flush finds its way back into the process, or out the gland plate.
|Designers have gone one step further, and realizing that accidents do happen, like losing the packing flush, they have added a "packing sleeve" over the shaft to protect the shaft. One of those sleeves is illustrated in the cross section of a pump in the picture above, and also is shown in the picture to the left.
|So how does something so simple go so wrong? How can I screw that packing up? Let me count the ways. Oh, by the way, I'm not making this up. I don't have that kind of imagination.
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