Reliability Engineering Snapshot TM

Illustrated Case Studies in the Maintenance Reliability Engineering World of Failure Analysis, Predictive Maintenance, and Non Destructive Evaluation



Machine Design - Case No. 20: Thirteen Ways to Ruin Pump Packing


Pump Stuffing BoxPump 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.]

I.D. Packing Ring Showing Score MarksOf 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.
Pump Shaft Packing SleeveDesigners 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.
  1. Don't provide enough packing flush. It'll overheat and self-destruct in short order.
  2. Forget to put the lantern ring back in the stuffing box. Instead, put in a 6th packing ring. This blocks the flow of flush to the packing. Again the packing overheats and self-destructs in short order.
  3. Save money up front (Ha Ha) and use a plastic lantern ring instead of a more expensive bronze lantern ring. The plastic ring deforms from the heat and collapses. The packing loosens up and begins to leak. Then the pump mechanic pushes in a 6th packing ring. The flush is prevented from distributing and the packing overheats and self-destructs in short order.
  4. Push the gland plate in until there is no leakage at all. No leakage means NO lubrication. Water can't get between the shaft and packing if there isn't ANY space. Again, the packing overheats and self-destructs in short order.
  5. When maintaining the packing, don't remove all of the packing rings, just shove more rings into the stuffing box. How does one guess where the exact location of the lantern ring is with respect to the flush port? This also puts more work on the 4th and 5th packing rings since the first three are actually worn down past their effectiveness. The new rings get compressed more and generate more heat. This is what happened in the failure shown above. Click on the 4th and 5th rings from the right to see close ups of the ring inner diameters. CAUTION: This is not for the faint hearted.
  6. Use a smaller size packing, and just shove more in the stuffing box. Again, where's the lantern ring?
  7. Use a larger size packing and FORCE it into the stuffing box. Again, no space for flush lubrication.
  8. After losing the packing for one of the reasons listed above, don't replace the shaft sleeve. Go to the shaft sleeve shown above and find the two links that will take you to the close up pictures of this damaged shaft. Then ask yourself how can a braided material withstand the sawtooth action of a ruined shaft sleeve.
  9. Be conscientious and put the lantern ring back in place, after adding an additional 3rd ring for "good measure." Again, the location of the lantern ring is off, and will not get any flush water to distribute. That's what happened in the failure shown above (see both pictures); the lantern ring was in the wrong location.
  10. When adjusting the packing by tightening the gland plate, bear down on just one adjusting nut more than the other adjusting nut. this causes misalignment of the packing gland plate and puts an eccentric load on the packing. Parts of the packing are being crushed while other parts are essentially unloaded.
  11. Don't bother adjusting the packing. Just let it leak product out. This does wonders for the packing rings and the shaft sleeve, especially when the process is gritty in nature. Everything gets abraded.
  12. Adjust the packing gland plate BEFORE putting the pump into service and then don't check it after the pump is running. A distinct burning smell indicates the pump packing is too tight. Higher amps on the motor is also a dead give-away.
  13. Mix packing brands. This is one case where mixing is not good. Packing is quite an engineered product. Crush capacity, distortion, heat capacity. It's all figured into the material.
  14. I'm getting depressed, I'm going to end it here.

All Pictures and Text Copyright © 1999 - 2016 Contact Mr. Adler Adler Engineering LLC of Wyoming USA


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Mr. Eric Adams - FEA Analyst