The 5 Whys of a mechanical seal failure

A new way to troubleshoot centrifugal pumps and mechanical seals

One of the U. S. based Japanese automobile manufacturers has a unique method of troubleshooting any type of mechanical failure. The system is called the?“Five Whys”. It is a simple but powerful concept, nothing has been solved until the question “why ?” has been asked at least five times and a sensible answer has been given for each of the “why” questions. As an example:

1. Why did the seal fail?

• The lapped faces opened and solids penetrated between them. (solids can’t get in until the faces open)

2. Why did the faces open?

• The set screws holding the rotary unit slipped due to a combination of vibration and system pressure.

3. Set screws are not supposed to slip. Why did the set screws slip?

• The seal was installed on a hardened sleeve.

4. Why was the seal installed on a hardened sleeve?

• This was a packing conversion and a stock sleeve was used.

5. Why couldn’t the mechanic tell the difference between a hardened sleeve and a soft one?

• They were both stored in the same bin.

6 Why were they stored in the same bin?

• Because they had the same part number.

7. Why did they have the same part number?

• They should have had different part numbers. Once that problem is corrected, the failures will stop.

Now you get the idea! Needless to say you may have to go further than just five “whys”. Let’s try another example:

1. Why did the seal fail?

• The pump was cavitating and the vibration caused the carbon face to crack.

2. Why was the pump cavitating?

• It did not have enough suction head.

3. Why didn’t it have enough suction head?

• The level in the tank got too low.

4. Why did the level in the tank get too low?

• I don’t know.

You have not finished five “whys” so you better go find out why the level in the tank got too low, or the problem is going to repeat its self. In the above example the float became stuck on a corroded rod, giving an incorrect level indication.

One more example should do it:

1. Why did the seal start to leak?

• The elastomer got hard and cracked.

2. Why did the elastomer get hard and crack?

• It got too hot.

3. Why did it get too hot?

• The pump stuffing box ran dry.

4. Why did the stuffing box run dry?

• It was running under a vacuum and it was not supposed to.

5. Why was it running under a vacuum?

• A Goulds pump impeller was adjusted backwards to the back plate and the impeller pump-out rings emptied the stuffing box.

6. Why was it adjusted backwards?

• Most of the pumps in the facility are of the Flowserve (Duriron) brand and they normally adjust to the back plate. The mechanic confused the impeller adjustment method. He has since been retrained