GENERAL TROUBLESHOOTING GT001
In this course we will be troubleshooting:
- Auxiliary seal equipment ST018-4
- Bearing troubleshooting, TBB001
- Leak paths in the mechanical seal, ST002
- Pump troubleshooting, PT001
- PD pump troubleshooting PT016
- Seal repair S026
- Seal specifications, how to write them S028
- Seal troubleshooting, S015
Before we get too wrapped up in the troubleshooting procedures there are some questions we have to think about:
- Is this a brand new application we’re troubleshooting? In other words, do we have experience with this type of failure, in this model of pump, handling this product, at these temperatures, speeds and pressures?
- Is the failed item a new design or model? Maybe there’s a design problem
- Is the failed item working satisfactorily in a similar application? If it is, you can probably rule out a design problem and concentrate on what is different in the two systems.
- Does the failure occur only at certain times?
- Does the seal failure happen soon after the pump starts? If a product has solidified or crystallized in the system during a shut down the seal has to “break away” at start up.
- Does the failure happen only during certain times of the year? Different seasons can produce different ambient temperatures.
- Does the seal failure occur within two weeks of the system being flushed with a cleaner or solvent of some type? It takes about two weeks for a solvent to attack the elastomers we use in mechanical seals.
- Has the failed pump or seal been rebuilt recently? Was it rebuilt by the original manufacturer using the same materials? Remember that rebuilding does not always remove rub marks and evidence of damage. These previous rub marks and damage might confuse the troubleshooter
- Has the failure rate increased after the system was altered? Changes in the piping, valves and fittings will change the friction head of the system that can cause shaft deflection problems. Shaft deflection can cause premature seal and bearing problems
Whether you are going to be troubleshooting mechanical seals, bearings or individual parts of the pump, it’s important that you remember several things about centrifugal pumps in general:
- The centrifugal pump always pumps the difference between the suction and discharge heads. If the suction head increases, the pump head will decrease to meet the system requirements. If the suction head decreases, the pump head will increase to meet the system requirements.
- A centrifugal pump always pumps a combination of head and capacity. These two numbers added together must remain a constant. In other words, if the head increases the capacity must decrease. Likewise if the head decreases the capacity must increase.
- The centrifugal pump will pump where the pump curve intersects the system curve.
- If the pump is not meeting the system curve requirements the problem could be in the pump, the suction side of the pump including the piping and source tank, or somewhere in the discharge system.
- Most pumps are oversized because of safety factors that were added at the time the pump was chosen. This means that throttling is a normal condition in most plants, causing the pump to run on the left-hand side of its curve.
- Most pumps come into the shop for repair because of two reasons:
- Something is wrong with the bearings.
- The pump is leaking too much.
There are two main reasons why bearings fail prematurely:
- Overheating. Over lubrication is the most common cause of overheating.
- Contamination of the bearing oil. Water or moisture is the biggest problem
Most of your seal problems are caused by the fact that you’re trying to install a mechanical seal in a pump that was designed for packing. There are two main reasons that seals leak prematurely:
- One of the seal components becomes damaged.
- The lapped seal faces open.
One of the Japanese automobile manufacturers has a unique method of troubleshooting any type of mechanical failure. The system is called the “five whys” and it is worth learning. It is a simple but powerful idea based on the concept that 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 asked. As an example, let’s look at some premature seal failures:
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 standard 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 parts bin.
6. Why were they stored in the same parts 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”. In this case it took seven. 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 this example I learned that the indicator float got stuck on a corroded rod giving an incorrect level indication.
One more example should do it. I ran into this one at an Opal factory in Germany.
1. Why did the seal start to leak?
- The dynamic elastomer (O-ring) became 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 Duriron brand and they normally adjust to the back plate. The mechanic confused the impeller adjustment method. He has since been retrained
This is a powerful trouble shooting technique. I hope you make good use of it.
In the next paragraphs we will be looking at some of the different practices that cause premature seal and bearing failure. We will be looking at:
- Specification practices
- Operation practices
- Maintenance practices.
PUMP SPECIFICATION PRACTICES THAT CAUSE SEAL AND BEARING PROBLEMS. GT002
PUMP OPERATION PRACTICES THAT CAUSE SEAL AND BEARING MAINTENANCE PROBLEMS GT003
MAINTENANCE PRACTICES THAT CAUSE SEAL AND BEARING PROBLEMS GT004