To avoid mechanical seals failure prematurely
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The benefits of installing mechanical seals correctly
Lower the maintenance cost for your pump
Save money and resources by reducing power consumption
Reduces the pollution of leakage
Let’s have a look at the cross-section of a typical centrifugal pump. To understand the operation of a centrifugal pump will help us to know the function of mechanical seals.
The pumping media enters through the impeller with lower pressure, and up the impeller vanes with low velocity. When the motor being spinning, the pressure and velocity inside of housing will increase when the media passed through the volute. The pressure is at the highest point when media exits through the discharge. The pump fluid carry energy and fly.
The three main aspects of mechanical seal failure
When a centrifugal pump failed, it could be coupling problems, it could be bearings problems, but 69% of all pump failures come from the failure of mechanical seal.
1. Installation problems
- The first thing you have to pay attention to is Dry running.
- Dry running can be seen commonly in a vertical pump since the mechanical seal is positioned on top. If you have improper venting, air can get trapped around the seal and won’t be able to evacuate the stuffing box. The mechanical seal will soon fail if the pump continues to run in this condition.
- Overheat seal faces will cause failure which can be very costly. External flush to seal can illuminate this problem when the pumping media is not suitable for flush the seal such as no lubricant features or the temperature is too high as well as it is too dirty containing abrasive small compressed mass.
- You need a proper seal flush arrangement, along with sufficient cooling. Dual systems have barrier fluids; the auxiliary seal pot needs to be in the right location, with the correct instrumentation and piping. You need to take the Length of Straight Pipe at Suction into account. After the pump is installed, don’t misalign pipes bolted to it, you will impart a lot of strain on the pump. Some older pump systems that often came as a packaged skid include a 90º elbow at suction right before the flow enters the impeller eye. The elbow causes a turbulent flow that creates instabilities in the rotating assembly. All the suction, discharge and bypass piping needs to be engineered correctly as well, especially if some pipes have been repaired at some point over the years.
- We will talk about external flush plan in the next following articles.
- Shaft misalignment and coupling imbalance, and impeller imbalance can all contribute to mechanical seal failures. Before install the mechanical seal, check the bearings. If the tolerance of the bearings wears thin, the shafts will move and cause vibrations in the pump.
- Poor handling and dropping the seal will damage the polished seal faces which is crucial parts of mechanical seal. The transmission of dirty stuff by installing contact can be embedded in the seal face, which will damage the seal face when running.
- Ensure seal faces not to contact grease lubrication during installation, and keep the seal faces clean. The external flush or pumping media will lubricate the seals face.
- In addition, You also need to avoid a bad base. Is the base secure? Is it grouted properly? Do you have a soft foot? Is it bolted correctly?
2. Choosing a proper mechanical seal for your application
- Do you have a good tribological (the study of friction) pair? Have you chosen the correct facing combinations?
- For example, when a carbon rotary face works with silicone carbide stationary, the outer diameter of silicone carbide lapping face should be bigger than carbon’s. The main reason is that the hardness of silicone carbide is much more harder than carbon’s. Think about Carbon VS Ceramic; Carbon Vs Sic; Carbon Vs Tungsten carbide; Sic Vs Sic; Ceramic Vs Sic; Tungsten carbide Vs Tungsten Carbide; Ceramic Vs Ceramic. Also the lubricant features of these material combination.
- What about the seal face material quality?
- Are your materials appropriate for your specific application?
- Have you selected the proper secondary seals, such as gaskets and o-rings, that are prepared for chemical and heat attacks? Do you require 304 stainless steel, 316SS, Dulpex stainless steel or even cast iron for the metal parts?
- Corrosion, abrasive, temperature or chemical attack by pumping product due to incorrect selection of build materials and elastomer’s.
- Your springs should not be clogged or your bellows corroded.
- Last but not least, keep an eye out for face distortions from pressure or heat, since a mechanical seal under great pressure will actually bow, and the skewed profile can cause a leak. To check with seal manufacturer and install mechanical seal according to the dimension chart and pay attention to the total working length. To ensure the working pressure is under allowance. You can click here to check those important sizes of a mechanical seal.
3. Pump operational Failures
- Neglecting the Best Efficiency Point:
- Pump is designed at the very first day with a performance curve. And it has a specific efficiency point. If the pump is operated beyond that region for a long time, the unstable flow will cause the system to fail.
- Insufficient Net Positive Suction Head (NPSH):
- If you don’t have enough suction head to your pump, the rotating assembly can become unstable. It will cause cavitation which will result in a seal failure.
- Operating Dead-Headed:
- If you set the control valve too low to throttle the pump, you can choke the flow. Choked flow causes recirculation within the pump, which generates heat and promotes a seal failure.
- Low Vapour Margin:
- These are flashing fluids; hot hydrocarbons will flash once exposed to atmospheric conditions. As the fluid film passes across the mechanical seal, it can flash at the atmospheric side and cause a failure. This failure often happens with boiler feed systems — hot water at 250–280ºF flash with the pressure drop across the seal faces.
Other miscellaneous factors
These factors account for only about 8 percent of all failures.
For example, auxiliary systems are sometimes required to provide an acceptable operating environment for a mechanical seal.
For reference to dual systems, you need an auxiliary fluid to act as a barrier that prevents contamination or process fluid from spilling into the environment.
However, for most users, addressing one of the three categories will check out a solution they need. Hope this article will give some ideas.
Ok, let’s all for today.
See you next time.
Wayne Liao-Mechanical Seal Maverick