I’m looking for a good seal for low-speed rotating shafts
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It is a low-speed shaft with a diameter of 180, rotating at about 20 revolutions per minute. The original design called for a packing seal; the packing was made of acid-resistant hemp wrapped around graphite. The operating temperature is 80 degrees, and the medium can be either liquid or gas. The pH value of the liquid medium is between 4.0 and 4.5. After operating for roughly 5 days, leakage would occur, so the seal would be tightened before continuing operation. The packing needs to be replaced again within a month. Since the entire ring of sealing rings has to be inserted from the shaft end inward, the disassembly and assembly process is quite labor-intensive; therefore, no improvements have been made to the mechanical seal for now. The difficulty in modification lies in the high acidity of the medium when it is in liquid state; moreover, when it is in gaseous state, dry wear on the packing due to rotation can be significant. One of my ideas is to find a better material for the packing, and the second idea is to add springs at the bolts that secure the packing, so that they can exert a certain self-tightening effect when the packing becomes loose. I was wondering if anyone could recommend good seals suitable for similar working conditions – what are the best materials and seal types available?
Changing to magnetic drive will do; there’s no need for a mechanical seal either
For your operating conditions, you can replace the packing with one made of PTFE. In fact, no matter what type of packing is used, it only provides a temporary solution – wear will still occur rapidly. It’s better to opt for a mechanical seal; using a 212 single-face seal along with an oil cup would work well. The lubricant should be chosen based on your material, as long as it’s dye-free. This approach is feasible provided that the deflection at the shaft seal isn’t too high, as high deflection can cause severe wear on both the stationary and rotating components. If the deflection is low, such a modification won’t require replacement for a year or two. Replacing it when necessary isn’t very troublesome, and the cost isn’t high either; a separate unit rather than a modular one is sufficient. There will generally be a need for maintenance within 2 years, at which point simply replace it with a new one. The money saved in labor costs far exceeds the expense of the mechanical seal. This is just my personal opinion; feel free to disagree if you wish
This post was last edited by small pad on 2020-4-2 03:00. Your approach is correct. However, there is one problem with using mechanical seals: sometimes the operating condition involves a gaseous state, and when it is in gas form, there is negative pressure inside; moreover, the material is in the process of drying and is very sensitive to contamination by external flushing liquids. Let’s take a walk around among the mechanical seal manufacturers – can a mechanical seal withstand 3 hours of operation in conditions of negative pressure and dry grinding? The only thing that’s slightly better is that the rotation speed is very low. After 3 hours of gaseous operation, it is followed by 2 hours of liquid operation at an acidic concentration of 0.2 Mpa, in a repeating cycle. Switch to dry gas; the rotation speed is too low to create a gas film.
Regarding this operating condition, dry grinding is no problem. If there is negative pressure inside the tank, it’s not possible to use lubricant – it has to be pure dry grinding. However, there is one issue: dry grinders require a high level of stiffness from the shaft seal; it’s not clear whether your current equipment can meet this requirement. In domestic equipment, mixing is generally achieved using a single-point support structure, with a bearing box in the middle of the frame. The drive shaft and the shaft sleeve within the bearing box are connected by a sliding fit, which results in low precision. This makes it difficult to ensure the necessary stiffness of the seal, leading to uneven grinding. As for the liquid mixing at 2 kilograms of pressure you mentioned, that shouldn’t be a problem since the pressure is very low. If you want to extend the maintenance interval, you can discuss with the manufacturer of mechanical seals the possibility of making the worn surface of the moving ring thicker than usual, thereby extending its service life as much as possible. The problem at hand is serious, and there aren’t any other good solutions available. This is just my personal opinion
After reviewing what you’ve described, there’s no problem with modifying the mechanical seal. I work at Zhangjiagang Seth Mechanical Sealing Co., Ltd. If you need anything, feel free to add me on WeChat: 17714292800