AS-C401 Customized Double Cartridge Mechanical Seal

This seal is dual seal in cartridge mechanical form made as per drawing.

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Background Knowledge：

How does a factory reduce the probability of mechanical seal leakage by adding a throttling bush?
Due to the particularity of the process medium in the synthetic butyl rubber device, the mechanical seal of the reactor or mixer frequently leaks. This article describes how a factory reduced the leakage rate of the mechanical seal by adding a throttling bush at the front end of the mechanical seal and arranging a reasonable flushing scheme.

Double cartridge mechanical seals have been widely used in agitator equipments in chemical production. Compared with single mechanical seals, double mechanical seals have better sealing performance. They can effectively prevent the leakage of dangerously toxic, flammable and explosive substances thus have an insurance effect.

In the synthetic butyl rubber device of a certain factory, the process medium in the agitator has the characteristics of deep cooling (about -100°C) and containing suspended particles (colloidal particles), which puts high requirements on the performance of mechanical seals. .

Phenomena analysis：
1. The polymerization kettle, degassing kettle, and stripping kettle in this plant are stirring shaft-bottom-inserted agitator), and its mechanical seal is EKATO back-to-back double mechanical seal S034GD series. The basic parameters of the said agitator are as follows.

2. The mechanical seals of all above-said agitators did not leak frequently at the initial stage of operation.
3. Most of the mechanical seals installed in the agitators have such phenomenon: The primary seals of the mechanical seals failed before the secondary seals did, and after dismantling, it was found that the rotary faces and stationary seats of the mechanical seals of the polymerization kettles were broken, and the sealing surface of the rotary faces often carried glue; the front end of the degassing kettles' mechanical seals were glued, and their rotary faces and stationary seats were broken.
4. The O-rings of the mechanical seals of the polymerization kettles had been replaced with low-temperature-resistant materials many times before, but the leakage rate did not significantly reduce after such measures were taken.
5. Leakage often occurd during the time when the agitators wre stopped & restarted, and there was often a temperature change with the media in the agitators.

Cause Analysis:
1. Due to the long-term operation of the polymerization kettles, the auxiliary seals at various points on the front axis of the mechanical seals were worn, and the leakage of feed materials under the action of gravity gradually increased. When the PLAN32 solution was unable to prevent the cryogenic polymer feed material from leaking along the shaft with the designed flushing volume, the deeply-cold feed materials would accumulate , the first ring material of the mechanical seal would become cold and brittle, and finally broke together with the secondary seal under the mechanical load generated by mechanical seal when undergoing stop to restart or vice versa.
2. Due to the long-term operation of the degassing kettles and the stripping kettles, the seals of the shaft cap wore with the rotation of the shafts, and finally the inner diameter increased, and the leakage channel increased. The colloidal water finally leaked along the shafts to the end faces of the primary seals of the said mechanical seals. Colloidal particles accumulated on the end faces of the first rings, and even blocked the PLAN flushing pipelines. In the end, the poorly lubricated primary seals' rotary faces and stationary seats wore or broke, resulting in mechanical seals leakage.
In short, the cause of damage can be summarized as follows: due to the particularity of the process materials, the designed flushing scheme did not achieve the ideal cleaning and lubrication effects, which deteriorated the sealing environments of the primary seals and eventually led to mechanical seal failures

Tackling Measures:
On the basis of not changing the mechanical seals' structures, make after measurement some respective throttling bushs on the outer ends of the mechanical seals' gland plates to form restricted tight gaps around the sleeves (or shafts) and arrange clean flushing materials.
The requirements for the layout of the flushing materials were: 1. The flushing materials were compatible and close to the materials in the kettles, and mostly were chosen from reaction solvent or diluent.2. The temperatures of the flushing materials were similar to the temperatures of the materials in the kettles, so that after PLAN32 were put to function, there will be no strong mass transfers or heat transfers, which will affect the reactions. 3. Check whether the matching clearance between the throttling bush and the shaft and the flushing volume of the flushing materials were appropriate. The greater the clearance between the throttle bush and the shaft, the greater the amount of flushing required. And on the contrary，too smaller matching clearances may easily cause wear of the shafts and bushings or increase the flushing pressures of PAN32s. In addition, the operating temperatures and the material selections of the throttling bushes also affect the selection of the matching clearances. 4. Throttle bushing can be made into labyrinth seal or spiral seal. The following table shows the selection plans of the factory mentioned above for the flushing materials and the matching clearances between the throttling bushes and the shafts.

Effect after modification:
1. The modifying measures by adding the throttling bushes greatly reduced the leakage rates of the mechanical seals of the above-said agitators and prolongd the service lives of the mechanical seals. In 2012 (before transformation), the polymerization kettles leaked 13 times, and in 2013 (after transformation), it was 2 times; The degassing kettles and the stripping kettles leaked 7 times in 2012 (before the transformation), and in 2013 (after the transformation) no leakage occurred.
2. The reduction of maintenance times reduced the maintenance costs. Comparing maintenance costs of the polymerization kettles, degassing kettles, and stripping kettles in 2012 and 2013 including the costs of disassembly and assembly of the polymerization kettles for cold preservation and the maintenance & replacement of the spare parts of the mechanical seals of each kettle, the cost saving was about 630,000 yuan.
3. For polymerization kettles, degassing kettles, and stripping kettles, the maintenance method of adding a throttling bush is much easier to overhaul the pumping shaft and replace the shaft cap seal, thus improving the maintenance efficiency.

Through repeated practice, we realized that the root cause of the leakage of the mechanical seals of the plant was the deterioration of the lubrication and cleaning conditions of the primary seals. The simple method of adding a throttling bush and adding flushing before the mechanical seal solves this problem, and it has been popularized in the stirred tank of highly corrosive materials in the halogenated butyl units.