Was ist eine gute Dichtung?Fact: Any conventional seal around a rotating shaft is prone to leakage!
In a domestic environment this is a minor problem easily fixed by a local technician. In a chemical manufacturing process it can lead to leakage of toxic gases, dangerous chemicals, harmful powders or slurries that require immediate process shutdown, expensive cleanup and costly production downtime. In a steam-driven power generating plant, downtime from a leaky boiler feed or condensate pump can be a costly nightmare.
This article discusses a variety of conventional sealing measures and introduces a revolutionary sealing technology solution for a wide range of sealing problems.
1. The Problem:
Leakage is when a gas, liquid, powder, or slurry leaks or evaporates from a process or a mechanical device such as a pump. Leakage in rotary equipment such as a pump is a well known occurrence and since 1870, the technology used to reduce the leakage known as Compression Packing is continuously being developed.
Leakage through a seal around a rotating shaft is caused by the rotational moment of the shaft causing the shaft to bend and vibrate. Over time, this movement creates gaps in the seal allowing material to escape into the environment.
(Fig 1) Diagram of a shaft rotating in a sealed environment:
Amount of leakage corresponds to the size of the gap, the process pressure, gap geometry and the physical characteristics of the production process. Leakage can cause harm to humans, animals, the environment and damage to the machine itself.
2. What is a good seal and why is it vitally important?
Although the answer seems to be simple "it should seal" there are a variety of important aspects that should be addressed when defining what a good seal is. A better question should be - "what is the TARGET performance of the seal?" Obviously, the target is to maintain uninterrupted production with a minimum of interference to the process and to the environment.
Aspects of determining a good seal:
1. Sealability - Very few sealing methods guarantee zero leakage. In many cases the application itself does not require a 100% seal or the plant operator may not need such ability. Generally - A good seal provides the smallest leakage possible. (Less leakage -> Better seal)
2. Operational Flexibility - The ability to maintain partial production even if the seal damaged or the working environment goes outside of the limits of the designed seal capabilities. Such a seal will enable the operator to complete the production quota.
3. Independent - Ability to work without reliably without impacting the process.
4. Predictability - Ability to provide early warning when maintenance is required thus preventing costly downtime due to seal failure.
5. Cost - Minimum total cost of:
* Failure - The most important parameter. What are the direct and more importantly the indirect costs when a seal in the production process fails? If downtime is critical to the success of a production plant an optimum sealing solution should be urgently employed.
* Purchasing - The price of the seal and its accessories.
* Installation - Direct cost and installation downtime cost.
* Operation - Waste of human resources, production time and liquids required to flush the process, plus the cost of extracting the flushing liquid from the process.
* Maintenance - Direct cost to maintain the seal to prevent future downtime cost.
* Replacement - Direct cost of removing the seal to prevent future downtime cost.
* Stock - How many seals should be held in stock in order to maintain continuous operation of the plant?
The above five aspects (S.O.I.P.C) define the cost-effectiveness of a sealing solution.
3. Solutions in the market
There are several technologies that can be employed to deal with leakage problems: Braided packing, Mechanical sealing, Injectable packing and for powder - air chamber.
a. Braided packing
The oldest solution, unchanged since 1870, is based on a braided material like Teflon, Graphite, Kevlar, etc., (or combination of these materials) that surround the shaft in a sealing area called a stuffing box. The braided material called Packing is pressed against the shaft. (See Fig 2) by a bolted flange.
(Fig 2) Packing structure: Friction from the pressure of the packing against the rotating shaft generates heat that can damage the shaft and the packing and cause and erosion of the shaft. (See Fig 3)
(Fig 3) Erosion caused by Packing: To control the heat, a flushing ring is often added so that water or other material can be fed to cool the seal. This often leads to leakages and contamination of the process. Generally, Packing can be considered more as a leakage control tool than a seal.
*Basic Problem: Not a Zero Leakage solution.
b. Mechanical seal
The most common sealing solution, mainly for pumps, was introduced by the John Crane Company about 70 years ago and continues to be improved. The basic concept is based on two hard and well-polished materials pressing against one another to form a seal. One face is stationary and the other face rotates against the stationary face. (See Fig 4)
The distance between the two faces stationary and rotating, is very small and thus leakage is theoretically prevented. However, due to the friction between the two faces, heat may increase to the point where the seal becomes damaged. To prevent this, a small film of fluid is maintained between the two faces to reduce the friction and transfer heat away. If the lubricating film is lost for whatever reason the seal may instantly become damaged and rupture.
Since the two faces must always remain polished to reduce friction and transfer heat away from one another, they are generally made from Silicon Carbide or Tungsten Carbide. These materials are brittle, and limit the seal from working with wide shaft run-out applications. To keep the faces constantly pressed against one another, a spring is added making the structure of this mechanical seal reasonably complicated and inefficient for slurries and wide run-outs.
There are many types of Mechanical seals; one of the most popular for gas applications is a seal with a cooling liquid system between the two internal faces. This structure is completely independent from the seal and does not impact upon the process it is designed to cool.
* Basic Problem: Fragile, Sensitive to slurry and run-outs.
c. Air Chamber
This method works mainly in dry powder applications. It is based on an area surrounded by Teflon or Graphite V-rings and into this area (Chamber) air is constantly circulated. The pressure in the chamber expands the V-rings and presses them against the shaft to limit the entry of the powder.
Actually, this solution is very limited and in some cases can even be dangerous. The rings wear down quickly and in some cases where the rings are made of rubber (Meco) they can actually wear down the rotating shaft.
The moment the air pressure is stopped, the chamber fills with the powder. As soon as the air pressure is restored, all the powder remaining in the chamber vents to the atmosphere around the process. Powder in the air can be extremely dangerous in a working environment.
* Basic Problem: Short time and Limited solution
d. Injectable packing
This method started about 20 year ago. The basic concept is to inject a material (Sealant) made of Teflon fibers, Graphite and other combinations, into the stuffing box using a hand-operated or electrical injector.
Since the injected material is flexible, it can track the shaft vibrations more closely and minimize the openings that were normally created in a regular packing solution.
The main problem in this approach is the inability to control the actual pressure in the stuffing box, as described in Fig 7.
Fig 7 - Pressure over Time graph in a regular inject able system
Point A - The operator detects a leak and starts to inject a sealant.
Point B - The pressure inside the stuffing box enters the Optimal Zone where there is no leakage and no extra heat. The operator continues to inject the material in order "to make sure" that no leak will develops.
Point C - The pressure enters the Over Heated Zone, where the over-pressure creates heat that the system cannot extract. In many cases, the heat burns and destroys the sealant. In some cases it can overheat the shaft and destroy the bearings. Such damage is not easy to repair.
Point D - End of sealant injection. A regular stuffing box cannot hold the pressure and the pressure immediately drops.
Point E - The pressure drops to Optimal Zone again. If the sealant and the system are not damaged, it starts to seal in the working operation.
Point F - The fail point, the pressure drops to the leakage area and the operator has to re-inject. If the sealant was damaged due to over-compression, the operator will have to replace all the sealant.
The period of time that the seal will remain in the optimal zone pressure is unknown due to the fact that the regular stuffing box is not designed to hold the sealant pressure in a steady state.
* Basic Problem: Inability to control the actual pressure
4. MTZ Technology
MTZ technology combines all of the benefits of the previous methods to create a synergetic solution that contains eight layers of protection against leakage. The result is Zero leakage even under extreme conditions of slurry and run-outs.
a. Eight layers of protection:
* Layer 1: Face-to-Face sealing - The front sealing rings are pressed against the sleeve shoulder to become a Face-to-Face sealing solution similar to a mechanical seal.
* Layer 2: Radial sealing - The sealing ring is attached to the radial sleeve similar to a packing seal.
* Layer 3: Sealant pressure -The Booster holds the sealant at a constant, positive, Optimal Pressure against the process. The sealant is injected into the assembly and the cartridge seal is plug-and-play.
The seal is maintained at the Optimal Pressure zone where zero-leakage is maintained without heating the system.
6. Layers 4, 5, 6, 7: Crenellated sleeve with 4 contact areas - The special and patented sleeve designed to maintain contact with the sealant, even under extreme run-outs.
7. Layers 8 : Back sealing rings
b. Sealant stock
- MTZ technology adds a sealant stock to the injectable sealant. This new approach brings two main improvements: 8. The maintenance is done automatically by the booster. When the operator needs to add more sealant, it is added to the stock while the machine is running. This translates into absolute minimum downtime for maintenance
9. Sealant meters provide the operator with an early warning of how much sealant stock remains. It enables them to forecast when to refill the sealant before the booster gets to its empty position thus preventing any pressure drop and potential leakage.
c. Cooling Jacket
The Cooling Jacket enables the seal to work efficiently without affecting the process. The cooling jacket acts like a dual-mechanical seal. The cooling water is separated from the process by an O-ring inserted at both ends.
5. MTZ seal in the S.O.I.P.C test
a. Sealability - The MTZ seal with eight protection layers provides zero leakage even under extreme run-out and slurry environments.
b. Operational Flexibility - The MTZ seal does not include any ceramic or silicon materials and it is an almost infallible solution. Even when the environment goes way beyond the limits of the seal, the seal will continue to work even if in a partial manner to prevent toxic emissions and costly downtime.
c. Independent - The MTZ seal is a dry-running seal and uses a separate cooling jacket for cooling. It operates without impacting the process it is designed to protect.
d. Predictability - The MTZ seal has a built-in mechanism that provides operators with an early warning of around 3 - 4 weeks before a sealant refill is required. As long there is some sealant in the stock, a positive pressure is maintained preventing pressure drops and leakage. Adding sealant is done on line and does not require production shut down.
e. Cost - The cost of the MTZ seal is within the range of a mechanical seal. However, the huge advantage of the MTZ seal is its ability to minimize production down-time, adding only a very small maintenance cost. This combined with dry-running operation, minimal on-site maintenance means an excellent cost-effective sealing solution.
6. About Tamar Technologies ( www.tamar-tech.com)
Tamar Technologies specializes in the research, design and production of sealing technology for rotating machinery. The company can also provide engineering and technical assistance, upon request, as part of our customer service. Knowledge accumulated over a period of years in the realm of seals for rotating machinery enables the company to offer the best and most reliable solutions for most kinds of machinery.
The MTZ technology is patented in US, Europe, China, India and Australia and South Africa.