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| The Problem |
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No seal manufacturer can possibly know all media and their variants, yet some still try to impress customers by publishing long lists of chemicals with corresponding seal recommendations.
This information is completely irrelevant for the user, however, because a change of just 10 % in concentration or 10 °C in temperature can result in utterly new behavior patterns with different adverse effects on the seal performance.
Then there are countless mixing ratios or chemical additives which can lead in addition to changes in the way the basic chemical reacts.
Can a seal manufacturer issue a useful recommendation if he has no details of all the critical, individual aspects of the medium in question?
The answer is no!
Is it possible nevertheless to help the user select seals, seal configurations and seal materials himself?
| The Solution |  |
DEPAC has developed a new strategy for a simpler selection process. Different seal criteria are defined on the basis of general behavior patterns of diverse liquids.
Rather than ask which liquid has to be sealed, we ask how does the liquid affect the seal?
An ideal liquid will not have a negative effect on the seal's functionality. It will be cool and clean, and it will have lubricating properties. With such a medium, every stationary mechanical seal would score good marks for service life. Unfortunately, "ideal" media are rare. What is necessary, therefore, is to analyze how diverse liquids might affect the mechanical seal in worst case scenarios. Then you can weigh up the options for preventing the problems arising on the seal in the first place, for counteracting the problems when they arise or for minimizing their effects as far as possible.
PLAN Numbers
The following aids, so-called PLAN numbers, are available:
| PLAN Numbers |
ANSI |
API |
| Recirculation flushing | Nr. 7311 | Nr. 11 |
| External flushing | Nr. 7332 | Nr. 32 |
| Quench | Nr. 7362 | Nr. 62 |
| Double mechanical seal with barrier fluid | Nr. 7352 | |
| Double mechanical seal with thermosyphon system | Nr. 7354 | Nr. 52, 54 |
| Heating / cooling the seal chamber | Nr. C |
Liquids / Media
Only 9 different factors need to be taken into consideration when selecting mechanical seals:
1. Media Containing Solid Materials
If a medium contains solid matter but is otherwise chemically neutral, cool and lubricating, it is irrelevant what the pumped
medium is called or which solids are contained in the medium.
The crucial point is that the solid matter can get in between the seal faces and destroy them.
No doubt there will be other aspects of seal design to take into consideration, e.g. dynamic configurations, springs in the medium,
O-ring movement toward the solid matter, etc. But to begin with, stationary seals should be given preference, and only then are
the following PLAN numbers to be considered as an additional aid:
a) FLUSHING PLAN No. 7332 with Base Bushing
Whatever other criteria there may be, it is crucial to keep the solid materials away from the mechanical seal. External flushing
with PLAN No. 7332 plus base ring should be used for this purpose. For the seal faces you should select wear-resistant materials
(e.g. silicon carbide / tungsten carbide). Only a small quantity of flushing liquid is required (3 - 10 liters/hour), but if even this is
not available or permissible in the process then a double seal is the only option.
b) DOUBLE MECHANICAL SEAL PLAN No. 7352 / 7354
For use in tandem-active arrangements and with a higher barrier pressure.
c) If a) and b) are not possible, the only option is a single mechanical seal with hard faces. In this case an accordingly shorter
or unpredictable lifespan must be taken into the bargain.
2. Crystallizing Media
For crystallizing media it is again completely irrelevant what the name of the medium is. The crucial question is why it crystallizes.
A fluid can crystallize for example when it comes into contact with the atmosphere or when the temperature changes.
When this happens, crystals form on the atmosphere side of the mechanical seal because this is where the pressure gradient between
the seal faces naturally has its zero point. The crystals grow, forcing the faces apart and causing the seal to leak.
The most important rule for crystallizing media is to use hard and durable material such as silicon carbide for all seal faces which
come into contact with the medium.
The following possibilities exist for preventing the seal faces being forced apart due to the formation of crystals:
a) FLUSHING PLAN No. 7332 with Base Bushing
In this case the crystallizing liquid in the seal chamber is replaced by flushing with a non-crystallizing liquid (3-10 liters/hour,
1 to 3 Gallon/hour).
b) QUENCH (Steam) PLAN No. 7362
Either the warm and moist atmosphere on the outside of the mechanical seal prevents crystallization in the first place or the
low-pressure steam removes any crystals which have already formed.
c) DOUBLE MECHANICAL SEAL PLAN No. 7352, 7354
This option should be considered if a steam quench is impossible.
d) If a) to c) are ruled out, a single mechanical seal with hard faces will have to be used but seal life will be neither optimal
nor predictable.
3. Sticking / Hardening Media
It is irrelevant what the medium is called. The crucial question is why it becomes sticky or hard.
If a medium is found to become sticky for example through contact with the atmosphere or a change of temperature, then steps
must be taken to ensure that no atmosphere reaches the working seal and that a specific temperature is maintained on the seal.
The following options are available:
a) FLUSHING PLAN No. 7332 with Base Bushing
If possible, this is always the best solution.
b) DOUBLE MECHANICAL SEAL PLAN No. 7352 / 7354
The double seal is supplied with a barrier fluid at a higher pressure than the pressure of the medium from the pump. The
atmosphere is kept away from the seal which is in contact with the product, and if necessary the temperature on the seal is
controlled by the barrier fluid.
c) HEATING / COOLING PLAN No. C, QUENCH PLAN No. 7362
If the medium becomes sticky or hard for example due to changes of temperature, steps must be taken to control the temperature
on the seal so that sticking and hardening are ruled out. PLAN No. C is recommended for this.
At all events you must use tungsten carbide faces because any remaining sticky areas will try to hold the faces together and brittle
materials such as silicon carbide often crack at the anti-rotation pins when the pump is started and stopped. Brittle materials such as
silicon carbide should not be used in spite of their hardness.
If there are other reasons for the sticking or hardening, their remedies have to be assessed and selected on the same basis.
4. Vaporizing or Gaseous Media
Obviously you must try to reduce the temperature on the seal or hold clean, cooling and lubricating liquid in contact with the seal.
The following alternatives exist:
a) FLUSHING PLAN No. 7322 with Base Bushing
b) HEATING / COOLING PLAN No. C
c) DOUBLE MECHANICAL SEAL PLAN No. 7352 / 7354
5. Media with Extreme Temperatures
Again the alternatives are:
a) FLUSHING PLAN No. 7322 with Base Bushing
b) DOUBLE MECHANICAL SEAL PLAN No. 7352 / 7354
c) SPECIAL MECHANICAL SEALS
Special mechanical seals can be developed and offered on the basis of the customer's specifications after due discussion.
All seal materials - and particularly those of the secondary seals - must be selected with due consideration to the temperatures
(including friction temperatures).
6. Hazardous Media
The user generally knows how hazardous a medium is. At the same time there is an increasing number of statutory regulations and
directives, e.g. "TEA Luft" in Germany, which must be observed by both the manufacturers and users of technical equipment.
The goal must always be to prevent such hazardous materials from getting into the atmosphere.
One possibility is to use an electromagnetically driven pump and do without mechanical seals altogether. The disadvantage of this option
is its extremely high level of energy consumption and the fact that such a system cannot be used for all media and sizes.
If it is at all possible to use mechanical seals, then DOUBLE or even TRIPLE MECHANICAL SEALS with PLAN No. 7352 or 7354 will
be given preference.
7. Media with Extreme Pressures
Either special mechanical seals are configured or the user can consider reducing the pump pressure on the seal chamber in steps by
means of DOUBLE MECHANICAL SEALS (PLAN No. 7352 or 7354).
8. Chemically Highly Aggressive Media
In this case it is important to select seal materials which are resistant to the chemicals in question. The choice of PLAN numbers is based
on the criteria mentioned above.
9. Special Equipment Features
Mechanical seals have been specially developed for equipment with either high shaft speeds, large shaft diameters or large shaft
deflections (see e.g. DEPAC TYPES 326/365). These versions should be used for such cases. If the requirements are too extreme to be
met by these purpose-designed seals, special seals will have to be ordered.
All the media requiring the use of a mechanical seal belong in one of the above categories or a combination of them. For combinations of categories, each criterion has to be considered separately one after the other. At the end you are left with the final selection of seals and any essential auxiliaries.
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