I will explain the flushing plan for mechanical seal.
A liquid that is handled in large quantities at a chemical plant. About 50% of the pumps used to send the liquid are centrifugal pumps.
Centrifugal pumps are always accompanied by mechanical seal problems.
Even for mechanical seals, flushing is related to piping design .
If you don’t decide how to do the flushing, the piping design will stop.
We have summarized the minimum information that you should know as a mechanical engineer of a batch chemical plant.
If you know this much, you can use it in practice.
Whoever controls the mechanism controls the centrifugal pump…maybe
API682
API stands for American Petroleum Institute.
Petrochemicals is the most basic industry in the chemical industry.
America has the most advanced technology in the world.
The standards of the American Petroleum Institute can be considered as standards in the chemical industry.
Among the standards of the society, code 682 defines the standard for mechanical seals for pumps.
If you are interested, please check the following site.
Plan01 Internal flushing
Internal flushing is the most basic flushing method.
This is a method of returning the high-pressure liquid from the discharge port of the pump to the mechanical seal.
Since the pressure at the mechanical seal is lower than the pump discharge port , the liquid can flow due to the differential pressure .
The pressure at the mechanical seal is said to be 2/3 or 3/4 of the pump discharge port .
This method is sometimes referred to as self-flushing or self-flushing.
Self-flushing may be confused with Plan11, so be careful
Plan01 creates a liquid path inside the pump casing.
Plan11 self flushing
Self-flushing is almost the same idea as internal flushing of Pan01.
The difference is that there is no liquid path inside the pump casing, but an external pipe is attached to return the liquid to the mechanical seal.
Install a plug at the pump discharge port, take the dedicated pipe outside, and connect it to the cooling port of the mechanical seal.
At the factory I belong to, the difference is whether or not there is a possibility that the dedicated piping will be clogged .
If there is a small chance of blockage , use Plan11.
If the possibility of blockage is high , there is also the possibility of using a different plan.
In this case, the factory to which I belong takes countermeasures such as attaching a strainer to the suction pipe instead of trying to do something about it on the pump side.
Although the liquid is almost clean, the possibility of clogging is still not zero, so we use Plan 11 to increase the degree of safety.
That’s my impression.
With this method, the external piping can be connected to a cyclone or heat exchanger, expanding the applicability .
There is almost no need for batch-type chemical plants, but…
Since the pipe can be removed, it has the advantage of being easy to unblock.
Plan32 External Flushing
External flushing is a method of cooling by injecting external liquid into the mechanical seal.
This method is used for slurry transfer pumps, etc.
The liquid path of Plan01 and 11 has a very small diameter and is easily clogged.
Even if you try to use Plan 01 or 11 for slurry transfer, the flushing line will be clogged, and you cannot attach a strainer to the piping.
In order to cool the sliding heat generated by the mechanical seal, the slurry liquid in the process is insufficient and an external normal cooling liquid is required.
This is Plan32’s external flushing.
There are several drawbacks and caveats.
construction costs
The obvious disadvantage of Plan32 is the cost.
Since you have to pull the piping, construction costs will occur.
When considering energy-saving operation, it is necessary to have a mechanism such as injecting liquid only while the pump is in operation, which incurs costs such as automatic valves and sequence control.
If you want to raise the cost, you can raise it as much as you want.
Injection type
In Plan32, you have to consider the type of injection.
- water
- co-liquid
The structure of water is simple, but the structure of chemicals becomes complicated.
cooling with water
Consider using water as the external injection fluid .
See the diagram below.
Whether water can be used depends on the properties of the process fluid .
- Aqueous process liquid
- Although it is a solvent system, it has little effect even if some water is included.
- drainage
It is necessary to think from the natural point of view that there is no problem even if water is mixed in the process liquid .
Since it affects various conditions such as reaction and crystallization, it cannot be considered simply.
It is important to have an understanding with the process engineer.
Since it is a mechanical seal, it should be considered on the premise that water will mix with the process liquid when cooling with water .
Let’s not wishful thinking that water won’t get mixed in unless the mechanism leaks .
In chemical plants, it is almost impossible to use centrifugal pumps for process liquids to prevent leakage, and they will be limited to slurry liquid transfer.
That’s why I don’t like water contamination.
As a type of mechanical seal. Coolant is often used for flushing or quenching purposes in single mechanical seals.
You can also run the cooling water directly into the pump.
Double mechanical seals can be used, but are less effective if water is allowed to escape into the process.
Cooling with co-liquid
We will introduce the case of using co-liquid for external injection.
Co-liquid refers to the process solvent.
process fluid = process solvent + reactants
Considering this simple composition, a clean process solvent is called a co-liquid.
The idea is to use a solvent that seems to have no effect even if mixed with water.
The flow in this case is as follows.
The method of directly flowing the process solvent into the pump, such as self-flushing, is not normally used.
Normally, a double mechanical seal is used and a co-liquid is used for the intermediate cooling liquid.
We want to minimize the consumption of solvents so as not to waste them .
The co-liquid is used as a buffer solution for the double mechanical seal, and after receiving the sliding heat of the pump, it is cooled by an external cooling heat exchanger .
Water is ultimately used as a cooling source for sliding heat, but there is a decisive difference in whether or not water enters the pump .
wasting energy
The biggest problem with external water injection is wasted energy.
- water consumption
- Electrical for control monitoring
- Cooling of sliding heat
These elements are extra.
Water consumption is easy to understand.
In the case of cooling with water, it is common to drip water all the time . Same idea as the ground seal.
This amount of water really does make a difference.
Even if the water is constantly flowing at 5L/min, it will be 7200L in one day .
That’s an unimaginable amount for a family…
Automation can be considered to reduce water consumption .
- Open the automatic cooling water valve before turning on the pump , and after a certain period of time, the pump will turn on.
- Close the automatic cooling water valve after a certain amount of time has passed after the pump is turned off .
The process will be like this.
Naturally, energy related to control is required.
If the cooling water is constantly released into the natural environment , thermal energy is unnecessary, but if circulating water is used as the cooling water, energy is required to cool the circulating water.
You may need an extra pump.
reference
There are surprisingly many mechanical engineers who want to study mechanical seals as a specialty.
However, as a chemical plant, it is only one element of the centrifugal pump, so it would be better to study the centrifugal pump first.
I recommend the following books.
lastly
I explained the flushing of mechanical seals within the scope of use in batch chemical plants.
The three basics are internal, self, and external.
Unless there is a serious reason, the self is normal, and the external is only considered for slurries and the like.
Please feel free to post your worries, questions, and questions about the design, maintenance, and operation of chemical plants in the comments section. (Comments are at the bottom of this article.)
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