Utilities related to instrumentation in chemical plants are electricity and instrument air.
Utilities for the entire factory include water and steam , but only electricity and air are included in the instrumentation .
Some instrumentation engineers are interested in air , but think that electricity is something that electrical engineers supply.
Therefore, mechanical engineers and electrical engineers can work even if they are not interested in air .
What is instrument air ? even if asked
It’s beautiful air, isn’t it?
The air is too clean
It’s not uncommon for engineers to have only limited information.
Since that would be a waste, I have compiled information on instrumentation air as reference information.
If you know it, you may think that you are an engineer who can do it.
- air cylinder actuation
- Properties required for instrument air
- Equipment for making instrumentation air
- Air 3-piece set
- Nozzle Flapper (Challenge)
air cylinder actuation
Instrument air is used to actuate pneumatic cylinders .
Air cylinders are driving mechanisms for instrumentation equipment such as automatic valves and regulating valves .
Electricity can also be used to move things, but in chemical plants where explosion -proofing is severely constrained, moving things with air occupies a certain position.
The idea is that air is sufficient for equipment that does not require much power, such as instrumentation equipment .
In addition to air, utilities related to force include electricity and oil. For your reference.
Properties required for instrument air
The mechanism of the pneumatic cylinder is to inject compressed air into a sealed container and move the cylinder with the force of the air.
Any foreign matter or debris in this air can clog the cylinder and jam it.
In the unlikely event that the automatic valve stops operating, there is a possibility of a runaway explosion in the process, and extremely high reliability operation is required.
Therefore, the air also needs clean air .
In order to have clean air, we have to remove garbage.
I usually use filters.
Filters that remove dust particles larger than several μm are common.
Filters tend to be installed appropriately, but they are surprisingly deep.
If the air contains a large amount of moisture, various malfunctions will occur in the cylinder.
- Moisture causes the iron in the parts to rust, leading to blockages.
- Water collects and blocks air movement.
There are two main problems that arise.
As the problem grows, you will get stuck in the cylinder either way.
By the way, there is a word called dew point .
The dew point is the temperature at which water begins to condense from moist air.
Instrumentation air should have a dew point that is a few degrees below the minimum ambient temperature of the installation site.
In Kyushu, the lowest temperature is around 0°C, so should the dew point be set at around -4°C?
In order to set the dew point to -4°C at an air pressure of 0.7 MPa, it seems that the dew point under atmospheric pressure is -28.5°C. It can be calculated using the dew point conversion table.
From this, the actual moisture content in the instrument air can be calculated to be approximately 0.4g/m3.
This is dry air with a relative humidity of about 3% when considered as air at atmospheric pressure and 10°C.
Oil may also be considered a foreign object in instrument air.
However, it is useless even if there is no oil completely.
If anything, it feels like it’s no good if there’s too much oil.
The oil mentioned here is hydraulic oil.
Hydraulic oil is better to have in order to move the cylinder.
Now, the performance of the cylinder has improved, and hydraulic oil may not be required…
It is better to use as little organic solvent as oil as possible.
It is better to use air from a clean place where organic solvents are not retained as the source air for instrumentation .
Design elements such as the location of instrumentation air generation equipment and piping openings are considered.
Equipment for making instrumentation air
Let’s look at an example of a flow that produces instrument air.
As usual, the textbook ends with this description.
Almost no pattern describes the flow in words.
So let’s verbalize it!
① Suction filter
The raw material for making compressed air is atmospheric air.
It is preferable to use clean air that contains as little dirt, dust, organic solvents, etc. as possible.
First there is the filter to take the dust out of that air.
Physical filters come first and foremost in process equipment.
Compress the air from which dust has been roughly removed in ① with a compressor .
This increases the pressure to a point slightly higher than the specified pressure.
When the air is compressed by the compressor in ②, the temperature of the air rises rapidly.
This process is called adiabatic compression.
As a general rule, the following is fine.
Energy is added to the air using the power of a compressor to change atmospheric pressure air into a state of high pressure air .
Then the energy of the air increases and the temperature rises .
Hot air cannot be used in process equipment and must be cooled.
Use a cooler there.
④ Drain separator
Cooling in ③ generates drainage.
The warmer the air, the easier it is to hold moisture.
The amount of saturated water vapor is proportional to temperature .
Moisture contained in high-pressure air condenses as water as the temperature of the air drops.
If water enters the pneumatic cylinder as instrument air, it will cause malfunction, so it must be removed.
Removing the drain in (4) is a method of simply removing the liquid from the state where gas and liquid are mixed.
This will remove most of the liquid that should be removed.
However, some liquid remains in the air as saturation .
If the temperature drops even a little, drain will be generated again.
For example, let’s say you made compressed air in the daytime and didn’t use it much at night.
Condensate is generated again from the compressed air whose temperature has dropped due to the influence of the outside air.
Of course, if you can supercool in ③, the problem will not occur.
If cooling tower circulating water is used as cooling water, subcooling will not be reached because the circulating water is dependent on the outside air temperature.
That is why dehumidifiers are sometimes used to remove moisture to a level slightly below the saturated water vapor level .
In the ② compressor and ⑤ dehumidifier, the air passes through the device.
Any dirt or dust inside this equipment will flow into the process instrumentation.
Therefore, at this stage, it is passed through a filter as the final dust removal.
Even if the filter is clogged, it is normal to install it in parallel so that the operation will not be hindered.
⑦ Capacity tank
Also called a buffer tank .
Store compressed air here.
Even if the compressor breaks down or a power outage occurs and it is no longer possible to generate compressed air,
The idea is to use the air in the capacity tank to do the minimum possible treatment.
Air 3-piece set
Cleanliness is required for instrumentation air.
Since we rarely see the air, it is difficult to judge whether it is clean or dirty.
But the air you’re breathing right now could be dirty, too!
In order to stably carry out the chemical reaction, keep the instrumentation air clean.
Although the instrumentation air is already clean at the stage of generating it, there are cases where additional equipment is added just before the handling equipment to further ensure cleanliness .
That is the air 3-piece set.
The air 3-piece set consists of a filter, regulator, and lubricator.
A filter literally removes foreign matter.
In general, you would assume solids such as dirt and dust in the air.
However, in the air 3-piece set filter, liquid systems are also assumed.
This filter also removes common moisture and oil.
The pot that removes and stores water and oil is much larger than the filter itself.
In other words, it would be more correct to call it a filter and drain pot .
Since this filter comes into direct contact with the air in the area where it is used, the frequency of filter replacement depends on the cleanliness of the air in the area where it is used.
So, let’s make the air intake port of the air 3-piece set a place where clean air can be taken in as much as possible.
A regulator is a device for reducing pressure to a specified pressure.
A pressure reducing valve would be easier for mechanics to understand.
For example, 0.7 MPa air is reduced to 0.4 MPa with a regulator.
This depends on the requirements of the pneumatic cylinder of the automatic valve in which it is used.
There is a pressure required for stable operation.
There is a regulator to make that happen.
Compressed air is generally produced by compressing it with a compressor.
The air supplied by the compressor is used in each pneumatic cylinder of each automatic valve in each plant.
Pneumatic cylinders are very numerous.
I don’t know when it will work.
Not all automatic valves will operate suddenly, but a certain amount of automatic valves will operate.
If too much air is consumed there, the supply air pressure will drop and other automatic valves may not operate.
Therefore, the compressor is compressed higher to give a margin, or an air holder is provided.
A regulator is attached to decompress high-pressure compressed air.
A lubricator is a device that disperses oil.
The dry air on the primary side is literally sprayed with oil.
It looks like the image below.
The oil is pushed up by the pressure of the air, and the oil is stirred and mixed by the power of the air while dropping the oil under its own weight.
In many cases, a check valve is attached to form the flow of oil and air.
In any case, it is a device that sprays by itself with air .
The oil is turbine oil and generally about ISO VG32 .
The oil film is on the order of several μm.
It can be said that the management of the amount of oil and the management of the check valve are almost everything .
By adding oil to the dry air, the purpose is to smooth the operation of the pneumatic cylinder.
In order to spray the oil, you need an oil pot, as not all of the spray will dissolve in the air.
However, in recent years, there are many patterns where this oil pot is unnecessary.
Is it because the air cylinder side has high performance?
Therefore, the air 3-piece set is sometimes called the air 2-piece set.
Nozzle Flapper (Challenge)
I will introduce the nozzle flapper as knowledge related to air.
This is a story of the old days when it was controlled only by air, but now that electric control is possible, it is a story that you do not need to know as knowledge.
Since it is classical knowledge, it is a world that can be understood if you know the basics of physics,
It’s more geared towards mechanical engineers than instrumentation engineers.
Instrumentation engineers in the olden days are derived from mechanical engineers, so I listed it to make it easier for mechanical engineers to be interested in instrumentation if they follow the same path.
It’s completely superfluous, so feel free to skip it.
It’s a little difficult, so it’s a challenging content.
The nozzle flapper is the following device.
Let me explain this principle.
Convert flapper-to-nozzle spacing X to back pressure P
This is the most important principle, but textbooks neglect it.
The flapper is the detector, for example, the liquid level information itself.
The flapper transforms left and right.
Moving the flapper to the left increases the interval X, moving it to the right decreases it.
If this X changes, P will change.
If the P signal is propagated to the outside, the air transmitter is completed.
The principle is simple.
If X=0 then P=P0
First, let’s consider X=0, that is, the flapper is in contact with the nozzle.
This is with the nozzle capped.
Air is supplied as P 0 , but since the nozzle is covered, the part that detects the back pressure P is also the pressure of P 0 .
This is the first.
If X = ∞, reduce pressure by pressure loss of throttle B
Conversely, let’s consider X = ∞, that is, the state without a flapper.
In this case, air with a pressure of P 0 is released to the outside after being decompressed by the amount of pressure loss from a throttle B with a diameter of db and a nozzle with a diameter of da.
$$ P = P_0 – ΔP_b $$
The back pressure P detects the pressure that has decreased by the pressure reduction at the throttle R.
Since the design is made with db < da, the pressure loss at the nozzle is assumed to be zero.
Given X=0 and X=∞, the rest is just connecting them.
If you think about it simply, you might think that it will be a linear relationship.
This is not really linear, the pressure drops as soon as the flapper opens.
This is due to the small contact area between the flapper and the nozzle.
Since the contact area is small, the pressure loss is high and the back pressure P is high in the area where X ≒ 0, where the flapper is close to the nozzle.
However, because the contact area is so small, the pressure loss drops dramatically even if the flapper comes off just a little.
A possible solution would be to change the shape of the nozzle.
It can be expected that the performance will change when the nozzle outer diameter is changed.
force balance formula
The force balance equation is illustrated in the figure below.
There are several components, but the two most important are:
- Forcebar fixed point
- Nozzle flapper position
Let’s look at the working principle of the force balance formula.
- Process information (liquid level, pressure, flow rate, etc.) is added as input
- When pressure is received at the input part, the left side of the fixed point of the bar is lowered.
- The right side rises by being caught from the fixed point of the bar
- Smaller nozzle flapper gap
- Nozzle flapper back pressure increases
- High pressure air is generated at the output and propagates to the outside.
I interpret it as a force balance formula because the force bar in the flapper is always in a state of equilibrium due to the spring .
Displacement equilibrium formula
The displacement equilibrium formula can be explained as shown in the figure below.
The position of the flapper is fixed.
The flapper back pressure is fixed.
The bar drops when pressure is applied to the input.
The bar on the output side is pulled up and narrows the output pressure tank.
The result is higher output pressure.
Since the displacement of the flapper is fixed, I interpret it as a displacement equilibrium formula .
Knowledge about pneumatic pressure is to the extent that a machine shop should know to some extent, and you will probably ask an instrumentation shop.
It is enough to be able to imagine the flow and control method from the circuit diagram, so if even a machine shop knows it, the range of knowledge will expand and it will be possible to apply it.
Information on instrument air for chemical plants is summarized.
Instrumentation air is used for air drive. Air that is clean and free of moisture and foreign matter is required,
Compressors, coolers, separators, etc. are used to create instrumentation air.
Add oil according to the place of use.
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.)
*We will read all the comments and reply seriously.