Automatic valve and control valve tend to be the exclusive domain of instrumentation engineers.
In highly automated chemical plants, automatic valves and regulating valves affect the operation.
Unlike manual valves, it is highly specialized equipment, so it ranks among the top points of equipment-related concerns for the manufacturing department, which is the user.
Mechanical engineers also deal with manual valves and are involved in the creation of P&IDs.
I’m not interested in automatic valves and regulating valves.
If you have an attitude like this, you will not be able to consult with those around you as the years go by, and the unfortunate result that you will not be able to answer even if you do ask for advice is waiting for you.
If you know this much about automatic valves and regulating valves, you will pass as a mechanical engineer! Summarized only to the extent that
Types of automatic valves and control valves
There are various types of automatic valves and regulating valves.
If you are a mechanical engineer, it is quick to understand based on manual valves.
Keep the following five in mind.
- ball valve
- butterfly valve
- gate valve
- globe valve
- sanders valve
ball valve
Ball valves are also commonly used as manual valves.
Most commonly used in chemical plants.
The same is true for automatic valves.
Process liquid transfer, jacket utility switching…
Use it almost anywhere you want to shut off .
butterfly valve
When automating butterfly valves, there are many powder lines .
In the gas line, the valve opening of the butterfly valve is almost unchanged once it is fixed.
On the other hand, the powder line involves opening and closing.
A butterfly valve with a small face-to-face distance is preferred for powder lines in order to minimize the piping distance.
There are many large-diameter lines to avoid powder clogging, and it is difficult to manually open and close large-diameter lines.
Therefore, it is often automated.
Due to the structure of the butterfly valve, there are parts where powder remains, and the sealing performance of the valve body is also questionable.
The current situation is that there is no other good valve.
gate valve
It is used less frequently in batch chemical plants.
Since gate valves are used for large-diameter lines , opening and closing work is difficult.
There are many parts that are touched only a few times a year.
As long as it is used for utility large-diameter lines, it is only used for starting and stopping utilities.
It is used when starting up and stopping the plant.
globe valve
Regulating valves are easier to understand than globe valves .
As long as it is a control valve, it is used frequently.
sanders valve
It is a valve that is frequently used in batch chemical plants.
It has a similar structure to a globe valve and is used as a control valve .
valve seat
Valve seat problems always occur in regulating valves.
If you think that a regulating valve is the same as a manual globe valve, you will get hurt.
There are many variations of control valves.
I tried to draw a simple outline around the valve seat.
The mechanism of this control valve can be expressed as follows.
The valve body moves up and down by the force of the air, and the flow rate is adjusted by adjusting the valve opening, which is the gap between the valve seat and the valve body.
From the machine shop’s point of view, it is OK if you understand it as a mechanism similar to a globe valve.
Single-seat and Double-seat
Control valves can be divided into two types, single-seat and double-seat, according to the structure of the valve seat.
A single seat has one valve seat, and a double seat has two valve seats.
The difference between single-seat and double-seat is mainly as follows.
Proper use of single-seat and double-seat
- Differential pressure is not a problem for single seats , and we want to reduce leakage.
- Double seat is concerned about differential pressure , but leakage is not a problem.
Differential pressure depends on the flow area of the valve seat
What does the valve seat of the regulating valve depend on?
Depends on flow area.
The flow area is determined by (the distance between the valve seat and valve stem) x (the diameter of the valve seat) .
None of these are fundamentally changeable once the valve size is determined.
The only factor that can vary greatly is the number of valve seats .
Single-seat valves have a small number of valve seats and a small flow area, resulting in a large differential pressure .
Leakage depends on the contact area of the valve seat
Regulating valves can leak.
This leaks when the contact area of the valve seat is large.
It is natural that the single-seat leaks less and the double-seat leaks more .
number of ports
As for the number of ports, the two patterns shown in the figure below are normal.
single port
double port
Single port and double port .
This is a 1:1 relationship between single-seat and double-seat.
When viewed from the valve stem side, there is a difference in driving force between single port and double port.
Singles requiredrive force , doubles less .
This is because the single has a higher pressure loss at the valve opening.
Unless the supply air pressure is extremely increased, the only way to increase the driving force is to change the size of the air pressure receiving part of the air cylinder.
There are not only air cylinders but also diaphragm systems, but the basic idea is the same.
port size
Port size generally indicates the size of the opening of the valve seat.
For example, a 50A valve refers to the size of the connection between the valve and the pipe, generally the flange size.
When we talk about a 50A valve, we tend to assume that everything, including the contents of the valve, is equivalent to 50A.
Such a valve is called a full port size valve.
Valves other than full ports are called reduced ports .
The port inside the valve is smaller than the flange size .
- Full-port valves…ball valves, gate valves
- Reduced port valves …Butterfly valves, globe valves, Saunders valves
A ball valve seems to be fixed with a full port instinctively , but surprisingly it cannot be said so.
It will be easier to understand if you imagine a plug valve.
A ball valve is a full port , whereas a plug valve is a reduced port .
The gate valve is a typical full port .
A butterfly valve is equivalent to a reduced port because the valve stem is fixed inside the valve.
Globe valves and Saunders valves are also equivalent to reduced ports in terms of valve structure .
Piping design may also require attention when using reduced port valves.
Actuation (DA and RA)
There are two types of valve actuation : DA and RA .
It is complicated because DA/RA is set for each valve and actuator.
DAs
DA stands for direct action.
This is an operation in which MV works positively when deviation SV-PV is negative.
PV is the current value, SV is the set value, and MV is the adjusted value. You shouldn’t be too conscious of it.
Minus and plus? The person who wondered is amazing.
Let’s look at a concrete example.
This is putting liquid in the tank,
If the indicated value of the liquid level gauge is higher than the set value, the valve opening is increased.
It works like this.
If SV-PV is negative, MV is positive
That’s what it means.
The fact that SV-PV is negative is academic.
I interpret it this way.
If the current value PV exceeds the set value SV, the direction in which the valve opens
The content is the same, but the expression is slightly from the site.
RA
RA is the opposite of DA.
If SV-PV is negative, MV is negative
A classic example of this would be temperature regulation.
- Tank temperature is high → Throttle the steam valve → Decrease the amount of heat
- The temperature inside the tank is low → Open the steam valve → Increase the heating amount
The same is true for the heating adjustment side and the cooling adjustment side.
This type of operation is more common in batch chemical plants.
DA/RA at 2 sites
DA is direct action and RA is reverse action.
This DA/RA applies to the actuator and valve respectively.
Let’s take a quick look at each mechanism.
Drive part
DA/RA in the drive is defined as follows.
When air pressure is applied to the actuator of the DA , the valve moves in the closing direction.
RA is the opposite.
This is determined by the air inlet to the drive and the shape of the spring, as shown in the diagram above.
Simple in mechanism.
Although we call it a valve here, it actually refers to the valve stem. ”
valve
DA/RA in valves is defined as follows.
DA works in the direction of closing the valve by lowering the valve stem .
RA is the opposite.
This is determined by the shape of the valve casing.
Whole valve
Combining the DA/RA of the driven valve and the valve, the DA/RA of the entire valve can be divided as follows.
| Drive part | valve | Whole valve | Usage | |
| ① | DAs | DAs | DAs | General DA |
| ② | DAs | RA | RA | Common RA |
| ③ | RA | DAs | RA | Special |
| ④ | RA | RA | DAs | rarely use |
① is a very common valve. This is most common in chemical plants, as direct actuation is common.
② is also common when selecting for reverse action.
The idea when choosing ③ is when there is a commitment to want the valve to be DA.
For example, for gate valves and Saunders valves, the shape of the casing is fixed, and the operation of the valve cannot be RA. ”
The idea is that if you try to realize RA with this valve, you have no choice but to change the drive part to RA .
Drive when air stops
How does the control valve operate when the air stops?
The following three patterns are possible.
- regulating valve opens
- regulating valve closes
- Maintain the status quo for the regulating valve
The fact that the regulating valve opens when the air stops means that the valve closes when the air is supplied.
So the whole valve is DA.
What does maintaining the status quo mean?
This is because the air cylinder is a double-acting type.
We discussed the operation of the drive part on the premise that the air cylinder is single-acting, but double-acting types also exist.
I will introduce the details of this area on another occasion.
Here, we will only introduce that there are two types, the single-acting type and the double-acting type.
Never be conscious of DA/RA
There is no need to worry about DA/RA in terms of driving control.
Only the instrumentation shop is aware of this.
As long as we procure automatic valves and design circuits for the purpose of control,
A mistake in this combination is fatal .
Cv
The CV value is a coefficient used to calculate the capacity of the control valve .
Q = Cv × √ΔP
This stipulates the relationship between the flow rate Q and the differential pressure ΔP across the valve.
CV is determined by the shape and size of the control valve .
Simple pressure loss calculation
If you are a machine shop or a process shop, this area will be your specialty.
Looking at the above formula, it is self-evident that it is a pressure loss calculation .
This is equivalent to performing valve pressure loss calculations in order to select a regulating valve with an appropriate CV value .
In that sense, it can be called valve capacity design or capacity calculation , but CV calculation is easier to convey to instrumentation engineers.
The instrumentation shop doesn’t have the ability to see the whole process, so it’s easier to talk about it if you match their expressions.
Capacity and control
CV calculations are performed for process pressure calculations.
Select a regulating valve that satisfies the process requirements, that is, the pressure loss in the system and allows the specified flow rate to flow.
From the perspective of the whole process, the regulating valve must meet the following two requirements.
- Must be within the pressure loss that can be assigned by the regulating valve
- The valve opening should operate within the most appropriate range for the regulating valve.
There are usually multiple valves that satisfy the specified flow rate and pressure loss .
Which size to choose among them largely depends on the design concept of the company.
It is often designed so that the valve opening is always in the range of 20-80 % or 30-70% .
In general terms, valve size and controllability are related as follows:
- Large valve size , → CV value increases.
- Large CV value → Larger flow rate with the same pressure loss .
- The flow rate that the valve can flow is large → The required flow rate cannot be controlled unless the valve opening is lowered.
Smaller than pipe diameter
When calculating CV, the size of the regulating valve is usually smaller than the size of the pipe diameter.
The instrumentation engineer selects a small regulating valve and requests a reducer, without thinking about the mechanical engineer.
There is actually a meaning in not thinking about anything…
The instrumentation shop really doesn’t give any thought to the plumbing design that the mechanic shop does.
As a common example, let’s take a look at a control valve for highly corrosive chemicals.
Corrosion resistance is also required for piping.
If it is SGP or stainless steel piping, thin piping can be handled to some extent.
However, when it comes to fluorine resin lining and glass lining, the situation changes completely.
Fluororesin lining and glass lining are almost impossible for 15A piping.
Nevertheless,
Instrumentation shop: 15A is required as a result of CV calculation, so please add a reducer.
There are many instrumentation makers who throw the whole thing to the machine shop just saying that.
I decided to go with 15A without even giving room for discussion on whether 20A is bad or not, so leave the rest.
There are many instrumentation shops who only communicate decisions without consulting them.
He didn’t oversee the entire process, so he listened to the site’s requirements and left the rest of the adjustments to the machine shop.
There are really many.
solenoid valve
Solenoid valves are used to turn on and off air piping by transmitting electrical signals from the DCS to the site.
When an electric signal is applied to the solenoid valve, the parts of the solenoid valve move.
This is called excitation/de-energization in technical terms.
You can understand that excitation is ON and de-excitation is OFF.
In the example above, air flows from IN to OUT when the solenoid valve is turned ON .
When air flows into the air cylinder and the cylinder moves, the port of the automatic valve moves and the automatic valve opens and closes.
This is the basic movement.
When the solenoid valve is turned off, the air in the air cylinder is discharged to the outside through the OUT→EXH route.
Push on site! I think there are many opportunities to hear the sound of the air coming out.
This is proof that the automatic valve is working.
Basic 2 ports
A solenoid valve can be thought of as a mechanism that uses an electromagnet to open and close a valve.
There are various types depending on the shape, but first of all, the basic 2 ports.
As shown in this figure, when an electric current is passed through the coil and the electromagnet is turned on , the valve opens .
By opening the valve, flow can flow from the primary side to the secondary side.
When the current in the coil is stopped, the electromagnet is turned off and the valve is closed .
This is the basic.
3 port valve
2 ports have two fluid paths.
3 ports have 3 fluid paths.
The fluid path can be switched by turning the solenoid valve ON and OFF .
In a chemical plant, there are two options to turn air on or off.
5 port valve
A 5-port valve has 5 fluid paths.
It will not function unless air flows in and out of the cylinder as the solenoid valve opens and closes.
For this to work, we achieve that we have 5 ports.
Solenoid valve position
It is normal to think that the solenoid valve can operate in two patterns with ON/OFF position.
However, positioning is possible up to 3 patterns.
It can be done with two electromagnets.
However, two patterns of ON/OFF are enough for chemical plants.
Limit switch
A limit switch is used to transmit the open/close status of the automatic valve to the DCS .
The limit switch itself is a mechanical switch.
The principle is exactly the same as the home switch.
If a mechanical part moves and you press a switch, an electrical signal will be transmitted, so you can send an electrical signal to a distant place.
Explosion-proof construction is a special feature so that it can be used in harsh environments such as chemical plants.
A solenoid valve sends a signal from the DCS to the automatic valve, and the result is returned to the DCS using a limit switch.
This completes the basic loop of the automatic valve.
air cylinder
single acting cylinder
The basic cylinder is a single-acting cylinder as shown in the figure below.
By inflating air and applying air pressure to the pressure receiving part, the shaft moves.
When the air runs out, the force of the spring will return it to its original position.
If the air is not released here, it will not return to its original position due to the force of the spring.
Therefore, a solenoid valve needs a pattern to let air in and a pattern to let out air.
This is possible with a 3-port solenoid valve.
With 2 ports, the pressure will not escape.
A single-acting cylinder means that there is only one air inlet and the movement is only on one side .
It is a device that is very often used in automatic valves in chemical plants.
This is because automatic valves, especially ball valves, are called on-off valves, and they only need to be able to control two patterns: completely open and completely closed .
A valve that can precisely stop or stop the flow of liquid is more reliable than a valve that stops at a halfway open.
If the actuator is DA, the direction in which the valve closes when air is introduced, and vice versa if the actuator is RA.
double acting cylinder
A double-acting cylinder has two air inlets.
Does not contain a spring like a single acting cylinder
When moving the cylinder, the area on the non-inflated side must be deflated.
- Inject air into the primary side → Remove air from the secondary side
- Inject air into the secondary side → Remove air from the primary side
A minimum of 4 ports is required to implement this pattern.
1 air inlet and 1 air vent when open, 1 air inlet and 1 air vent when closed.
This function alone is the same as a single acting cylinder.
The advantage of a double-acting cylinder is that it can be stopped anywhere .
In a double-acting cylinder, if air cannot enter and exit from either the right or left side, the cylinder will be in equilibrium at a certain position.
To achieve this mode, we need a port to keep the air out, for a total of 5 ports.
Parallelism is important
An air cylinder is a machine that reciprocates a rod using the power of air pressure.
It is important that the parallelism of this rod matches the parallelism of the cylinder.
If they are parallel, the rod and cylinder will move smoothly.
If it is not parallel, it will interfere with the cylinder when the rod is at the end.
If the rod is operated repeatedly in this state, the piston that directly receives the air pressure will wear out.
When the piston wears out, the sealing performance between the piston and cylinder is lost.
If the seal is lost, air will leak out even if you try to increase the air pressure.
Positional relationship with automatic valve
Check the positional relationship between the position of the air cylinder and the position of the automatic valve.
There is an air cylinder just above the stem of the automatic valve.
The distance between the action point of the air cylinder and the automatic valve is zero
I can say that.
Deflection of automatic valves is almost never a problem in chemical plants.
In general, the air cylinder is often installed horizontally with the valve stem pointing upwards.
This creates a cylinder deflection problem.
If so, turn the cylinder upside down , that is, turn the stem sideways to solve the problem.
This kind of thinking is sometimes used for large automatic valves.
There are almost no automatic valves over 100A in batch-type chemical plants, so they are rarely talked about.
If the valve stem is turned sideways, the gland seal will be turned sideways, making it easier for the liquid inside the valve to leak.
If you absolutely dislike leakage, give priority to pointing the stem upward.
In this case, we may consider supporting the cylinder so that the air cylinder does not flex .
Precautions in the mechatronics system
Compared to automatic valves, when using air cylinders in mechatronic equipment,
Cylinder and working point are misaligned
That’s what it means.
If the distance from the cylinder to the point of action is long, something like a carriage is required.
Generally not used in batch chemical plants
In order to organize the equipment compactly, the cylinder is installed in the vertical direction, and a gear is used to change the direction .
This is because if the cylinder is long as shown in the figure above, I feel uneasy during long-term operation.
At least not in batch chemical plants.
positioner
A positioner is used as a control valve.
A solenoid valve alone can only switch ON/OFF , and cannot be stopped halfway .
With a positioner, the difference in current value of the electrical signal is detected as displacement and the air pressure is changed accordingly.
In principle, it can be installed not only on regulating valves such as globe valves and Sanders valves, but also on ball valves and butterfly valves.
In fact, in special places, ball valves are sometimes equipped with positioners.
speed controller
When the solenoid valve is turned ON/OFF, the valve opens and closes rapidly.
In cases where this is an issue, add a speed controller.
However, in reality
Simply attach a muffler-like resistance to the outlet EXH of the solenoid valve .
It has a simple structure that only suppresses the air discharge speed in the air cylinder by providing pressure loss.
The advantage of this is that the liquid inside the jacket of the reaction vessel is drained .
The jacket of the reaction vessel is heated with hot water and cooled with cold water.
We may heat using a large quantity of steam, and drain may occur in large quantities.
It is common to use an automatic valve when discharging these liquids.
You don’t want human intervention in the middle of the response sequence several times a day.
When the heating process is completed under certain conditions, it automatically shifts to the next process. For that purpose, the liquid in the jacket is discharged with an automatic valve.
There are many such scenes.
Here, the jacket contains liquid with a certain pressure, and when the automatic valve is suddenly opened, the water is rapidly discharged.
There is a risk of burns if hot water is suddenly discharged and overflows from the piping.
To avoid this, add a speed controller to slowly open the automatic valve.
reference
Related article
Related information
lastly
Information on automatic valves and regulating valves for chemical plants is summarized.
Valve type, valve seat, operation method, CV value, solenoid valve, limit switch, air cylinder, positioner, speed controller
It’s a lot of volume, but even if you’re a machine shop, knowing it as peripheral knowledge will be very useful in practice.
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