字幕表 動画を再生する 英語字幕をプリント Welcome to lesson 25 on Flow Control Valves of the course on Industrial Automation. Flow control valves are very important, so after learning the lesson, the student should be able to describe the importance of flow control valves, they are found everywhere in process industries. Learn the structure of major types of flow control valves, learn about the their flow characteristics, because that is very important in designing the applications. And finally, the how to actuate these valves and how to affect their characteristics to achieve a certain characteristic of the process control loop, so these are the topics, that the student is expected to learn from this lesson. So, the first of all, let us have a look at, the importance of flow control, flow control is probably the most important control in a process control application, and as we shall see, during our process control module, that flow control loops, form a part of most type of control loops. For example, they are parts of flow loops, where directly flow has to be controlled, flow is a final objective of control, they are parts of temperature loops, because temperature is generally controlled by controlling, flow of either a coolant or and let us say steam, for heating, this is not stream, this is steam. Of course, for level loops, because by integrating flow only you have level, so all level control is essentially flow control. Similarly, pressure loops, because again pressure control is achieved by using flow control, and composition loop, because compositions of products, are typically dependent on the compositions of the components in a let say a reactor. So, if you want to control the composition of a particular product, flow control is often a very important part of that, control applications. So, we see that, for most types of control applications, flow control is a part, and the element that finally, achieves the control is the flow control part. So, it is importance, cannot be overstated and as we shall, as we need to mention again slight spelling mistake. So, this is a valve flow is actually a function of valve, the pressure drop across the valve and this, and the stem position, as we shall as we perhaps know that by Bernoulli's equation. The flow of a, flow through a, through an orifice, of flow control valve is essential in orifice and it is the dimensions of the orifice, which are varied is proportional to, proportional to a root over of delta P. Delta P is the pressure difference across the valve, and K is proportionality constant, which contains among other things, a what we, what we call as discharge coefficient or C v, so the flow the inflow control valves, it is this K or this discharge coefficient of the valve which is changed, by changing the orifice dimensions, so that is the way, we achieves flow control. Now, so first of all we see, the various kinds of valves and the first kind of valve that we see are globe valves. Globe valves are so before we must understand the various parts, so I am going to hatch it, so this is the, these are the ports, this particular flow control valve, this is inlet port, this is outlet port, this is another component of the body, not this one, not I am sorry, not this one, not this one, this part, this part, this is the body, the fluid in fact, there are, this is a top and bottom guided. Top and bottom guided means the basic valve assembly movement is guided at in the top and at the bottom. And, it is a double seated globe valve, so there are two seats, one seat is here, another seat is here. So actually the fluid enters through this and will go through this, when this valve will rise, when this valve will rise, it will go through this and will flow out, similarly, it will go through this, it will go through this path and go out. So, since there are two seats, it is a double seated globe valve, one of the advantages of double seating, is that the force as you can see, that the fluid when it flows through the valve, it actually exerts a pressure on, this valve mechanism, this is called the stem and these are called the plugs, these are the plugs. So, the plugs actually come and this is the seat, and the plug actually comes and sits over the seat, and seals the, seals the orifice and when the valve opens, this plug goes up, so the fluid flows through the orifice. And, this plug movement is actually realized, by moving the stem, to which the plug is connected, so obviously, there is the fluid, exerts force on the plug and plug sometimes has to work against this force. So, to reduce for double seated valves, although they are not so popular now a days, but double seated valves, one of the biggest advantages of double seated valves is that, since the force, when the liquid is flowing in this direction and the force that the liquid exerts in this direction are opposing each other, so the net force on the stem is actually small. So, therefore, it requires a smaller capacity of the actuator, to make a movement, but still nevertheless these valves are not so popular, because of mainly two reasons. Firstly, that single seated valves are can be realized with a much smaller size number 1, Number 2 is that, because of you know slight mechanical problems, it is very difficult to ensure that, both the plugs actually seal the, seal the orifice at the same time, and therefore, often you have problems of leaking through the valve, that the shut off of the valve is not so tight. So it is for this reason that people, nowadays prefer single seated valves, so this is a single seated valve, you know you this is the plug, this is the plug you can see that. This is the seat on which the plug sits, this is the seat, this is the stem, this is the, these are the bodies, this is the body. So, the fluid actually flows like this, like this, like this, so this is the fluid path, when the valve opens, this is the inlet port, inlet and this is the outlet port. So, this is a top entry, top entry because the valve stem enters from the top, top guided here there is only one guidance, one guiding piece, that is top guided not, not top and bottom guided, single seated, because there is only one seat globe valve. So, these valves are one of the most common types of valves used in the process industry. Next are ball valves, these valves have, the in the previous case, the stem actually moves in a linear fashion up and down, and for these valves the stem actually rotates, so it is a so it requires a rotary actuator, it can be directly coupled to a motor. So, you see that, actually you have a ball, a ball like structure, through which there is a hole, so you can see, the hole, this is the ball, these are ball valves and this is the hole through the, this is the hole through the ball. So, now suppose, so this is the hole suppose, so when the ball is in this position, then you can understand, that this is the inlet port and this is the outlet port. So, when the suppose the fluid is coming like, this is the inlet port and this is the outlet port, so when the hole is aligned with the inlet port and outlet port holes, then the fluid can flow from inlet to outlet. On the other hand if the ball rotates, then the flow is blocked, so it is by rotating the ball, that various amounts of flows can be realized, so this is the basic principle of a ball valve. For example this is a multi-port ball valves, so you can see the ball, this is a cross section, so the ball is you know like this, semi cylindrical ellipsoidal, and these are the holes. So, the in this case, this has this can take care of three ports, so you can see that, in various positions of the ball, if the ball is aligned like this, then liquid can flow from here to here, if it is aligned this way, it can flow from this to this or this to this. So, under the various positions of the ball valve, you can have various kinds of, various ports can be connected to various others. This is a T ported ball valve we can have an angle ported valve, ball valve and things like that, so this is the basic principle of balls valve. This is this picture shows how when a ball valve rotates, then how the flow throttling takes place, so you see, that as it is rotating. So this, the effective area of flow, they gets reduced, so as it rotates slowly the effective area of flow, will get reduced and therefore, the flow will get reduced, so the flow gets throttled. This is another, kinds of ball valve, where the ball is of a certain shape, so it is called a characterized ball valve. So, here you can see that, as again as it rotates this surface, slowly comes and closes the flow, and therefore the flow the flow can be throttled or it can be completely shut off, so these are this is another kind of ball valve called the characterized ball vale. The third kind of valve, actually there are various kinds of valves, we are going to only talk about some major ones, but there are at least ten, fifteen different types of valve, which are, which are used in various kinds of applications in the industry diaphragm valve, pinch valve a sliding gate valve, etcetera, etcetera. So, this is another kind of valve, which is called a butterfly valve, so basic idea is that, this is butterfly valves are used in large pipes, they are also used for the apart from you know, applications in let us say, a liquid applications like water, water flow control etcetera. They are also used in gas applications, like they are used in a, heating ventilation, air conditioning applications of large buildings, where the airflow needs to be controlled. so in such applications butterfly valves are also used. So, basic idea is that, in all valves there has to be an variable obstruction right, so it is this disc, which is the, which creates the obstruction, and there is a shaft or a pin about which, so you can understand, that you can understand that this is a butterfly valve and there is basically a shaft runs across it and this shaft is driven. So, this is valve is actually, stuck to this and if you rotate this actuator, then this valve can be either in this position or in this position. So, if you have pipe here, if you have a pipe here, then if you connect in this position then it is open, if you connect at this, if you put it in this positive then it is closed. So, exactly that is the position, so the these two positions are shown, so this is the open position of the disc, open position and this is the closed position of the disc, both positions are shown closed position. And this is the shaft or pin, which is driven to move the disc, various shapes of discs are used to you know again, to reduce the torque requirement on the shaft or to reduce noise, of these such big discs, when you have a fast flowing fluid can sometimes vibrate and create noise. So, this is the picture which shows that, so look from a side, when the disc is, in this position then the damper or then the damper is perpendicular to flow and the valve is closed. When it is moving in and throttling or controlling the flow and when it is in this position, then when damper is parallel flow, then is completely open. So, there are various kinds of disc, which are used as I said to take care of various factors like torque and noise. Now, we so we have seen three different types of valves, characterized in terms of construction. Now, we shall characterize valves in another way, depending on their flow characteristics, so depending on their flow characteristics, valve can be generally characterized, in into three different classes. One is butterfly valves are typically of equal percentage type, that is why and butterfly was written, so one is this equal percentage,. so another is linear and the third one is quick opening. So, this equal percentage valve is you can see, equal percentage means, that if you have a this is percent lift, percent lift means, the stem if it is lifted by a certain percentage, this the stem is moving, so percent lift or percent stem position it, this it may not be, though it is called lift, it may not be always a lift you know, sometimes it may be a rotation also. Basically means, that percent of the total stem movement, so it says that, if you increase the stem movement by x percent, then y percent of the current flow will it, so the flow will increase by y percent of the current flow. So, if you make x percent change, if you make a delta x, x percent of full scale, so if you make a 20 percent change here, then may be 5 percent of the current flow, which is here will take place. On the other hand, if you make a 20 percent change here, then 5 percent of the current flow, which is here will take place, if you make twenty percent change here, then 5 percent of the current flow which is here will take place. So, you see that for the same 20 percent change, at 20 percent, 40 percent, 60 percent, 80 percent the change in flow is going to gradually increase, giving rise to this characteristics. So, an equal percentage of the current flow will take place, if you make a certain, a certain fixed percentage of lift change, that is the reason, why these valves are called equal percentage. So, you can easily analyze, you can easily understand, that this sort of characteristic exponential kind of characteristic arises, so on the other hand, we have linear, which is obvious, that is for a certain percent of lift change, a certain fixed percentage of the total full scale change, not current flow will take place, so it is a linear it is added by constant. Actually, the linear and the equal percentage are mostly used in process applications, quick opening valves are you know like our bathroom taps are typically quick openings. So, you must have seen that, if you the almost full flow is realized by, a may be, even one turn or one and a half turns of the tap, while if you move it more and more, then not much flow increase takes place. So, these walls there is a quick, increase of flow and then for the rest of the movement there is very little flow. So, it is a kind of opposite of the equal percentage and they are typically used more in you know, on off kind of applications or some certain special kinds of process control applications, but most of the control applications, they use linear and equal percentage parts. Remember one thing, that these characteristics have assumed, that this characteristic are called inherent characteristic and are provided by the manufacturer, inherent characteristics of the valve and are provided by the manufacturer, under conditions that the pressure across the valve is constant. So, they actually maintain the pressure across the valve and then they characterize this curves, so this is important to understand. And, now how are these characteristics realized, they are realized by various profiles of the plug, so in the case of the globe valve here, say we have there are three kinds of, these are three plugs, which realize equal percentage, linear or quick opening characteristics. Now, it turns out one must realize, that if you actually put the valve, into an application and connected up, with you know other components pumps systems pipes etcetera, then the inherent characteristic will not be realized. So, the pressure flow characteristic of the actually the rather the stem lift versus flow characteristic of the valve, which is provided by the manufacturer, which is the inherent characteristic will not be realized, because of the fact, that delta P will not remain constant. So, how does that happen, so you see that, when you are connecting, so this goes to the system wherever, you want to send this flow and we are just you know, arbitrarily assuming that the head of the, that the system takes a particular kind of static head. So, what happens is that during flow there are actually pressure drops, so there is some pressure drop at the inlet of the pump, then the pump rises, the pressure, that is the job of the pump, it creates a pressure head. Then this flows through the pipe, so again there is some friction loss and there is some pressure head. Then there is a drop across the valves, because all, because all valves will have a, you know if it has flow through an orifice, there has to be a delta P, then again there is a drop at along the pipe and then the available pressure at the system is there, so this is the way the pressure drops and actually as we shall see now. That now, as now as we know, that these pressure various pressure drops, vary with flow itself, so for example, the pipe friction pressure loss, will also rise with flow. Similarly, if the pump head, because there are pressure losses inside the pumps, so the pump head available, the pump head that will be generated, will also be lower. Similarly, here we have assumed a static head pressure, it may be constant or in some cases, even this for example, if the fluid is a you know, kind of heat exchanger, then again the heat exchanger is actually nothing but, a intertwined length of pipe. So, basically the pressure head across the system will also increase with flow, so eventually what happens, is that see the pump is the prime mover right. So, the total pump head available is this one, and that must be equal to the sum of the drop in pipes, drop in the valves, plus drop in the system. So, as the drop in the pipe and the drop in the system rises, there is less and less delta P available, across the valve and, so the flow actually reduces. So, the operating point that are established, will always have delta P falling, so the valve differential pressure available, actually falls quite sharply with the flow, so it is not constant. In effect what happens is that, for example, this is the case of an equal percentage valve, at some delta P, so you see that the inherent characteristic is almost like an equal percentage nearly. On the other hand, if you put the valve, that valve into along with a pipe and a pump and a system, then initially, there is a lot of fresh delta P available, because there is hardly any drop, in the flow is low, so there is hardly any drop in the system. So, the pump, so the valve flow with change in lift, because delta P available across the valve is now quite high at this stage, so there is a, for a sudden change in lift, there is a good change in the flow, so the rate remains high. On the other hand here, you see that in this part, for the inherent characteristic the rate of flow change is