Placeholder Image

字幕表 動画を再生する

  • 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