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