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Top control valves interview questions and answers to help you ace your next interview. Learn about types, applications, and common issues.
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0:30
The first asked control val question is
0:32
what is choke flow in control valve. So
0:34
let us look into that. If you have to
0:36
understand this concept, we can say that
0:37
if we have a valve and we keep the
0:39
upstream pressure as 24 bar. So we keep
0:41
a constant upstream pressure and the
0:44
valve opening is kept constant. For
0:45
example, let's say 50 percentage and we
0:48
keep increasing or decreasing the
0:50
downstream pressure like example 20 bar,
0:52
19 bar, 18 bar and we try to plot a
0:55
graph we'll be getting a linear
0:57
characteristics. Eventually it could
0:59
happen that you reach to a point where
1:01
the pressure difference is not allowing
1:03
the flow to increase. And this point is
1:06
called as the delp choked. And this is
1:08
against the ideal characteristics where
1:10
the flow should increase. And this
1:12
phenomenon is called as choke flow. The
1:15
next most asked question in interview is
1:18
what is balanced and an unbalanced
1:21
stream. So let us look into that. Let us
1:23
take this example of an unbalanced
1:25
valve. So here's your cage plug-in seat
1:27
assembly. The flow is going from the
1:28
inlet to the outlet. And here you have
1:31
the force P1 which is acting from the
1:32
fluid. On the other side there is force
1:34
P2 from the actuator. Let us understand
1:37
this amazingly with an example of a
1:39
seessaw. So here imagine you have your
1:42
plug and the seesaw has from one side
1:45
the pressure which is P1 or the force
1:47
which is acting from the fluid. On the
1:49
other side what is there to balance it?
1:51
The answer to it is nothing else except
1:53
the force of the actuator. But let us
1:56
see a balanced design. Now in a balanced
1:58
design one of the most amazing thing is
2:00
the simple hole. What it does is when
2:03
the flow is flowing it will also go to
2:05
the opposite side of it. So here let's
2:07
imagine this is the force P1 acting and
2:09
here this force P2 acting. So if you
2:12
take the same example of seessaw on one
2:14
side is the fluid force acting P1 but on
2:17
the other side approximately the same
2:19
force which is P2 is acting on the trim.
2:22
Now what happens here is actuator has to
2:24
put very little force in order to move
2:26
the plug assembly. The third most asked
2:29
interview question is what are the
2:31
failure modes in control valve. So
2:33
without further delay let's look into
2:34
that. So in order to understand these
2:37
failure modes let us try to dissect each
2:40
and every failure symbol. So if you see
2:43
here for example FC stands for fail
2:46
close. So if the flow of air is stopped
2:49
or there is some electrical issue the
2:51
fail close would be the valve's default
2:53
action. So the valve will get into its
2:56
closed state. Similarly, FO stands for
2:58
fail open. So basically, in any loss of
3:01
air or in terms of any electrical issue,
3:03
the valve is going to go in its open
3:06
state. Similarly, fail lock. So whatever
3:09
is its last position, it will stay
3:11
locked in that position. It will not
3:13
move. Either if it is in the halfway
3:15
position, it will remain in the halfway
3:17
position. This is achieved sometimes by
3:19
a check valve. Now FLDO stands for fail
3:23
last drift open. Remember this is
3:26
different from fail lock. Why? Because
3:29
in fail last position it will just
3:31
remain wherever it was and eventually
3:34
with the help of flow it will drift to
3:37
its open position. Similarly flc stands
3:40
for fail last drift close. Here the
3:42
valve is going to remain again in
3:44
whatever last position when the air
3:46
supply was cut off and then it will
3:48
drift to its closed position eventually.
3:50
the flow will try to push it in such a
3:52
way that it leads the plug into its
3:54
closed position. The most common type of
3:58
failure modes that have been used in
4:00
control wall is either FC or an FO. A
4:02
fail close valve and a fail open valve.
4:05
Remember this information is very
4:07
important while designing a control
4:09
valve. So when you find it on PN ID, you
4:11
can maybe you know mention in the
4:12
remarks column of the index etc. But
4:15
please make sure to capture this
4:17
information because this is very
4:18
important for sizing and for
4:21
understanding a control valve.
4:24
Meet Warren and Elon. Warren has been
4:28
giving a lot of interviews lately in a
4:31
variety of different companies but the
4:33
output of all of them is the same.
4:35
Rejection, rejection, and rejection. On
4:39
the other hand, we have Elon. Elon has
4:41
already cleared the interviews and he's
4:44
got four job offers from various
4:46
multinational companies. So let us try
4:48
to learn what is the secret from him. So
4:50
he says that giving an answer is very
4:54
easy but giving a convincing answer is
4:59
difficult. A lot of interviews the
5:02
interviewer is not interested to get a
5:04
mugged up answer. What they're
5:06
interested is are the fundamentals and
5:09
the concepts clear of the candidate and
5:12
one of the most important topic for an
5:14
interviewer is control valves. So in
5:17
this video we'll try to learn what is
5:19
the logic behind the questions that are
5:21
asked and especially what is the
5:25
definition after that. So you can give a
5:28
combination of the logic plus the
5:30
definition together so that your answer
5:32
is more convincing. The first question
5:35
which is one of the most asked question
5:37
is what is CV in a control valve. So we
5:41
look at the concept first and then the
5:43
definition. So let's get into it. The
5:45
first thing is that whether you have a
5:47
temperature control valve, pressure
5:49
level or flow control valve. Actually
5:52
they're nothing but a simple control
5:54
valve which has only one basic function
5:58
which is to control the flow within the
6:02
valve. Now how does CV help in that is
6:05
in the most simple terms we can say that
6:09
CV is nothing but a tool so that you can
6:13
compare flow capacity from any valve
6:16
throughout the world. But let us dig
6:18
into this concept in such a way that
6:20
we'll remember this concept forever. As
6:23
we had initially discussed imagine that
6:25
for valve A and for valve B we have a
6:29
different flow rate. Valve A has a flow
6:31
rate of 10 gpm and valve B has a flow
6:34
rate of 5 GPM. Here as we had said we
6:37
cannot say that this means that valve A
6:39
has the higher flow capacity. Why?
6:42
Because there could be the case that
6:44
both the valves are made of the exact
6:46
same construction same size but the
6:49
pressure drop across the first valve is
6:51
15 PSI while the pressure drop across
6:54
the second valve is just 1 PSI. So we
6:56
know that as you increase the pressure
6:59
drop across the valve the flow through
7:02
the valve increases. So this flow
7:05
increases just because of pressure drop
7:07
and not because of the valve size. So
7:10
here if we want to compare two valves
7:13
flow capacity we have to keep them under
7:15
the same pressure. So let us take a
7:18
standard of 1 psi as pressure between
7:20
the two valves. again. Now what I do is
7:24
I'll measure the flow between the two
7:26
valves. But here I see the flow between
7:28
the first valve was 10 gpm and the flow
7:31
with the second valve was 4 gpm. Why?
7:35
There could be the case that for the
7:36
first valve the fluid was water and for
7:40
the other valve the fluid was honey. Now
7:42
we all know in such cases that honey is
7:45
very dense. So it will have a lower flow
7:47
rate as compared to water. So even
7:49
though the valves are made of same
7:51
construction, we have to also ensure
7:54
that the liquid between them is the same
7:56
if you want to compare the flow capacity
7:58
between two walls. So let us select
8:01
water because water is one of the most
8:03
available substance and very easy to be
8:06
found at any site or at any vendor
8:08
location. Now let's keep water for both
8:11
the valves. I've kept the same pressure
8:13
drop. I have kept the same liquid.
8:15
Ideally I should get the flow rate to be
8:17
same but for this valve I get 10 gpm as
8:20
the flow rate and for this valve which
8:22
is valve B I'm getting 12 gpm. Now what
8:26
is the issue here? There's another
8:28
parameter which comes into play which is
8:30
the temperature. So both water being the
8:34
same fluid might have different
8:35
temperatures. So first may be at 60°
8:38
Fahrenheit and the other one might be at
8:41
150° F. So we know as the temperature
8:43
increases there's again a difference in
8:45
flow rate. So we will have to maintain a
8:48
constant temperature as well. So we'll
8:51
select 60° F. A lot of people ask that
8:55
in CV definition why 60° Fahrenheit is
8:58
specially taken. The answer is because
9:00
the specific gravity of water is 1 at
9:02
60° Fahrenheit. So this will help
9:05
greatly when we are doing CV
9:07
calculations. So we'll have three
9:09
standard parameters which is the PSI
9:11
drop is 1 psi, the water is the fluid
9:14
which is taken and the temperature 60°
9:16
Fahrenheit. That being the case, we can
9:19
say the definition of CV is as follows.
9:22
CV is the number of US gallons of water
9:26
that can flow through a valve with one
9:29
psi pressure drop at 60° Fahrenheit for
9:34
1 minute. So this is the definition of
9:36
CV. Also, I want to share one more thing
9:38
that I produce a new video every
9:40
Saturday. So, if you want to learn
9:42
something new every Saturday, please
9:44
click on the bell icon and subscribe so
9:47
that you can learn a new video. The most
9:49
asked question, the all-time favorite of
9:52
any interviewer which has been asked in
9:55
the last three decades is what is
9:58
cavitation, flashing and choke flow. So
10:01
use the graph which will be shown now so
10:04
that you can give a more convincing
10:05
answer and that would help in your
10:07
explanation. So imagine this is your
10:10
valve put in a line. Now with the flow
10:12
there is some restriction put. So you're
10:14
going to have a DP or a differential
10:15
pressure created to it. So the upstream
10:18
pressure is P1 and the downstream
10:19
pressure is P2. Now imagine that this is
10:21
your vapor pressure curve. So what
10:23
happens when the fluid is going to be at
10:25
this particular uh region? the fluid is
10:29
going to change from liquid state to
10:31
vapor state and this stage is called as
10:35
the point where the liquid changes to
10:37
vapor phase. Now at the exact opposite
10:40
side if you notice the the vapor is
10:43
going to turn back into liquid state.
10:46
Here what is going to happen is the the
10:48
bubbles are going to burst to come back
10:51
to liquid state which is called as
10:52
popping which has very high velocities
10:56
that can damage the valve and the piping
10:59
downstream. This entire phenomenon is
11:02
called as cavitation.
11:04
Now we look into the next case which is
11:07
when there is flow to the valve but what
11:09
happens is the pressure downstream does
11:11
not recover. This happens when the
11:14
pressure downstream is still below the
11:16
vapor pressure curve. This phenomenon
11:18
here makes the liquid to still stay in
11:21
the vapor pressure phase in the
11:23
downstream. And this phenomenon is
11:24
called as flashing. What happens here is
11:27
imagine that this is your valve and this
11:29
is your pressure drop happening. We are
11:30
very sure with the concept that if we
11:32
increase DP there is going to be an
11:34
increase in flow. But we keep increasing
11:36
DP at a point of time flow will not
11:39
increase. This point is called as choked
11:42
flow. For this third question, now the
11:44
interviewer is looking for thought
11:46
process and usually this question is
11:48
asked which is usually can be used for
11:50
variety of instruments that is material
11:53
selection. So we look into packing
11:55
material selection as an example but you
11:57
can use this concepts of the pressure,
11:59
temperature, chemical compatibility etc
12:02
which is to be used while an engineer
12:04
comes towards selection of the material.
12:06
So let's get into it. The first thing is
12:09
imagine this is water and here's our
12:11
boat placed into it. The first thing is
12:13
you need an engine to run the boat and
12:15
there has to be a rod that has to be
12:18
penetrated through the boat and here
12:20
would be a propeller which would help
12:22
the boat to go forward. Now here's an
12:24
interesting thing. If you look at this
12:26
point, water can enter through this
12:29
location into your boat. And if the
12:32
water enters, you know what's going to
12:33
happen, right? The boat is going to
12:35
sink. So for this concept what will we
12:37
do? We add something called as a
12:39
stuffing box. Now stuffing box as the
12:42
name suggests is basically you're
12:43
stuffing something inside so that the
12:46
water cannot penetrate through this
12:48
barrier and come inside the boat. Sounds
12:51
very simple right? And the first thing
12:53
is the more we stuff the more safe we
12:55
are that the water will not enter inside
12:58
the engine or the boat. But if you see
13:02
here, if you're putting a lot of
13:04
pressure, what is the adverse effect
13:06
that can happen? So if it's very tight,
13:09
then there is no movement that the
13:11
propeller has so much friction that it
13:13
might not be able to move. Okay? So we
13:16
might keep it loose, right? But if we
13:18
keep it very loose, the other issue is
13:20
that the fluid will enter the engine. So
13:23
this is another issue. This same concept
13:26
also applies for a control valve. Here
13:28
let's take the same example of boat and
13:31
now let's put a control valve. So both
13:33
these places require a packing here.
13:36
Same is for control valve. But for a
13:39
boat the concept is still little simple
13:41
because the liquid is finally what?
13:43
Water. Even if little bit enters what's
13:45
the issue? But the control valve has to
13:48
go through a lot of services corrosive
13:50
erosive toxic etc. So it's not that easy
13:54
as it looks like. Okay. Now you would
13:56
ask me let's get to the basics. So for a
13:59
simple control well what are the basic
14:01
packing materials that we use? Usually
14:03
these two materials are one of the most
14:06
used materials. The first one being is
14:08
PTFE and the second one being is
14:10
graphite. These are actual pictures of
14:12
how a PTFE and graphite looks like. So
14:14
if you see the picture one PTA is around
14:17
something you could say whitish
14:19
complexion and graphite is shiny
14:22
blackish in color which is also
14:24
sometimes referred to as flexible
14:26
graphite. Now with respect to chemical
14:29
compatibility PTFE is the most
14:32
compatible with almost majority of the
14:35
services. But then why do we have
14:37
graphite? Because of temperature
14:39
limitation. So PTFE usually as a thumb
14:41
rule you can say suggest up to 200°C
14:45
while graphite can go up to 600°C.
14:49
However, for graphite also you need to
14:51
ensure from the material certificate
14:53
what is the maximum limit. But as a
14:55
thumb rule you can consider this also
14:57
notice that is it compatible with the
15:00
service. Now is it only these two
15:03
factors nothing else is required for
15:04
packing. Let us look at an interesting
15:07
case. Now imagine that you have a valve
15:09
put in a line and there is a site person
15:12
who inspecting the valve. Now the valve
15:14
has slight leakage. Okay, it's very
15:16
slight but theage point is maybe H2S
15:19
which is leaking or maybe some very
15:21
toxic service or lethal service. There
15:23
are certain services which if the
15:25
operator or the plant person just
15:27
inhales for a few seconds they could
15:29
die. It's that grievious. Also if you
15:33
have for example a 100 valves in a plant
15:35
or 500 valves which are continuously
15:38
emitting these toxic gases in the
15:39
environment then even that's very
15:41
harmful right so you have something
15:43
called as fugitive emissions where the
15:45
authorities give you a certain limit
15:47
beyond which your valve should not be
15:49
allowing leakage but how do you ensure
15:52
that you are able to meet such criterias
15:55
engineers have come up with some very
15:57
interesting concept in order to meet
16:00
such stringent criterias. as let's look
16:02
at the first one which is called as live
16:05
load packing. Now this concept we'll try
16:07
to understand with an hypothetical valve
16:09
example. So here's my packing material
16:12
which is put in the valve. Now the valve
16:14
in normal operation will keep on
16:15
operating throttling the valve and the
16:18
flow through it. But if you notice
16:21
eventually the packing is going to get
16:23
worn out and maybe it's not able to
16:25
provide that much pressure. So engineers
16:27
came up with a spring which is usually
16:30
used and that creates a positive
16:32
pressure on the packing material. If you
16:36
want to see an actual real life example,
16:37
see here the white thing is nothing but
16:40
your packing which seems to be PTFE as
16:42
we said it's white in color and you can
16:44
see a spring assembly also. So it kind
16:47
of puts a positive pressure to keep the
16:49
packing in place. Interesting. Right now
16:53
this is not it. Still there could be
16:55
certain issues like the spring has
16:56
failed or the spring is itself the
16:58
tension is reduced. Do we have another
17:00
amazing way? Yes, we do have it. Let's
17:03
look at the next amazing. The next
17:04
amazing way is something called as
17:06
bellow seal. This is a level up even the
17:09
life loading. But how does it work?
17:11
Let's see. Here's your standard packing
17:13
which is available. Now here are your
17:14
bellows and here's something called as a
17:16
leak detection port. We'll look into it
17:18
at the later part of it. But right now
17:20
let's first focus on the bellow. Bellow
17:22
is an uninterrupted tube and if you see
17:25
it has no place for the leaks to be
17:27
developed. Also the bellows are
17:29
extremely flexible. So neither there is
17:33
any leakage chance nor is going to
17:35
create any friction because it's
17:36
completely flexible. The only one issue
17:39
is what if the bellow ruptures. So for
17:42
that case here at the leak detection
17:43
point we can put a pressure transmitter.
17:45
So anytime the bellows fail there's a
17:47
rupture the pressure at this chamber is
17:49
going to increase and we can get an
17:51
alarm but the leakage is greatly reduced
17:54
by bellow seals.
17:57
[Music]