Choice of control valve trims

1. Control Valve Selection
Control Valves provide the final element in controlling a process plant. They have a
very large influence on how the plant responds to changes and upon the overall
operability and efficiency of the operation.
No matter how accurate or modern the DCS and field instruments may be, unless the
control valve is efficient enough to match the requirements of the control system and
of the correct design to handle the process condition in an effective way, the overall
plant operation will not be as efficient as it could be. Money will be wasted. Time will
be expended. Operability will be compromised.
Correct selection of control valves is often overlooked during plant modernisation
programmes and on new plant builds, decisions are often made based on initial capital
cost with little or no long term operability and efficiency thoughts. It is not
uncommon on new project for valves to need replacement after a short period of time
due to incorrect control valve choice initially.
At this point, I would like to distinguish between “conventional” control valves and
those deemed to be “Engineered valves.
Engineered Valves are usually required on difficult or critical duties Although
conventional valves can also be specified incorrectly, most problems occur on
applications where engineered valves are required.. These errors are usually due to
the use of a conventional valve when an engineered solution is required, or the use of
the incorrect engineered valve. In most cases, errors develop because of a basic lack
of understanding of the process requirements and operational modes. Some of these
will be discussed later in this article.
I will now go on to discuss some of the more common valve related problems and the
mistakes that are commonly made. It is fair to say at this stage that the larger the
selection of valve trims and body combinations a valve vendor can offer, the more
likely it is that he can supply the correct solution to any problem. With a limited range
of trims etc, a manufacturers options are more limited, leading to compromise. No one
trim technology can provide the best solution for liquid applications, high pressure
gas letdown applications and erosive service applications for example. However, a
single solution is sometimes suggested for all three by some vendors, usually due to a
limitation in their range of available trims. There are better, individual solutions
available, tailored to the specific problem.
CAVITATION
The simple words contained in a specification “Cavitation must be eliminated
wherever possible” seems at first glance to be a very sensible statement. However,
upon close examination, we find it can result in a much higher than needed
expenditure on control valves AND in many cases the purchase of a valve which does
not work particularly well. To explain…
There are three main approaches to applications where cavitation may be a problem…

2. 1 Use of harder materials to lengthen the period for which a valve will work
before cavitation damage begins to cause problems. Brute force approach.
2 Use of designs which allow small amounts of cavitation to occur but control
the location of the cavitation such that it does no damage. I.e. the bubbles
formed by cavitation collapse (and this is where the damage is done) in the
middle of the fluid stream and well away from any trim or body parts. Hence
no damage occurs. Single Drilled hole cage (DHC) valves can be very
successfully be used for smaller amounts of cavitation whilst for moderate
amounts, double DHC cage designs can be used. There are literally thousand
so such designs from many suppliers working well in the field over many
years.
3 Cavitation elimination. This approach invariably calls for multi-stage valves.
This approach is required where higher levels of cavitation can occur. These
designs are more expensive than DHC valves and if used for lower amounts if
cavitation provides no additional performance but will increase valve costs.
If labyrinth type valves are used on cavitation control services, there is a chance of
blockages occurring if the fluid is dirty (grit, sand, impurities, weld slag, chemical
coagulation etc). If a multistage design is required, it is far better to use a design
which has no small holes (as found n labyrinth designs). Designs are available (such
as Masoneilan LincolnLog) which are extremely trash tolerant and have no small
holes which may block.
Valve manufacturers will (or should) provide advice as to which is the most cost
effective trim for all applications. Again, the wider the range of trims available, the
more likely the correct solution will be found.
SEAT POSITION
On cavitation service it is important that the seat in the valve is placed whenever
possible in the HIGH pressure area of the valve. This is because a seat placed in the
Low pressure area (i.e. at the last pressure reducing stage) is much nearer to the point
on the pressure profile where cavitation damage will occur. A seat placed at the HP
area of the valve will not suffer cavitation damage.
NOISE
Noise is measured in DbA where the A stands for Acoustic, i.e. noise that can be
heard by the human ear. All manufacturers use a principle of moving the noise out of
the audible spectrum to reduce the noise measurement value. This is achieved by
splitting the flow in the valve into a number of small volume jets at a higher
frequency than fewer larger jets. It is important to keep these high frequency jets
separate and not allow them to combine. To achieve this, holes in low noise designs
must be accurately spaced to avoid recombination of the HF jets into a larger LF jet.
Similarly labyrinth designs which are excellent for noise control, must be designed
such that the jets of gas exiting the labyrinth stack maintain separation. Different
manufacturer’s designs vary considerably in this respect and therefore exhibit very
different noise reduction capabilities.
Beware of over specifying. Whilst labyrinth designs are excellent for noise control,
they are not always required and are expensive. Fig 5 shows the relevant noise
reduction capbilitesof three valves. The purple line shows a 2 stage DHC valve. Blue

3. indicates an 8 stage labyrinth design whilst the red line indicates a 16 stage labyrinth
design. The interesting portion of the graph is marked A-A. In this region, the 2
stage DHC valve is actually providing a better noise reduction than an 8 stage
labyrinth valve (at a much lower cost!) It can be seen that at higher pressure drop
ratios, the effectiveness of the DHC valve declines whilst the 8 stage labyrinth valve
becomes more effective. Hence, at higher pressure drops, labyrinth valves are
required but not at lower pressure drops. It is surprising how many times we are
requested to supply labyrinth valves when a large cost saving could be made
(especially in larger sized valves) by using DHC trims as the pressure drops do not
warrant a labyrinth design. Consult us if you have any queries on this type of
application!
EROSIVE SERVICE
All erosive services are different whether they have sand content, catalyst being
carried around in the fluid or any other medium. We have dealt with sand, catalyst,
even metal particles in fluid. In many cases we have developed valves for particular
processes and are specified by licensors. Each application, whether it be a choke
valve or a process valve requires a different design of valve. There are two
approaches to erosive service; either use hard materials to avoid erosion, or reduce the
fluid velocity to reduce the erosive effect. In practice most erosive service valves use
a combination of these two approaches. Masoneilan has a large range of erosive
service valves using metallic trim, Tungsten Carbide, Ceramics etc. Valves are
developed specifically for such difficult applications as coal slurry, Hydrocracking
applications with high catalyst content etc. Again, I stress it is important to contact
the manufacturer at an early stage such that we can give advice as to valve type….
We have seen most problems before! On a erosive service, blockages can usually be
of concern…avoid small hole designs at all costs.
EMMISSIONS
As producing fields become more difficult, higher pressure, larger and more sour,
emissions from control valves become more important. Fields with percentage H2S
content up into double figures are being planned. It is obviously important to keep
such dangerous components inside the valves and not allow them to leak out through
the stem glanding. A number of methods can be utilised to achieve this but it is
important to realise that the design of a low emission valve is not just a case of
redesigning the packing…it is a total valve design since stems may get to be a larger
diameter (to avoid stem vibration and flow induced forces), hence friction increases,
hence actuators change in size etc. Usually, to reduce friction, low emission designs
have a highly polished stem. A common problem occurs when such valves are
overhauled by someone other than the manufacturer who does not appreciate the need
for the extremely high quality of finish. If a stem is replaced with a “standard” item,
the valve will never again achieve the required standard of low emission and will
suffer sticking problems etc.
HIGH SPEED ACTUATION
Especially important on antisurge valves which are critical in protecting multimillion
dollar compressors. It is now possible to achieve stroking speeds of 1 second
comfortably using pneumatic controls, even on relatively large valves. Advances in
instrumentation have made this possible. Masoneilan for example is unique in that it
has a specially developed programme which can accurately model the behaviour of

4. any valve system before it is built. This includes modelling the performance of
boosters, dump valves, positioners, even the size of tubing and the ambient air
temperature. This enable accurate graphs to be produced showing not only fast
opening, but response to a step change showing minimal overshoot etc, often a factor
ignored in specifications for fast acting valves. This accurate prediction technique is
of use to compressor manufacturers and also compressor control system vendors.
Control systems produced by this method are also MUCH easier to tune in site. On
the subject of positioners, the “dead time” or “thinking time” of the SMART
positioner used is very important. Thee “dead times” vary considerably between
manufacturers with the fastest acting being the SVI IIAP from Masoneilan being
specially developed for such fast stroking systems. 200 ms additional lag does not
sound a long time, but it is 20% of a required 1 second stroking speed!.
To obtain the correct solution to a valve problem, or to select the correct valve for a
particular application, I cannot stress enough how important it is to ask a specialist
valve vendor, such as Masoneilan, for advice in the very early stages. We are often
asked too late in the design stage when piping has been fixed and necessitates a globe
valve for example when an angle valve would be the bet solution. Pipe sizes are often
fixed before the valve is given any thought, a situation which either leads to
compromise or expensive changes. Incorrect valve selection can be extremely
expensive as the following example shows.
Underground Gas Storage applications.
Due to the increasing demand for gas, supplies in winter time are often too low to
meet demand whilst the opposite is true in summer. Hence large underground gas
storage systems, using old salt domes or depleted systems are used to store excess gas
in summer; this gas is then brought back and fed into a grid during the peak demand
winter months.
This gas usually has hydrate formation problems when it is brought back above
ground. These problems can usually be eliminated completely without the need for
gas heating or treating if the valves are correctly chosen as the following actual
example shows.
Example
A large US gas storage system was installed with approx 62 injection and producing
systems. The installation was as shown in Figs 1 & 2. Pressure drop on production
was taken over two single stage valves to cope with the pressure drop. Separate valves
were used for injection. To allow this system to work correctly, a gas heating plant
was required to heat the gas to prevent hydrate formation in the valves on the
production system. This system was very expensive to install and ongoing operation
costs were expensive in terms of fuel gas and maintenance. After a short time in
operation, Masoneilan were asked to look at the system to try and provide a better
solution.
The result which was implemented on all 60+ wells (as well as new future
installations) is shown in Fig 3.

5. The two production valves were replaced by a single VLOG labyrinth design valve.
By the use of the correct multistage design, the temperature in the valve was kept
above the temperature at which hydrate forms, WITHOUT the need to heat the gas in
any way. (Fig 4) Hence the gas heating plant and its associated costs were eliminated.
If the correct valves had been chosen at the outset, a large capital cost would have
been eliminated.
As an added bonus, we were able to design the multistage valves to be bi directional
with a large rangeability (to cope with vastly different pressure and flow conditions
during injection and recovery) which enabled the same valve to be used for injection
and withdrawal of gas.
The end result was the use of one valve instead of four (with a reduction in isolation
valves, piping, tees etc) and elimination of the gas heating plant. This was achieved
simply by correct control valve choice. Payback period was less than 6 months. Had
a specialist control valve vendor been approached at initial design stage then the
saving would have been much higher.
Conclusion
There is no point in spending large sums of money on new instrumentation of the
control valves do not perform correctly. Control valve selection is a complex matter
with a huge range of trims and body configurations being available. It is essential that
the control valve designer is fully familiar with the mode of use of the valve (this is
often not fully specified to a vendor at RFQ stage) It is very easy to specify the
wrong valve, or to spend more money than is necessary on control valves.
Masoneilan has experts who have seen most problems before and have vast
experience. Early contact with the control valve vendor is essential on difficult or
special applications. Advice is usually without obligation and is free. Do not be tied
to one particular design of valve for numerous applications. Each application has its
own requirements and therefore different valve designs are required to cope with
these.

6. The two production valves were replaced by a single VLOG labyrinth design valve.
By the use of the correct multistage design, the temperature in the valve was kept
above the temperature at which hydrate forms, WITHOUT the need to heat the gas in
any way. (Fig 4) Hence the gas heating plant and its associated costs were eliminated.
If the correct valves had been chosen at the outset, a large capital cost would have
been eliminated.
As an added bonus, we were able to design the multistage valves to be bi directional
with a large rangeability (to cope with vastly different pressure and flow conditions
during injection and recovery) which enabled the same valve to be used for injection
and withdrawal of gas.
The end result was the use of one valve instead of four (with a reduction in isolation
valves, piping, tees etc) and elimination of the gas heating plant. This was achieved
simply by correct control valve choice. Payback period was less than 6 months. Had
a specialist control valve vendor been approached at initial design stage then the
saving would have been much higher.
Conclusion
There is no point in spending large sums of money on new instrumentation of the
control valves do not perform correctly. Control valve selection is a complex matter
with a huge range of trims and body configurations being available. It is essential that
the control valve designer is fully familiar with the mode of use of the valve (this is
often not fully specified to a vendor at RFQ stage) It is very easy to specify the
wrong valve, or to spend more money than is necessary on control valves.
Masoneilan has experts who have seen most problems before and have vast
experience. Early contact with the control valve vendor is essential on difficult or
special applications. Advice is usually without obligation and is free. Do not be tied
to one particular design of valve for numerous applications. Each application has its
own requirements and therefore different valve designs are required to cope with
these.