Valves
A valve
is a mechanical device that controls the flow of fluid and pressure within a
system or
process.
A valve controls system or process fluid flow and pressure by performing any of
the
following
functions:
·
Stopping and starting fluid flow
·
Varying (throttling) the amount of fluid flow
·
Controlling the direction of fluid flow
·
Regulating downstream system or process pressure
·
Relieving component or piping over pressure
There are
many valve designs and types that satisfy one or more of the functions
identified
above. A
multitude of valve types and designs safely accommodate a wide variety of
industrial
applications.
Regardless
of type, all valves have the following basic parts: the body, bonnet, trim
(internal
elements), actuator, and
packing. The basic parts of a valve are illustrated in Figure.
Body:
The valve's body is the outer casing of
most or the entire valve that contains the internal parts or trim. The
bonnet is the part of the encasing
through which the stem (see below) passes and that forms a guide
and seal for the stem. The bonnet typically screws into or is bolted to the
valve body.
PSV valves
Introduction
Pressure Safety Valve (PSV) is one of safety devices in oil and gas production facility, which ensure that pipes, valves, fittings, and pressure vessels can never be subjected to pressure higher than their design pressure. Therefore, the selection of PSV to be installed must be conducted in a careful and proper manner.
Pressure Safety Valve (PSV) is one of safety devices in oil and gas production facility, which ensure that pipes, valves, fittings, and pressure vessels can never be subjected to pressure higher than their design pressure. Therefore, the selection of PSV to be installed must be conducted in a careful and proper manner.
These are the questions worth
to be asked when you are going to specify details of PSV.
·
What
type of PSV we will have for our process requirements?
·
Is
there any easier way for PSV sizing (PSV calculation) rather than calculate it
manually?
·
What
kind of material shall be chosen for our process requirements?
Prior to the PSV selection, it
would be better if we know how the PSV works which will lead us in
understanding of critical parts of PSV. Then, the PSV selection process can be
done with awareness of some strong points.
Pressure
Safety Valve by definition
Cited from API 520 part 1 (Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries; Sizing and Selection) about Safety Valve definition: “A safety valve is a spring loaded pressure relief valve actuated by the static pressure upstream of the valve and characterized by rapid opening or pop action. A safety valve is normally used with compressible fluids.” Figure 1 shows Conventional PSV, which is purposed for description only.
Cited from API 520 part 1 (Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries; Sizing and Selection) about Safety Valve definition: “A safety valve is a spring loaded pressure relief valve actuated by the static pressure upstream of the valve and characterized by rapid opening or pop action. A safety valve is normally used with compressible fluids.” Figure 1 shows Conventional PSV, which is purposed for description only.
Figure
1. Conventional Pressure Safety Valve (Taken
from API 520 part 1)
How
does it work?
How does the PSV work? Figure
2 is a simple sketch of pressure relief valve which shows the disc held in the
closed position by the spring. When system pressure reaches the desired opening
pressure, the pressure force of the process fluid pass through the inlet and
then it is acting over Area A1 equals
the force of the spring, and the disc will lift and allow fluid to flow out
through the outlet. When pressure in the system returns to a safe level, the
valve will return to the closed position.
Certain area of the disc and
nozzle will allow certain amount of the gas/liquid volume. The area of the
nozzle (so called as “Orifice”) needs to be calculated in order to have proper
amount flow of the process fluid. This certain area has been standardized in
API 526 (Flange Steel Pressure Relief Valves) and designated into certain
alphabetic as shown on Table 1.
Since PSV will most likely to
be in closed position, it is a good idea to choose some kind of “seal” between
disc and the nozzle to keep the process fluid from leaking to the outlet of the
PSV.
Conventional,
Bellows or Pilot type?
Backpressure
considerations
Types of PSV are created due to existence of backpressure. The effect of backpressure can be depicted by Figure 3 which incorporate forces from spring (Fs), process fluid from the pressurized system (PVAN), and backpressure (PBAN). The PV is the pressure due to the changes over the pressurized system, and the PB is the pressure which exist in the outlet of the PSV, we recognize this as a back pressure. As you may see, that the spring – denotes with the Fs – is having main contribution to the force balance, and have a positive direction along the PB. The overpressure in the pressurized system will increase the magnitude of the PV, and eventually it will affect the balance of the pressure force, and hence the sum of the PBAN and the Fs will be less than the PVAN. The spring, which holds the disk and isolates the pressurized system into the outlet of the PSV, is moving upward and the disk will not contain the pressurized system anymore.
Types of PSV are created due to existence of backpressure. The effect of backpressure can be depicted by Figure 3 which incorporate forces from spring (Fs), process fluid from the pressurized system (PVAN), and backpressure (PBAN). The PV is the pressure due to the changes over the pressurized system, and the PB is the pressure which exist in the outlet of the PSV, we recognize this as a back pressure. As you may see, that the spring – denotes with the Fs – is having main contribution to the force balance, and have a positive direction along the PB. The overpressure in the pressurized system will increase the magnitude of the PV, and eventually it will affect the balance of the pressure force, and hence the sum of the PBAN and the Fs will be less than the PVAN. The spring, which holds the disk and isolates the pressurized system into the outlet of the PSV, is moving upward and the disk will not contain the pressurized system anymore.
Figure
3. Effect of Backpressure to the set pressure (Taken from API 520 part 1)
An extreme example, in the
closed position, if backpressure is high enough to compensate the force
pressure of process fluid, the force resultant will be zero, in other words the
PSV will remain close. In this condition, the PSV is not successfully to
fulfill its function. We will examine types of PSV.
Conventional type
This type of PSV is the simplest one as you may see on Figure 4. Usually, this type of PSV is used whenever the existence of back pressure is relatively small (less than 10% of set pressure), or nearly zero. Due to its low immunity to back pressure, the conventional type outlet is vented into atmospheric, and mostly, the fluid to be vented is non-hazardous fluid i.e.: water steam.
This type of PSV is the simplest one as you may see on Figure 4. Usually, this type of PSV is used whenever the existence of back pressure is relatively small (less than 10% of set pressure), or nearly zero. Due to its low immunity to back pressure, the conventional type outlet is vented into atmospheric, and mostly, the fluid to be vented is non-hazardous fluid i.e.: water steam.
Figure
4. Conventional Pressure Safety Valve (Taken
from API 520 part 1)
Bellows type
PSV with bellows type or balanced-bellows type is used when the backpressure does not exceed than 50% of set pressure. This type of PSV is almost the same with the conventional ones, but there is additional bellows in it as you may see on Figure 5. The bellows itself has a function to reduce the effect of the backpressure force (PBAN) over the disk as you may clearly see on the forces diagram on Figure 3. The bellows contained the upper side of the disc and the rod which connected to the spring from pressure force of process fluid/pressurized system – in which connected through PSV outlet – and the inside chamber of the bellow will be vented to the atmospheric, which obviously has constant pressure. Commonly, this type of PSV does not have a wide range of PSV, hence, it is not so flexible in alteration of set pressure.
PSV with bellows type or balanced-bellows type is used when the backpressure does not exceed than 50% of set pressure. This type of PSV is almost the same with the conventional ones, but there is additional bellows in it as you may see on Figure 5. The bellows itself has a function to reduce the effect of the backpressure force (PBAN) over the disk as you may clearly see on the forces diagram on Figure 3. The bellows contained the upper side of the disc and the rod which connected to the spring from pressure force of process fluid/pressurized system – in which connected through PSV outlet – and the inside chamber of the bellow will be vented to the atmospheric, which obviously has constant pressure. Commonly, this type of PSV does not have a wide range of PSV, hence, it is not so flexible in alteration of set pressure.
Figure
5. Bellows Pressure Relief Valve (Taken
from API 520 part 1)
Pilot type
A pilot-operated pressure safety valve consists of the main valve, which normally encloses a floating unbalanced piston assembly, and an external pilot as shown on Fig.6. The piston is designed to have a larger area on the top than on the bottom. Up to the set pressure, the top and bottom areas are exposed to the same inlet operating pressure. Because of the larger area on the top of the piston, the net force holds the piston tightly against the main valve nozzle. As the operating pressure increases, the net seating force increases and tends to make the valve tighter. This feature allows most pilot-operated valves to be used where the maximum expected operating pressure is higher than 90% of MAWP
A pilot-operated pressure safety valve consists of the main valve, which normally encloses a floating unbalanced piston assembly, and an external pilot as shown on Fig.6. The piston is designed to have a larger area on the top than on the bottom. Up to the set pressure, the top and bottom areas are exposed to the same inlet operating pressure. Because of the larger area on the top of the piston, the net force holds the piston tightly against the main valve nozzle. As the operating pressure increases, the net seating force increases and tends to make the valve tighter. This feature allows most pilot-operated valves to be used where the maximum expected operating pressure is higher than 90% of MAWP
The pilot type has a sensing
line and its function is transmitting the built-up pressure that may exist in
the pressurized system to the pilot valve. As the pressure in the pressurized
system is increasing and reaching the set pressure, the pilot valve will
actuate the PSV spring inside the main valve to pop up the PSV. Due to the
actuator has no direct contact with the venting system the valve will not
relatively be affected by backpressure. Moreover, this type of PSV has a wide
range of spring setting, it will be an advantage if we want to change the set
pressure on a wide range alternatives.
Figure
6. Typical pilot-operated valve
Multiphase Fluid
How about if we need to release multiphase fluid? Is there another type of PSV which is able to handle that kind of case? Well, it is good question actually. If we are using conventional PSV, we will have big problem in the backpressure consideration if we do have large backpressure or even a variation of backpressure.
How about if we need to release multiphase fluid? Is there another type of PSV which is able to handle that kind of case? Well, it is good question actually. If we are using conventional PSV, we will have big problem in the backpressure consideration if we do have large backpressure or even a variation of backpressure.
Another option is pilot. It
also has a week point which is critical on multiphase handling since there will
be possibilities that the sensing line will be plugged with non-clean fluid.
None will guarantee whether or not the process fluid is “clean” (containing of
liquid and gas only). They may have little solids or debris which eventually
plug the sensing line.
The last option is the bellows
type, since it is relatively unaffected by the backpressure and it has no
sensing line like the pilot type has. We will choose this last option, because
we only have three available type in the market. It is obvious now that every
possible case is not available in ready-on-stock PSV type, we have to conduct
an engineering judgment on any possible case within available type.
For comprehensive
understanding between types of PSV, Table 2 is describing the advantages and
disadvantages each one of them.
What
are required for PSV Sizing?
After we have selected the
type of the PSV, we should calculate the size of the orifice. Of course this is
one of the important step to select PSV. Why do we have to calculate the PSV
anyway? If you don’t calculate your PSV, you’re not really sure whether the
size is adequate or not to handle the fluid relief. The main principle of PSV
sizing: it is fit for purpose. Smaller size of PSV means smaller capacity of
the valve and also, bigger size of PSV means bigger capacity of the valve.
The application of the smaller
capacity of PSV than its design capacity shall be avoided. Because if the PSV
is unable to allow the process fluid to be released, then the pressure in
pressurized system is tending to increase and adjacent parts of the pressurized
system will be burst or rupture. In other words, the PSV is unable to fulfill
its main function.
It is almost similar to the
application of bigger capacity of PSV than its design capacity. The bigger
capacity from its design capacity means PSV is allowing the process fluid “too
much”. If we have pressurized system to be in overpressure condition, the set
pressure of the PSV is reached and the process fluids will be vented through
the outlet. Due to its large capacity, the pressure in the pressurize system
will be decreased rapidly and then the PSV will re-close. But, as the PSV is
closing, the pressure in the pressurized system is increasing again and the set
pressure of the PSV is reached again, and the PSV will open again. This is what
people called as “chattering”, and most of cases the chattering itself is more
like to be a rapid vibration. This is an example of bad sizing of PSV because
the PSV will be damaged after a chattering. In other words, the PSV is unable
to fulfill its main function again.
As a basic of PSV sizing,
these following process data as shown on Table 3 shall be provided to calculate
the orifice designation.
PSV Sizing using
Software
Is there any chance that we
can size PSV easier? The answer is “yes”. But you must be careful then, wise
people said that: “it’s not about the gun, it’s about the man behind the gun”.
Software is only calculating what is coming through it, and do what we told. In
another word: garbage in, garbage out.
You can use specific software,
which made special for it. The useful software tool for PSV sizing I ever had
is Instrucalc Version 5.1, the user interface is as shown in Fig.7. I will use
Instrucalc Version 5.1 as description-purposed only, even there are other
software which have the same capability.
Figure 7. Instrucalc version
5.1 for PSV sizing.
This software is non-vendor
oriented, since its calculation relied on API-520 and ASME Sect.VIII, and
almost all vendors are taking reference to those two standards. Instrucalc is
best on describing the size of orifice designation, inlet and outlet size and
maximum capacity of the valve could handle. Moreover, for Gas Relief and Liquid
Relief case, the calculation result of Instrucalc and vendor software is most
likely to be the same, that would be a reason for choosing Instrucalc as a
general calculation software.
However, for some specific
types of PSV from certain vendor, I would rather choose vendor software which
is able to calculate various outputs based on their PSV models, especially when
reviewing vendor’s proposal. For an instance, Instrucalc will generate certain
size of inlet and outlet, which any vendor does not have that size of
inlet/outlet. If there is discrepancy with Instrucalc, it doesn’t mean that
vendor calculate incorrectly, they just don’t have that size, as Instrucalc has
calculated. As long as the size and liquid/gas capacity from vendor proposal is
adequate with our technical data, that would be all good.
For some reasons, certain
vendor is not allowing their software to be installed side by side with other
vendor’s software in a computer. This is a difficult problem since the
software’s bugs were intentionally “created” by vendor, which eventually we
cannot fix. In case you’re facing this problem, consult your vendor
representative for more assistance.
Proper
material for parts
Compatibility with the process
fluid is achieved by careful selection of materials of construction. Materials
must be chosen with sufficient strength to withstand the pressure and
temperature of the system fluid. Materials must also resist chemical attack by
the process fluid and the local environment to ensure valve function is not
impaired over long periods of exposure. The ability to achieve a fine finish on
the seating surfaces of the disc and nozzle is required for tight shut off.
Rates of expansion caused by temperature of mating parts is another design
factor.
Comparison
among Vendors
We have some basic knowledge
about basic of PSV selection, let’s do some real job here.
Correctness of
calculation
We require to pay attention for process data. Mostly, they are root cause of incorrect calculations, wrong data will lead you to some confusing results, so be careful then. Having the process data correctly, we need to see the result and compare them (vendor’s and ours), are they different badly? We need to see, whether the discrepancies are critical or not. As example, the calculation of orifice area from each vendor can be different with the same process data and method of calculation (API-520), but you must pay attention that vendors will refer to the same orifice designation. The same way if vendors offer 1.5 inch of inlet size, while according to our own calculation we need 2 inch. That would be fine if the valve capacity is capable to handle our data process with the size of inlet/outlet pipe is not too large or too small compared to our own calculation.
We require to pay attention for process data. Mostly, they are root cause of incorrect calculations, wrong data will lead you to some confusing results, so be careful then. Having the process data correctly, we need to see the result and compare them (vendor’s and ours), are they different badly? We need to see, whether the discrepancies are critical or not. As example, the calculation of orifice area from each vendor can be different with the same process data and method of calculation (API-520), but you must pay attention that vendors will refer to the same orifice designation. The same way if vendors offer 1.5 inch of inlet size, while according to our own calculation we need 2 inch. That would be fine if the valve capacity is capable to handle our data process with the size of inlet/outlet pipe is not too large or too small compared to our own calculation.
Material
Material is another important issue since we need the PSV to be “seated” for some years and most probable to handle “bad” fluid process characteristics.
Material is another important issue since we need the PSV to be “seated” for some years and most probable to handle “bad” fluid process characteristics.
The most critical parts are
the spring, seat and disc. We need to pay attention on their material to be
proposed by your vendors. The internal part of the PSV is shown in Figure 8.
Spring’s material is one of
the important consideration, since it is “muscle” of the PSV. There are many
alternatives for the spring’s material, i.e : chrome steel, inconel. Different
material will be impacted to the overall price, you should select the material
properly.
Seating surface – or seat for
short – has a function to contain the pressurized system and the vented system,
since it is “clutching” the disc. Usually, we have a soft seated and hard
seated options. The hard seated means that it is made from the metal material,
i.e : steel. While the soft seated means that it is made from the non-metal
material, i.e : kalrez, viton. The advantage of having soft seated that it will
have a good isolation, because it is “softer” than the hard seated, so its
shape is more flexible to clutch the disk, which the disk is commonly made of
stainless steel.
The most exposed part to the process fluid is the disk. That would be a reason that we have to choose a good material of it. Usually the disk is made of stainless steel because of its properties to be able stand on the harsh environment.
The most exposed part to the process fluid is the disk. That would be a reason that we have to choose a good material of it. Usually the disk is made of stainless steel because of its properties to be able stand on the harsh environment.
Price criteria
In most cases, money talks. High price means high quality, low price means low quality, but you should remember, it is not always true. You shouldn’t believe, for instance, with the low price of the PSV also will has low quality, either with the high price. There must be some overheads over the price components or even low quality of the materials. You should examine vendor’s proposal very carefully and thoroughly, you must go into as detail as possible. If you have any doubt about some points, you must ask to vendor for explanations until you have satisfaction on the answers and you have confident to determine whether or not you are going to accept vendor’s proposal.
In most cases, money talks. High price means high quality, low price means low quality, but you should remember, it is not always true. You shouldn’t believe, for instance, with the low price of the PSV also will has low quality, either with the high price. There must be some overheads over the price components or even low quality of the materials. You should examine vendor’s proposal very carefully and thoroughly, you must go into as detail as possible. If you have any doubt about some points, you must ask to vendor for explanations until you have satisfaction on the answers and you have confident to determine whether or not you are going to accept vendor’s proposal.
