Jumat, 02 Juli 2010

PSV Selection for Beginner

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.
Figure 1. Conventional Pressure Safety Valve (Taken from API 520 part 1)

How does it work?

Figure 2. Sketch of Pressure Relief Valve
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.
Table 1. Standard effective Orifice Areas and Letter Designations

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.

Sketch of Pressure Relief Valve
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.
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.
Figure 5. Bellows Pressure Relief Valve
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
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
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.
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.

Table 2. Comparison between types of PSV

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.
Table 3. Process Data for PSV Sizing
 Table 3. Process Data for PSV Sizing

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.
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.

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.

Figure 8. Internal part of the PSV (Taken from API 520 Part 1)
Figure 8. Internal part of the PSV (Taken from API 520 Part 1)

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.

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.

References :
1. Crosby, Pressure Relief Valve Engineering Handbook
2. API 520 Part 1, Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries; Sizing and Selection
3. API 526, Flange Steel Pressure Relief Valves
4. Ken Arnold, Maurice Stewart, Surface Production Operations,Vol.2, Design of Gas Handling Systems and Facilities,2nd edition,1999,

sumber : Mas Webby

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