PSV Selection for Beginner « the Weby’s

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PSV Selection for Beginner

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.

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.

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1 of 14 9/2/2011 11:50 AM

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.


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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.

Figure 3. Effect of Backpressure to the set pressure (Taken from API 520 part 1)


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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.


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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.


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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.

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 dont calculate your


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PSV, youre 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

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: its not about the gun, its 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.


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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 vendors 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 doesnt mean that vendor calculate incorrectly,

they just dont 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 vendors

software in a computer. This is a difficult problem since the softwares bugs were intentionally created by

vendor, which eventually we cannot fix. In case youre 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, lets do some real job here.


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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 (vendors 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.

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)

Springs material is one of the important consideration, since it is muscle of the PSV. There are many

alternatives for the springs 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.


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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 shouldnt 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 vendors 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 vendors 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,

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Tulisan ini dikirim pada pada Rabu, Oktober 3rd, 2007 9:10 pm dan di isikan dibawah Rupa-rupa. Anda dapat meneruskan melihat

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14 Responses to PSV Selection for Beginner

ayodya mengatakan:

15 Oktober 2007 pukul 11:39 am

Papa..aku pusing baca artikelnya..

nanti papa ajarin aku yaaa..

Balas

1.

ayodya mengatakan:

2 November 2007 pukul 9:47 am

Papa.. katanya mau banyak nulis di blog? aku udah banyak nulis nih. Papa kasih komentar dong.

Balas

2.

roeddyes2001 mengatakan:

3 Juni 2008 pukul 9:30 pm

excellent article mas webby go ahead publish some more article its help a lot of young engineers to

advance in practical knowledge and a must knowledge as a design engineer, process engineer a like

Balas

3.


10 of 14 9/2/2011 11:50 AM

Anto mengatakan:

6 Agustus 2008 pukul 5:10 pm

Dear mas Weby,

I am still strugling to figure out what is the difference/s betwen semi nozzle and full nozzle? when semi

or full nozzle PSV is selected ? thanks in advance.

Regards,

Suprianto Kamijaya

Balas

4.

farghmee mengatakan:

2 September 2008 pukul 1:15 pm

Mas Weby,

A very nice article for beginner like me. quite confusing though reading the API520

TQ Mas!

Balas

5.

birru mengatakan:

20 September 2008 pukul 9:14 am

Mas Weby,

Waaa..bagus bgt artikelnya.. Lagi nyari2 info tentang PSV eh ketemu blognya Mas Weby.. Makasiy

banyak mas.. Engineer sejati deh emg..hehehehe

Balas

6.

monique mengatakan:

5 November 2008 pukul 4:36 pm

thank you for the posting mas webytapi masih sedikit confused ni

btw mas webby, fire size gas expansion sama pake yang gas relief-known flow tapi ASME sect

fire.bedanya apa ya?

Balas

7.

kl mengatakan:

18 November 2008 pukul 8:39 am

thanks mate. great info.

Balas

8.

FANDI mengatakan:

22 Oktober 2009 pukul 10:39 am

9.


11 of 14 9/2/2011 11:50 AM

tq, mas ijin copy yo

Balas

rixxaman mengatakan:

24 Maret 2010 pukul 11:53 am

Mas Webby.

Nice article..

Please allow me to put it in my blog as an article.

Thanks

Rixxaman

weby: silahkan pak/bu Rixxaman

Balas

10.

andy mengatakan:

27 Maret 2010 pukul 3:25 am

Sorry, I have questions about PSV, that i still confused about that..

If we have PSV with DP 1000 psi, Can we change the setting until 500 PSI without change the spring?

How much persen the limit of the spring force from the Design Pressure? n im hopeless for the

answer, thx before.

Balas

11.

Manuj mengatakan:

24 Juli 2010 pukul 12:42 pm

There is a great calculation tool available at a Mekaster Engineering Ltd website. You can take print of

data sheet and then of GA drawing drawing also.

Balas

12.

kamran mengatakan:

24 November 2010 pukul 11:11 pm

can you tell me who is that psv that have a spring or ded waight

Balas

13.

iwan mengatakan:

24 Maret 2011 pukul 4:52 pm

Salut buat yang nulis,

karena sampean menulis, maka saya membaca:)

saya pikir background anda adalah chemical engineer,

tapi ternyata

Wawkeren artikel ini.

14.


12 of 14 9/2/2011 11:50 AM

Beritahu saya balasan komentar lewat surat elektronik.

Beritahu saya tulisan baru lewat surat elektronik.

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PSV Selection for Beginner « the Weby’s

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