- 2 days ago
In this session you learn
1. How to calculate orifice size for Pressure relief valve
2. Liquid Sizing
3. Vapor Sizing
4. Steam sizing
1. How to calculate orifice size for Pressure relief valve
2. Liquid Sizing
3. Vapor Sizing
4. Steam sizing
Category
📚
LearningTranscript
00:00Hi friends welcome to the channel we are in module 1 for pressure relief falls and this is session 4
00:06where we are going to understand how to calculate the orifice size for pressure relief falls so we will see
00:12the orifice sizing for various cases what we for for which we have calculated the relief load in session 3
00:20and session 2 so let's start disclaimer for our channel and the content shown in this videos
00:30to start before starting the orifice sizing important terminology has to be understand so what is critical flow it is
00:41a compressible gas is expanded across a nozzle orifice or end of the pipe its velocity and specific volume increases
00:50with decreasing downstream pressure
00:53so when any high pressure gas releases its velocity and specific volume increases with decreasing pressure in the downstream which
01:01is happening also in the discharge of pressure safety wall
01:06for given set of upstream condition the mass flow rate through the nozzle will increase until the limiting velocity reached
01:15in the nozzle
01:17so it can be shown that the limiting velocity in the is the velocity of sound so that limiting velocity
01:25till the flow rates or mass flow rate continuously increases the velocity of sound flowing for the flowing the fluid
01:33the flow rate that corresponds to limiting velocity is known as the critical flow
01:39so when any high pressure ratio so when your any fluid passes at the any fluid passes at the passing
01:46through the nozzle or end of pipe or to the orifice at sonic velocity or sound velocity at that time
01:56the flow rate the flow rate is a critical flow rate
02:03what is critical pressure ratio what is critical pressure ratio the absolute pressure ratio of the pressure at the nozzle
02:09exit at sonic velocity to the inlet pressure is called a pressure ratio
02:14so when at the sonic velocity or at the speed of sound if anything is exist from the nozzle or
02:26an orifice
02:28what is critical flow rate to the inlet pressure that is called a critical pressure ratio
02:35so what is critical flow pressure the critical flow pressure ratio is absolute unit may be estimated using the idle
02:43gas relationship in providing the expansion law PVK is equal to constant
02:50how to calculate critical flow pressure ratio
02:56so the formula is critical pressure divided by upstream relieving pressure is equal to within the bracket 2 by K
03:06plus 1 to the power K by K minus 1
03:09so K is the ratio of specific heats for an idle gas
03:14under critical condition the actual pressure at the nozzle exit of the pressure relief device cannot be fall below the
03:22critical flow
03:23pressure pressure even if much lower pressure exist in the downstream so in actual if the pressure at the exit
03:34of pressure relief wall cannot be fall below the critical pressure ratio
03:40critical pressure even if the lower pressure is available so critical pressure comes when the critical flow passes through that
03:48line which is the sonic velocity
03:54the ideal gas specific heat ratio is independent of pressure
03:59most process simulators will provide the real gas specific heat which should not be used in the above equation
04:06because the real gas specific heat ratio because the real gas specific heat ratio does not provide a good representation
04:11of the
04:11isentropic expansion coefficient
04:14that is why you have to refer the API 520 part 1 table 7
04:20so Kb
04:21capacity correction factor due to back pressure
04:23as in earlier lecture
04:25or a session we know that
04:27the back pressure is impacted on the conventional relief wall
04:31so Kb
04:31or a correction factor capacity correction factor to be used in
04:35conventional walls
04:38this can be often from the manufacturing literature or
04:41it can be often from API 520 part 1 figure 30
04:45the back pressure correction factor is applies to
04:48conventional walls only
04:50not for the
04:52balance below or pilot operated wall
04:53for this Kb can be used as 1
04:58Kd
04:58effective coefficient of discharge
05:010.975
05:03when PRV is installed without
05:05a ruptured disk in combination
05:07so generally ruptured disk
05:09if it is not installed
05:11then it can be taken as 0.975
05:16Kc
05:18is in the
05:19is in the combination correction factor for the
05:22installation of ruptured disk upstream of the PRV
05:26it can be taken 1
05:27when the ruptured disk is not installed
05:30it can be taken as 0.9
05:31when the ruptured disk is involved
05:33in combination with the PRV
05:44sizing for vapor
05:46or gas relief
05:47so basically type of orifice sizing
05:50we have to size
05:51orifice sizing can be done for the
05:53vapor or gas relief
05:55in that
05:56sizing for critical flow
05:58and subcritical flow
06:00then sizing for steam relief
06:02we have to do
06:04sizing for liquid relief
06:05we have to do
06:06and in liquid relief
06:08PRV requiring capacity correction
06:10and PRV not requiring capacity correction
06:12this is based on the ASME
06:15sizing for two-phase
06:17liquid and vapor relief
06:20there are two methods
06:22there are two methods
06:22homogenous equilibrium method
06:23and omega method
06:24so we will go
06:25generally followed is the omega method
06:27more rigorous
06:28so these are
06:29these are five
06:30four types of
06:32orifice sizing we have to do
06:36so first is orifice sizing for vapor or gas relief
06:42size equation for the
06:43vapor or gas railing falls into two categories
06:46first if the pressure downstream of the nozzle is less than or equal to the critical flow pressure
06:52then critical flow will occur and equation for sizing to be used is a critical flow equation to be used
06:58if PSV back pressure is less than critical pressure flow then the critical flow to be used
07:11if downstream pressure exceeds the critical flow and which means the PSV back pressure is greater than your PCF
07:20then subcritical flow to be used equation for subcritical flow to be used
07:27so what are the equation for critical and subcritical flow
07:30so sizing equation for critical flow
07:32it is in SI unit A is equal to W upon C into KD into PR into KB into KC
07:42into square root of T into Z into M
07:46so where is the required orifice discharge size
07:49W already calculated it is a relief flow or required flow rate
07:55in KG per hour
07:57C is the coefficient of coefficient due to K
08:00that is the CP by CP
08:02CP by CV value
08:04and how to calculate it C
08:05we will see in the next slide
08:08KD is the effective coefficient of discharge which is 0.975
08:12which is with or without rubsidisc
08:16KD will be 0.975
08:21PR is the relieving pressure that is the set pressure plus over pressure plus atmospheric pressure
08:28KB is a capacity correction factor due to back pressure
08:32KC is a combination correction factor
08:36T is a relieving temperature in Kelvin
08:38Z is a compressibility factor
08:40and M is a molecular weight of a vapor or a gas relieving
08:45So, when you have put all these value in this
08:48you will get your orifice size
08:50So, first we have to understand how to calculate the coefficient due to K
08:57So, how to calculate K
08:59Coefficient due to K
09:00CP by CV is
09:02in SI unit C is equal to 0.03948
09:06square root of K
09:07K into 2 by K plus 1
09:10to the power K plus 1 by K minus 1
09:15So, K is again your CP by CV
09:18Tidal gas specific heat ratio is independent of pressure
09:22Most process simulator will provide real gas specific heats
09:26which should not be used
09:27the above equation
09:28this I have been checked in the previous slide as well
09:33The value of C can be obtained from
09:35APF I-20 part 1 figure 32
09:38or table 8
09:40for idle gases
09:42where K cannot be established
09:44it suggested that the conservative value of C value is 0.0239 to be used
09:50If you are not able to
09:54establish the K value
09:56then you can use a conservative value of 0.0239 for C
10:02and then you can get a orifice size for the critical flow condition
10:08Unit of C is
10:11little bit difficult
10:12So, you have to remember this
10:14under root of Kg into Kg minus mol into kelvin divided by
10:19mm square into hour into k pascal
10:28orifice size for the critical flow
10:30there are other equations available in API
10:32which is A into 2.676 into V by Tz dam
10:38C by Kd, V1, Kb and Kc
10:41So, C has to be calculated based on
10:44previous slide
10:46So, A is required effective area
10:47B is a
10:49required volumetric flow rate
10:51Here it is a volumetric flow rate
10:53so unit has to be seen
10:54C is coefficient of coefficient due to K
10:57which is all we have seen in previous slide
10:59Kd is effective coefficient of discharge
11:03and all this Kb is capacity correction factor
11:05T is relieving pressure
11:06Z is compressibility
11:07GV is a specific gravity of gas
11:09standard condition
11:10referred to air standard conditions
11:15So, sizing for subcritical flow
11:20we have to use this formula
11:21A is equal to
11:2217.9 into W divided by F2 Kd Kc
11:27square root of Zd by M into P1
11:30into the bracket P1 minus P2
11:33So, what is A?
11:34A is orifice area
11:36W is the
11:38relief load in Kg per hour
11:40F2 is a coefficient of
11:42subcritical flow
11:42So, that we will see how to calculate F2 in next slide
11:46Kd is again the effective discharge
11:48P2 is relieving pressure
11:51P2 is a back pressure
11:52Kc is a combination factor
11:54T is relieving temperature
11:56Z is compressibility
11:56M is molecular weight
11:57So, based on this
11:59you can put this
12:00in an excel sheet
12:02so, you can have a
12:06readyment available
12:08but however, you have to check the units
12:09unit has to be taken correctly
12:12best that is why units are highlighted in this
12:16So, how to calculate F2 coefficient of subcritical flow
12:19The formula is F2
12:23square root of K by K minus 1
12:25R to the power 2 by K
12:29into
12:301 minus R by 1 minus R
12:321 minus R to the power K minus 1 by this
12:35So, these are very typical formulas
12:39So, there is nothing to explain that
12:42this formula has to be used as it is
12:45however, the unit and the values has to be taken properly
12:50otherwise, you will get the wrong output
12:53So, R is equal to the ratio of back pressure to upstream relieving pressure
12:56P2 by P1
12:58K is the specific heat ratios
13:04Sizing of subcritical flow
13:05other equation
13:06So, there are few other equations also
13:10In this also, we have to calculate F2
13:12F2 will be calculated based on the earlier slide
13:15So, these are the typical formulas
13:17these are the SI unit formulas
13:21Orificizing for steam relief
13:24The formula is A is equal to
13:26190.5 into
13:28W divided by P1
13:29Kd, Kb, Kc, Kn and Ksh
13:32So, where all these are
13:35we know only Kn is a correction factor for the
13:38Napier equation
13:41and W is your relieving load
13:43So, in this particular
13:46the fourth session
13:47we have lot of formulas for the calculation of orifice
13:50which are the very simple
13:51which are taken from the API directly
13:53and if you use this formula
13:55you will get the orifice size
13:56nmm square
14:01How to calculate Kn
14:02Coefficient of Subcritical Flow
14:04So, formula is
14:05Kn is equal to
14:060.02764 into
14:09P1 minus 100
14:10and 0.03324
14:12divided by into
14:13P1 minus 1061
14:15So, where P1 is
14:19If the P1 is
14:21greater than
14:23100339 Kpi
14:24and less than
14:2622056 Kpi
14:27then Kn can be considered as 1
14:29straight away
14:31Ksh
14:32Ksh is a supercritical correction factor
14:34this can be obtained from
14:35EPI part 1 table 9
14:38for the saturated steam
14:39at any pressure
14:41Ksh is 1
14:43For the temperature above
14:441200 F
14:46use the critical vapour sizing
14:47equation to be used
14:49so, if it is a steam pressure
14:50is more than 1200 F
14:52then straight away you have to use
14:54critical vapour sizing equation
14:56instead of
14:57steam relief equation
15:01Orifice sizing for the liquid relief
15:04Assume that the liquid is incompressible
15:06and the density of the liquid does not change
15:08as the pressure decreases
15:10for relieving pressure
15:11to the total back pressure
15:13So, as per ASME code
15:15that requires a capacity correction
15:18to be obtained
15:19for PRV design for a liquid service
15:24The procedure of obtaining the capacity certification
15:27includes a testing to determine the rated coefficient of discharge
15:31for the liquid relief
15:33PRV at 10%
15:36or office sizing for the liquid relief
15:38and where the
15:39where the PRV is required
15:41capacity certification
15:42based on ASME
15:44then the formula to be used is
15:45A is equal to
15:4611.78
15:47into Q divided by
15:48Kd, Kw, Kc, Kv
15:50square root of
15:51into square root of G1
15:53by P1 minus P2
15:55where all the
15:56Kd, Kc, Kw
15:57we have already seen
15:58Kv is a correction factor
15:59due to viscosity
16:00which we will see in the next slide
16:02how to calculate it
16:06Kv
16:07how to calculate Kv
16:08correction factor
16:09due to viscosity
16:10this is the formula
16:11Kv is equal to
16:120.9935287
16:15and with the
16:16divided by Reynolds number
16:17square root of Reynolds number
16:18so Re is a Reynolds number
16:21when PRV size for the viscous liquid service
16:24then during calculation
16:26you have to consider non-viscous application
16:28and calculate the
16:31PRV size
16:32and Kv can be considered as 1
16:35Kw is a correction factor
16:37due to the back pressure
16:38if the back pressure
16:39is atmospheric
16:39use the value of
16:41Kw as 1
16:43conventional wall
16:44is a back pressure service
16:45required a correction factor
16:46determined by
16:47EPFI 20 part 1
16:49figure 31
16:50balance below and pilot operated walls
16:52does not required any special correction
16:55this shall be noted
16:58so continuing the orifice size for liquid relief
17:02PRV
17:02which is not required
17:03capacity correction
17:04the formula is
17:06A is equal to
17:0711.78 into
17:08Q by Kd
17:09Kc
17:10Kw
17:10Kv
17:11and this
17:11this method assumes the
17:13effective coefficient of discharge
17:15Kd is 0.62
17:16and 25% over pressure
17:18an additional capacity correction factor
17:20Kp is
17:22needed for relieving pressure
17:24other than 25% over pressure
17:29so
17:29A is
17:30again your
17:31orifice area
17:32Q is flow rate
17:33Ps is set pressure
17:34P2 is
17:35total back pressure
17:37Kd is effective coefficient of discharge
17:39Kc is
17:39combination correction factor
17:40Kw is a correction factor
17:42due to back pressure
17:43Kv is a correction factor
17:44due to the viscosity
17:45G is the specific gravity of the liquid
17:47at flowing temperature
17:49referred to the water standard condition
17:51so
17:53in earlier slides we have
17:54we
17:55we understood that
17:56how to calculate the orifice size
17:58for the various cases
18:00now there is a
18:01API standard
18:02orifice designation
18:03an effective area
18:05given in API 526
18:07based on that
18:09table
18:09the orifice designation is
18:11start with
18:12D
18:13E
18:13F
18:14G
18:14H
18:15J
18:16K
18:16L
18:17M
18:17N
18:18P
18:19Q
18:19R
18:20and T
18:20the last one is T
18:22if your
18:23orifice size
18:25is more than T
18:28then you can go for the
18:30two walls
18:32and then you have to see the size again
18:34or
18:36but
18:37API is not allowing the size more than T
18:39so effective area in inches square is also given for the each
18:43if
18:43anywhere you see that the
18:45orifice designation is T
18:47it means the orifice size for that particular PSV is
18:500.11
18:53inch square
18:53or
18:5471 mm square
18:56similarly
18:57if any
18:57any orifice size if you take
18:59like K
19:00or anything
19:01the
19:01inches area
19:03or mm square area
19:04remains constant
19:06for any orifice
19:07which designated with API
19:10526
19:10and
19:12shows as a K
19:14or any
19:14any
19:15orifice designation
19:17so
19:18what is E
19:19E
19:20is the effective area
19:21this area is
19:22used in the API 520 sizing calculation
19:25it calculated by
19:27dividing the
19:27walls
19:28actual orifice area
19:30by the
19:30tested coefficient of discharge
19:33sizing rule
19:34when you are required to calculate area
19:37and falls between
19:38two standard API effective areas
19:41you must always select the next larger standard orifice area
19:45suppose if you are
19:46you calculating the area and your area comes between the 0.4
19:50so 0.4 comes between F and G
19:53okay
19:53your required orifice area comes as 0.4
19:56then in that case you have to go for G
20:00which is 0.503
20:03also
20:03also
20:04you should not select like if you are
20:07for the
20:09lowest size is D
20:10then that can be selected if you have very low
20:14relief load
20:16but
20:17if you have calculated
20:19orifice area is like 0.2
20:22sorry 0.05 or something
20:25then you should not select E
20:27because
20:27then it is a very high capacity
20:29because you are all
20:31the
20:31downstream or you can say
20:33inlet and outlet
20:34line sizing to be
20:35to be decided based on the
20:37actual orifice area
20:38not on the calculated orifice area
20:40so
20:41that is why we should not
20:42select a very high orifice area
20:45like if
20:45now 0.4 is there
20:46you should not select H
20:48it should be the next
20:50closest one
20:51instead of going jumping to the
20:53bigger one
20:53one
20:53and thinking that okay
20:54I will design a orifice
20:56a better orifice
20:57but that will create a different
20:58problem
20:59if it is oversized
21:02then you are
21:02relieving load
21:04actual relieving load
21:05when the PSV gets popped up
21:07is very high
21:08and then your downstream
21:10will be affected
21:12API 526 versus 520
21:15API 526 defines the physical dimensions
21:18and materials for the walls
21:20including the standard orifice sizes
21:23this tables comes from
21:24API 526
21:26and API 520 part 1
21:28provides the sizing equation
21:30that utilize
21:31this area
21:34conclusion for this video
21:36key sizing principles
21:38we have successfully translated
21:40the required capacity
21:41that is
21:41relief load required
21:43W required
21:44into a physical area required
21:45that is orifice area
21:47A required
21:47tackling the unique formulas
21:49for each phase
21:50vapor gas sizing
21:51defined by the flow regime
21:53use critical flow
21:54for most
21:55conventional PSV
21:56applications
21:58relying primarily
21:59on upstream pressure
22:01only use the
22:02non-critical flow
22:03when the pressure ratio
22:04dictates
22:05subsonic conditions
22:06the
22:07crucial take way
22:08is that
22:09the
22:09calculated
22:10area required
22:12is
22:12theoretical minimum
22:14we must select
22:15the next standard
22:16larger orifice designation
22:17from
22:17API 526 tables
22:20it may be
22:21DEF
22:22G
22:23etc.
22:24to ensure the wall
22:25has sufficient capacity
22:26under all conditions
22:28so
22:29trust the calculation
22:30but verify the factor
22:31orifice sizing
22:32is
22:33where the fundamental
22:34meets the hardware
22:34hit the subscribe button
22:36to ensure your fundamentals
22:37are always in focus
22:39effects
22:40and
22:41just a note
22:41a
22:42two-phase
22:43flow calculation
22:44or two-phase sizing
22:46I have
22:46I have covered in
22:47a next lecture
22:49so
22:49watch that lecture also
22:51because that is little bit
22:52lengthy
22:52so I have segregated
22:54the
22:56the
22:56flow calculation
22:58for
22:58orifice calculation
22:59for steam
23:00liquid
23:01and vapor gas
23:02and the next session
23:03will be on the
23:04two-phase
23:09so thank you very much
23:10write your question
23:11and comment
23:12I will be happy
23:13to answer it
23:16you can reach us on
23:19conceptengineering2025
23:19at gmail.com
23:22links and
23:23links for the other sessions
23:25are given in
23:25this
23:26description