- 2 days ago
In this video you will learn
1. Different scenarios of overpressure
2. how it occurs
3. what is fire case
4. how to calculate relief load
1. Different scenarios of overpressure
2. how it occurs
3. what is fire case
4. how to calculate relief load
Category
📚
LearningTranscript
00:01hi friends welcome to our channel today we are going to see pressure relief
00:05all scenarios of over pressure and guideline for fire case so let's start
00:10disclaimer for our channel and the content you shown in this videos
00:21assume assumptions while accessing the various scenarios see there are some assumptions has
00:27to be understand because we want to understand the fundamentals because we cannot go straight
00:32away to the formula and how to calculate the fire load or what are the different scenarios
00:36so first we have to assume we have to understand what are the assumptions it is assumed that
00:43the train operator operates the plant it is not like that anybody goes in the process
00:48plant and he starts operating he is a well trained operator so he will not he will listen
00:55to alarms and other things so that part to be understand simultaneous occurrence of a two
01:02or more condition that could result in over pressure will not be considered if the cause
01:07are unrelated so double geo party here we talking about the double geo party in the process and
01:13mechanical or electrical commonality exist among this cause so we cannot say that the earthquake
01:21and tsunami can come together so we have to take each occurrence independently double geo party to be
01:29avoided otherwise we will at the end we will design a very over design or over design for that pressure
01:39safety wall so that has to be avoided the opening and closing action of a control wall and the automatic
01:48start of the equipment will not be considered as a suitable pressure relief device for equipment protection
01:54because the supply of these items is an emergency and is not regard it is regarded as a reliable
02:02as general rule final over pressure protection is to be provided utilizing the mechanical pressure relief device
02:12and the equipment not affected by utility failure begin to evaluate will be considered to remain
02:20in operation so the equipment which are not affected by utility failure that has to be considered as
02:29operating safely and the automatic wall wearer the instrument air and control walls are there
02:36they are considered as can be filled can be filled so that the failing of that equipments which are
02:43connected to instruments or electrical the filler can be considered so these are the four assumptions
02:52flow rates through the equipment when we are calculating any block outlet case or the flow rate
02:57through the equipment or other conditions during emergency will be assumed to be a normal rate
03:02except where the particular preliminary emergency cause under the consideration would alter the flow
03:09so unless any other emergency continuation of one emergency to another case comes till that time
03:17we have to consider the normal flow rates for the calculation otherwise again we will over design our PSV
03:25in case of fire the flow is assumed to have stopped and been contained within a defined system so this
03:33also for the fire case because we are
03:37going to see that how to calculate a leaf load for fire case so for fire case we have to
03:41assume that it is getting isolated
03:44the possibility of operator intervention opening or closing of any wall
03:49or taking any incorrect action in a wrong sequence or at the wrong time will be considered
03:55so operator uh inadvertently open some walls or he misunderstand because when when uh there are
04:05when operator works and he got a 10 hundred alarms together he get panic and maybe there is a chance
04:12he can
04:12uh he can wrong sequence or some wrong action he can take so that's why operator uh failure has to
04:22be considered
04:24however the block walls electrical switches and equipments items like car seal walls etc uh will not be
04:31considered involved in any cases or operator error so uh it will not be considered that car seal wall
04:38somebody gets wrongly open due to the panic situation no
04:42that that that that assumption we cannot take that for calculation
04:50so overpressure scenarios in process industries so there are few overpressure scenarios decided
04:58listed by the apis as well
05:00so first is external fire which is the most important then exit block or blocked outlet so some
05:10if the block outlet will see each scenario how what it means and then we will go we will see
05:17how to
05:18calculate relief load for each scenario the third one is cooling or column reflux failure
05:24or pump around failure or pump around failure then tube rupture in shell and tube exchanger
05:31control valve failure hydraulic and thermal expansion power failure instrument air failure loss of fan in
05:41air cooled exchangers or tripping of fan cooling water failure
05:49ambient heat input to revoilers uh abnormal heat input to revoilers
05:57check valve valve operation and loss of heating series fractionation system and liquid
06:06overfill these are the these are uh these are the scenarios to be considered
06:13so first scenario we will see the external fire
06:18so suppose if any vessel you have seen this picture so what is external fire generally external fire or fire
06:26happens on the surface
06:28it will not happen on the platforms or where it is a checker split it can happen on a
06:34you can say in a building or a where the concrete foundation or a solid foundation is available
06:41you cannot consider the fire on any checker plates or any platform open platform which
06:48where the drains you cannot say drain it is open platform and we have to understand that flame height
06:57based on the studies it has been it can go up to the 25 feet from the ground
07:04so or a 7.6 meter or generally it takes as a 8 meter so if you see this is
07:10the ground elevation
07:11and this is the fire so fire flame height can go up to 25 feet or a 7.6 meter
07:19so up to this point only we have to consider our weighted surface area so liquid in this
07:26area will get boiled because of the external fire heat
07:33so this is the fire case and if any vessel which is above this if if suppose this vessel having
07:41a skirt
07:42support and the skirt is up to some 8 meter like some columns will have the bigger
07:50height of the skirts so if this height is more than 25 feet then this liquid liquid available in the
07:59column
07:59will not come as a weighted surface area you can straight away say that because the height of the vessel
08:06or the bottom of the vessel is above the 25 feet so it is not qualified for the fire case
08:17there are some other other overpressure scenarios so we will see one by one so suppose this is in a
08:24process plant this is a stripper where the steam is getting entered into the column and feed coming
08:30from the top so this is a stripping action and then the vapors or lighters goes on the top which
08:39will
08:39be cooled to the overhead condenser then there is a separator and separated liquid will be partially
08:47sent for the reflux and partially it will be taken as a product it is mentioned as a naphtha here
08:55and whatever the flashing vapors are going like lpg vapors these are the off gases from this separator
09:02and the bottom there is a bottom product which is a product diesel or bottom product you can say
09:11so loss of fan in air cooled exchanger so if this fan get tripped so when this fan gets tripped
09:20this is an over pressure scenario for this column so we have to this is a one case which we
09:27which we
09:28have listed then control all failures it can be common it can control all failure can be anywhere
09:35so if this in the vapor line if this control all fails to close then the pressure in this vessel
09:42will
09:43increase and this increasing in this pressure will directly or indirectly increasing the pressure of
09:50the column because column pressure is controlled by this control wall only so this is a control wall
09:58this type of a control failure control wall failure can be considered in here as well it can be
10:03considered here as well anywhere control failure control failure case means control wall is
10:09in a safe position if it is a fail open and the control failure case comes like
10:15whether when he wants that wall should be open however that wall is getting closed
10:23column reflux failure because we know that the column is required reflux
10:30to cool down the upcoming vapors so if this
10:34either this wall get closed or this pumps get tripped so these both are comes under column reflux failure
10:43liquid overfill suppose if this outlet wall get closed or this wall can be here as well
10:50if this wall get closed so liquid
10:54coming down
10:57it's continuously coming down so it will get overfilled so liquid on this column will start building
11:02and this will eventually there will be no vapor space to come down so eventually this vapor will
11:08start increasing the pressure in this column so these are the four cases cases can be understand from
11:16this these are the four core pressure scenarios
11:23also this is also a methanol unit where two columns are there then there are some
11:33if you see these are the compressor or turbines are there
11:37okay and there are exchangers separator exchangers the complete system is there cooling water is also
11:44supplied to the few of the so so few are the coolers these are the coolers where the cooling water
11:49has
11:49supplied so first scenario is cooling water failure if the cooling water can be failure based because of the
11:58dripping of the cooling water pump at the cooling tower or closing of any walls in the individual
12:05exchanger so that cooling water has to be considered as
12:09to be seen that it is a local scenario like somebody has isolated the cooling water
12:15wall for the whole plant or some exchanger so based on that cooling water
12:19filler case has to be considered or or the pump in the cooling tower got tripped
12:26power failure if the compressors are running
12:29and if the power which are the motor driven if if that compressor trips so it will
12:36it will it will that power failure case will compressor trips and then it will not able to take out
12:44pull
12:44out the vapors from the column or so that will increase the pressure
12:54like in a vacuum column ejectors are there if that failures that is also increasing the pressure of the
13:00column then liquid overfill we will see there also if if the outlet or any control wall in this section
13:08get closed this liquid start building in this vessel tube failure this is in a methanol synthesis reactor
13:17which is having the tubes and shell so if tube failures happen then there is a chance of
13:24that the high pressures fluid will travel towards the low pressure fluid and if the low pressure
13:33system is like shell is designed for lower pressure then
13:37it has to be protected shell has to be protected from for the tubelic case
13:43then abnormal heat inputs suppose these are the boilers which is taking heat from the steam and
13:50there is a steam control wall in this line so if this control wall gets face open so this
13:56reboiler will get additional it's abnormal heat which is not required additional heat it will get
14:02so because of that there may be generation of additional vapors and which will and you do not have the
14:09sufficient cooling in the overhead so then that vapor additional generated vapor will
14:16increase your pressure so that is also one scenario so we will start to start with the external fire how
14:26to calculate relief load for the external fire case which is very important
14:35so fire may occur in a gas processing facility
14:39and create a greatest relieving requirement so if it is happening in a gas processing
14:46then it is very difficult to manage but generally we have to consider a
14:52pool fire instead of a jet fire for fire case means if the pool fire is happen pool fire means
14:59a liquid pool will be burned and then that flame gets
15:06flame may be heating the vessel if it is if if there is no liquid available in that process unit
15:13then the pool fire is not possible the following equipment not required to consider in fire case
15:21so before so as we want to understand the fundamentals so first thing we should know that what we do
15:27not
15:28need to consider a vessel which is normally contain no liquid as a failure of vessel shell due to overheating
15:37would probably occur before pressure relief wall protect the vessel so if the vessel is not containing
15:43any liquid what will happen it will over it because of the fire and metal get brittle and it get
15:51damaged
15:52before it it reach to the pressure pressure of a set point pressure so in such cases we have to
15:59be a depressurization system
16:02not the pressure relief wall vessel or a drum with two feet or less in diameter constructed of pipe
16:10pipe fitting or equivalent do not require pressure relief walls for protection against fire unless
16:17these are stamped as coded vessel so if the vessels if the very small vessels of two feet and if
16:24it is not
16:25stamped as a coded vessel then relief wall is not required heat exchanger do not need a separate pressure
16:33relief wall for protection against the fire exposure since they are usually protected by the pressure
16:39relief wall in the interconnected equipment or have an open escape path to the atmosphere
16:46so generally exchangers are like if reboiler is there it is protected by the column psv itself so it is
16:53a
16:54not an additional fire case has to be considered however the if the uh heat exchanger comes near the
17:01column and it comes in within that height of 25 feet then the wetted surface area has to be considered
17:08for
17:08heat exchanger but a separate psv is not required for heat exchanger
17:14pressure vessel filled with both a liquid and a solid such as molecular sleeve catalyst not required
17:21or pressure caused by fire so if any vessel or any reactor which is filled with the catalyst
17:27it has to be considered it has not to be considered for fire case it has to be considered for
17:33depressurization
17:35not for the psv calculation
17:42in calculating the fire load for individual vessel assume that the vessel contain liquid at operating
17:48condition and vapour are generated by fire exposure and heat transport so we should not assume that
17:55the liquid is already got heated and then there is a fire also liquid we have to consider as operating
18:03at
18:03operating conditions only determine the pressure relief device capacity for several interconnected vessel
18:12each vessel should be calculated separately rather than determining the heat input on the basis of
18:19summation of the total weighted surface area so if the two or three vessels together expected to be
18:25come in the fire zone and fire can be happened then individual vessel has to be considered instead of
18:33summation of the total weighted surface area
18:37individual vessel has to be calculated independently for the weighted surface area
18:46so it's a very simple formula to calculate the relieving load
18:50the w is the relieving load q is the heat load and l is the
18:55latent heat so w is the relieving load capacity in kg per hour q is the total heat of absorption
19:00kilo cal per hour and l is the latent heat of but this formula is so simple but
19:08to know that what is the heat of total heat of absorption or how to calculate the weight
19:15relieving capacity
19:21q the first question
19:26as per the api the adequate if the adequate drainage and fire fighting equipments are exists
19:32means the draining for the water a fire water to drain properly like
19:36uh you can say
19:40proper slope on the floor and there is a channels which can take the water out and firefighting
19:48equipment like fire extinguishers are there in nearby area so it's called as adequate drainage for
19:53firefighting so if it is available then q can be calculated as
19:58units 21 000 into f into a to the power 82 for british unit and q is equal to 27140
20:07into f into a to the power 82 for metric units
20:12similarly if inadequate drainage or you don't know if the drainage is available or not
20:17and firefighting equipments are there or not then you can take
20:21the credit and uh you can assume but you can put that assumption your cal in your calculation and
20:27then you can use this formula q is equal to 34500 into f into a into a to the power
20:3382 for
20:34battery unit and 61 000 into f into 0.82 for the metric unit
20:42so where in this formula f is the environmental factor and a is the total weighted surface area in meter
20:48square
20:50so next question comes
20:54how to calculate the weighted surface area
20:59so weighted surface area used to calculate heat absorption due to the fire within the 25 feet
21:07or 7.6 meter or generally it takes as a 8 meter above the grade usually referred to the ground
21:14level or a
21:14solid platform as we seen in the earlier slide it should be as either it should be from the ground
21:20level or it should be a solid platform it cannot be from any checkers plate or any open platform
21:28weighted surface area has to be calculated for the horizontal vessel and drum vertical vessel and drum
21:32storage spheres storage tanks other than atmospheric tanks
21:39fractionation and other towers if it is a fractionator uh for the these vessels if these vessels are coming
21:46in this 25 feet of area then the wetted surface area has to be calculated
21:51for fractionator and other towers an equivalent tower dump level in is calculated by adding the liquid holdup
22:00on the tray the liquid at height high high liquid level holdup at the tower bottom and the surface
22:09that is weighted by the equivalent level which is 25 feet above the grid and the level of the re
22:17-boiler
22:17is to be included in the re-boiler included in the calculation so so tower high level has to be
22:24considered
22:28included including the liquid holdup on the tray if the trays are coming in between this
22:338 meter area then liquid holdup on the each tray has to be considered and the highest level of the
22:39at the bottom that will be considered also if the re-boiler comes in that area within the 25 feet
22:47of height also then it has to be considered
22:49temperature for fin fan coolers air cooler exchanger located within the fire zone area if suppose any
22:58fin fan cooler which is at the lower and if we're generally in the refineries or in
23:02big petrochemical plants they are on the pipe rack and they are fell above the 8 feet sorry 8 meter
23:10but if suppose it didn't in any existing plant in all plant if it is there and you want to
23:16read it
23:16or you want to check that psv calculation again then then you can consider this case
23:23then then the air fins located directly above the pipe rack are also normally excluded as i said
23:29because the pipe rack height is more than 8 meter then it can be directly executed excluded and also
23:35because it is a sheltered by the radiation from by the piping because it is on the pipe rack and
23:41there
23:41are a lot of if there are a lot of pipes on the pipe rack already then heat will be
23:47shielded it will be
23:47given to it will taken by that pipe line and the metal which is inside
23:56so that's why and if we have to consider the air fin fan cooler we have to consider the
24:01bare area of the tubes only not with the fins because fins will be get damaged very quickly once
24:08it comes under the fire so only bare pipe area has to be considered for the calculation
24:19in air cooled exchanger are considered separately since unlikely shell and tube unit there are
24:26heat transfer surface is exposed directly to the fire they are designed for designed for ambient inlet
24:34air conditions and rapidly loss of air cooling and condensing ability when they are exposed to the
24:41fire so when we are considering if we know that the a fin fan cooler comes under the area of
24:47height of
24:488 meter and it coming in the fire zone then it has to be considered considered
24:56because it can be it is designed for the only for the air design temperature so
25:00it's chance of getting hitting is very quickly the relieving load could be calculated using the
25:09bare tube area as we just in the last slide we know that we have to consider the bare tube
25:18area only
25:19instead of fin tubes and if the condensing without sub cooling is there that if that cooler is condensing
25:27only without sub cooling means just and condensation means from latent heat transfer then weighted surface
25:33area is equal to 0.3 times of the bare tube area based on the bottom 30 percent of the
25:40circumference
25:42being weighted by the condensed condensate layer so if it is condensing means it is the liquidates at
25:48saturation temperature or it is just getting condensed it is not it remains at the saturation only so in the
25:56top of the
25:57top tubes of the fin fan cooler there will be a vapors only and some at the bottom only there
26:05will be a liquid
26:07because condensation happen in the bottom tube may be on the second pass or third pass depending on the
26:11construction of the air cooler so that has to be considered that's why the weighted surface can be
26:17considered as equal to the 0.3 times of the bare tube area condensing the sub cooling
26:24the condensing section should be threatened as the item and sub cooling section so weighted surface
26:30are equal to the bare tube area if it is a condensing with sub cooling means you can consider all
26:35the tubes
26:36but the bare surface area without fins if gas cooling is there surface area is equal to the bare tube
26:44if it is liquid cooling also it is a bare tube only and one thing needs to be understand sometimes
26:51there
26:52is a guideline that if the fin fan gets stripped you can consider some natural draft cooling but in fire
27:00case we cannot consider the natural draft cooling is happening in the fin fan cooler so this is a caution
27:09point
27:11it is applicable for other place but not for fire in the fire case
27:19so gas cooling service there will be no vapor generation due to fire and the tubes are likely
27:25to fill due to overheating so if it is a gas cooling service so before getting more vapor vapors are
27:34already
27:34there so then there is a chance of failure because it's designed for the air cooling temperature only
27:41normal condensing load for air cooler condenser must be added to the calculation fire load and other
27:48surfaces so heating and as well as the water the queue of that exchanger to be considered for the
27:57relief load calculation in the wetted surface area causes by air fan cooler the area term a would be taken
28:06to an exponent to an exponent of 1 instead of 0.82 we seen that the 67000 into f into
28:15a to the power
28:160.82 in in in to calculate the wetted surface area for the air fin fan cooler we instead of
28:220.82 it can be taken as
28:231 so it will be a conservative calculation also the because fin fan coolers are not anywhere is the
28:34insulated so environmental factor also has to be considered one
28:43so now we understand how to calculate the wetted surface area for the air fan fan cooler for the
28:48column for the vessels for the exchanger now the second part of that calculation is how to know the
28:54environmental factor how to calculate the environmental factor
28:58so for insulated vessel the environment factor for insulation becomes based on the following formula
29:06so f is the environmental factor k is the thermal conductivity of insulation we have to
29:15check the units BTU into ft square by f
29:21t is the thickness of insulation and the capital T is the temperature of the vessel containing the
29:27living condition in degree Fahrenheit it shall be noted that if the pressure relief facility are
29:34based on the environmental factor with insulation this facility should be rechecked for environmental
29:39factor of 1 if the insulation is removed so as I said if you do not know that insulation is
29:46properly
29:46there or not or if it is removed then you have to consider the environmental factor as 1 otherwise
29:51you can if you are sure that or if if you are designing it for the new unit and then
29:56you know that
29:56the insulation will be properly insulated that vessel will properly insulated and insulation is not
30:03damaged so then you can use this formula
30:11also there is also there is also there is a guideline available in api
30:17that for the bare vessel we have to use one and based on the insulated vessel
30:23within the insulation conductance value these are the conductance value for the insulation if you know that
30:30the conduction conductance value for the insulation then you can take up
30:34environmental factor accordingly if the conduction value is 4 actually the conduction value are the
30:39computed value from the equation are based upon the insulation having thermal conductivity of 4 btu
30:47to 4 btu at 1000 degree Fahrenheit and correspond to the various thickness of insulation between 1 inch to 12
30:55inch
30:55so this is a computed value given in api based on the thermal conductivity between 4 btu at 100 degree
31:06Fahrenheit
31:07so if you have available this information you can use this environmental factor also for water application
31:16facility on bare vessel environmental factor to be considered one depressurization and
31:21emitting facility you have to consider
31:25environmental factor as one earth covered vessel
31:29so which are the buried vessels you can consider the environmental factor as 0.03
31:33and below grade vessel no need to consider anything
31:38so no fire case for the underground vessels or buried vessel below the great vessel
31:51so now we found out that how to get the wetted surface area then how to get the
31:58environmental factor and now the last point we based on the wetted surface area and the environmental
32:05factor we can get the q value now we want to know the what is l which means the latent
32:11heat
32:14so for single component system the term lambda or latent heat equal to the latent heat of vaporization
32:21at relieving condition when the it is a single component system like a propane only or butane only
32:30but generally this not happen or any like water only water only steam only whatever depending on the
32:37or process a single component but this should be single component it may be determined from the
32:42flash calculation as a difference of specific enthalpies and vapor liquid phases equilibrium
32:49with each other it may be obtained from api api rp 521 appendix a figure if one this needs to
32:58be
32:58checked because api is getting if getting updated and if the maybe number may not change but if any changes
33:05are there but this is for only for the single component system which is available in api the peak
33:11relief load will always occur at the start of the fire when the wetted surface area is area is a
33:19and
33:20consequently the heat input is q are both at the maximum so peak load had starts at the start
33:29and if if the relieving pressure is beyond the critical pressure use 50 btu or 27.8 kilo cal per
33:36hour as a latent heat or some places if you do not able to calculate the the latent heat you
33:46can
33:46consider this 27.8 kilo cal per hour for the conservative calculation or a 50 btu per lb as a
33:54conservative
33:54if you do not have the software to calculate the flash calculation then this is this can be but this
34:02has to be written in your basis perfectly so because based on this you may be get some over design
34:09in
34:09your psv so accordingly you have to see your inlet line size and other things will should not impact
34:15but if you do not have any latent heat available to calculate you can consider this
34:21as a for the preliminary calculation like in feed if you do not have the during feed calculation or
34:29in the initial state of project if you do not have the more information you can go with this number
34:39so otherwise this is the graph which is available in api you can take it you can check this for
34:46the
34:51single component for the multi component so mixture of generally hydrocarbons or in refinery we have a we
34:58do not have a single component it's a mixture of components so using the composition of a residual
35:03liquid inventory in the vessel perform a bubble point flash at the accumulated pressure in doing this flash
35:10the flow rate the flow rate will be flow rate can be anything or you can take any unit number
35:15because
35:15it's a we have to calculate we have to divide it by per unit we have to calculate so any
35:21unit you can take
35:22you can consider a thousand unit of mass flow rate for doing the bubble point color of flash for the
35:29multi component system so flash liquid from from the preceding flash at constant pressure and the weight
35:39percentage vaporized equal to one to five so this you can do in the high c simulation
35:45for the liquid you can vaporize it for the one to five percent of flash the multi for the multi
35:52component only
35:53divide the heat duty calculated for the flash by mass flow rate of a vapor generated so the result will
36:02be heat of absorption per unit mass of vapor generated so when you you have to how to do we
36:08have to take
36:09a component we have to take a stream we have to put all the components in that and then you
36:14just flash
36:15you vaporize one or one to five percent of the you have to put in the vapor fraction as 0
36:21.05 or 0.01 then
36:24when the result comes it will give the vapor flow rate and liquid flow rate as well so you have
36:29to
36:29divide that vapor flow rate so what are the duty heat duty comes you have to divide it by mass
36:35flow rate
36:35so that will give you the the heat of absorption per unit of mass generated
36:44however it should be noted that in generally this wall this value will not equal to the latent heat
36:51of vaporization nor equal to the difference in vapor and liquid specific enthalpies in fact the value
36:58thus calculated will be generally exceeding the latent heat of vaporization especially in case of
37:04wide boiling mixture and the reason for a reason is that the significant portion of the heat absorbed
37:11goes into the rising the temperature of the system means the sensible heat the most of which is a
37:18residual liquid at the point and to the equilibrium temperature of the flash that is a sensible heat
37:23because lot of heat will go for the increasing the temperature to get it saturation temperature
37:28and then it will change the phase so that will be the latent heat
37:37so if this is this is the calculation for the wetted surface area but if you as we talked in
37:44the
37:45earlier slides unwetted surface area is also there so unwetted wall vessel are those in which the internal
37:53walls are exposed to gas vapor or a supercritical fluid so in such cases we have to use this formula
38:01weighted surface area or the sorry relieving load for the unweighted surface area is 0.1406 into
38:10under root of m into p to the power of 1 into the bracket a t w minus t 1
38:17divided by t 1 with this
38:19so w is a relieving capacity m is the molecular weight p 1 is the relieving pressure it is a
38:27set pressure
38:28and allowable pressure and atmospheric pressure it is a relieving pressure we have to consider
38:32then a is the exposed surface area you have to check that if it is coming in
38:42it is coming in that 25 feet area or not and based on that surface area we have to calculate
38:46it is
38:47and that exposed surface area then t w is a vessel wall temperature vessel wall temperature is this is the
38:54maximum temperature of the vessel or wall which vessel can sustain it is not the operating temperature of
39:03the wall it is a so at the whatever the temperature comes the worst temperature which that material can
39:13handle like carbon steel or any any material or ss t 1 is the gas temperature absolute in ranking at
39:25the
39:25upstream relieving pressure determined from the relationship so according to api recommendation
39:32recommended practice sizing selection and installation of pressure relief device in refineries
39:38the conventional assumed maximum wall a vessel wall temperature for typical carbon steel is
39:451100 fahrenheit or equal to 593 degree c so that tw if it is a carbon steel vessel you can
39:50take as 593
39:52and then minus the operating temperature of the gas this value is used for the specific api
39:59521 formula which is this this is api formula only for calculating the required relief capacity for
40:06gas filled unweighted gas filled unweighted vessel exposed to the fire
40:16so in unweighted relief load unweighted surface area relief rate calculation the api formula which
40:25which have seen in the earlier slide for the relief load is an unweighted fire case is based on the
40:32heat
40:32transfer from the hot vessel wall to the content gas okay tw and t1 using a very high maximum temperature
40:40of tw ensuring the calculated relief rate accounts for the worst case heating scenario
40:47so material failure concerns comes there in unweighted fire case where there is a no liquid to absorb the heat
40:55and keep the wall cool the vessel wall can heat up significantly faster than the internal gas
41:02pressure increases so before increasing the pressure
41:06your vessel can heat up significantly which and it can be get damaged
41:11so vessel rupture versus PSV activation so we have to see that before putting a PSV on
41:17a unweighted vessel the primary risk of unweighted vessel is that the wall material will become a severely
41:27weakened by the high temperature causing the vessel to rupture due to loss of tensile strength before the
41:35pressure reach the relief wall set set point therefore the maximum wall temperature assumed for carbon
41:42still is 1100 degree Fahrenheit is also related to the temperature which common vessel material
41:48lose the significant strength to contain the pressure so because before it reaches to the
41:56set pressure of the PSV it will lose the tensile strength and it can be damaged
42:02because of this material failure risk api 521 often recommended deep pressure agent system
42:08or a water spray as an additional layer of protection for unweighted vessel exposed to fire as PRV alone may
42:17not be sufficient so when somebody asks you to put a PSV or design a PSV for unweighted surface area
42:25you
42:25all these four points you have to explain them and then if there is a better option is to go
42:33for the
42:34depression system or water spray if it is available depending on the typical situation
42:44so we the conclusion of this session we have seen the different scenarios of overpressure and then
42:52how to calculate relief load for fire case for weighted surface area and unweighted surface
42:59this so we have established the sizing for fire case is not merely an exercise in mathematics but it
43:06a critical matter of a system integrity and safety because we do not want to design a pressure safety
43:13wall which will not popped up before and before that your vessel or a system or your pipeline will get
43:21damaged the key principle to take away from this the conclusion of this understand what is the
43:26wetted surface area that is the 8 meter height and it is from the ground and all from the solid
43:32platform
43:32not from any other platform the unweighted thread which we seen in earlier slides for gas filled vessels
43:40focus shift danger is not just a pressure but a material failure so it has to be understand that it
43:47is not a
43:47uh increasing the pressure it is damaging the or failure of the material
43:54also apf i29 gives us a guideline not the guarantee ultimate ultimately the goal of the calculation is
44:02not just to comply with code but guarantee that our final safety device the pressure relief wall is
44:09sized correctly to protect the vessel and the environment under the most extreme life threatening scenarios
44:16so that is our aim it is not to comply only api or any guideline but here all the guideline
44:24has to be
44:25follow whatever the guidelines are given they are based on the apis and the standard books so
44:33that are the valid guidelines which are followed for the various calculation
44:39if this fundamental calculation helps you in mitigate the risk of your design hit subscribe button for more
44:47critical engineering insights a quick share ensure every engineer is equipped with this fire case scenario calculation
44:59so thank you very much write your question and comment i will be happy to answer it
45:06you can reach us on conceptengineering2025 at gmail.com
45:12links and links for the other sessions are given in this description
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