00:01hi friends welcome to the channel we are in module 1 for pressure relief fall and
00:06this is a session 3 on relief load for non-fire contingencies and this is a part 1 before that
00:13we have completed the relief load calculation for fire case so let's start disclaimer for
00:21our channel and the content you shown in this videos over pressure scenario in process industry
00:29these are listed here and which we are seen in detail in earlier session and we have covered
00:35the relief load calculation for external fire case and now we will see for the other cases
00:42control wall failure
00:47control wall generally installed with a fail open fail close or fail safe position
00:53but when we are calculating the relief load irrespective or regardless of its position
00:59safe position it has to be considered for a failure like if it is a fail say a fail closed
01:05then it has
01:06to be considered as fail open for the calculating the relief load
01:11also when we are calculating the relief load we have to consider the installed cv
01:17or the coefficient of discharge for the control wall
01:22and the maximum flow through the installed cv the installed cv generally given by the vendor and
01:29the on the given installed cv we have to calculate the maximum flow for the relief load
01:38so when we are calculating the relief flow on cv we have to consider we have to calculate it on
01:46minimum pressure or the permanent pressure drop not on the normal pressure drop it should be the
01:51minimum pressure drop so this is a general schematic of control wall some control walls will have a bypass
02:00so once we it have a bypass it has requirement to calculate the relief load calculation in case of
02:07failure of control wall as well bypass there is a different requirements
02:12so when control wall is having bypass the cv for the bypass wall all should not in should not be
02:22higher
02:22than the cv of the control wall and it shall it can be considered as the same cv of the
02:28control wall
02:34so in some cases uh it has considered that the control wall will fail open as well as the bypass
02:41wall will
02:42partially open so in that case we have to consider the total flow through the control wall
02:48on the installed cv with the minimum pressure drop as well as 50 percent of the bypass wall cv which
02:59is
02:59generally equal to the 50 percent of your installed cv of the main control wall
03:06that is regardless of actual size of bypass wall and if the bypass wall considered to be wide open
03:13with the control wall closed or blocked in then relieving rate in that case has to be calculated
03:19on the wide open bypass wall and we have to consider the bypass wall installed cv generally
03:26the normal wall or a manual wall having a higher cv than the control wall so this case has to
03:32be
03:33considered separately for the relief load calculation the manual wall should be considered
03:40are passing its capacity at full open position with the pressure in the vessel at relieving condition
03:46so whatever when we are calculating the relief load the relief load has to be calculated on the
03:51relieving condition not on the operating condition of the wall or not on the maximum pressure of the wall
03:56it should be on the relieving condition if control wall operates at 10 bar but however the set pressure
04:01of the psv is at 15 bar the relieving condition has to be calculated on the 15 bar
04:07so to calculate the flow rate through the control valve for the control valve failure case
04:13control valve will be used for either liquid or vapor or steam so to know the flow rate from the
04:20selected
04:21cv or the installed cv if we have a cv value then we can calculate through the formulas given in
04:28this
04:28slide so for the liquid flow rate the relief load will be 27.3 into installed cv or a selected
04:38cv given on
04:39the data sheet of existing wall into density of liquid into a square root of density of liquid into
04:49inlet pressure of the control valve and p2 is the relieving pressure of or the set pressure of the
04:58relief wall for the vapor flow rate if vapor is critical flow the critical flow defined as
05:07when p2 is less than or equal to 0.5 of p1 means if the in if the inlet pressure
05:16is 50 bar and outlet
05:17pressure is 20 or your psv set pressure is 10 bar or 20 or 15 bar which is less than
05:24half of the
05:26inlet pressure inlet pressure so in that case your flow vapor flow will be the critical so for critical
05:33flow to calculate the relief rate on the selected cv is 56.9 into installed or selected cv into p1
05:45square root of molecular weight by inlet temperature t1 and for non-critical vapor where the inlet pressure
05:56is or you can say outlet pressure is more than half of the inlet pressure if inlet pressure is 50
06:04bar
06:04and outlet pressure is 35 bar or a 30 bar then this is a non-critical vapor vapor flow so
06:11for that
06:12you can use the this to any two of the formula if you know the density you can go with
06:16the first
06:17formula if you know the molecular weight you can go with the second formula
06:21so in this formula w is the relieving capacity cv is the control valve coefficient which is the
06:30this is the selected one i am repeatedly telling not the normal or required cv it is the installed cv
06:40p1 is the pressure at control valve inlet based on the normal operating pressure p2 is the pressure of the
06:45control valve at outlet that is equal to the psp relieving pressure and unit also has to be
06:52considered it's all in absolute m is the molecular weight t1 is the temperature at control valve inlet
07:05so based on these formulas we can calculate the relief load for liquid in from the control valve as well
07:15as the vapor for critical and non-critical both the cases and if it is a steam then
07:23for stream for critical steam flow again the critical steam flow means your p2 is less than or equal to
07:320.5 of p1 which is same if it is a 50 bar inlet pressure and outlet was 15 or
07:39less than 25 then it
07:41is a critical flow so for that critical flow we have to use the formula for really flow w is
07:4611.76 into
07:48selected cv or installed cv into inlet pressure divided by 1 plus t into TCH and for non-critical flow
07:59rates
07:59we have to this TCH is the steam degree superheat which is the superheated temperature minus saturated
08:08temperature at that for the steam for that condition it is in Kelvin so for so for critical steam
08:17flow and non-critical steam flow we have to use this formula to calculate the relief flow from the
08:21control valve
08:25there is a one alternate method for calculation the cv which is when we we initially starting
08:33the psv calculation and we do not have the control valve information we only have the control valve
08:39calculated cv and we do not know what vendor is going to give us
08:44in such cases in such cases when we are working in the feed stage of the project or early design
08:49stage of the project
08:52so to calculate the process require so for that what we have to do we have to calculate the cv
08:57based on the process requirement suppose that calculated cv comes around 100
09:07then the selected cv to be considered as 200 percent of the calculated cv so you have to consider
09:15200 as a selected cv or a installed cv which is going to be installed on vendor which is
09:22which will be given by the vendor this is for the earlier calculation and if you have to consider
09:29the bypass for that control valve and your scenario is like control valve failure as as well as bypass
09:37valve is partially open in that case you have to consider 300 percent of the calculated cv so this
09:44is the calculated cv 100 is our calculated cv this is our installed cv
09:51so in case of bypass the cv will becomes 300 so when you calculate the flow rate considering the cv
10:02value
10:02of 300 in earlier formulas for liquid critical vapor non-critical vapor or steam or non-critical steam
10:10then you will get the relief load for the during the design stage
10:16uh design stage itself and that can be used to calculate the orifice size for the psv
10:25so in simple way 100 percent cv is the pressure required uh process required cv then 200 percent
10:32is the maximum cv value or you can say installed cv value and three percent c 300 percent cv is
10:38the
10:38maximum cv plus bypass valve open so in this way we can calculate uh relief load during the early design
10:45stage of the project or during the feed stage of the project the second scenario non-fire case is
10:53blocked outlet or a closed outlet so suppose in this schematic this is a column and column overhead
11:02having the vapor which is getting condensed in air fin fan cooler then uh water cooler and then it is
11:08getting separated separated in a separator reflux drum and vapors or lighters are going to further
11:15processing and then reflux is sent back to the column and overhead product is taken out parallelly
11:24so suppose this pressure control wall on the uh reflex drum get closed
11:33or this will be considered as a block outlet so it is blocking the vapor pass out of the reflux
11:40drum
11:40so what will happen it start increasing pressure in the reflux drum and that will govern your pressure
11:46of the column as well so that's why we have to install a pressure relief wall and we need to
11:53get the relief load for this pressure safety wall
12:01so the capacity of generally in the relief wall in a block outlet case is at least the capacity of
12:07the
12:07source pressure so whatever the my source pressure is the same capacity i have to use for the
12:13relief load calculation in block outlet the quantity of material to be relieved should be determined
12:20at the condition that correspond to the set pressure plus over pressure instead of normal operating condition
12:28so basically when it is a block outlet case liquid relief you have to consider the maximum liquid
12:33pumping rate and for vapor relief the total incoming stream vapor generated during the relieving condition
12:41and all the so in simple way from the block outlet case you have to consider the maximum inlet flow
12:49from
12:49that line or from that system or from that from that vessel to be considered however the relieving however
12:59the properties or the rate has to be calculated on set pressure plus over pressure instead of normal
13:08operating pressure that points has to be understood
13:12so third case is the cooling water or column reflux or a pump around failure
13:18so when reflux flow failures the failure cause the flooding of the condenser which is equivalent to
13:26the pressure relief wall capacity so when the reflux flow closed it will have a it will give the impact
13:34it will flood the condenser and the total capacity of your condenser is the your relief load for that scenario
13:42however it has to be noted that when the composition which is coming out of a condenser is different in
13:53normal
13:53operating case and when it is in a failure case of reflux your composition will be changed in the column
14:01itself
14:01because we do not have there may be some heaviest also traveling towards the vapor side because you do not
14:10have the reflux
14:10so compositional change has to be considered and to be taken care of case by case
14:17usually pressure relief size for the tower head with adequate conditions but each case must be examined
14:24what i said now if the pump around failure case basically what is the pump around in a column
14:32when there is a lot of heat in the field is available it will be cooled uh to like preheating
14:40for the preheating
14:41for the other uh feed or the product and the column uh it will be taken out of the column
14:49it will be cooled
14:50and again sent back on the top trays of the column so that will remove your uh that will reduce
14:56your load on
14:56the condenser and this will uh help in uh recovering the heat that's why pump around are given to the
15:05column
15:06however uh if the pump around duty is very high the functional high and highly superheated then pump if
15:16the loss of pump around will uh will increase your uh reduction in tower cooling and result in dry
15:24running of the tower so that case to be considered the potential dryout should be evaluated this tray
15:30will be completely get dry if your pump around stops and the relief load due to the fractionated
15:38dryout is usually the sum of your vapor entering to the column which is to the feed itself
15:44and plus if you are putting any stripping stream into this or you have a reboiler in this so whatever
15:51the duty of this reboiler or vapor generated through this reboiler or you have a stripping stream
15:58and the vapor available in your feed that the total will be your relief load so that's why in
16:07uh non-firing fire case scenarios we do not have much calculation but we need to engineering judgments
16:14to be taken the relief flow requirement in the vaporization rate caused by an uh amount of heat
16:22equal to the removed by the pump around so latent heat of vaporization will be corresponding to the latent
16:27heat under relieving conditions so uh when this vapors heat load we have to consider the latent heat
16:35uh for vaporization will be corresponds to the latent heat under relieving conditions so
16:40when we are calculating all the relieving cases it should be noted that all the relieving should be
16:46calculated on the relieving pressure only not on the operating conditions
16:52if it is a total condensing the relief load requirement is equal to the total incoming vapor
16:57rate of the condenser so whatever vapors coming from the condensers or the tower
17:02it will be your relieving rate and similarly you have to calculate the composition and the
17:13composition based on the relieving condition not on the operating condition
17:20and if the search capacity of overhead generally search capacity remains around 10 minutes
17:25in the enured condenser so your failure is more than 10 minutes it means you already lost complete
17:32reflux so whatever the vapor coming from the tower that will be your relieving load
17:39in partial condensing basically in partial condensing what happen uh vapor are not getting completely condensed
17:47so incoming is vapor and outgoing is two phase flow maybe partially liquid and partially vapor so in that case
17:56you have to take difference like uh 100 kgs or 100 kg per hour vapors are coming and 50 kg
18:04are getting
18:06condensed and 50 kg are still vapor so the load for the relief in this case has to be considered
18:12for the 50 kg per hour
18:14only but that also on the relieving conditions
18:23louver closer so louvers are installed on the thin fan coolers generally if the louvers get closed which
18:29are on the top of here if the louvers get closed you are not able to pass the air through
18:34the thin fan
18:35cooler so in that case it has to be considered as total failure of the coolant and whatever the load
18:42vapor load coming to this cooler has to be considered for the relieving capacity
18:48top reflux failure
18:52it is the uh the total incoming steam vapor plus generated within the relieving condition
18:58less the vapor condensing by the side stream refluxes
19:02so basically top reflux failure means
19:05it has to be again the total vapor coming which is getting condensed and
19:11whatever the only ref because of the reflux whatever the it means the condensers are working
19:17however the reflux pumps has stopped so whatever the cooling happen by the reflux that has to be minused
19:23from the and that to be considered for the relief load not the complete condenser load
19:32if the side stream reflux failure difference between the vapor entering and leaving to that
19:36section has to be considered if fan failure considered for this fan no louvers close nothing
19:43only the fan get tripped then we can have we can consider some uh partial condensation because of
19:50the natural convection and that that remains around 20 to 30 percent suppose the the load of this is
20:00100 kilojoule per kilo cal per hour so 20 to 30 percent of it so you can say 80 percent
20:07will uh suppose if it we consider the 20 percent margin so 80 percent 80 kilo cal per hour should
20:14be your relieving rate in that case due to we will get the margin of 20 percent based on the
20:20natural draft convection
20:24solution thank you very much and follow our channel like and subscribe it and if you have any comments
20:31uh we are we are happy to answer it you can write us your comment on conceptengineering2025gmail.com
20:40link for other sessions are also given in the description box so
20:43please go through the other lectures as well thank you
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