00:00hi friends welcome to our channel we are in module 1 last session session 6 pressure relief wall and
00:07in this session we will see how to size the orifice for the two phase flow this is little bit
00:13critical
00:13session and little bit theoretical so it is divided into two parts so this is the first
00:19part and the second part will be the next video so watch it carefully and let us start
00:28disclaimer for our channel and the content you shown in this videos
00:35so sizing for two-phase relief liquid and vapor so basic thing is what is the two-phase flow and
00:41what
00:41it means in a psv a pressure relief device handling liquid or a vapor at its equilibrium or a mixed
00:51phase fluid will produce a flashing with the vapor generation as fluids passes through the psv or a
00:58psv device the vapor generation will reduce its effective mass flow rate
01:07that's why balanced or a pilot operated psvs are required because there is when the flashing
01:13happens it increase the your back pressure so conventional wall cannot be used so that's why
01:19if you need to have a balanced or a pilot operated walls when you expecting any two-phase flow at
01:24the outlet of the psv and the actual flow rate through the psv will can be of many more times
01:32higher than
01:32what you predicted because if when the two-phase forms it flashes and it will give you at lower density
01:38the
01:39volumetric flow rate will be increased maybe the actual mass flow remains same but the volumetric flow changes
01:47and also when the two-phase happen and if it is flash you have to see the auto refrigeration that
01:54may
01:54arise and that will reduce the temperature at the outlet of the psv and and that's why your outlet lines
02:03shall be designed for the lower temperature conditions and also this may form any blockage because of
02:11in the network if there is a there is any icing or any blockage can be formed because of this
02:18auto
02:19refrigeration one thing also needs to be noted that no pressure relief device with certified capacity for
02:25two-phase flow is available as there is no testing method available for this certification so two-phase
02:33ps two-phase flow psvs are not certification certification for that is not available
02:41there are two main models for the two-phase sizing the first one is a homogeneous equilibrium model
02:47and second is a omega model homogeneous equilibrium model uses the isentropic flow equation where the
02:53it considers the entropy remains constant and it integrates the pressure drop from inlet to the throat of
03:01the nozzle to determine the maximum flux and it also assumes the insogenous flashing
03:08equation where ever omega this is advanced model for two-phase two-phase method and it also depends on
03:15principle of integrated isentropic nozzle flow however it introduces an additional omega term
03:21which modifies the equation and its account for the complex changing properties
03:26in the mixture at the nozzle of the throat
03:33homogeneous equilibrium model one things to it shall be noted that this model
03:36equilibrium models and the omega model these are to get the properties at the two-phase it is not the
03:43calculation it is not giving you the formulas it tells about the properties method so homogeneous equilibrium
03:51model it assumes that the fluid mixture behaves as a pseudo component
03:55means the one component with the density that volume average of the density of the two-phase so
04:00and it considers it has a pseudo single phase and density will be the average of the density of two
04:07phases these methods assume that the thermal and mechanical equilibrium exists
04:12as the two-phase fluid passes through the prv so it's considering the thermal and mechanical equilibrium so
04:17there is no change in pressure when it passes through it and there is a thermal equilibrium as well
04:25the homogenous equilibrium model assumes that the liquid and vapor phases are traveling at the same
04:30velocity this is very important and are perfectly thermodynamical equilibrium at the point at any
04:37instant flashing and it goes for the instant flashing and usually yield in the low that's why it is usually
04:43yields in the lower area required it also assumes that liquid does not flash during very brief transit
04:51so it not flash means it its pressure remains constant as well and there is no mass transfer as well
05:00and homogeneous equilibrium model it's a simplest and most common starting for starting modeling for
05:07the two-phase flow however because of the flashing process is not instantaneous in reality a basic
05:14HMA often underestimate as i said it may give you a lower area requirement
05:21meaning it is often used with a multiplier or a correction factor so super and also it can be
05:27superseded by the rigorous method like omega method for the complex flashing mixtures
05:35the omega model this method is non-equilibrium model that accounts for the complex thermodynamics
05:43behavior of the fluid as it expands and flashes through the relief wall basically it's a it's it's
05:48calculate the omega which is a single parameter which represents the pseudo molecular weight of two-phase
05:54mixture at nozzle throat this is in a rigorous analytical non-equilibrium model it accounts the
06:02complex thermodynamic changes and the fact that the liquid may not fully flash during the brief residence
06:09time of nozzles so it's account that this is a dimensionless parameter that characterizes the expandability
06:18or compressibility of a two-phase mixture as phase through the wall orifice so like omega when you
06:25calculate omega omega is zero then it's a pure liquid flow incompressible when omega is one it's a pure vapor
06:31flow
06:31and omega when it is in between or less than one the flashing two-phase flow that indicates a flashing
06:37two-phase flow
06:40the omega parameter is calculated using a specific volume
06:44or a density of a fluid mixture at inlet and at the and slightly reduced pressure
06:52suppose around 90 percent at inlet pressure only so at these two places it's a two-point method basically
06:59omega method is a two-point method
07:04there is an additional method which is similar to homogeneous equilibrium method called as homogeneous
07:10frozen flow model basically this this is a liquid and vapor phases travels at the same velocity the same
07:18assumption which is taken in homogeneous equilibrium method is also follow the same assumption
07:23but there is a no mass transfer
07:27or as the fluid passes through the wall orifice and mix the mixture composition is frozen at the inlet
07:34condition this is the assumption
07:35for this homogeneous frozen flow model this model typically use the scenario where the pressure drop
07:42is very small through the wall or the through the road the wall opening time is extremely brief
07:49so it's considered the wall opening time is also extremely which it's usually yields larger or more
07:55conservative area than the homogeneous equilibrium method as ignore the density reducing effect
08:01of flashing so it is it's not considering the reduction of uh density in the flashing and it considers the
08:09conditions are frozen uh whatever given in the inlet condition so that's why it will give a very
08:14little bit higher uh orifice area so this is an additional method
08:21as we seen the three models homogeneous equilibrium model
08:26omega model and homogeneous frozen model so how to select the correct model
08:33so homogeneous equilibrium model used when what is the assumption is a perfect equilibrium assume the
08:40liquid flashes instantly and fully to equilibrium state uh what is the result it's it's least conservative
08:51and used theoretical baseline often considering non-conservative unsafe for many real world
08:57flashing flows omega model it is used when the non-equilibrium
09:02analytical it used an empirical parameter which is called omega and analytically account for a
09:10partial or delay flashing effect
09:12effect it is a it is a balanced or regress method you can say typically falls between
09:18hhf hff and hem which is homogeneous frozen flow and homogeneous equilibrium model
09:26it is most recommended method widely used in rigorous methods complex for two-phase flashing
09:31flow in industry and commercial softwares homogeneous frozen flow method it's a non-equilibrium method again
09:40zero flashing it's assumed no liquid flashing into vapor in the wall so walls when the it passes it's
09:47assuming that there is no flashing happens the most conservative one it gives the largest area
09:54when it calculate and used for highly viscous fluid or or the when the required by a standard for maximum
10:01safety margin example for a small systems then this hff is used however you can find in
10:09high cis you will find the homogeneous equilibrium model is used majorly but these models with the
10:17mathematical formulas or with the mathematical formulas we have to calculate the two-phase flow
10:23we will see in the next coming slides
10:28so different methods of two-phase flow calculation the method one is direct integration isentropic nozzle
10:34flow method second method is sizing for two-phase flashing and non-flashing
10:38flow through the prv method and third method is the sizing subcooled liquid at prv inlet using
10:45the omega method so these are the three main methods for calculating the two-phase flow calculation
10:51this each method we will see in the next video for because it's take a lot of formulas and we
10:59have to
10:59calculate the maximum flux and then from maximum flux for this first method isentropic nozzle method for the
11:08second method sizing two-phase flow we have to calculate omega factor because it's take the omega model
11:14so omega factor we have to calculate and based on omega factor you can calculate g max which is
11:20the maximum flux and from maximum flux you can calculate the required orifice area similarly for
11:25the method 3 sizing subcooled liquid it is also calculating the omega and after from the omega g max and
11:32then from g max you can calculate the
11:34uh orifice area so we will see in the next session
11:39in the last slide we've seen the three method method one direct integration of the isentropic nozzle flow
11:45then method two is sizing for two-phase flashing non-flashing flow and method three was sizing for
11:50subcooled liquid at the prv inlet so when these three methods are used just a brief table is given here
11:58so when the two-phase liquid or vapor relief scenario example when the two-phase system
12:04enters prv and flashes no non-condensable gas presents also include the fluids both above and below the
12:11thermocritical properties
12:15thermocritical point in condensing of the two-phase if this case is there
12:19then you can use method one or two so what is the example of it when the saturated liquid vapor
12:24of
12:24propane system enters the prv and the liquid propane flashes so this case is when such type of case
12:33comes you can use model method one or method two method one means you can use the direct integration
12:40direct integration of eccentric nozzle method you can use or either sizing for two-phase
12:45flashing and non-flashing flow through prv you can use the second case when the two-phase system
12:52highly subcooled and either non-condensable gas and condensable vapor both enters prv and does not flash
13:02so when it does not flash two-phase system with a subcooled so that time also you can use more
13:11method one and
13:11method two which is direct integration method is entropic nozzle or uh sizing for two-phase flashing
13:19non-flashing so example is highly subcooled propane and nitrogen enters the prv and the propane does not
13:26flash so when mixture of nitrogen which is non-condensable generally considered and propane
13:32which is a subcooled so when it pass through the prv the propane will not flash only non-condensable
13:37nitrogen will be there similarly third case when the two-phase system which contains the vapor at the
13:45inlet and non-condensable gas and the liquid is either saturated or subcooled so in this case liquid
13:52is either saturated or subcooled and there is a non-condensable also present enters the prv and flashes
13:59so non-condensable gas enters the prv in this case also you have to use method one or two
14:05so example is saturated liquid vapor
14:10propane system and nitrogen enters the prv and the liquid propane flashes so in this case in the
14:17second case propane was not flashing in the third case propane was flashing still we can use
14:22method one or method two and the last method is a last point is subcooled liquid enters the prv and
14:31flashes no condensable or non-condensable gas enter the prv so in this case subcooled propane enters the prv and
14:38flashes
14:39even its subcooled propane still it once it enters it flashes in that case you have to use method three
14:44which is
14:45sizing the subcooled liquid of prv inlet using omega method so only for that case you have to use omega
14:52method
14:53method three so we will see how to all these three methods this is just introduction like
14:59which method to be used when and then we will see in each method how to calculate or face size
15:04for the two
15:05phase system so thank you very much write your question and comment i will be happy to answer it
15:15you can reach us on conceptengineering2025 at gmail.com
15:21the links and links for the other sessions are given in this description
15:26you
