00:00hi friends welcome to our channel most of the people know the inlet line pressure drop for a
00:06pressure relief wall is three percent but why it is three percent limit to three percent
00:13most of the people are not knowing so we'll find out the answer in this today's video so let's start
00:21disclaimer for our channel and the content you shown in this videos so pressure drop criteria
00:28for psv as we know the three percent rule is non-negotiable requirement mandatory by the
00:34apf i 20 sizing selection and installation for the relieving device which governs the process
00:42equipment safety worldwide so that's why we have to follow this rule three percent rule so maximum
00:48pressure drop should be less than or equal to three percent of your set pressure
00:56so why this criteria is critical the single most important reason is for three percent limit is to
01:03prevent the wall from chattering so this criteria used for reducing the chattering or you to avoid
01:11the chattering so what is chattering chattering is a rapid unstable cyclic wall repeatedly lifting and
01:18closing or relifting it's extremely destructive and can severely damage your wall so to avoid the
01:25chattering and chattering is the continuous opening and closing or lifting and closing of your pressure
01:30relief wall so how does this inlet pressure drop cause the chattering when the system pressure increases
01:39and set pressure of the prv begins to open the lift when the system pressure reaches to suppose p1
01:46which is your set pressure will and your uh set pressure from this spring load is also considered as p1
01:54only
01:56so as soon as the wall start flowing the velocity in the long undersized inlet pipeline increases and
02:03causing the friction losses so once this pressure reaches just above the your set pressure of p1
02:08greater than your p1 the uh you can say this is p2 which will be the greater than your p1
02:15of your
02:16set pressure then psv lifts and it starts discharging so once it starts discharging the the fluid flows
02:23through the inlet line long inlet line it and because it is not designed properly it will give you a
02:29more
02:29pressure drop suppose the more than three percent like five percent or ten percent so because of that drop
02:37we'll see due to this friction loss the pipe pressure and actual inlet flange walls falls rapidly so
02:44because of this friction drop this p2 whatever previously is higher than your set pressure it
02:48starts dropping so p2 starts dropping again back because of the frictional pressure so drop in inlet
02:55pressure reduces the lifting force acting under the wall disc and closing force of the spring then becomes the
03:03movement really dominant so when is this p2 decreasing when the p2 get reduced lesser than your p1
03:13p1 p1 is your set pressure so this spring pushes this disc down and wall start closing
03:22so spring force overcome to reduce the lifting pressure and causing the disc to slam back and
03:27reduce the lift so it start closing the wall and this is a cyclic action and it's repeating so inlet
03:36the inlet flow stops because this wall disc goes down inlet flow stops pressure quickly recovers so
03:44whatever the pressure because of the some upset of the pressure process this pressure against because
03:50you have stopped the flow rate going out of the psv so again this pressure start increasing because of
03:57you have stopped sending it out so this increasing pressure quickly
04:01and the wall relifts so because this pressure again then p2 is again going higher than your or you can
04:08say greater than your p1 and again this psv starts open so this way the cyclic action of opening and
04:15closing
04:16which is called chattering will starts and and then wall clamps and shut down again this when this flow
04:23started out of this you have a friction uh frictional drop higher frictional drop then again p2 will be
04:29lower than your p1 again it start closing so this way this inlet pressure drop creating the chain
04:34chattering in the your psv so by limiting limiting the pressure drop or friction losses to 3 percent
04:45you are ensuring that uh the pressure drop is at the inlet of the wall is minimized and ensuring that
04:52the pressure is maintained above the blow down pressure as you as we have seen in earlier
04:57our session the blow down pressure is generally remains the 7 percent so if if you increase the
05:06your pressure drop higher then it's continuously start chattering so wall will be get closed and open
05:12continuously so in summary 3 percent rule is for the stability not for the capacity limit in ensure
05:19that the prv opens cleanly stay open stable and function as designed without a self-destruction because
05:27of the chattering so to uh avoid this 3 percent more than 3 percent pressure drop engineers may often
05:36only use a larger pipe size or shorter run of your of uh of your inlet line so that will
05:43other othering
05:45you the 3 percent limit so in this way this 3 percent pressure criteria is very much important for the
05:54bsv inlet line so thank you very much write your question and comment i will be happy to answer it
06:04you can reach us on conceptengineering2025 at gmail.com
06:10links and links for the other sessions are given in this description
06:15for the other session
06:15for the next session
06:15the time
06:16You
Comments