Hi,all
I am very confused about relief load analysis on open system .In my opinion, this problem will be dicussed on two case, one is with umbrella fitting, the other is not with an umbrella fitting, when with umbrella fitting,there are two forces ,called "Thrust at Pipe/Vent Pipe Interface" and "Thrust at Vent Pipe exit", the CAESAR II's help file also said:"If the vent stack is hard piped to the relief valve piping, then this intermediate thrust is balanced by tensile loads in the pipe and can be ignored", Dose this mean when vent stack is hard piped to relief valve pipe , thrust at Pipe/Vent Pipe Interface can be ignored? only thrust at vent pipe exit should be considered?
Now, in a project, for case of not with umbrella fitting , owner ask me add the thrust at pipe/vent pipe interface at the valve,in the axial direction of discharge tube,against the flow direction,is this right? he said"This force is due to short period transient due to flow passing by the orifice of the PRV", if this statement is ture, is this force equal to "Thrust at Pipe/Vent Pipe Interface"?
Look forward to anyone's reply
Many thanks

Hi,all
I have another doubt about this question.
For example:
Use CAESAR II's "Relief Load Synthesis",
Line temperature:100℃,
ID of orifice: 72.4mm,
ID of relief piping: 202.6mm,
ID of vent stack: 438mm,
length of vent stack:5627mm,
ratio of gas:1.424,
gas contant: 358,
N,
N,

Mass (Kg/hour):91508,
Thrust at valve pipe/vent pipe interface(N):24187.779,
Thrust at vent pipe exit(N): 1720,
.....,
.....,
.....,

...,
Vent Pipe exit gas condition..(P,Vel,T) 49.8(KPa--abs) 373664(mm.s)..,


From result above, the P of exit is 49.8KPa which is below the atmospheric pressure,in my opinion, this is imposible. is the P real?
I made a calculus with parameters above:
W=91580 Kg/hour=25.44Kg/s,
V=373.664m/s,
P=49.8*1000Pa,
Pa=101*1000Pa,
A of exit=0.1506m²,
gc=1.0013 kg.m/N.s2
The thrust at vent pipe exit=W*V/gc+(P-Pa)A
=25.44*373.664/1.0013+(49.8-101)*1000*0.004114 = 9506+（-51.2*1000*0.1506） =9506-7710=1796N=value above table
Is this to say, The P(49.8)used in thrust at vent calculation ,and generates a negative force(-51.2X0.1506*1000N) which cause the thrust is small? is this real?

Really looking forward to your reply.


Regards,

I haven't the article "Steam Flow through Safety Valve Vent Pipes" in front of me; as I remember, a vent pipe outlet pressure greater than atmospheric is one of the limits that the article imposed.

mariog,
Thanks for your reply, Do this mean something wrong in
CAESAR II's "Relief Load Synthesis"?

I realized I posted that old article in http://65.57.255.42/ubbthreads/ubbthreads.php?ubb=showflat&Number=45565
At page 4 in pdf document (202 in print) the last statement is "Other limits include a vent pipe outlet pressure greater than atmospheric pressure [...]"