Team,

18" steam line having the safety valve(6"*8")( Inlet pressure of 780psig, Discharge pressure of 95 psig & the set pressure is 680 psig). The discharge goes to silencer & the Maximum design back pressure at silencer inlet is 9.1 psig. I have considered this is an closed system and done the analysis. Since the discharge is connected to silencer which is open to atmosphere why can't we consider this is an open system? Kindly advise me.

Regards,

Rams.

yes

SUPERPIPER,

EXPECTING YOUR RESPONSE ON MY QUERY.

REGARDS,

RAMS

Rams,

agree with superpiper. yes.

Yee,

You Mean that, it has to be considered as open system !!

Rams

Rams,

I would understand a closed system to contain all the relief outlet flow in a storage tank or vessel. No fluid is released to the environment. A closed system might be used for a highly toxic chemical, a radioactive material, or a high value substance, catalyst, etc. The closed system analysis has to include the variation of backpressure, which increases as the storage vessel is filled, and flow also decreases as vessel back pressure increases. It is not a simple problem to analyze.

The steam relief system that is described has flow to the atmosphere. Hence it would be considered an open system. Now, since there is the back pressure of the silencer, the pressures in the discharge piping will be at a higher value than discharge directly to atmosphere. The thrust forces at the final outlet will be the F = P x A, P= internal pressure 9.1 psig of silencer multiplied by exit area of silencer outlet. This asssumes outlet to be straight without elbow or other angled fitting. For a outlet size Nps-12 on the silencer the thrust force would be F= 9.1 x 113.1 in2 = 1029 lbs. during the steady relief flow condition. If the silencer outlet is a much larger Nps-24 (considering 95 psig inlet pressure ratio to 9.1 psig outlet pressure) then outlet thrust is F = 9.1 x 424.5 = 3863 lbs.

Thanks Yee!!

Richard,

The normal definitions are ... an open PSV system discharges directly to atmosphere. A closed system may be as you describe, but will most commonly define typical refinery and petrochem facility relief systems, where a collection header connects all PSV's & BDV outlets etc. to a knock-out drum, and thence to a flare or vent stack in a safe location.

what you said, although i treat it as free volume.

ie a 2" line relieving into a 12" header is pretty much free volume

To MoverZ and SUPERPIPER,

The relief collection headers and Flare system lines are extensive for a refinery and would be a large volume to consider for relief flow analysis. The initial few seconds of relief flow would be like an open flow situation, I agree.

There are also flare header flow situations where the unit is in startup, and there is continuous venting to the flare for hours if not days. The steady state flow could be analyzed for the flow pressure drops in each part of the system. Any other department with a relief flow would have to contend with the backpressure in that portion of the flare header.

The story goes about the Saudi refineries having their flare header and system designed for blowdown of the complete plant to get all of the H2S out of the equipment and piping to flare or burn pit - within 1 minute, in event of an alert of missile or bomb strike. If the H2S was released by a vessel / piping rupture, then the released H2S cloud would have wiped out the plant's personnel staff. That's an open flow system!

Yee,

9.1 psig Back pressure is almost above the atmosphere(1.6 bar), in that basis can we consider this is a open system?

How to consider the force due to back pressure, 12.7 KN for 20" header line(silence inlet)apart from the psv reaction force. In other case will it create a siginificant effect for the psv discharge line is concerned?

In practical, why can't we consider that the first rest support from the silence will take the reaction force due to back pressure!

please advise.

For Your Information.[ 2 psv outlets(12") connecting to 20" common header which is connected to the silencer inlet} .. psv reaction force is 36 KN.

regards,

rams.

rams,

The question of open or closed flow system would determine whether the relief outlet pressures are steady state (open system), or pressures are incrementally increasing (closed system) with decreasing flow. I think we have a difference of perspective. Some might be viewing the effects of pressure and flow for short time at component level ( the discharge elbow, tee, silencer), or for extended time at the system level (entire collection header, flare system). Either way, the laws of physics apply for forces and unbalance between elbow pairs in the straight runs of pipe. Where there is an unbalance of forces due to 1) pressure difference, 2) area difference 3) lack of closure at end of pipe - that is an 'OPEN' pipe condition - then those are the locations to be concerned about force to apply for the loads analysis. The force action and reaction occurs in both closed and open systems - you need to simplify the forces by cancelling out forces in the straight run pipes with elbow pairs. If the pipe size changes within the run of elbow pairs, or the pressure changes between the elbow pair ends, then there is a net force to be calculated. Otherwise, if there is no net force, then a support at first rest point or else where will not serve a purpose except for momentary restraint of dynamic motion (or other load cases).
If the silencer has an outlet direct to atmosphere (up), then the silencer supports will have the unbalanced force added to the weight of silencer. If the silencer oulet is piped away to a different location, then there is an elbow to cancel the pressure area of silencer outlet. There will still be the force unbalance due to pressure drop thru the silencer - whether it is an open system or a closed system.