16 replies to this topic

[CSNK]

Dear all,

My doubt is that in one of my PSV sizing - i m getting inlet line losses of around 9% of set pressure of PSV. (3% allowable by code)
In fact orifice area required for my PSV is quite low compared to what is provided.
Required area of orifice is ~ 20 in² and available orifice area is ~ 32 in².
Since my inlet line losses are on higher side - what should be my preferred solution in such case?

What i think is that since area available is quite high i can ignore the higher inlet line losses -- am i right?

Thanks.

[flarenuf]

your available orifice area is greater than a T orifice (26 in2) so do you have 2 valves in parallel?
with a calculated at 20 and an installed of 32 you run the risk of severe valve chattering here

[fallah]

chiks,

Please specify if 9% inlet line pressure drop corresponds with rated relief load or required one.

Fallah

[Olidin]

Look like you may have 2R? If you happen to have two orifices, just make sure the pressure drop is calculated using the rated capacity of EACH valve instead the total relief load. I had a case like that where we overestimated the inlet pressure drop.

If there were two valves, dropping the valve sizes to 2 Q orifices would probably help. Or shorten the piping but I wouldn't suggest that first since piping might be pretty big (8" or more?) considering it handles such high flow.

[CSNK]

Dear Flarenuf and olidin

I have two 'R' - orifice as rightly said by olidin,
To olidin - Yes i have done pressure drop calculation based on rated flow only.
Reducing the orifice size to 2 Q wont help in reducing by inlet line losses (since with required flow also inlet line losses are ~ 4.5%)

Dear Fallah,

9% line losses corresponds to the rated flow per PSV, and even with required flow i am getting line losses of ~ 4.5%.
Can we say that with higher inlet line losses - PSV rated capacity will decrease.

[fallah]

Dear Fallah,

9% line losses corresponds to the rated flow per PSV, and even with required flow i am getting line losses of ~ 4.5%.
Can we say that with higher inlet line losses - PSV rated capacity will decrease.

chiks,

Actually at first you should specify maximum operating pressure, PSV blowdown and PSV type.
And yes we can say with higher inlet losses PSV capacity will decrease.

Fallah

[CSNK]

PSV set pressure is 12.0 kg/cm²-g and the size is 6R10 with two inline and one standby PSV.
And yes PSV is designed for Blocked outlet scenario with two phase relief (since PSV is at the downstream of fired heater).
PSV is balanced bellow type PSV.
PSV discharge is routed to flare via blowdown column.

As you said with higher inlet line losses - PSV capacity will decrease. -- so from my 1st post can i say that i can ignore higher inlet line losses and say that PSV will be ok with existing conditions.

[paulhorth]

Chiks,
In my view, you really need to do something to reduce the inlet line losses to nearer 3% (0.36 bar, I know that is not very much).You can consider making the section of line at the outlet from the heater a larger diameter than the main process line, and connect each PSV directly to this line (not via a tee) using lines of 10 inch or 12 inch ( ie significantly larger than the PSV inlet). The inlet isolation valves will have to be this size also. Route the inlet lines as directly as possible.
You could also consider using pilot operated PSVs with the pilot tappings on the main process line - then the DP from there to the PSV will not affect the PSV opening pressure.

Paul

[fallah]

As you said with higher inlet line losses - PSV capacity will decrease. -- so from my 1st post can i say that i can ignore higher inlet line losses and say that PSV will be ok with existing conditions.

chiks,

You fail to submit maximum operating pressure and PSV blowdown...

Anyway the first priority is lowering inlet loss. If it isn't possible might with such high inlet loss a modulating pilot operated PSV would do the job but it should be studied in detail.

Reducing capacity due to high inlet loss in order to compensate increased capacity of oversized PSV could be considered if PSV blowdown (in percent of set pressure) to be so high (higher than inlet loss in percent of PSV set pressure) will not result in PSV chattering.

Fallah

[CSNK]

Dear Fallah,

MAWP is 20 kg/cm²-g and Blowdown of PSV is 10% of set pressure of PSV.

Thanks.

[chemsac2]

chiks,
I am not very sure if capacity would decrease due to higher inlet pressure drop.
When PSV opens first time, it is at set pressure and would relieve rated capacity as no flow-no pressure drop. Since rated capacity is much more than required capacity, pressure in system would drop below blowdown value (around 93% of set pressure) and PSV would reseat.
When valve opens next time (chattering), it is again at set pressure and would relieve rated capacity only.

Hence, I would not take credit of reduced capacity at higher inlet pressure drop. I would still meet 3% inlet loss criteria.
Options suggested by Paul in his post namely increased inlet line size and use of remote sensing pilot PSV should be considered.

I also see MAWP as 20 barg and PSV set pressure as 12 barg. Such large difference in PSV and MAWP seems little odd. If you set PSVs at 20 barg, you will have increased inlet pressure drop available.
Also, blowdown of 10% would make it possible to allow 7% of inlet pressure drop (+/- 3% for PSV tolerance).

I was going through article in Hydrocarbon processing, November 2011 on PSV 3% inlet line loss. Authors contend that 3% inlet line loss is related to pressure wave travel from PSV to source and back. With 2-phase flow in your case, pressure wave velocity is high and hence longer PSV inlet line length can be tolerated for same PSV opening time.

Hope it helps.

Regards,

Sachin

[fallah]

MAWP is 20 kg/cm²-g and Blowdown of PSV is 10% of set pressure of PSV.

chiks,

Still you haven't specified maximum operating pressure!

Anyway 10% blowdown indicates that the PSV reseats at 10.8 kg/cm² means the maximum operating pressure is lower than this value. Please check this matter and come back with the result.

Sachin,

You mentioned: "When PSV opens first time, it is at set pressure and would relieve rated capacity as no flow-no pressure drop." then please specify when there is no flow how you would have relieving?!
In fact, at the moment the PSV opens and start to relieve the pressure at PSV upstream would reduce from set pressure and the reduced value depends on the inlet line size and configuration. Obviously higher inlet loss will result in lower PSV upstream pressure hence rated capacity reduction.

Fallah

[chemsac2]

Fallah,

By that statement I meant during 1st opening of PSV, there would be simmering, partial lift of PSV disk and then pop action of PSV. Flow would no where be near to rated capacity during simmering or partial lift and hence pressure at disc would be same as vessel pressure.

PSV is not like control valve that starts opening at set pressure and fully opens at relieving pressure. Simmering and initial partial lift would give a small flowrate and probably negligible inlet pressure drop. Pressure drop exceeding 3% pressure drop in Chiks case would occur only at the pop action and pressure at disc would be same as vessel pressure.

If valve does not reseat due to difference in required and rated capacity, only then disc would see reduced pressure at its inlet and give lower capacity than required.

In chiks' case difference in rated and required capacities is expected to be significant (20 in2 against 32 in2) and hence most probably valve would reseat after initial pop-up. This can be checked by doing an energy/material balance around system and equation of state.

Regards,

Sachin

Edited by chemsac2, 25 October 2012 - 10:30 AM.

[fallah]

Sachin,

We are not discussing about transient states like PSV simmering before opening or partially lifting before full opening; the main point i hope to be clarified for you is that with higher inlet frictional loss than, let say, 3% of the set pressure in full open situation rated capacity will be reduced due to decrease in PSV upsteam pressure.

Fallah

[CSNK]

Dear Fallah,

You are right that my maximum operating pressure is around 9.5 kg/cm2-g and yes it is less than PSV reseat pressure.

Yes from this i can conclude that:
MAWP is quite higher than PSV set pressure and so in case of blocked outlet condition, even with high inlet line losses, since PSV area is more, there would be no concern of required capacity (since rated capacity is quite high).

Thanks for a good discussion on this topic.

[fallah]

chiks,

Be careful! It can be a wrong conclusion and might lead to a severe PSV chattering because you are simultaneously facing with two malfunctions in PSV operation: Oversized PSV and High inlet loss

Fallah

[Tsquare09]

My understanding so far is that you are using the rated flow to calculate inlet line pressure loss for each PSV which is wrong.

Since you have two installed PSVs online (2R) you need to confirm that your rated flow is divided equally with half of the rated flow going into each PSV.


Stephen