11 replies to this topic

[Mohamed Soliman]

Hi
We are working on sizing a relief valve with:
vapor relief load of 250000 lb/hr ,
MW=112
set pressure=75 psig.
Relief temp= 160 deg C

The calculated relief size is 8T10 and we asked the vessel vendor to provide 8" nozzle for the relief valve. the inlet line to the relief valve is 12' long , 8" diameter and it has one elbow and one "flow into a pipe". the calculated pressure drop for maximum relief load of 300,000 lb/hr is 4.5 psi which is greater than 3% of the relief valve set pressure.

elbow pressure drop=0.89 psig
12' pipe pressure drop=1.27 psig
Flow in to a pipe =2.43 psig
The pressure drop in the fitting " Flow in to a pipe" alone is 2.43 psig which is greater than 3% of 75 psig

Do we have to increase the safety nozzle in the vessel. The vessel is in the manufacturing process ?

Api recommends the pressure shall not exceed 3%, is there any exception to this rule?

Thank you very much

Edited by Mohamed Soliman, 20 February 2006 - 03:59 PM.

[kdal]

Am sure you will get a number of replies to this question, so here's my two penny worth...

No you don't have to get a bigger relief valve. This would only make the problem worse because the rated relieving flow would have higher and hence the inlet pressure drop due to friction would also be higher. The PSV would also then be oversized which could lead to chattering and mechanical damage.

First thing you should do is to look at increasing the diameter of the inlet line. This should dramatically reduce the inlet line losses. You could also look at moving the PSV slightly closer to the vessel.

Alternatively since you are very close to the API recommended limit of 3% (you're at 3.24%) you could ask the PSV vendor whether your current pressure drop is acceptable for his valve type. The 3% limit is somewhat arbitrary (refer to other posts on this subject) and API says that an 'engineering analysis of the valve performance at higher inlet losses may permit increasing the allowable pressure loss above 3%'.

Hope that helps (I wait to be shot down in flames!)

[kdal]

sorry - as soon as I posted I spotted my mistake.
Ignore the bit about being close to 3%. If you sum all your losses youre >6%, so I would definitely look to increase inlet line size.

[Mohamed Soliman]

The pressure drop should be calculated from the source including entrance to pipe losses as API 520 part II page 10 figure 8


Kdal,
I am not looking to increase the safety valve size but I was looking to avoid increasing the safety valve nozzle size on the vessel it self.
Thank you


Mohamed

[kdal]

Have you tried leaving the vessel nozzle size the same and simply increasing the size of the PSV inlet pipe?

Yes, we tried that but the pressure drop in the Vessel nozzle alone is 3%.

[djack77494]

Mohamed,
I think the best solution would be to locate the PSV directly on top of the vessel. Avoid the piping and elbow you now have planned. In thinking about your situation, the "flow into a pipe" pressure drop seems excessive. For example, if you locate the PSV directly on the top center of the vessel, you might approach something closer to what Crane TP410 calls a "well rounded entrance" rather than something like a "sharp edged" or, worse yet, an "inward projecting pipe entrance". K factors can vary from 0.04 to 0.78 depending on what you think best represents your situation. Maybe you should strive to install something resembling the well rounded entrance. This is an interesting problem since many of us face PSV sizing issues often and with high losses like yours, perhaps some would have strategies to reduce the problem.
Doug

[pleckner]

Yes, this is a very common problem in PSV sizing. Notice I said PSV sizing and not line sizing for a PSV. This is because the sizing of a PSV is not comple until you take into account the inlet and discharge piping. In a plant design, the PSV is usually specified and could even be bought way before the piping designers finish the piping design. So how do you not fall into this problem?

1. Try to arrange to put the PSV directly on the vessle (nozzle) as has been suggested.

2. Make the vessel nozzle for the PSV one size larger than the PSV. You are not allowed to make the inlet smaller than the inlet size of the PSV but nothing stops you from making it larger. The only caveat is dealing with a reducer, but again this usually works.

Will this always solve the problem? It will for the most part but "always" is a horrible word to use. The next step is to evaluate the blowdown of the valve with the valve manufacturer. This way you can exceed the 3% rule with confidence that everything will work out.

[djack77494]

I do not agree with "Guest" who said,

The intent behind the 3% figure is to put a limitation on the overpressure in the vessel/equipment being protected (10% or 21%). Hence, if same can be ensured, the excess pressure drop in the inlet line could be considered acceptable, as per one interpretation of a design practice.

The 3% limitation for the inlet piping has nothing to do with limiting overpressure to 10% to 21%. Rather, the intent of the 3% limitation was as a guideline to avoid valve chatter. With low values of blowdown, the valve could chatter (or open and close at rapid intervals) as follows:
1) Valve is subjected to high vessel pressure and opens to relieve the excess pressure.
2) As soon as the valve opens, there is flow through the inlet line and into the valve.
3) The flow results in hydraulic losses (friction and acceleration) which cause the pressure at the valve to drop.
4) Since the pressure is now lower, the valve closes.
5) The cycle repeats at small intervals (rapidly).
6) The valve may fail due to being exercised excessively.

This problem is somewhat alleviated by providing a high blowdown setting. Many valves permit blowdown adjustments via an internal threaded ring. If there is a high blowdown setting, then more pressure will be vented before the valve reseats; conversely, with low blowdown, the valve reseats at just under set pressure.

Hope that helps,
Doug

[rxnarang]

I do not agree with "Guest" who said,

The intent behind the 3% figure is to put a limitation on the overpressure in the vessel/equipment being protected (10% or 21%). Hence, if same can be ensured, the excess pressure drop in the inlet line could be considered acceptable, as per one interpretation of a design practice.

The 3% limitation for the inlet piping has nothing to do with limiting overpressure to 10% to 21%. Rather, the intent of the 3% limitation was as a guideline to avoid valve chatter. With low values of blowdown, the valve could chatter (or open and close at rapid intervals) as follows:
1) Valve is subjected to high vessel pressure and opens to relieve the excess pressure.
2) As soon as the valve opens, there is flow through the inlet line and into the valve.
3) The flow results in hydraulic losses (friction and acceleration) which cause the pressure at the valve to drop.
4) Since the pressure is now lower, the valve closes.
5) The cycle repeats at small intervals (rapidly).
6) The valve may fail due to being exercised excessively.

This problem is somewhat alleviated by providing a high blowdown setting. Many valves permit blowdown adjustments via an internal threaded ring. If there is a high blowdown setting, then more pressure will be vented before the valve reseats; conversely, with low blowdown, the valve reseats at just under set pressure.

Hope that helps,
Doug

To avoid the kind of problems Doug mentions above, the line losses are kept at 3% which is about half of the differnece between blowdown and set pressure. Typically the blowdown is 92-93% which is 6-8% below set pressure. Hence line losses are half of this viz 3%. This value can go up, if one lowers the blowdown. But you can try a non-flowing, remotly sensed pilot valve too. The valve dimensions will also be a lot lower than a conventional valve, and piping will love you for that.

Regards
Rajiv