4 replies to this topic

[bernath]

Hi All,

I'm currently designing a relief system for a 3 phase separator. The company only permit me to have one relief valve to protect the vessel. Therefore I have to choose the governing scenario a design basis for my relief valve.

The considered scenarios would be closed outlet and fire case. The load for fire case usually is found to be smaller than the closed outlet which lead me to decide that the governing scenario should be closed outlet instead of fire case.

I was choosing the bigger size orifice area based on closed outlet scenario and ready to release the data specification to the vendors. But I'm quite confused when it comes to the maximum accumulated pressure (i.e. relieving pressure) of this relief valve, whether to employ the overpressure of 110% MAWP for non-fire case or 121% MAWP for fire case. According to API 520, the set pressure for both case should be the same while the maximum accumulated pressure differs. I need this "110%/121%" to find my relieving temperature.

Please kindly advise.


Next question, I realize when I choose the bigger orifice, there's possibility that my relief valve will be oversized and lead to chattering. I did some digging on local forum and found some interesting explanation.

The relief valve will undergo chattering problem only if,

1. There is an excessive pressure loss of inlet piping of relief valve. The pressure drop should be below 3% of set pressure, otherwise chattering will occur.

2. The required rate is below 25% of the rated capacity of the relief valve. The required rate should be at least 25% of rated capacity, otherwise chattering will occur too.

Is this above 2 statement is correct? I'm quite sure the 3% rule is okay, but the 25% rule is kind of new to me. Please someone clarify this.


any comments will be highly appreciated

thank you
regards,
bernath

[paulhorth]

Bernath

You state in the first sentence:

"The company only permit me to have one relief valve to protect the vessel. Therefore I have to choose the governing scenario a design basis for my relief valve."

This is really scary because it shows that you do not understand what you are doing. You really need to get some senior supervision.

Considering just this sentence you must try to understand this:
You ALWAYS have to work out and prove (not choose) the governing scenario, regardless of whether there is one PSV or fifty. The reason for using more than one is not so that each relief case has its own valve, that is simply not how they work, but so as to provide a 100% spare valve to allow you to test and maintain a valve without shutting down the vessel.

Now, moving on to the next part of your post:

If the governing case is blocked outlet then the allowed accumulation is 110%. End of story. The 121% is for fire cases only.
Restricting pressure drop in the inlet line to 3% of set pressure is to avoid chattering. Correct.
I don't know a rule of thumb about chattering below 25% of rated capacity, but your selected valve capacity should not greatly exceed the calculated relief rate. You should not end up with a valve 4 x the required capacity.

Paul

[bernath]

Thanks Paul. It's been hectic weeks and I'm 'all by my self' for the last few weeks. You're right, I need to seek supervision. Thanks again for providing me by such excellent answer. I'm still green at relief system subject trying to capture the concept and do my best to solve my problem.

regards,
bernath

[Erwin APRIANDI]

Dear Bernarth, protection of separator by using one relief valve is mostly enough, but it will be depend on your maximum flow and your sparing philosophy. Which mean if the calculated orifice size required is greater than what is available on the market you may use two valve to met the requirement or sometimes company required two, because it is sometimes mandatory for PSV to be certified every year and also sometimes if the test equipment is available nearby, you may required to test the PSV for certification somewhere else. Therefore you need spare, to protect the separator, but in this case mostly one of the valve inlet will be isolated if it is not in use.

For the second question, I believe Paul has explain it very clear. Before you made any datasheet a calculation is required and shall be done. Based on this calculation you have to ensure how much is your maximum relieving rate based on the possible considered case.

And regarding to chattering, relief valve can be chatter if (possible causes)
1. There is excessive inlet pressure drop, means the pressure inside the separator will be higher than the pressure upstream of the relief valve (for conventional PSV or PRV 3% is the maximum limit cansidered)
2. Excessive built-up back pressure
3. Oversized valve, you must flow at least 25% of capacity to keep valve open (rule of thumb) to be confirm by valve manufacture
4. Valve handling widely differing rates, you have to check the rates of other possible cases if it is largely different you may consider installing two smaller valve instead of one large valve set point of using two relief valve to be evaluated further

enclosed I'm attaching some refference to help you in understanding the concept

Attached Files

•  Pressure Relief Devices Scott Ostrowski.ppt   1.72MB   326 downloads

[fallah]

Two important points should be considered as below red color comments!:

Hi All,

I'm currently designing a relief system for a 3 phase separator. The company only permit me to have one relief valve to protect the vessel. Therefore I have to choose the governing scenario a design basis for my relief valve.

The considered scenarios would be closed outlet and fire case. The load for fire case usually is found to be smaller than the closed outlet which lead me to decide that the governing scenario should be closed outlet instead of fire case.

What would determine the governing scenario isn't the relevant load that to be higher! The governing scenario would be determined based on higher calculated orifice area using relevant relief load!

I was choosing the bigger size orifice area based on closed outlet scenario and ready to release the data specification to the vendors. But I'm quite confused when it comes to the maximum accumulated pressure (i.e. relieving pressure) of this relief valve, whether to employ the overpressure of 110% MAWP for non-fire case or 121% MAWP for fire case.

Relevant allowable overpressure (10% and 21% respectively for blocked outlet and fire cases) should be considered separately in orifice area calculation for each scenario and then comparing the calculated areas the governing case would be specified.

According to API 520, the set pressure for both case should be the same while the maximum accumulated pressure differs. I need this "110%/121%" to find my relieving temperature.

Please kindly advise.


Next question, I realize when I choose the bigger orifice, there's possibility that my relief valve will be oversized and lead to chattering. I did some digging on local forum and found some interesting explanation.

The relief valve will undergo chattering problem only if,

1. There is an excessive pressure loss of inlet piping of relief valve. The pressure drop should be below 3% of set pressure, otherwise chattering will occur.

2. The required rate is below 25% of the rated capacity of the relief valve. The required rate should be at least 25% of rated capacity, otherwise chattering will occur too.

Is this above 2 statement is correct? I'm quite sure the 3% rule is okay, but the 25% rule is kind of new to me. Please someone clarify this.


any comments will be highly appreciated

thank you
regards,
bernath