10 replies to this topic

[boilermaker]

Dear all,

I am a mechanical engineer, working for an oil and gas exploration and production company. I need to make a manual calculation (for the benefit of my own understanding) regarding the required rate of relief for a PSV installed at one of our in-operation two-phase horizontal separators. The case can be either blocked discharge or fire case.

The process guys can use plenty of softwares to calculate this; but I need to know how one can get an estimate of the required relief rate through the use of manual equations. The separator operates at around 1,275 psi while its design pressure is 1,785 psig which is also the set pressure of the PSV. The vessel's normal operating temperature is -2 ^oF and it separates the incoming stream into hydrocarbon liquid (condensate) and gas.

1. I need to know how I can go about doing a calculation to finding the relief rate required for the PSV for fire case and blocked discharge.

2. In addition, could anyone please let me know how to convert lb/hr to MMSCFD ???

Best Regards

[Art Montemayor]

Boilermaker:

First, welcome to our Forums. We Chemical Engineers can all profit from the experience and expertise mechanical engineers can bring to our forums. We hope we can be of help to you on this topic.

There are no arcane or secret equations for calculating the required relief rate for a safety relief valve. Most, if not all of the information required is found in 3 basic documents:

1) API RECOMMENDED PRACTICE 520
Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries
Part I - Sizing and Selection

2) API RECOMMENDED PRACTICE 520
Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries
Part II - Installation

3) API RECOMMENDED PRACTICE 521
Guide for Pressure-Relieving and - Depressuring Systems

You should not only have access to the latest editions of these documents, but you should also have previously studied and thoroughly digested the material found in these documents. These are the documents upon which the design should be based and calculated – regardless of engineering discipline. These documents should be freely discussed and employed by both Mechanical and Chemical Engineers alike. They contain the basic engineering design theory well-known to both disciplines alike.

You will find that often, such as in the case of the blocked discharge scenario, there are no calculations involved. Rather, what the engineer takes into account is the maximum, possible, and credible amount of fluid that could be required to be relieved under normal steady-state conditions. The maximum amount of fluid might already be identified by previous design or calculations – for example, if there is a control valves upstream of your 2-phase separator, the C_V of the control valve determines the maximum flow rate through the valve at the flow conditions. Even if the calculation has not yet been done, it is no big deal to calculate it yourself or have your instrumentation engineer or process engineer calculate it for you.

The Fire case is well defined in the API documents and, again, is easily calculated following the prescribed graphs and empirical equations given therein. These are really simple, down-and-dirty calculations that are routinely done by process and instrumentation people.

The method used to convert lb/hr of gas or vapor flow to volumetric flow rate is a very easy one also:

1) Obtain the molecular weight of the vapor (usually a mixture of various components). This information is to be found in a Process Flow Diagram’s Mass Balance table showing all the flow streams and their respective properties at normal design flow rates. If it doesn’t exist or you don’t know how to calculate the molecular weight, then have a process engineer do it for you. Again, this information is common and well-known for design purposes.

2) Use a spreadsheet and divide the vapor mass flow rate by its molecular weight. The answer is lb-mols of vapor /hr. This is so because the units of molecular weight are Lbs/lb-mol.

3) Every lb-mol of a gas (or vapor) occupies 379.49 cu. ft. at 14.696 psia and 60 ^oF. Note: these are conveniently considered as “Standard Conditions” (SCF) by such references as the GPSA Engineering Databook. Knowing this, multiply the lb-mols of vapor/hr by 379.49 and the product will be Standard Cubic Feet / hr. If you multiply the SCF / hr by 24 hrs/day and divide by 1,000,000 you will obtain millions of standard cubic feet per day (MMSCFD). It is that easy. All you need is the molecular weight.

I hope this helps you out.

One more tip from an old engineer: Be sure to download a free version of Harvey (Katmar) Wilson’s excellent engineering units conversion program called “Uconeer”. You can obtain this free download at Harvey’s website: http://katmarsoftware.com/

There is no better conversion program to be found for engineers.

[boilermaker]

Dear Sir,

I have read a lot of your posts and for a person of your stature to answer mine is just an honour i must say.

thanks a lot sir, and i will get back after digesting the above documents.

[ankur2061]

boilermaker,

Besides the cases of a blocked discharge and external pool fire another very credible case would be to check for gas blow-by if your 2-phase separator is connected upstream to a source of 2-phase fluid at a higher pressure and the liquid level is lost in the separator. In many cases where there is a train of separators, the separators downstream of the 1st separator operating at a lower pressure can have a gas blow-by due to loss of liquid level and this may turn out to be the governing case for the relief valve sizing.

Regards,
Ankur.

[boilermaker]

Dear Ankur, i understand your point and from what i have read on the gas blow-by scenario, i find that while it might be applicable for trains of seperators (as you correctly advised) it might not be applicable in my case since the seperator i am working on is on the downstream of a slug catcher and the gas seperated by the slug catcher is flowing into the seperator without any inline PCV. Hence my assumption that this case maynot be applicable to me.

Please let me know if the above seems erronious to you.

Thanks for your inputs though, i have made note of them and shall use them if an occasion so arises.

Best Regards

[fallah]

boilermaker,

If you send a simple sketch of your system, the matters such as applicability of gas blowby scenario can exactly be specified.

Fallah

Edited by fallah, 13 June 2012 - 02:38 AM.

[boilermaker]

Dear Fallah,

i have uploaded the simple schematic showing our slug catcher (fingre type, 3-phase seperator) from which the gas splits equally between two verticle 2-phase seperators.

Attached Files

•  simple schematic for sepertor and slug catcher.pdf   86.18KB   85 downloads

[fallah]

boilmaker,

The gas (may be a little bit two phase) is normally blown between slug catcher and separator!, hence in your case as you mentioned the gas blow-by couldn't be an process upset needed to be considered in relieving scenarios. It could be applicable between separators and down stream equipment provided that a control valve located inbetween in order to adjust the level in the separator.

One more thing: You mentioned in your first post that the separators are horizontal while in 7th post, and also as per the sketch, they are described as vertical!

Fallah

Edited by fallah, 13 June 2012 - 05:16 AM.

[boilermaker]

oops!!!! yes i did make that blooper, they are in fact verticle. sorry for that

thanks for your inputs though

[boilermaker]

Dear people,

I need to ask the following concerning external pool fire case scenario in API 521

1. section 5.15.2.2 refers to heat absorbtion equations for vessels. It has two sub-sections one for heat absorbtion to vessels containing liquids and the other for heat absorbtion to vessels containing gases/vapors. under which category shall my two phase seperator fall which seperates the incoming gas stream into both liquids and gas???

best regards

P.S: I am talking about the same seperator as being discussed above.

[fallah]

boilermaker,

As long as the vessel containing liquid, the section 5.15.2.2.1 (Heat absorption to liquid) and when all liquid is vaporised and the vessel just contains gas/vapour the section 5.15.2.2.2 (Vessels containing only gases...) would be applicable for determination of relieving rate in fire case.

Fallah