Safety Relief Valve Sizing
Safety Relief Valve Sizing
(OP)
I am trying to size a relief valve on a small mixing tank for a pilot plant operation. The worst case is for a very viscous material (65,000cP) and is blocked in fire case. Everywhere I have looked about finding the capacity of valve wants to know the orifice area. I thought that was what I was trying to determine! Can someone please help me determine the necessary GPM that the relief valve needs to accomodate? Any help at all would really be appreciated....thanks!





RE: Safety Relief Valve Sizing
Makesure the equations match the stated size in the US its either ASME or API, each one gets the same results but the eqautions/orifice sizes are diffferent.
RE: Safety Relief Valve Sizing
In order to size a relief valve you need to know the "required relief rate" and conditions. You take that information and plug that into the relief valve sizing equations such as provided in API RP-520 "Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries, Part I" or as provided by a particular valve manufacturer. When it comes time to purchase the valve, complete a relief valve specification sheet such as found in API RP-520 and give that to a valve manufacturer/supplier and they generally will perform the calculations to verify your request before giving you a quote.
Now you say you know the worst case but I have some question that you do because if you have not been able to size the valve yet how do you know which overpressure case is the worst case. The exception, I suppose, would be if blocked-in fire is the only case.
You state that everywhere you look about finding the capacity of a valve you need to know the orifice area and that is a correct statement. The capacity of a valve is determined not only by the orifice area installed in the valve but also by other aspects of a particular valve design. But don't confuse the capacity of the valve with the "required relief rate". "You" have to determine the required relief rate and conditions before you can select a valve of sufficient capacity and that is compatible with your service.
Off hand, I'm not sure what "relief valve" would be suitable for relieving a liquid at 65000 cP. I don't have experience with such material but I suspect the viscosity correction factor that would apply for such a fluid would suggest that some other type relief device would be indicated, possibly a rupture disc instead of relief valve. The viscosity issue is something you probably need to investigate straight away with a relief valve manufacturer such as Farris, Consolidated, Anderson-Greenwood just to name a few.
Okay, so as far as getting help to determine the necessary "required relief rate" and relief conditions, that is something that cannot easily be provided through a forum. Your best bet would be to seek the assistance and direction from an experienced designer at your location. I could make a guess as to what you might need to do but, without knowing all the details of your facilities, most likely it would be an incomplete guess at best.
If blocked-in fire was the only case and, if your system is liquid full and, if the material that was contained in your mixer would not vaporize at the relief set point and, if the material was not prone to decomposition at elevated temperatures, then you might get by with determining the required relief rate as if for liquid expansion due to the heat input from the fire. Hopefully you get my point about all the "ifs".
If you want to start learning relief system design, my suggestion would be to start with reviewing API RP-520 (mentioned above) and reviewing API Standard 521 (formerly RP-521) "Pressure-relieving and Depressuring Systems". API 521 includes discussion about various overpressure cases that should be considered in order to determine your worst case event and does give guidance for determining heat input from fire exposure as well as a means to evaluate the liquid expansion case. The AIChE publication "Guidelines for Pressure Relief and Effluent Handling Systems" is another good reference.
RE: Safety Relief Valve Sizing
RE: Safety Relief Valve Sizing
Sizing the PSV is the easy part as you've found out; it only takes inputting values into typical equations. Finding out which equation to use and what these values are is the hard part. Depending on the system, you need to have a good working knowledge of pumps and how to read a pump curve; you need to know hydrauilcs, i.e. pressure drop and line sizing; you need to have a good working knowledge of thermodynamics, e.g. vapor-liquid equilibrium; you need to have a good working knowledge of heat exchangers, e.g. maximum Q based on the clean coefficient; is there a temperature pinch?; you need to have a good working knowledge of separations systems, e.g. distillation, stripping and absorbtion systems; and etc.
Having a novice doing PSV relief sizing is just asking for trouble as this is not something you can afford to "get wrong". I don't think I really need to tell you that if you do get it wrong, there is a substantial risk to not only material damage in your facility but loss of life as well.
You would be well advised to follow the recommendation given to you by @EGT01 above in the last paragraph of his post.
RE: Safety Relief Valve Sizing
Yup. A process engineer (often a chemical engineer) will need to determine what happens to materials as they start to heat up in a fire case. Most "fluids" are also not a pure substance (eg oil) and does different things at different rates/temps/pressures/etc.
I would suggest going back to whoever asked you to do this and let them know you can't do it, and get some help to determine the relieve capacity required.
"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
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RE: Safety Relief Valve Sizing
RE: Safety Relief Valve Sizing
Not "sadly". It is only sad that your boss put you in this situation. At least you knew enough that you needed help. Hopefully, you will find it.
There are lots of things you will not know in your career. The trick is to recognise when you don't know something and need help. The fact that you can recognise this is saying quite a bit.
Good luck.
"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
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RE: Safety Relief Valve Sizing
RE: Safety Relief Valve Sizing
RE: Safety Relief Valve Sizing
Darby,R. and Kamyar Molavi, "Viscosity Correction Factor for Safety Relief Valves", Process Safety Progress, Vol. 16, No. 2, Summer 1997.
This is a publication of the AIChE, Safety Division.
RE: Safety Relief Valve Sizing
It may have become lost in my lengthy previous post, but what you're seeing is precisely one of the things I was trying to warn about.
Though I don't have the specific article PLeckner refers to, I have seen that article referenced in many other papers so certainly it wouldn't hurt to get a copy and I'm sure that will help to better design a relief valve for your system.
But, considering that you are only talking about a pilot plant with a relatively small volume, I wonder how important it is to have a reclosing type relief device and you might be better off with looking at a non-reclosing device such as a rupture disc.
For more information on rupture discs, one very good reference can be found here...
http://www.cheresources.com/asiseeit3.shtml
...you might recognize a name there.
By the way, I've often wondered about those API Pressure Relieving courses. I see that through Equity Engineering, there was a 3 day course June 19, is that the one you attended? What did you think about it?
RE: Safety Relief Valve Sizing
RE: Safety Relief Valve Sizing
For instance, I have precious little data for sizing a PSV on a knock-out drum. In the absence of hard information, how would one go about calculating the worst-case flow rate? The Process Engineer provided the S/N on the upstream PCV (All written data lost in flood; mfr could only provide the maximum flow rate for the valve series.)
Is it reasonable to use Bernoulli to find velocity based on the pressure differential of the upstream vessel's PSV set pressure and the vessel in question's PSV set pressure? Then Continuity to calculate Q, using the area of the PSV nozzle?
RE: Safety Relief Valve Sizing
Introduction, Basic Terminology, PRV Design and Operations, Codes and Standards, PRV Sizing and Selection, Two-phase flow sizing, Rupture Disks, Installation of Pressure Relief Devices, Relief System Design Philosophy, Overpressure Scenarios, Atmospheric Storage Tanks, Power Boilers, MOC and Documentation, Specifications/Data Sheets, PRV Inspection, Testing, and Repair.
Nearly each section had "sample problems" that we worked individually then discussed as a class. Also, the course description says that you should bring API 520 P1, API 520, and API STD 2000, however I only looked at these once during the course and I really didnt need to do that. They give you all the equations, tables, and figures they refer to in the discussion and problems. Of course you will eventually want these as a resource, but I had to hurry to get them because I registered so late, and they really were not necessary.
As far as you question goes, I can't definitively give a suggestion. It sounds like your assumptions are reasonable, however I'm still pretty new to this and I don't want to make a recommendation. Maybe someone with more experience can comment.
RE: Safety Relief Valve Sizing