design part 3 – sizing orifices and piping
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Introduction to Pressure Relief ValveDesign Part 3 – Sizing Orifices and Piping
2009 November 21tags: accumulation, car sealed close, car sealed open, choked flow, overpressure, pressure
relief valve, relief valves by admin
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This is the third in a set of articles introducing the basics of pressure relief valve design from a process designer’s viewpoint. Read Part 1 here and Part 2 here.
Orifice Sizes
Once you have all of the scenarios that can cause your relief valve to open (see Part 2), and
all the key fluid data for each scenario, you can size the relief valve orifice. This is the sizeof the opening the fluid passes through within your relief valve. In general, a relief valvevendor will have several standard orifice sizes and you will pick the one that best fits your
need.
API 520/521 has some equations to determine the minimum orifice size you need, as well
as good advice and factors to put into their equations when you don’t have informationfrom the valve manufacturer yet. There are many programs and spreadsheets out there tosize the orifice, so find out what your office uses. By carefully reading the standard and an
example problem or two done by your office’s methods, you should find the actual“sizing” of the relief valve is relatively easy.
One key factor that any calculation procedure will have you do is check for choked flow /critical flow. Choked flow is when a fluid is going so fast that it reaches sonic velocity:after that, it cannot go any faster no matter what the downstream pressure is. You should
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know the approximate inlet and outlet pressures of the relief valve, so you can check if you
will reach choked flow. If it is choked, then that changes your results. API discusses thisand gives different instructions and equations for chocked vs. non-chocked flow.
Once you have several orifice sizes calculated, you will select the relief valve orifice size
just a bit higher than the maximum area that you calculated. So if you calculate 2.0 squareinch for the fire case, 0.675 sq in for the cooling water failure case, and 0.5 sq in for
thermal expansion, you’d probably take the L orifice which is good for up to 2.853 sq in.After that, you look at a catalog of relief valves and you can see what valve sizes areoffered for that relief valve orifice size. For example, a 4 N 6 valve means a 4” inlet
flange, 6” outlet flange, and a N sized orifice. I would not expect to find a 1 N 2 valvethough, because an “N” orifice is relatively large and a 1″x2″ relief valve is somewhat
small. API 526 also has tables you can look at showing typical valve sizes for each orifice.
Accumulation: pressures over the set pressure?
You may notice some discussions of accumulation in the sizing standards: in API 520,
figure 1 shows that you can have 10% accumulation in most cases with a single relief valve, 16% in most cases if there is more than one relief valve, and 21% for a fire case.
What does this mean?
Well, let’s say the relief valve set pressure is 100 psig. The valve you bought is going tostart opening at 100 psig. But it will not fully open immediately and magically vent
absolutely everything to a safe location in an instant. Instead, the pressure will probablyget a little above 100 psig as the valve opens; it will open fast, but not instantaneously. An
allowable accumulation (also known as allowable overpressure) of 10% means that the pressure in the vessel can get as high 110 psig at some temporary point in the process of the relief valve opening. The relief valve vendor is going to look at your set pressure, look
at the flowrate for the worst relief scenarios, and then make sure the valve acts fast enoughthat things never ever get worse than 110 psig. (Or 121 psig for a fire).
Inlet and Outlet Piping
Design of the inlet and outlet piping of relief valves must be done carefully. For a start,you want low pressure drops. High inlet pressure drops upstream of the valve can cause
pipe stress / vibration, and also a high pressure loss can “hide” the real pressure in thevessel from the valve. A high downstream outlet pressure can result in back-pressure: thehigh outlet pressure “pushes” your relief valve and makes it harder to open or stay open. In
general, the inlet pressure drop from protected equipment to relief valve should bemaximum 3% of set pressure (in gauge pressure). Meanwhile, the outlet pressure of the
relief valve should be a maximum X% of the set pressure; 10% is the most typical. Youwill check that by starting at the final relief destination (flare stack / vent location / etc.)and doing hydraulic calculations backwards, until you determine the pressure at the outlet
of the relief valve. Also, because relief scenarios can put a lot of strain onto pipes, youshould work with your piping designer to minimize pipe stress and have a mechanically
robust layout.
Tip: “balanced” and “pilot” relief valves are two special types of relief valves that mayhelp if you cannot meet these rules. They have less stringent requirements.
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When designing inlet and outlet piping, the flowrates you use are often NOT just the relief
scenario flowrates. Rather, they are the flowrates multiplied by the actual orifice size / thecalculated orifice size. This is called the “rated” flowrate.
(Example: I complete calculations for all the relief scenarios, and the largest load is from
the fire case. I have a fire case generating 2000 lb/hr, and I calculate I need a 2.000 sqinch orifice using my API rules. But the actual orifice I buy is going to be the closest
orifice size I can find that is equal to or greater than the calculated size; probably theclosest size I can find is 2.853 sq inches. My calculations told me to buy 2.000 sq inch but
I actually bought 2.853 sq inch. Therefore I design all the piping and the flare header as if
there were 2000 x 2.853 / 2.000 = 2853 lb/hr at relief.
The relief valve is set at 500 psig, so I aim to keep the inlet pressure losses from the
vessel/pipe to the relief valve below 500 x 0.03 = 15 psig. I need to design the inlet piping such that 2853 lb/hr will not cause a pressure drop over 15 psi. I must also design theoutlet piping to ensure that relief valve outlet pressure shall be less than 500 * 0.10 = 50
psig. If I am discharging to atmosphere (0 psig), that means I can have up to 50 psig pressure drop in my outlet lines. (0 psig pressure + 50 psig line losses = 50 psi, just barely
meeting my 10% rule). Because it is a fire case, as discussed in Part 2 , the fire could affect several vessels simultaneously. I must check if the main flare headers may be receiving loads from several relief valves at once, because the extra fluid from several valves at once
will affect the outlet pressure profile.
Lastly, all my other relief scenarios will be checked and scaled the same way, with the
flowrates and minimum orifices sizes calculated by API. If a cooling water failure caused 500 lb/hr and required a 0.675 sq inch minimum orifice size, I will have to check my
pressure drops against 500 * 2.853 / 0.675 = 2113 lb/hr of the relief fluid generated by a
cooling water failure)
In some cases, when sizing common flare header lines that serve as a main multiple for
several PSVs, it is acceptable to use the normal loads when considering multiple valvesrelieving at once. (But you would still consider rated flows for any scenarios where asingle valve pops). Also, sometimes the “normal” load is used for non-conventional valves
like pilot valves. Check API and the rules of your company. If you are dealing with a lot of PSVs interacting, like say a main flare header in a plant of some kind, it’s recommended
you get a commercial package specializing in these types flare/header designs. They willhelp you keep track of multiple relief scenarios and deal the complex hydraulics.
One last point: recall that you checked for choked flow in the orifice sizing. Choked flow
is OK in the valve, but it’s a bad idea for the piping: it can increase vibrations and stresseson the pipes and the noise can be so loud it breaks safety regulations. To avoid dealing
with these complexities, many companies have a rule of thumb like “keep relief valveoutlet pipe velocity below X% of sonic in all relief scenarios.” Where X might be 60-80%.Larger outlet pipes will help you avoid sonic flow.
(For a fluid, Sonic velocity (ft/s) = 68.1 x root(k * P/ rho), where k, P (psia), and rho
(lb/ft3) are evaluated at the actual fluid conditions. Alternatively, API 520 Part 1, Section3.3.3.1.3 has a calculation to help you do a quick check whether your outlet line will choke
or not).
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Odds and Ends
Some miscellaneous thoughts to close this relief valve series and encourage further reading:
• Iteration: You notice that to size the orifice you need fluid properties at the inlet of the valve, and you don’t technically know precisely what they are until you calculatethe inlet pressure drop. (So that you can know the exact inlet pressure). Sometimes
you can avoid this by just assuming inlet pressure = set pressure, or inlet pressure =set pressure – 3%. That guess may be close enough. If you cannot make anassumption, you may need to iterate. Other steps may also require design iteration:
doing your relief design may stimulate you to change the design of the protected process system to make the relief valve cheaper. Also, you may need to do some
relief design to get scenarios and rough flowrates before you start the flare header design, and then change parts of your relief system as you run through the flaresystem. (Recall that normally you want to limit outlet piping pressure drop, and that
the relief header can face several relief loads at once in some cases like power failures or large pool fires…sometimes in the flare header design you find reasons to
go back and change the relief valves)• Two-phase flow: Is common and makes this all more complicated. API 520
Appendix D has some advice. Information from the Design Institute for Emergency
Relief Systems (DIERS) may help if you need to get into real details• Choose the right set pressure: Don’t make the set pressure unnecessarily low “to
be safe.” Each time a relief valve opens you are losing product. And now someone’sgot to go make sure it closed properly, and test it…you want to design your systemso that the relief valve is never used.
• Remember that relief valves are safety devices, not control devices. Use a normalcontrol loop to control pressure during normal operation• Chattering: You may wonder why we don’t just add really huge relief valves with
huge orifices to be extra safe and to avoid a lot of designer’s grief. One reason is tosave money, of course, because bigger valves cost money and also generate larger
releases, which increases the size of the outlet piping. Another reason is you need todetermine these loads anyway to design a flare system. A third issue is chattering:recall that relief valves will close after the pressure in the vessel drops. If you have a
huge relief valve, then as soon as it opens the pressure in the protected vessel willdrop rapidly and the valve will slam shut. If you had a huge relief valve popping up
but the overpressure cause that rebuilds the pressure again and again after each
release, then the valve will keep opening, spewing all the fluid, and slamming shut.This cycle of jolting open and slamming shut wrecks the valve. You would rather
have the valve smoothly open and gradually close. I think pilot valves (a type of relief valve) may help in some cases where chattering is unavoidable.
• Multiple relief valves: Sometimes several relief valves are a good idea. Why?Maybe it is too expensive to get one giant relief valve, but you can buy two smaller valves that together provide the orifice area to do the job. In some cases you have
two relief scenarios at vastly different flowrates: you can buy two relief vales, set thesmall valve to open first to deal with the small scenario, and the big relief valve to
open at a slightly higher pressure (so that if the small valve can’t cut it, the pressurerises more, and the big valve steps in to save the day). That way when you get the
small relief scenario the small valve handles it, and you don’t have some huge valveexperiencing dangerous chattering.
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• Offline valves/maintenance: Normally it is OK to have one valve, or one set of
working valves, covering a service. But what about maintenance of the valves, youask? Well, it is nice if the relief valve inspection and maintenance schedule syncswith the equipment maintenance schedule, then you can take the equipment and the
valve(s) offline for service at the same time. If the maintenance schedules do notsync up, then normally a careful procedure can be followed where the relief valve is
quickly serviced while the pressure in the vessel is carefully watched and someone isready to take manual action in the event of a high pressure.• Installed spares: In some critical services people like to spend more and have spare
of relief valves kept “offline.” This could be one valve + one spare, or if two relief valves are called for in the design, have three valves with one kept offline. Some
companies are willing to spend and make spare relief valves the standard procedureto use in almost all cases – it costs more but makes maintenance easier and safer.You can easily pull a valve for maintenance and have the spare perfectly placed to
take over. But remember, do not have the offline valve “in service” (connected andwith the isolation valves open) while you have with the other standard valve(s) also
connected, because then you risk chattering. 2 valves is not safer than 1, if 1 valve
was sized to do the job. Keep the spare valve(s) closed off from the process until youneed them
• Locked open (LO), locked closed (LC), Car sealed open (CSO), and car sealedclosed (CSC) valves: You may see gate valves marked LO and LC on the P&ID
drawings. These valves can only be opened and closed with a special key, whose useis tightly controlled. The plant will have procedures so that the key has to be signedout, and can only be taken after a formal work plan has been made. Basically, these
valves are a real pain for maintenance and operations to use, which prevent someonefrom casually and accidentally turning these valves. Spare relief valves are acommon place to use such valves: you do not want spare valves open to the process
UNLESS the main valve is taken offline, and you definitely do not want to
accidentally close all the gate valves so that no relief valve is connected to thesystem. You use LO/LC to put isolation valves around the relief valves, whileminimizing the risk that people accidentally open or close them incorrectly. Theisolation valves are only turned after a formal work plan has been written and people
have thought through the consequences. (If there is only one relief valve, preferred practice may be to have NO isolation valves, or it may be LO/LC valves…it depends
on how the valve and system will be maintained). CSO & CSC are “car sealedopen/closed” valves, which are a similar idea but replaces the key with a plastic sealyou have to cut open after filling out a plant paperwork procedure. One cannot turn
the valves without breaking the seal. Reference.• Boilers: They have special rules. Check ASME I or whatever the appropriate
standard is for you.• Revamp projects: Whenever an existing plant or process is modified it is called a
revamp. Most revamps intend to expand the capacity of the existing plant, through a
combination of installing new equipment and creatively re-using what is alreadythere. In any modification, the relief valves have to be checked to make sure they
can still be used in the new service, and this is no easy task. However, there is a bigsavings when an existing relief valve can handle the new service, compared to theexpense of installing a new valve. Sometimes companies will invest in fancy
dynamic simulators and other tools to try to re-use existing relief valves. By“sharpening your pencil” with these tools and doing very precise scenariocalculations, you may find that the design margins and assumptions in the original
sizing calculations were so high that the old relief valve can be reused for larger revamp flowrates. This is a big savings! Also, a warning: often in revamps the
hardest part can be gathering the data: finding the original datasheets and
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calculations available for valves and vessels, learning the layout, waiting for the
records guy to get back to you, etc. The actual calculation may be relatively easycompared to the difficulties in getting data to work with
• Relief valves in equipment packages: Sometimes vendors who sell “packages” of
equipment can design relief valves for you. For example, vendors selling a pressurereducing valve or a chemical injection system may have relief valves perfectly
customized to suit their equipment. This can save you time and trouble• Relief valve vendors: The good ones are quite knowledgeable. Ask for their helpand advice. Ask about things like balanced and pilot relief valves if you are worried
about challenges like back pressure, chattering, etc.
This closes the introduction. Return to Part 1 here or Part 2 here. More topics and advice
may come in subsequent posts under this category. Use the sidebar to browse posts bycategory.
Edit 2010-04-22: Minor rewrite for clarity, added note on offline valves/maintenance.
Edit 2010-11-04: Expanded example, added note on Car Sealed Open / Closed valves.
Edit 2011-04-26: Noted that in some cases it is acceptable to design some flare header
lines for normal flow through several relief valves, rather than the rated (scaled-by-orifice size) flow.
Edit 2011-09-14: Edited definition of locked open/closed vs. car sealed open/closed
valves.
Edit 2011-12-06: Improved description of outlet line sizing.
Popularity: 65% [?]
Related posts:
1. Introduction to Pressure Relief Valve Design Part 2 – Relief Scenarios and theRelief Rate
2. Introduction to Pressure Relief Valve Design Part 1 – Types & Set Pressure
3. More help finding salaries and control valve pressure drops4. Assign Control Valve Pressure Drops5. Introduction to Process Hazard Safety Meetings: Part 1 Concepts and Worksheet
from → Relief & Safety
4 Responsesleave one →
316SS Pressure Regulators
Gas and Hydraulic Pressure Service Forward Reducing and Back Pressureww.pressure-tech.com
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1.reducing valve permalink
December 9, 2009
Thanks much for that useful post.
Reply
2.John S permalink June 1, 2010
Thank you, this has cleared up a lot of confusion I had regarding relief systems.
Reply
3.
Amit Bansal permalink December 19, 2010
AWESOME ARTICLES !!!
Hi, I read all the way you article for relief valve. I am sizing relief valve from
approx last 2 years and I found what you wrote here is very very useful for newEngineers.
Warm Regards
AB
Reply
4.admin permalink *
January 3, 2011
Glad you guys found it so useful!
By the way, since a few people asked, the example numbers here (e.g. 2000 lb/hr fire case with 2.0 sq inch orifice size required and 2.853 selected) are just made upnumbers to illustrate the point, not based on actual calculations.
Reply
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