Design Part 3 – Sizing Orifices and Piping

8/3/2019 Design Part 3 – Sizing Orifices and Piping

http://slidepdf.com/reader/full/design-part-3-sizing-orifices-and-piping 1/11

Skip to content

Smart Process Design

Chemical Engineering Blog | Process Design | Project

Management |EPC | Business | Excel | Tools & Tricks

• Home• About/Terms

• Tools & Books

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

If you're new here, you may want to subscribe to my RSS feed or bookmark this site.You might find the answers to another problem down the road. Thanks for visiting!

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

safety relief valve-CNChina Professional making of Various valve,iso,api certification

www.taito-valve.com

Ads by Google Relief Valve Piping Pressure Sizing

Page 1 of 11Introduction to Pressure Relief Valve Design Part 3 – Sizing Orifices and Piping – Sm...

18/02/2012http://smartprocessdesign.com/introduction-pressure-relief-valve-design-part-3-sizing...



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.





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).

Rotary ValvesRotary Airlocks and Feeders for all Industries and Applications.

www.anval.net





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.





• 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





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





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

Leave a Reply

Name: :





Email: :

Website:

Comment:

Note: You can use basic XHTML in your comments. Your email address will never be published.

Subscribe to this comment feed via RSS

Preview

Click the "Preview" button to preview your comment here.

Notify me of followup comments via e-mail

Notify me of new posts by email.

Spam protection by WP Captcha-Free

Submit Comment

Free Updates

Get the latest and the greatest news delivered for free to your reader or your inbox:

RSS FeedEmail Updates

•

Smart Process Design's Suggested Books

Branan's Rulesof Thumb for Chemical

Engineers,Fourth...Carl R. BrananNew $91.49Best $85.46

Perry's ChemicalEngineers'Handbook,

Eighth EditionDon Green,Robert ...New $121.96Best $85.44

Working Guide toProcessEquipmentNormanLieberman, ...New $55.96Best $45.55

Rules of Thumbfor ChemicalEngineersInteractive CDCarl R. BrananBest $304.55

Privacy Information

• Google Search the Site





Search

• Search

type and press enter

• Learn about our site

◦ About/Terms◦ Tools & Books

• Sponsored Links

Emergency Shelter

•

• Profile Controls

◦ Register ◦ Log in

◦ Entries RSS◦ Comments RSS

◦ WordPress.org

• Topics

Select Category

• Archives

Select Month

• Recent Comments

◦ glacial acetic acid on 3 Top Blogs: Chemical Engineering and Life Hackingarticles

◦ arel on Aspentech’s Hysys: Fluid Package (Thermodynamics) Notes◦ Caustic soda manufacturer on 3 Top Blogs: Chemical Engineering and Life

Hacking articles

• Recent Posts

Ads by Google

Mechanical Design

Process Design

ASME Design





Units Converter

◦ Sulfuric Acid Manufacturing: Favored book and website

◦ Overspilling (Multi-Page) Datasheets in Aspen Basic Engineering◦ Recalled to life

• Popular Recently (90 Days)

1. 2%Hysys Tips and Tricks for Logical Units and Reactors2. 1%Recalled to life

3. 1%Sulfuric Acid Manufacturing: Favored book and website

• Polls

Climate Change: Which statement best reflects your opinion?

It's not happening, or not a concern

It's natural, cyclical changes. Not man-made

I'm not sure what to think It's one problem among many. Let's not panic A major problem the world isn't facing

Among the greatest threats in the world today

Vote

View Results

◦ Polls Archive

•

Conversion type:

Amount:

1.00

From:

To:

Result:0.00

Convert

• Blogroll

◦ Chemical & Process Technology

◦ Chemical Engineering Blog◦ Chemical Engineering Web 2.0◦ Chemical Professionals

◦ ChEng◦ ChENGINEER SPACE

Page 10 of 11Introduction to Pressure Relief Valve Design Part 3 – Sizing Orifices and Piping – ...




Copyright 2012 smartprocessdesign.com. By viewing this website you agree to the terms

at http://smartprocessdesign.com/about/

Vigilance Theme by The Theme Foundry

Page 11 of 11Introduction to Pressure Relief Valve Design Part 3 – Sizing Orifices and Piping – ...

Documents

Design Part 3 – Sizing Orifices and Piping