rupture disks for process engineers

Rupture Disks for Process Engineers - Part 1

This is a real story. A rupture disk manufacturer presented a seminar to a group consisting of junior and more senior level process design engineers (yours truly included) with a few instrument engineers thrown in. After about an hour of hearing terms such as bursting pressure, tolerance, manufacturing range, etc., and discussions on the mechanical aspects that differentiate the various types of rupture disks, the seminar ended with many of those attending just shaking their heads. Most of the attendees just wanted to learn how to specify this item so the instrument engineer can buy one or the manufacturer can tell you what is needed.

I eventually put together a

ads not by this site seminar on rupture disks for process design engineers that went over very well. This series of articles is taken from that seminar. Part 1 covers the whys and when to use a rupture disk. Part 2 covers how to size the rupture disk. Subsequent parts will include how to set the burst pressure, the Relief Valve/Rupture Disk combination, how to specify the device and some discussion on the type of rupture disks you can purchase.

Before I begin, let me point out that most of what is included in this series of articles can be found in API RP5201 and API RP5212, and ASME Section VIII, Division 13. Much of what is found in these documents can also be found in vendor literature.

Why and When to Use Rupture Disks

Why Do We Use a Stand-Alone Rupture Disk?

A rupture disk is just another pressure relieving device. It is used for the same purpose as a relief valve, to protect a vessel or system from overpressure that can cause catastrophic failure and even a death.

When Do We Use a Stand-Alone Rupture Disk?

Some of the more common reasons are listed below. You may think of others.

Figure 1: Basic Components of a Rupture Disk

1. Capital and Maintenance Savings: Rupture disks cost less than relief valves. They generally require little to no maintenance.

2. Contents will be lost, but who cares? A rupture disk is a nonreclosing device, which means once it opens, it doesn't close. Whatever is in the system will get out and continue to do so until stopped by some form of intervention. If loss of contents is not an issue, then a rupture disk may be the relief device of choice.

3. Benign service: It is preferable that the relieving contents be non-toxic, non-hazardous, etc. However, this is not a requirement when deciding to use, or not use, a stand-alone rupture disk.

4. Rupture disks are extremely fast acting: Rupture disks should be considered first when there is a potential for runaway reactions. In this application, relief valves will not react fast enough to prevent a catastrophic failure. A relief valve may still be installed on the vessel to protect against other relieving scenarios. Some engineers prefer to use rupture disks for heat exchanger tube rupture scenarios rather than relief valves. They are concerned that relief valves won't respond fast enough to pressure spikes that may be experienced if gas/vapor is the driving force or liquid flashing occurs.

Figure 2: Rupture Disk Mounted on a Vessel

5. The system contents can plug the relief valve during relief: There are some liquids that may actually freeze when undergoing rapid depressurization. This may cause blockage within a relief valve that would render it useless. Also, if the vessel contains solids, there is a danger of the relief valve plugging during relief.

6. High viscosity liquids. If the system is filled with highly viscous liquids such as polymers, the rupture disk should seriously be considered as the preferable relieving device. Flow through a relief valve will be very difficult to calculate accurately. Also, very viscous fluid may not relieve fast enough through a relief valve.

Cost Comparison Between Comparable Stand-Alone Rupture Disk and Relief Valve

Rupture disk manufacturers burst at least two disks per lot before shipping them to a customer.

As a consequence even if you want just one rupture disk you will be buying three. Therefore, the first usable rupture disk is comparatively expensive. Also for new installations, each installed rupture disk must be purchased along with a holder. However, the same holder may be used for replacement purchases as long as you buy the exact same rupture disk from the same manufacturer.

Below is a capital cost comparison between Continental Disc Corp. (www.contdisc.com) 3" Ultrx Hastelloy C rupture disks with holders and Farris Engineering (www.cwfc.com) 2600 series relief valves, based on a budget estimate in year 2001 dollars.

Table 1: Cost Comparison - Rupture Disk vs. Relief Valve

Basis: Continental DiscBasis: Farris Engineering

3" Ultrx Hast C Disc = $2,600 for 1st usable disk, then $870 each3" x 4" Hast C 26KA10-120 = $13,400

3" Ultrx Hast C Holder = $3,300 ea.

TOTAL for one pair = $5,900

TOTAL for three pair = $14,240TOTAL for three = $40,200

This capital cost comparison will vary considerably with size and material of construction but you get the point. However please note that everything has a value and the loss of contents should be considered in the overall cost difference between a rupture disk and a relief valve.

When Do We Use a Rupture Disk-Relief Valve Combination?

Rupture disks are often used in combination with and installed just upstream and/or just downstream of a relief valve. You may want to choose the combination option if:

Figure 3: Rupture Disk-Relief Valve Combination

1. You need to ensure a positive seal of the system (the system contains a toxic substance and you are concerned that the relief valve may leak). Application: rupture disk installed upstream of the relief valve.

2. The system contains solids that may plug the relief valve over time. Remember, the relief valve is continuously exposed to the system. Application: rupture disk installed upstream of the relief valve.

3. TO SAVE MONEY! If the system is a corrosive environment, the rupture disk is specified with the more exotic and corrosion resistant material. It acts as the barrier between the corrosive system and the relief valve. Application: rupture disk installed either upstream and/or downstream of the relief valve.

Below is a capital cost comparison between combination Hastelloy C rupture disks with stainless steel relief valves and three stand-alone Hastelloy C relief valves. Again, this is based on a budget estimate in year 2001 dollars using Continental Disc Corp. rupture disks and holders and Farris Engineering relief valves.

Table 2: Cost Comparison for Rupture Disk-Relief Valve Combinations

Basis: Continental DiscBasis: Farris Engineering

3" Ultrx Hast C Holder = $3,3003" x 4" Hast C 26KA10-120 = $13,400

3" Ultrx Hast C Disc = $2,600 for 1st usable disk, then $870 each3" x 4" SS 26KA10-120 = $4,300

Combination of Hastelloy C Diskand SS Relief Valve

Single Installation Total = $10,200

Total for three installations = $27,140

Three stand-alone Hastelloy C relief valves = $40,200Summary

A stand-alone rupture disk is used when:

1. You are looking for capital and maintenance savings

2. You can afford to loose the system contents

3. The system contents are relatively benign

4. You need a pressure relief device that is fast acting

5. A relief valve is not suitable due to the nature of the system contents

A rupture disk / relief valve combination is used when:

1. You need to ensure a positive seal of the system

2. The system contains solids that may plug the relief valve over time

3. TO SAVE MONEY! If the system is a corrosive environment, the rupture disk is specified with the more exotic and corrosion resistant material

References

1. API (www.api.org) Recommended Practice 520, "Sizing, Selection, and Installation of Pressure-Relieving Device in Refineries, Part 1-Sizing and Selection", 7th Edition (January 2000)

2. API (www.api.org) Recommended Practice 521, "Guide for Pressure-Relieving and Depressuring Systems", 4th Edition (March 1997)

3. ASME (www.asme.org) "Boiler and Pressure Vessel Code, Section VIII, Division 1" (1998)

Chemical and Process Engineering Resources


Nov 08 2010 01:30 PM | pleckner in Safety and Pressure Relief Share this topic:

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Part 1 of this series on rupture disks for Process Engineers covered why you use a rupture disk and when you might want to use this device. This part will discuss how to size the rupture disk. Subsequent parts will include how to set the burst pressure, the Relief Valve/Rupture Disk combination, how to specify the device and some discussion on the type of rupture disks you can purchase.

Before I begin, let me point out that most of what is included in this series of articles can be found in API RP5201 and API RP5212, and ASME Section VIII, Division 13. Much of what is found in these documents can also be found in vendor literature

Sizing

Sizing the rupture disk is a two-part procedure. First, determine how much flow the rupture disk needs to pass. Then determine how big it needs to be.

How much flow does it need to pass?Answering this question is the same as determining the required relieving rate for the system. There is no difference between determining the relieving rate for a rupture disk and a relief valve. They both require a set pressure (burst pressure for rupture disk), an allowable overpressure, an evaluation and calculation of the required relieving rate for each credible scenario and then choosing the flow rate associated with the worst-case scenario. Determining the controlling relieving rate is a paper in of itself and I will not attempt to get into details here.

How Big?There are two recognized methods that can be used to answer this question, the Resistance to Flow Method or the Coefficient of Discharge Method.

Resistance to Flow Method

The Resistance to Flow Method analyzes the flow capacity of the relief piping. The analysis takes into account frictional losses of the relief piping and all piping components. Calculations are performed using accepted engineering practices for determining fluid flow through piping systems such as the Bernoulli equation for liquids, the Isothermal or adiabatic flow equations for vapor/gas and DIERS methodology for two-phase flow.

Piping component losses may include nozzle entrances and exits, elbows, tees, reducers, valves and the rupture disk (note that the rupture disk and its holder are considered a unit). Let me emphasize that in this method, the rupture disk is considered to be just another piping component, nothing more, and nothing less. Therefore the rupture disk's contribution to the over all frictional loss in the piping system needs to be determined. This is accomplished by using "Kr", which is analogous to the K value of other piping components. Kr is determined experimentally in flow laboratories by the manufacturer for their line of products and is certified per ASME Section VIII, Division 13. It is a measure of the flow resistance through the rupture disk and accounts for the holder and the bursting characteristics of the disk.

Below is a list of some models of Continental Disc Corporation rupture disks with their certified Kr values4.

Table 1: Rupture Disks from Continental Disc Corp.

Rupture Disk (and holder) TypeMediaSize RangeKr

ULTRXGas, Liquid1" - 12"0.62

ULTRXGas only1" - 12"0.36

MINTRXGas, Liquid1"- 8"0.75

STARXGas, Liquid1" - 6"0.38

SANITRXGas, Liquid11/2" - 4"3.18

For comparison, the following is a list of some models of Fike rupture disks with their certified Kr values5.

Table 2: Rupture Disks from Fike

Rupture Disk (and holder) TypeMediaSize RangeKr

SRX-1" - 24"0.99

SRL-1" - 8"0.38

SRH-1 1/2" - 4"1.88

HO / HOV-1" - 24"2.02

PV, CPV, CP-C, CPV-C-1/2" - 24"3.50

If at the time of sizing the manufacturer and model of the rupture disk are unknown, there are guidelines to help you choose Kr. API RP5212 recommends using a K of 1.5. However, ASME Section VIII, Division 13 states that a Kr of 2.4 shall be used. Which one? Remember that ASME is Code (meaning LAW for the most part) and API is a recommended practice. In addition, as can be seen in the tables above, even ASME may not be as conservative as you may think. Therefore, it is in the engineer's best interest to determine ahead of time the manufacturer and model of the rupture disk that eventually will be purchased. This can be done without knowing the exact size, as Kr is more manufacturer and model specific than size specific (see above tables). If a number of manufacturers are on the allowable purchase list, then at the very least choose the most likely models you would buy from each manufacturer and use the largest Kr from that list. This will be a significantly better guess than just using guidelines.

Once the piping system is laid out and all the fitting types are known, including the rupture disk, the engineer can proceed with the calculations in the following manner (presented here as a suggestion, there are many ways to do it).

1. Known are the two terminal pressures, these being the relieving pressure (upstream) and the downstream pressure (a knock-out pot, atmosphere, etc.).

2. Also known are the fluid properties and required relieving rate (the flow the rupture disk needs to pass).

3. Choose a pipe size. This will be the size to use for all components, including the rupture disk.

4. For vapor/gas or two-phase flow, use one of the accepted calculation methods to determine the maximum flow through the system. The maximum flow through the system is commonly known as critical flow or choked flow. For liquids, use the Bernoulli equation to calculate the flow that will balance the system pressure losses.5. Per ASME Section VIII, Division 1, multiply this flow by 0.9 to take into account inaccuracies in the system parameters. Compare the adjusted calculated flow to the required relieving rate. If it is greater, then the calculation is basically done. However, the next smaller line size should also be checked to make sure the system is optimized; you want the smallest sized system possible. If the adjusted calculated flow is less than the required relieving rate, the pipe is too small, choose a larger size and repeat the calculations.

Why not just choose a large Kr? Isn't that more conservative?

Many times, relief is not to atmosphere but to some downstream collection and treatment system, e.g. knockout drums and flares or thermal oxidizers. These are more often than not specified at a time period in the design that predates the actual purchase of the rupture disk. The flow used to size this equipment will be based on the capacity of your relief system as determined above.

If the rupture disk contributes a significant portion of the frictional losses to the system, a fictitiously large Kr might result in an oversized piping system. Sounds all right on the surface but once the actual rupture disk is chosen, the calculation must be repeated with the "real" Kr and this may be a much lower value than originally used. More fluid will flow through the system than previously determined because there will actually be less resistance to flow. The result is that the downstream processing equipment may have been undersized.

The opposite is also true. An initial guess of a fictitiously small Kr might ultimately result in oversized downstream equipment and the excessive expenditure of a significant amount of money.

Atmospheric discharge must also be similarly analyzed because the flow capacity determined after rupture disk selection may have a major impact on the emissions reported for permitting if they were based on the initial value of Kr.

Coefficient of Discharge Model

The second calculational method is the Coefficient of Discharge Method. The rupture disk is treated as a relief valve with the flow area calculated utilizing relief valve formulas and a fixed coefficient of discharge, Kd', of 0.62. This method does NOT directly take into account piping so there are restrictions in its use. These restrictions are known as the "8 & 5 Rule" which states that in order to use this method to

size the rupture disk ALL of the following four conditions MUST be met3:

The rupture disk must be installed within 8 pipe diameters of the vessel or other overpressure source.

1. The rupture disk discharge pipe must not exceed 5 pipe diameters.

2. The rupture disk must discharge directly to atmosphere.

3. The inlet and outlet piping is at least the same nominal pipe size as the rupture disk.

A sketch of the "8 & 5" rule starting with a 2" nominal sized pipe is shown at the below.

The flow area calculated with this method is called the Minimum Net Flow Area or MNFA. The MNFA is the rupture disk's minimum cross

Figure 1: Diagram Showing the "8 & 5" Rule

sectional area required to meet the needed flow.

This is not the area (and thus the size) you specify. Just like a pipe with a nominal size and an actual inside diameter, the rupture disk has a nominal size and an actual Net Flow Area or NFA. The rupture disk purchased must have a NFA equal to or greater than the MNFA. The manufacturer publishes the NFA for every rupture disk model and size they sell. The NFA also accounts for bursting characteristics of the disk and the holder.

Below is a list of some Continental Disc Corporation rupture disks with their NFA4.

Table 3: Continental Disc Corp Disks with NFA

Rupture Disk (and holder) TypeNominal Size, inchesNFA,

in2

ULTRX1-1/2"2.04

ULTRX3"7.39

SANITRX1-1/2"1.18

SANITRX3"5.49

Once the actual NFA of the rupture disk is determined, the calculations must be repeated, basically for the same reasons discussed above for the Resistance to Flow Method.




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Why I Don't Like the Coefficient of Discharge Model

It's too restrictive! During the basic design phase of a project, actual piping configuration is unknown. You may think you are within the "8 & 5" rule at first but may not be when the final details are worked out. Remember, the "5" means 5 pipe diameters. For a 3" line, that is only a nominal 15". For a 6' vertical vessel with a rupture disk discharge being piped to a drain hub on the floor, the 15" maximum length is exceeded without even thinking.

Using the Resistance to Flow Method is valid for all cases. All sizing calculations can be standardized.

ads not by this site The Kr used in the Resistance to Flow Method is obtained by actual flow data for a given model of rupture disk and holder. Its use will provide a much more accurate calculation. The 0.62 coefficient of discharge used in the Coefficient of Discharge Method is very general and independent of rupture disk manufacturer model and type, holder, disk bursting characteristics and flow restrictions of the total relief system.

Two-phase flow can be a major concern when using this method. The coefficient of discharge was established mainly for true vapors. Its application to liquids is questionable and its application to two-phase flow is totally fictitious. Granted, for the Resistance to Flow Method the Kr is not particularly applicable to two-phase systems either but one can easily compensate for this in the system calculations. Also, the rupture disk is only a part of an entire piping system and its overall contribution to the system frictional losses may not be greatly significant. Therefore, errors in Kr may not be very significant. In the Coefficient of Discharge Method, it is the only device considered. If the coefficient of discharge is grossly in error, the MNFA calculated will also be grossly in error.

The same argument can be made for highly viscous liquid systems such as polymers.

In Summary

Obtain the required relieving rate using good sound "what can go wrong" scenario analysis.

Use the Resistance to Flow Method to calculate the size of the rupture disk (use the Coefficient of Discharge Method if you really must and you fall within the "8 & 5" rule).

For the Resistance to Flow Method, try to choose the manufacturer and model of rupture disk you intend to purchase ahead of time or at least have a list of acceptable manufacturers and a good idea of the model you intend to use from each.

For the Resistance to Flow Method use the ASME Kr value of 2.4 if you have no idea who the manufacturer(s) will be at the time of sizing.

References




4. Continental Disc Corporation (www.contdisc.com), CertiflowTM Catalogue 1-1112

5. Fike (www.fike.com), Technical Bulletin TB8104, December 1999

6. Another good rupture disk manufacturer to investigate would be Oseco (www.oseco.com).

7. A good reference source for calculating flow through the system for liquids and gas/vapors is CRANE Technical Paper 410, "Flow of Fluids Through Valves, Fittings, and Pipe"

8. A great source and one that I feel should be the bible on two-phase flow is: Leung, J.C. "Easily Size Relief Device and Piping for Two-Phase Flow", Chemical Engineering Progress, December, 1996




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Part 1 of this series on rupture disks for Process Engineers covered why you use a rupture disk and when you might want to use this device. Part 2 discussed how to size the rupture disk. In this part, I will cover how to set the burst pressure. Subsequent parts will include temperature and backpressure affects, the Relief Valve/Rupture Disk combination, how to specify the rupture disk and some discussion on the type of rupture disks you can purchase.


Problem

ads not by this site 1. What is the maximum allowable specified burst pressure?

2. What should the expected stamped (rated) burst pressure of the rupture disk be?

3. At what pressure(s) can we expect the delivered rupture disk to actually burst at?

4. What is the maximum allowable operating pressure in the vessel?

All these questions must be considered in order to properly set the burst pressure of a rupture disk.

What is the Maximum Allowable Specified Burst Pressure?

Burst Pressure

What do we mean by burst pressure? This is the pressure at which the rupture disk will open or burst. It is analogous to the set pressure of a relief valve and is specified by the process engineer.

Design Pressure

To find the maximum allowable specified burst pressure, the process engineer first needs to define a vessel design pressure. The design pressure is an arbitrary value above the vessel maximum operating pressure. One guideline used by many process design engineers is to increase the maximum operating pressure by 25 psig or 10% whichever is greater. For example, if the maximum operating pressure is 70 psig, then 25 psig should be added to arrive at the design pressure since 10% is only 7 psig. The design pressure would then be set at a nice round 100 psig. Other criteria to determine design pressure may be used but I recommend that the margins never be less than what I described above (the reason will become apparent later).

Maximum Allowable Working Pressure (MAWP)

The next step is to determine the Maximum Allowable Working Pressure (MAWP) of the vessel. A vessel specification stating design pressure, the coincident design temperature and other parameters is sent to the manufacturer. The manufacturer performs a series of calculations utilizing these parameters, amongst others, to determine material thickness for use in vessel fabrication. A standard material thickness (greater than or equal to what was calculated) is chosen. With the actual material thickness known, the true MAWP is calculated. The vessel design documents are then stamped (certified) at this pressure in accordance with code. However, for one reason or another, the MAWP calculation is not always done and the vendor will just stamp the vessel at the specified design pressure.

The Maximum Allowable Specified Burst Pressure

So, what is the maximum allowable specified burst pressure? Theoretically it is the MAWP. However, rupture disks are typically specified during basic engineering, which is performed way before the vessel is mechanically designed. This, combined with the fact that the true MAWP may never really be known (as mentioned above), the maximum allowable specified burst pressure will more typically be the vessel's design pressure.

Note if the rupture disk is to be used in conjunction with another relief device to fulfill the total required relieving capacity, the maximum allowable specified burst pressure could be 5% or even 10% greater than the design pressure (or MAWP). See ASME Section VIII, Division 1 paragraphs UG-125 and UG-134.

Also note that the specified burst pressure can be lower than the maximum allowable. Indeed, this is often the case if the rupture disk is used to protect reactor vessels against over pressure due to run-away reactions.

Stamped Burst Pressure

What should the expected stamped (rated) burst pressure of the rupture disk be?

What do we mean by "stamped or rated" burst pressure? Per code, the rupture disk vendor must provide a tag containing, amongst other things, the rated or what is typically called the stamped burst pressure. This is a guaranteed value so the user knows (within an allowable tolerance; more on this later) the exact bursting pressure of the rupture disk. Also this stamped burst pressure must never exceed the design pressure (or MAWP); except for the special case mentioned above.

So, the rupture disk vendor stamps the disk with the burst pressure specified by the process engineer? Not necessarily!

Manufacturing Range (MR)

Figure 1: Graphical Representation of the Manufacturing Range

A rupture disk is made out of a sheet of material, e.g. stainless steel, high alloys, ceramics, etc. Like all things in this world, this sheet of material is not perfect. To quantify the inaccuracies in sheet material thickness, the vendor uses what is called the Manufacturing Range (MR).

Figure 2A: Specified and Stamped Burst Pressure

The MR is expressed as % of the specified burst pressure. It determines the highest pressure above the specified burst pressure or the lowest pressure below the specified burst pressure that the disk can be stamped at. This is shown graphically in Figure 1.

Figure 1 shows the two extremes, a MR of 0% and a MR of some value%. Note that other combinations may be used such as + 0% and - some value% or - 0% and + some value%.

Let's look at an example. If the specified burst pressure is 100 psig with a MR of 0%, the stamped or rated burst pressure will be 100 psig (see Figure 2A). However, if the MR is +5% and - 10%, the disk can be delivered with a stamped burst pressure of 105 psig, 90 psig or anywhere in between (see Figure 2B). That's right, if the MR is anything but 0%, the user won't know the stamped burst pressure until the rupture disk is ready for shipment!

Figure 2B: Differences in Specified and Stamped Burst Pressures

Do you see anything wrong with this rupture disk as specified?

Remember, the stamped or rated burst pressure must never exceed the vessel's design pressure or MAWP (assumes a single device, no special cases). Since the specified burst pressure is the design pressure, this particular rupture disk is not acceptable because the delivered rupture disk may have a stamped burst pressure of 105 psig or 5 psig greater than design!

How can we avoid this problem? There are a number of ways.

The process engineer specifies the Manufacturing Range, not the manufacturer. You can ask for any range within the capability of fabrication including 0%. Considering the potential problems, why specify anything other than 0%? Cost. A MR of +5% and -10% can save as much as 40% off the cost of a similar rupture disk with a MR of 0%. Even if you demand +0% (which you should), you can still realize some cost savings if a stamped burst pressure lower than specified is acceptable (not always a good idea as will be discussed later). Note that code only affects the upper stamped limit, not the lower.

Another way to avoid the potential violation of code and still get a cheaper rupture disk is to specify a burst pressure that will be lower than the vessel design pressure. Thus, when the MR is added the stamped burst pressure will not exceed the design pressure. The maximum allowable specified burst pressure could be determined in the following manner:

Pspec_max = (DP) - (+MR/100) x (Pspec_max)Where DP = Design pressure

So:

Pspec_max = (DP) / [1+(+MR)/100]Since DP = 100 psig and the upper value of MR = +5%,

Pspec_max = 100 /[1+(+5/100)] = 100/(1+0.05) = 100/1.05 = 95.2 psig

This rupture disk would be specified with a burst pressure no higher than 95.2 psig while the stamped burst pressure may be as high as 100 psig.

Note that the standard Manufacturing Range for most manufacturers is 0% and this is reflected in the base price you will be quoted.




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Where Will the Disk Burst?

At what pressure(s) can we expect the delivered rupture disk to burst at?

Trick question? The answer should be the stamped burst pressure. But again the world isn't perfect.

Burst Tolerance

Figure 3A: Burst Tolerance

The Manufacturing Range is applied to the specified burst pressure but there is yet another unknown due to imperfections in the material used to fabricate the rupture disk. This is accounted for in the burst tolerance. Burst tolerance is applied to the stamped burst pressure and is set by code. For stamped burst pressures of 40 psig and lower, the burst tolerance is 2 psi. For stamped burst pressures above 40 psig, the burst tolerance is 5%.

Let's look at the examples again but apply the burst tolerance. For this discussion, I'm changing the specified burst pressure for the case of a rupture disk with a Manufacturing Range of +5% and -10% to 95.2 psig (see Figure 3B) so the stamped burst pressure can't exceed code.

The important thing to notice is that in both Figures 3A and 3B, the upper limit of the stamped burst pressure is equal to the design pressure but the maximum bursting pressure is 105 psig, or 5 psig over design pressure. Unlike the stamped burst pressure, which by code cannot exceed the design pressure (or MAWP), the maximum expected burst pressure can if it is caused by the burst tolerance.

Figure 3B: Specified, Stamped, and Maximum Burst

Maximum Allowable Operating Pressure

What is the maximum allowable operating pressure in the vessel?

Up to now, the discussions focused on the upper limit of the stamped burst pressure because this is governed by code. But the lower limit is extremely important to consider as well because of the possible affect it has on the maximum allowable operating pressure in the vessel.

Operating Ratio (OR)

The operating ratio is defined as the ratio of the maximum operating pressure to the lowest stamped burst pressure. The OR is used to protect against premature bursting of the rupture disk. If the operating pressure is too close to the lowest stamped burst pressure, or the system pressure cycles (pressure rises and falls during operation) too close to the stamped burst pressure, the material will fatigue and can

ads not by this site eventually loose its structural integrity. This is a classic reason for premature bursting of a rupture disk.

The manufacturer publishes the Operating Ratio for every rupture disk model they sell. For example, the Continental Disc Corporation's ULTRX rupture disk has an operating ratio of 90%4. This means the system pressure can operate to within 90% of the lowest stamped burst pressure without the fear of premature bursting. However, it's always best to operate as far away from the lowest stamped burst pressure as you can to avoid material fatigue.

From Figure 3B above, the lower limit or minimum stamped burst pressure is 85.7 psig:

Pstamped_min = (Pspec) - ABS [(-MR/100)] x (Pspec)Where ABS' stands for Absolute Value.

So:

Pstamped_min = (Pspec) x {1- ABS [(-MR/100)]}Since Pspec = 95.2 psig and the lower value of MR = -10%,

Pstamped_min = 95.2 x {1 - ABS [(-10/100)]} = 95.2 x {1-ABS [(-0.1)]} = 95.2 x (1-0.1) = 95.2 x 0.9 = 85.7 psig

Therefore based on an OR of 90%, the maximum allowable operating pressure should not be greater than:

Pop = Pstamped_min x OR = 85.7 x 0.9 = 77 psig.

Since our discussions have been based on a maximum operating pressure of 70 psig, this rupture disk is acceptable. But note that this 10% cushion exists only because of the design pressure margin used (25 psig). Had the margin been less, say only 10%, the rupture disk we would want to use would be unacceptable.

How to avoid this problem?

Set the design pressure appropriately

Choose a rupture disk with a MR of 0%

Choose a rupture disk with a OR of 90% (they don't really go much higher)

There is one more point to consider. Although I have never seen any mention of checking the maximum allowable operating pressure against the minimum expected burst pressure (arrived at by taking into account the burst tolerance), I think it only makes good engineering sense to do so. After all, if the disk can burst at this lower pressure, one certainly does not want to operate too close to it!

Getting back to our question, what is the maximum allowable operating pressure in the vessel? In this case, it is 77 psig.

ummary

What is the maximum allowable specified burst pressure?

- Design Pressure or MAWP if the rupture disk is the only relief device

OR

- For special cases, 105% (or even 110%) of design pressure or MAWP if the rupture disk is a secondary device

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What should be the expected stamped (rated) burst pressure of the rupture disk?

- As specified by the process engineer for a Manufacturing Range of 0%

OR

- As specified by the process engineer but could be adjusted per the Manufacturing Range if other than 0%

At what pressure(s) can we expect the delivered rupture disk to actually burst at?

- 5% of stamped burst pressure for stamped pressures greater than 40 psig

OR

- 2 psi for stamped pressures 40 psig and lower

What is the maximum allowable operating pressure in the vessel?

- Specified by the process engineer based on operating need but must be checked against the Operating Ratio of the rupture disk- I strongly suggest you also check against the minimum expected burst pressure as well.

Manufacturing Range is applied to the specified burst pressure

Burst Tolerance is applied to the stamped burst pressure

Set the design pressure appropriately

Choose a rupture disk with a MR of 0%

Choose a rupture disk with a OR of 90%

WARNING!Don't go running out and specifying a rupture disk just quite yet! We still need to consider the affects of temperature and backpressure and the relief valve-rupture disk combination.

References




4. Continental Disc Corporation, ULTRX Catalogue 3-2210-3




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Part 1 of this series on rupture disks for Process Engineers covered why you use a rupture disk and when you might want to use this device. Part 2 discussed how to size the rupture disk. Part 3 discussed how to set the burst pressure. In this part, I will discuss how temperature and backpressure affects the rupture disk design. Subsequent parts will include the Relief Valve/Rupture Disk combination, how to specify the rupture disk and some discussion on the type of rupture disks you can purchase.


Temperature and Backpressure Considerations

In Part 3, I discussed how to set the burst pressure of the rupture disk. However, the discussion is not complete without considering the affects of temperature and backpressure on the bursting pressure.

Temperature

The rupture disk manufacturer uses both the specified burst pressure and the specified temperature when designing and stamping the disk. (In this instance, I use the term design to mean arriving at the correct burst pressure, not mechanical integrity). However, it is more than likely that the temperature of the rupture disk will not be at the specified temperature when it is called into service. Why is this so?

ads not by this site The temperature most commonly specified is that of the relieving fluid coincident with the burst pressure, i.e. relieving conditions. Sounds logical, but remember that the disk is continuously exposed to the process stream for hours, days, weeks or even months before it may ever be needed. Or, the disk may be exposed to ambient conditions. Therefore, expect the disk temperature to be approximately equal to its environment during normal operation of the system. When a process upset occurs, system pressure rises until it reaches relief (burst). The temperature of the relieving fluid also rises per thermodynamics. However, the time interval between normal system operation and relief is usually so small that the rupture disk's temperature hardly has time to come to equilibrium with the higher process fluid temperature. Therefore the disk can actually be colder than it's specified temperature. The affects?

In general, burst pressure varies inversely with temperature. For some rupture disks, the burst pressure can be as much as 15 psi greater than stamped if the actual temperature is 100oF lower than specified, e.g. a disk specified with a burst pressure of 350 psig at a temperature of 400oF will actually burst at 365 psig if its temperature is only 300oF4. This doesn't sound like a big difference but if 350 psig were the design pressure (or MAWP) of the vessel, then a burst pressure of 365 psig would be in violation of code (LAW). The opposite is also true. A disk at a temperature hotter than specified when called into service will burst at a pressure lower than stamped. Although this is considered to be the more conservative approach because code can't be violated and there is no risk of catastrophic failure of the vessel, specifying too low of a temperature can lead to the not so desirable action of premature bursting.

The bottom line is that the specified burst temperature must be carefully considered. Specify the lowest temperature at the time the disk is expected to burst. Consider that this might be the normal process operating temperature or even ambient rather than the calculated relieving temperature.

Note that different materials and different types of rupture disks have different sensitivities to temperature. This is an excellent topic of discussion for your rupture disk manufacturer!




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Backpressure

A rupture disk is actually a differential pressure device where the specified burst pressure is equal to the difference between the desired upstream pressure (vessel) at the time of rupture disk burst and the downstream pressure (backpressure):

ads not by this site Pburst = Pvessel - PbackpressureOr alternately the desired upstream pressure (vessel) at the time of rupture disk burst is equal to the sum of the specified burst pressure and the downstream pressure (backpressure):

Pvessel = Pburst + PbackpressureEither way, it is apparent that the vessel pressure at the time the rupture disk bursts (commonly called the relief pressure) is directly dependent on backpressure.

When discussing relief systems, three types of backpressure are considered, these being constant, built-up and superimposed.

Figure 1A: Single Vessel, Single Rupture Disk Protection,Expected Constant Back pressure = 0 psig

Figure 1A shows a system comprised of a single vessel protected by a single rupture disk with a specified burst pressure of 100 psig. The relief pipe discharges a few inches below the liquid surface in a knockout drum, which is held at a constant 0-psig pressure. Therefore, the rupture disk sees a constant (fixed) backpressure of 0 psig. If the vessel were to go into relief, this disk will burst at 100 psig and the vessel relief pressure will be 100 psig (100 + 0 = 100).

Figure 1B: Single Vessel, Single Rupture Disk Protection, Actual Constant Back pressure > Expected

Figure 1B is the same system however for some reason the pressure in the knockout drum is to be maintained at 5 psig instead of 0 psig. The constant (fixed) backpressure against the rupture disk is now 5 psig. If the vessel were to go into relief, the rupture disk would still burst at 100 psig but the vessel relief pressure would now be 105 psig (100 + 5 = 105) rather than the 100 psig expected. This situation could result in a violation of code3.

Figure 1C: Single Vessel, Single Rupture Disk Protection, Actual Constant Back pressure

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