A Monthly Column by Content Manager, Philip Leckner "Process Engineering--As I See It"

Rupture Disks for Process Engineers
(From the Process Design Engineer's Perspective)
Part 3: How Do We Set the Burst Pressure?
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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. 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.

Problem 

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.

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)

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).

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!

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:

P_{spec_max} = (DP) - (+MR/100) x (P_{spec_max})

Where DP = Design pressure

So:

P_{spec_max} = (DP) / [1+(+MR)/100]

Since DP = 100 psig and the upper value of MR = +5%,

P_{spec_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.

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

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.

What is the maximum allowable operating pressure in the vessel?