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Rupture Disks for Process Engineers - Part 6

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. Part 2 discussed how to size the rupture disk. Part 3 discussed how to set the burst pressure. Part 4 discussed how temperature and backpressure affects the rupture disk specification and the relief pressure in the system. Part 5 discussed the Relief Valve/Rupture Disk combination. In this part, I conclude the series with a discussion of the rupture disk specification.

I will also touch upon 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.

We've answered the two questions required to size a rupture disk, how much flow and how big. Now it's time to specify the rupture disk so that it can be purchased for our process. Although API RP5201 provides a specification sheet that can be adapted by any company as a standard, there are fifty-three separate items asked for in this specification sheet. Much of what is on this specification sheet is not required by the manufacturer to be able to provide you with the correct disk. Let's look at the basic minimum information you, the Process Design Engineer must provide.

Must Haves

Project Identifier/Company Information/Device identifier/Number of Devices

The vendor will want to know who you are. It is also necessary to "name" the relief device for proper documentation. A unique instrument Tag number should suffice for each device ordered.

Code/Standard Requirements

Various codes and standards dictate how the rupture disk is to be marked and stamped.

Maximum Operating Conditions

Temperature

The maximum operating temperature is used to determine materials compatibility.

Pressure

The maximum operating pressure will be used with the stamped burst pressure to determine the Operating Ratio. The Operating Ratio will help determine the type of disk to purchase.

Rupture Disk Burst Conditions

Temperature

This must be coincident with the bursting pressure and will also be stamped on the disk. You will recall from Part 4 that this parameter is extremely important in making sure the disk will burst at the pressure you need it to burst, not less or greater. Also remember that it is not necessarily the same as the maximum operating temperature of the system.

Pressure

This is the pressure that meets system protection requirements, taking into account the Manufacturing Range. The vendor will stamp this value on the disk. It is also used with the Maximum Operating Pressure to determine the Operating Ratio.

Process Media (Liquid/Gas/2-Phase)

Some rupture disk models are designed according to the media in which they are used. Process media is also used to determine materials compatibility.

Backpressure/Vacuum

The manufacturer uses the backpressure to help determine disk type and how it is to be supported in the system. Vacuum service will either require the use of a special support for disk installation or even dictate the type of disk to use. Note that exposure to vacuum conditions must be considered both upstream and downstream of the disk.

Service Conditions (Status/Cyclic/Pulsating)

This typically refers to the upstream conditions. Cyclic service is considered to be large changes in pressure over a relatively long period of time. Pulsating service is considered to be small changes in pressure but occurring frequently or even rapidly. Both of these can have a major affect on the Operating Ratio. The manufacturer uses the service conditions to help determine disk type and how the disk is to be supported in the system.

Rupture Disk and Holder Material Requirements

Many installations require the rupture disk to be mounted inside a holder. The holder is then bolted onto a vessel nozzle or between pipe flanges. Make very certain the materials of construction of both the disk and its holder is totally compatible with the system media and operating conditions.

Disk Size

This is the nominal size you determined when answering the question, how big?

Flange Connection Details

These tell the manufacturer how big the holder needs to be (connection size), the pressure rating of the system it will be installed in (class) and the type of connection, e.g. raised or flat faced flanges, sanitary connections, etc.

The pressure rating or class can be a most confusing concept. This refers to the flanges in the piping system. More common flange ratings are 150 and 300 pounds (pressure pounds, not weight) but they can go very much higher. A major difference in these classes is the thickness of material, number of boltholes and the bolthole pattern you would get in the flange.

Required Accessories for Rupture Disk

Options can be added to the basic design. For instance to enhance corrosion protection, coatings or linings can be applied.  Some types of rupture disks can withstand upstream vacuum conditions without doing anything special to them others may need special supports.

Required Accessories for Holder

Options can be added to the rupture disk holder as well. For instance to enhance corrosion protection, coatings or linings can be applied. Tell-tales may be specified under this header or can be specified under the heading of "Special Considerations".

Other Special Considerations

You can specify just about anything under this heading including the need for a tell-tale. You may want to give more specific detail of a particular design item. You can ask for burst detection and alarms, etc., etc. and etc. The best reference source would be your manufacturer and/or their catalog.

Again, the above should be considered just the minimum amount of information the manufacturer needs to provide the proper rupture disk. Of course your particular manufacturer, or even your company standards, may require much more.

Should you stop here, perhaps not? Below is some information that I consider to be "should haves".

Should Haves

MAWP (or Design Pressure) of the Vessel or System

A vendor does not necessarily require this information (they were already told what to stamp the disk for). However a good vendor will actually be your second set of eyes and make sure that this, along with the other information given, is consistent with Code requirements.

Manufacturing Range

One would think that this should fall under the "must haves" but not really. When the burst pressure was specified in the "must haves", the manufacturing range had to be taken into account. All the vendor needs to know is what to stamp the rupture disk at and will therefore design the disk with the appropriate manufacturing range to accommodate. However, it never hurts to spell it out so there are no misunderstandings.

In Combination with a PSV

With this information, the rupture disk vendor will be able to recommend the proper type of rupture disk to use for this service. They will also be able to recommend proper installation techniques. And again, the vendor is your second set of eyes and may be able to tell whether your specification data is consistent.

Calculate and Report the Operating Ratio

I could never quite figure out why the vendor cannot just do the simple math but I've seen this as requested information on a number of vendor's specification sheets.

What about all the rest of the information usually included in many specification sheets, e.g. required relieving flow, molecular weight, specific heat ratio, specific gravity, compressibility factor, viscosity, etc.? These are definitely important, but really only to the Process Design Engineer. You need this information to answer the two questions, how much flow and how big? The vendor doesn't need these but we all seem to include them on our specification sheets nevertheless!

The best suggestion I can make is to talk to the vendor first, find out exactly what they need and provide it. But of course, never violate your own company standards.





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