SKR-U Reverse Buckling Disk

Introduction
The SKR-U™ rupture disk (bursting disc) is partnered with the UR-2 safety head to provide high integrity reverse buckling rupture disk (bursting disc) technology in a threaded union type holder. The SKR-U™ rupture disk (bursting disc) uses Saf™ technology (structural apex forming) to accurately control its burst pressure making it insensitive to the assembly torque variations that will occur in a threaded type holder.

The UR-2 safety head is available in three standard nominal sizes: 1/2 " / 15mm,3/4_" / 20mm, and 1" / 25mm. Standard inlet & outlet female threads are 1/2 ", 3/4 ", and 1" NPT. Custom thread configurations are available. The SKR-U™ rupture disks (bursting discs) are identified SKR-U™1/2" / 15mm, SKR-U™¾" / 20mm, SKR-U™ 1" / 25mm and are all supplied with permanently attached 3-D metal identification tags.

Table of MNFA, NRA, KR

Nominal Size	MNFA	KRg/KRI	NRA
1/2"	0.304	0.85/	1.96
¾"	0.533	0.73 / 1.1	3.43
1"	0.608	1.25 / 2.2	3.92

Table of Materials & Set Pressures

Rupture Disk Material	Minimum Burst Pressure	Maximum Burst Pressure
Nickel (alloy 200)	55 psi / 3.79 Bar	500 psi / 34.47 Bar
Stainless Steel	60 psi / 4.13 Bar	500 psi / 34.47 Bar
Inconel (alloy 600)	60 psi / 4.13 Bar	500 psi / 34.47 Bar
Hastelloy C-276 (alloy C-276)	60 psi / 4.13 Bar	500 psi / 34.47 Bar
Tantalum	55 psi / 3.79 Bar	500 psi / 34.47 Bar
Monel (alloy 400)	60 psi / 4.13 Bar	500 psi / 34.47 Bar
Titanium	90 psi / 6.20 Bar	500 psi / 34.47 Bar

Note: The SKR-U™ is a 2-part rupture disk (bursting disc) comprising a calibrated metal dome that faces into and responds to a source of overpressure, and a stainless steel hinge ring permanently fitted to the downstream side of the disk that works together with the safety head to retain the disk petal after opening.

These pressures are applicable to all sizes of SKR-U™ rupture disk (bursting disc).

Standard safety head materials are carbon steel & stainless steel.

Temperature
The design temperature for the UR-2 Union safety head is 700 deg F / 371 deg C. All of the standard holder and rupture disk (bursting disc) materials are available up to this limit except for Titanium & Tantalum. Titanium is limited to 572 deg F / 300 deg C, and Tantalum is limited to 500 deg F / 260 deg C.

Thread Connection Options
While the standard thread connections are _", _" and 1" NPT female inlet & outlet, the following alternatives are available within the same overall dimensions.

Burst Pressure Tolerances
BS&B applies both a burst tolerance and a manufacturing design range to the SKR-U™ rupture disk (bursting disc). Whether North American ASME, European Pressure Equipment Directive or ISO standards are followed, the approach is the same.

Burst tolerance is the +/- range of pressure over which a rupture disk (bursting disc) can be expected to burst. Manufacturing design range is a range of pressure, always applied to the minus side of the user requested burst pressure for the SKR-U™, that simplifies disk fabrication testing and provides economic benefit to the user where such an added tolerance can be accommodated by the application.

Burst tolerance is either +/-5% of burst pressure or +/-2psi / +/-0.138 bar for disks rated below 40 psi/2.76 bar.

Manufacturing design range (MDR) choices are provided for SKR-U™ rupture disks (bursting discs):

MDR = 0	where the user requires the tightest margin between normal service pressure and set pressure, a zero range disk shall be used.
MDR = -5%	applied to the requested burst pressure, a -5% range allows an additional 5% tolerance applied on the minus side of the user requested burst pressure.
MDR = -10%	applied to the requested burst pressure, a -10% range allows an additional 10% tolerance applied on the minus side of the user requested burst pressure.

MDR and burst tolerance are additive. In the European PED & ISO cases, the burst tolerance and MDR are combined. In the ASME case, the ‘marked burst pressure’ must be a value within the agreed manufacturing design range & then the burst tolerance is applied.

Safety Head Design – Rupture Disk (Bursting Disc) Installation
The UR-2 safety head contains 4 metal components: inlet, holdown ring, outlet, and hexagon assembly nut. The SKR-U™ rupture disk (bursting disc) is pre-assembled to the safety head between the inlet and the holdown ring using two capscrews. This locates and centers the disk, ensuring the correct direction of installation: these two capscrews and a single locating pin in the UR-2 inlet have an asymmetric layout that matches three holes in the SKR-U™ disk flange.

This pre-assembly feature ensures that the disk can be easily installed even where the piping flow path may not be ‘vertical up’. The inlet/disk/holdown subassembly is fixed to the UR-2 threaded outlet by a hexagon nut. It is the hexagon nut that provides the sealing load to the safety head assembly (the capscrews are for preassembly only).

As an option, the UR-2 safety head can be provided with a threaded connection on the outlet side to enable the connection of a ‘tell-tale’ assembly.

Torque Drive Adapter
Accurate torque control on a large hexagon can be difficult. BS&B Safety Systems can provide a Torque Drive Adapter for use with all UR-2 safety heads (one adapter for all sizes). This adapter permits the use of a standard torque wrench for control of installation load.

Leak Tightness
The SKR-U™ safety head incorporates a ‘bite-type’ seal. This concentrates the holder assembly load to cut an impression in the SKR-U™ rupture disk (bursting disc) flange that ensures a leak tight installation.

A gasket (compressed non asbestos fiber) is placed inside the UR-2 safety head between the holder outlet and the disk holdown ring to ensure a good seal on the downstream side of the installation after assembly of the two holder halves. This gasket is supplied with each SKR-U™ rupture disk (bursting disc), and should be replaced with each new disk.

Pressure Rating of Holder
The design pressure of the UR-2 holder is 1,000 psi / 68.95 Bar. This far exceeds the maximum burst pressure of the SKR-U™ rupture disk.

Codes & Standards
The SKR-U™ & UR-2 are manufactured in conformance with the following international codes and standards:
ASME Section III & VIII (UD stamped)
ISO 6718
BS2915
TUV AD Merkblatt A1
SVTI
Stoomwezen

Address, International logo, ISO & UD logos, printed in the USA, 9.2000

Products, specifications and all data in this literature are subject to change without notice.  Questions regarding product selection and specifications for specific applications should be directed to BS&B Safety Systems, Inc. or BS&B Safety Systems Ltd.