This valve-disk combination may have previously been considered by some merely as an added expense. It is now well accepted, however, that isolating a safety relief valve in this manner actually saves money in the following, five ways:
- Zero process leakage to the atmosphere;
- Longer periods between major overhauls;
- Valves can be checked in place;
- Less expensive valve trim material can be used; and
- Valve life is extended by isolating corrosive fluids from internal valve parts.
This bulletin has been published to give you the facts, the applications, and the advantages of using rupture disks (bursting discs) and safety relief valves in series.
Advantages of Rupture Disks Used in Combinations with Relief Valves
There are at least five advantages to using rupture disks (bursting discs) for isolating safety relief valves.
Advantage No. 1 Zero Process Leakage to the Atmosphere
The most important reason for isolating safety relief valves with rupture disks (bursting discs) is to prevent the process from leaking into the atmosphere. On conventional safety relief valves, API Standard 526 states that for an orifice size of F and smaller, the maximum allowable leakage rate is 40 bubbles/minute (approximately 6 cu. ft. per 24 hours).
A rupture disk (bursting disc) used at the inlet of a relief valve acts as a solid metal barrier between the process and the valve.
Not only does the disk prevent air pollution (thereby satisfying EPA regulations), it can save you money. Leakage that customarily wastes expensive product every hour of every day is stopped with a rupture disk (bursting disc)/safety relief valve combination.
Advantage No. 2 Allows Safety Relief Valves to be Tested in Place
When a rupture disk (bursting disc) is used to isolate a safety relief valve, the valve can be field tested in place. With a reverse buckling rupture disk (bursting disc) installed at the valve inlet, the safety relief valve can be tested on the spot by one man with a portable pressure source.
To accomplish this without removing the valve from the process (i.e., where one has a properly functioning relief valve), air or an inert gas such as Nitrogen is injected from an outside source into the chamber between the rupture disk (bursting disc) and the safety relief valve inlet when safe to do so (typically when the process is not running). Pressure is increased, to a reasonable degree, until a popping action is heard or the valve simmers.
Of course, one generally should not use air pressure exceeding 110% of the rated pressure marked on the reverse buckling disk tag.
Advantage No. 3 Valve Life is Extended
Safety relief valve life extension is the third major advantage for using a disk/valve combination. The rupture disk (bursting disc) acts as a solid metal barrier between the valve and the process. The disk prevents product buildup from adhering to mechanical components of the valve that otherwise could affect valve performance and the safety of the system. Because the process media will not come in contact with the internal surfaces and parts of the valve, it will remain in better condition until called upon to relieve pressure.
Advantage No. 4 Longer Periods Between Major Overhauls
Because the valve’s internals are not normally exposed to process contamination, they remain in a protected condition, allowing longer periods between major overhauls.
Advantage No. 5 Less Expensive Valve Material can be Used
The large initial cost of the safety relief valve can be reduced by ordering valves from less expensive metal and isolating the valve with a rupture disk (bursting disc). For example, if a Hastelloy* valve might normally be required, use a carbon steel valve with Hastelloy trim, resulting in an average 65% savings. The savings will more than pay for the rupture disk (bursting disc) plus give advantages 1 thru 4, above.
* Hastelloy is the registered trademark name of Haynes International, Inc.
Safety Relief Valve Isolation Applications
To isolate safety relief valves with rupture disks (bursting discs), one should use a reverse buckling disk. This disk can withstand pressures in two directions; thus allowing the valve to be field checked in place and eliminating the necessity of a vacuum support for vacuum process. The following is a guide to selecting rupture disks for each application.
1. NEW CONSTRUCTION -VALVE INLET ISOLATION
For new construction, the BS&B STA-SAF® System is recommended for relief valve inlet isolation. The STA-SAF® System is a proven concept in rupture disk (bursting disc) and safety head technology. The STA-SAF® rupture disk device fits between ASME – ANSI / EN / and JIS flanges, can be pre-torqued in the shop and assures correct installation by means of a J-bolt and disk centering pins. The STA-SAF® holder, type SRB-7RS™, is used with either the S-90TM, JRSTM, RLSTM , SKr™, LPS™ or Sigma™ reverse buckling rupture disks, depending on the burst pressure required and service conditions.
The STA-SAF® System covers most ranges of safety relief valve set pressures. For gas service, use any of the STA-SAF® System reverse buckling rupture disks in the SRB-7RS Head. For liquid service, use the type RLS™, SKR™, LPS™ or Sigma™ type rupture disks. Refer to the RLSTM reverse buckling disk for higher pressures. Specifications for each of these rupture disk types may be found on the BS&B web site.
The STA-SAF® System is ideal for isolating safety relief valves, with the following advantages:
- Available in sizes 1″ thru 30″;
- Designed for non fragmentation;
- Fail Safe; Low damage and reversal ratio;
- No knife blades to maintain; and
- Operating pressure up to 100% of the disk minimum burst pressure (SigmaTM); all disks may be used up to 90% of minimum burst pressure.
2. RETROFIT-VALVE INLET ISOLATION
For isolating safety relief valves already in service with rigid discharge piping, the SVITM Rupture Disk (Bursting Disc) Assembly is recommended. The SVITM is a reverse buckling S-90TM Rupture Disk (Bursting Disc) similar to the one described above, but welded into its own one-piece assembly that slips into the inlet piping. The SVITM is unique in that:
- No safety heads are required;
- It fits into existing piping;
- It has all of the advantages of the S-90TM Rupture Disk (Bursting Disc); and
- It is available in sizes 1-1/2″ thru 6″ (including 2-1/2″).
3. NEW CONSTRUCTION OR RETROFIT-VALVE OUTLET ISOLATION
With tighter EPA regulations causing safety relief valves to be manifolded rather than discharged to the atmosphere, it is important to isolate the safety relief valve outlet (as well as the inlet) with a rupture disk (bursting disc) to prevent “backflow” corrosion & to isolate valves whose set pressure is affected by variable back pressure.
The type AVTM Low Pressure Rupture Disk (Bursting Disc) is the perfect fit for this job.
The AVTM Disk requires no Safety Heads, comes with its own gaskets and fits between standard ASME – ANSI / EN / JIS flanges. Its advantages are ideal for valve outlet isolation:
- Low burst pressures, 1 to 15 psig;
- Bursts at equal pressure in both directions (1:1 Ratio);
- Cannot be installed wrong, as both sides are identical; and
- Available in sizes 2″ thru 36″.
For more specifics on materials, pressures, and other data on the above BS&B Rupture Disks for safety relief valve isolation, consult the following BS&B Bulletins:
|STA-SAF® and S-90TM or JRSTM
|STA-SAF® and RLSTM
|STA-SAF® and SKrTM
|STA-SAF® and LPSTM
|STA-SAF® and SigmaTM
Facts in Selecting a Rupture Disk Device (Bursting Disc) or Safety Relief Valve
To better understand the use of rupture disks (bursting discs) to isolate safety relief valves, compare the advantages and disadvantages of each relief device.
SAFETY RELIEF VALVE
- Metal seated valves leak to the atmosphere -loss of product as well as pollution of the atmosphere.
- High cost.
- High maintenance required-Since the process goes through the valve, it must be checked periodically for correct operation. The system must be shut down.
- Blow down – recloses after pressure relief.
- Reusable-recloses after each operation.
RUPTURE DISK DEVICE
- Leak tight.
- Low cost.
- Low maintenance.
- Fixed Setting.
- Disposable (rupture disk/bursting disc) -must be replaced after each operation.
RUPTURE DISK/SAFETY RELIEF VALVE COMBINATION
- Leak tight.
- Medium cost.
- Medium maintenance required.
- Blowdown-recloses after pressure relief.
- Valve is reusable, disk is disposable.
The ASME code requires that the space between a rupture disk (bursting disc) device and a safety relief valve be provided with a pressure gauge, tricock, free vent, or suitable telltale indicator.