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Technologies combine to deliver increased plant safety benefits

Technologies combine to deliver increased plant safety benefits
Geof Brazier of BS&B looks at the advantages of combining rupture disk devices with safety relief valves.

A Safety Relief Valve serves an important function in protecting pressurized process systems. To provide further protection, the combination of a relief valve with a rupture disk device is used to isolate the relief valve inlet from the process media.  This combination adds service life to the relief valve while preventing process leakage (important from safety, conservation and financial standpoints).
 
Isolating a safety relief valve with a rupture disk device prevents process from coming in contact with the safety relief valve under normal operating conditions (see figure 1). The rupture disk is used at the inlet of the relief valve acting as a barrier between the process and the valve.  This barrier stops the process from entering the mechanics of the relief valve, which keeps the relief valve from coming into contact with caustic materials. The rupture disk also protects from highly viscous materials that risk potentially "gumming up" the relief valve.
 
Another advantage is that the rupture disk barrier keeps process fluid from leaking into the atmosphere. On conventional safety valves, API standards allow for an orifice size of F and smaller to have the maximum allowable leakage rate of 40 bubbles/minute (approximately 6 CuFt per 24 hours and 2,190 CuFt per year). Unchecked, this leakage seeps into the environment, loses expensive product every hour of every day eroding profits, or requires the installation of a means to either recover or handle this leakage as waste. Rupture disks stop the leakage to protect the environment and to protect plant profits.
 
When rupture disks are used to isolate safety relief valves (figure 2), the rupture disk is first to open in the event of system overpressure; the vented process fluid then contacts the safety relief valve, which releases the fluid if the pressure is excessive. Once the pressure drops to a safe level, the valve reseats itself and continues to protect the system. 

Further advantages
Besides zero process leakage, other advantages of using a rupture disk at the inlet of a safety relief valve include:
  • Allows safety relief valve to be 'tested in place' - When a rupture disk is used to isolate a safety relief valve, the valve can be field tested in place. With a suitable reverse buckling rupture disk 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 opening up the process piping, air, Nitrogen or other acceptable fluid is injected from an outside source into the chamber between the rupture disk and the safety relief valve inlet. The test  pressure is increased until a popping action is heard from the valve. The test pressure observed shall be within the set pressure tolerance of the valve. Upon removal of the portable pressure source, both the rupture disk and relief valve are ready to immediately resume service. 
  • Valve life is extended - Safety relief valve life extension is a major advantage of using a disk/valve combination.  The rupture disk acts as a solid barrier between the valve and the process.  The disk prevents product buildup from adhering to mechanical components of the valve that otherwise would affect valve performance and safety of the pressurized system. Since the process fluid will not come in contact with internal surfaces and components of the valve, it will remain in pristine condition until called upon to relieve pressure.
  • Longer periods between major overhauls - Because the valve internals are not normally exposed to process contamination, they remain in "like new" condition, allowing longer periods between major overhauls.
  • Less expensive valve material can be used - The large initial cost of a safety relief valve can be reduced by ordering the valve from less expensive metal and isolating it with a suitable rupture disk.  As an example, if the process fluid requires that Hastelloy be the preferred material of construction for continuous contact, use a carbon steel valve with Hastelloy trim combined with a Hastelloy rupture disk device, saving over half the cost of the valve.

A question posed often is when to use a rupture disk by itself and when to use a rupture disk combined with a safety relief valve.  The benefits of using only a rupture disk begin with cost. Rupture disks are significantly less costly than safety relief valves particularly when constructed of exotic materials, and require little to no maintenance. Another benefit includes the quick burst of the rupture disk making it a first consideration when a potential for runaway reactions exist. Safety relief valves, by themselves, will not react fast enough to protect from the pressure of a deflagration or a detonation. Also to consider is that some liquids may freeze or cause icing under rapid depressurisation causing blockage within a safety relief valve rendering it ineffective. Highly viscous liquids, such as polymers, may not relieve fast enough through a safety relief valve and create a danger of plugging the valve.

Individually, a rupture disk is an excellent choice for overpressure protection when process contents are inexpensive, non-hazardous and environmentally safe or when hazardous material can be released to a safe recovery or waste station. With the availability of rupture disk technology capable of a 100% operating ratio and superior process control technology, the benefits of rupture disk devices can be fully realised. 

A rupture disk and relief valve combination will be the unrivaled choice when a leak tight seal of the pressurised system is needed combined with the conservation of product within the pressurised system (figure 3) because the system contains a corrosive, hazardous or expensive substance. The installation of a rupture disk device upstream serves as a barrier between the process fluid and the relief valve. The disk prevents product buildup from adhering to mechanical components of the valve. Since the process fluid will not come in contact with internal surfaces of the valve, the valve will remain in pristine condition until called upon to relieve overpressure.

The benefits of rupture disk isolation to a relief valve can also be applied to the downstream discharge connection of the valve. When discharging multiple relief devices to a common header this technique will prevent potential contaminants reaching the downstream components of all the relief valves connected to the header. For relief valves whose set pressure is influenced by back pressure, rupture disk downstream isolation will also prevent the momentary back pressure from an active relief device from affecting the performance of the other valves connected to the header system, thereby maintaining the intended safety of each of the pressurized systems connected to the header.

Even when the process fluid is not labeled as corrosive, hazardous or expensive, the arguments for the application of rupture disk and relief valve combinations make for compelling 'best engineering practice' with respect to both safety and economics. 

Capacity requirements
When sizing a relief valve, the engineer determines the required fluid flow capacity while analysing emergency scenarios such as fire, loss of process cooling, or equipment failure. The capacity requirements are then entered into a sizing equation to determine the relief valve area. In most cases, engineers can select the calculated relief valve area from relief valve manufacturer's data sheets which present information derived from ASME Code mandated capacity testing.  When sizing a relief valve and rupture disk combination the flow capacity of the combination must be confirmed to support the selection of both the valve and the rupture disk. 

A Combination Capacity Factor (CCF) is used in support of this design safety decision. The CCF is often determined from ASME certified capacity testing where first the capacity of the stand-alone safety relief valve is determined and second that of the rupture disk and relief valve combination is determined. The combination capacity factor is calculated as the ratio of the rupture disk and relief valve combination capacity to the stand-alone relief valve capacity. The CCF shall not be greater than 1. If the CCF is unknown, the ASME Code allows for a default CCF value of 0.9 to be used in place of a tested, certified value provided that the rupture disk device has a certified flow resistance value (KR) of equal to or less than 6. A low flow resistance value is indicative of a rupture disk device that provides a clear opening upon activation. 

What about the pressure drop between a vessel and a rupture disk isolated relief valve? The proper function of a relief valve requires that the pressure drop between the vessel it protects and the valve inlet is not more than 3% of its set pressure. When a relief valve is isolated by a rupture disk device, this contributes to the piping pressure drop. The certified flow resistance value for rupture disks that may be ASME "UD" stamped  (KR) is used to accurately calculate pressure drop. With many rupture disk devices having low flow resistance values, the pressure drop target is routinely achieved. 

The maintenance of a known pressure differential across the rupture disk device in a rupture disk and relief valve combination is conveniently achieved by the use of the 'tell tale' assembly shown in each of figures 1, 2 and 3. The tell tale assembly combines an excess flow valve to maintain atmospheric pressure in the space between the rupture disk and the relief valve with a pressure gage to provide local confirmation of pressure status. The tell tale assembly is a requirement of the ASME Code as it relates to rupture disk / relief valve combinations; other monitoring methods, such as a pressure switch that will generate a remote electrical signal, are permitted.
 
BS&B Safety Systems Ltd

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