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Combining the use of Rupture Discs with Relief Valves

| By Roger Bours,  Fike Corp.

Pressure-relief solutions are commonly used in the chemical process industries (CPI) to ensure safe working environments for personnel and to protect equipment and assets. Relief valves (RV) and rupture discs (RD) are the most commonly used pressure-safety devices and are generally specified and selected according to application-specific requirements. For pressure-system designers looking for viable pressure-relief solutions, the two device classes offer specific pressure safety features and require consideration of different factors.

Using rupture discs in combination with relief valves offers a wide range of benefits to end-users. Possible benefits include improved environmental performance, cost savings, better emissions control, higher safety and reliability levels and improved performance of plant safety systems. To realize the benefits, process system designers need to evaluate the effects of each of the devices and select the arrangement that works best, based on the individual plant requirements.

Various industry standards and legislation exist to help set up safe and effective solutions. In most applications, using rupture discs together with relief valves offers higher overall value and a host of worthwhile benefits.

Pressure control versus relief

Since the beginning of the industrial revolution, industrial processes have operated at pressures that differ from atmospheric pressures (both overpressure and vacuum). With the advent of pressurized processes the requirement to adhere to mandatory safety measures has also arisen. A body of national and international legislation has been developed and is in place for promoting pressure safety and reducing risk to personnel, the environment and to investments.

The first line of defense for pressure safety are typically pressure-control systems. These systems monitor the pressure changes in the process equipment and interact in a timely fashion with the process-control system to adjust the pressure to acceptable levels. Control and monitoring devices, which are not specifically a part of a safety system, are usually excluded from safety design standards, since they are typically active in advance of a safety system.

The efficiency of pressure-control systems depends on the input received from instrumentation devices, and requires extensive and validated reliability analysis, based on probability of failure on demand (PFD) or safety integrity level (SIL) assessment. Because pressure-control systems may not ensure the required level of reliability under all service conditions, the use of pressure-relief systems as a last line of defense is often required. In situations where the pressure-control systems do not achieve the required pressure-safety levels alone, dedicated pressure-relief devices must be installed to protect the process when the critical pressure threshold is reached. Figure 1 illustrates the correlation between pressure control-and-monitoring systems, and pressure-relief systems.

 Figure 1. Pressure-control systems may not ensure the required level of pressure safety, so dedicated pressure-relief systems are also needed in many situations

When designing an effective pressure-relief system, it is essential to consider the complete system holistically to maintain the full pressure-relieving capacity of the pressure-relieving devices and avoid having the operation of the devices interfere with each other. Operating problems within pressure-relief systems, when they are observed, frequently result from one or more of the following: incorrect device selection; improper pressure handling; incorrect device installation; or improper (or lack of) maintenance.

Reclosing versus non-reclosing

Pressure-relief devices are categorized as reclosing and non-reclosing types. Both offer unique characteristics for design engineers seeking to protect against pressures that exceed allowable levels. Most industry sectors traditionally work with reclosing relief valves or non-reclosing rupture discs to achieve pressure-relief action.

Reclosing devices — sometimes referred to as safety relief valves (SRV), pressure-relief valves (PRV) or relief valves (RV) — are designed to open at a selected set pressure. Opening allows the overpressure to evacuate and the pressure to return to an acceptable level, whereupon the valve recloses. Pressure-relief valves come as spring-operated or as pilot-operated units.

To protect installations against unacceptable vacuum pressures, the use of vacuum-relief valves (VRV) may be considered. These devices will similarly open and allow for atmospheric pressure to be re-established when the set-to-open vacuum pressure is reached.

Rupture disc devices are often preferred as a means to achieving instant and unrestricted pressure relief (both overpressure and vacuum pressure). They consist of a calibrated (metallic or graphite) membrane that ruptures when the set pressure is achieved. After activation, the membrane remains open, resulting in a complete discharge of the pressure in the installation.

The main properties of rupture discs and relief valves are summarized in Table 1.

Table 1. Rupture Discs versus Relief Valves
Properties Rupture disc Relief valve
Complexity of device Low High
Investment cost Low High
After activation Replace Reset
Protection against overpressure Yes Yes
Protection against vacuum pressure Yes No
Mounting-position restrictions No Vertical only
Installation cost Low High
Maintenance cost Low High
Requires regular recalibration No Yes
Affected by backpressure Yes Yes
Operational testing possible No Yes
Leak-tight Yes No
Selection of materials of construction Large Limited
Size range Large Limited
Change of set pressure No Yes
Suitable for gas-liquid two-phase systems Yes No
Reaction time Low High
Unrestricted opening Yes No

 

Depending on the equipment that needs to be protected and the required performance, reclosing and non-reclosing devices can be complementary, offering unique advantages and limitations. The appropriate selection of devices must be determined by the design engineer and end-user, depending on the needs of a specific application.

Figure 2. Installation of rupture discs upstream of relief
valves allows for in-situ calibration testing of relief valves
 
Figure 3. Rupture discs can prevent leakage through the
relief valve and prevent corrosion of relief-valve internals
when positioned on the process side of a valve

 

Complementary RDs & RVs

Using rupture discs in combination with relief valves can utilize the properties of both, often arriving at an optimal solution (Figures 2 and 3). Combinations can employ the RD and RV either in parallel or in series, offering a combination of features that achieves operating and safety objectives. The goal of design engineers and safety specialists is to determine which combination provides the desired features, while keeping the consequences of exceeding pressure limits in balance.

The two major standards governing combination of rupture discs and relief valves are the ASME Boiler and Pressure Vessel (BPV) Code (Section VIII Division 1, § UG-125 (c) (1)) and European Pressure Equipment Directive 93/23/EC (EN764-7 § 6.1.4, as defined in EN/ISO 4126-3). The requirements for the two are outlined in Table 2. The elements of API Recommended Practice (RP) 520 (Sizing, Selection and Installation of Pressure-Relieving Devices) are taken directly from the ASME BPV code, Section VIII, Division 1.

Table 2. ASME versus ISO Requirements for Combination RD/RV Systems
Requirement ASME Sect. VIII, Div. 1 (API) EN ISO 4126-3 Comments
Definition of an RD/PRV combination None Rupture disc is within five pipe diameters of the inlet of the PRV If the RD is not within five pipe diameters, then a combination capacity factor is not applicable
The three-percent rule Pressure drop between the vessel and PRV inlet, including the effect of the rupture disc, shall not exceed 3% of the valve set pressure at valve nameplate flowing conditions Pressure drop between the vessel and PRV inlet, including the effect of the rupture disc, shall not exceed 3% of the set pressure of the valve at maximum flowing conditions The difference between flowing at nameplate capacity or another maximum could be significant. That is, what if the PRV is set well below the MAWP (maximum allowable working pressure) but sized to prevent exceeding 110% of MAWP. It may be impossible to meet the ISO requirements in this situation
Certified combination capacity factor (CCCF) One-size method applicable to all sizes equal to and larger than the tested combination One-size method for a single size or three-size method to be applied to a family Pursuit of the ISO three-size combination capacity factors is cumbersome due to the cost and logistics. With a default of 0.9 the pay-back on three-size testing is minimal
Protrusion of petals into valve No specific requirement Petals shall not protrude into the PRV inlet unless the influence of the petals on the capacity and performance of the PRV has been assessed and proven to meet the requirements of Clause 7 (Combination Performance) Both codes use language prohibiting the RD to impair the performance of the PRV. The ISO document seems to take a firm stand on the petal protrusion issue but points to Clause 7 which allows a default CCF (Fd) of 0.9
Documentation of the combination Nameplate marking for the combination provided by the user, PRV, RD, or vessel manufacturer Supplier of the combination shall provide the nameplate, certification, assembly and installation instructions taking into account the results of a hazards analysis In both codes, there are gaps in these requirements. In practice these requirements are rarely followed

 

RD and RV in parallel

When using relief valves and rupture discs in parallel (Figure 4A), the main objective is to allow the relief valve to initially handle overpressure situations — bleeding the pressure until an acceptable, reduced pressure is achieved — while allowing the process to continue. When the overpressure cannot be effectively reduced by the relief valve (due to malfunction, blockage or generation of excessive pressure), the pressure may continue to rise until a higher set pressure of the rupture disc is reached. Upon activation, the rupture disc provides an additional backup relief path for the overpressure, resulting in a safer process.

When using RDs and RVs in parallel, a suitable margin of set pressure needs to be designed into the system to avoid premature failure of the rupture disc. This requires that the set-to-open pressure of the relief valve be below the burst-pressure range of the rupture disc, with a appropriate margin to separate them.

Regulatory and legislative requirements for pressure limitation and size determination, range from what is found in the ASME BPV Code — which says “Sizing of the secondary relief devices (the rupture disc) [should] be such that the pressure does not exceed 116% of the equipment design pressure” — up to the language in the European Pressure Equipment Directive, which says the “Maximum achieved overpressure [is] not to exceed 110% of the equipment design pressure.

Figure 4a and 4b. Rupture discs and relief valves can be used in parallel (left) and in series (right)

 

RD upstream of RV

Rupture-disc devices may also be installed either upstream or downstream of a relief valve (Figure 4B). Each geometry offers particular benefits for the user. Using rupture discs upstream of relief valves is a common practice to achieve one or more of the following, each of which is explained further below:

• Prevent plugging of the RV

• Prevent corrosion on RV internals

• Prevent leakage through the RV

• Allow for in-situ testing of the RV

Prevent plugging or gumming of the relief valve. Through the selected use of suitably designed rupture discs upstream, product buildup or polymerization can be limited. Most relief valves are not suitable for use with media that create a buildup layer, because it interferes with the ability of the relief valve to open. The use of an upstream rupture disc reduces the need for regular inspection, maintenance or cleaning of the relief valve, leading to increased productivity and more reliable safety.

Prevent corrosion of the relief-valve internals.When the process media require that specific corrosion-resistant materials are used, relief-valve options can be limited and those valves that are available could carry higher costs, longer delivery times and more difficulty obtaining spare parts. By installing a high-alloy rupture disc upstream of the relief valve, the valve is physically isolated from the process. Exposure of the valve to the process media is restricted to the overpressure event only. Until this emergency event occurs, the relief valve remains in pristine condition, unaffected by the process.

An in-series arrangement allows for the use of valves and related spare parts that are made from “standard” materials, resulting in substantial cost savings at the initial investment, a wider range of potential valve and parts suppliers, and shorter equipment lead times.

Prevent leakage through the relief valve.To achieve leak tightness, most spring-operated relief valves rely on special metal-to-metal sealing surfaces and on the applied spring-load force. Such systems inevitably result in some leakage, which increases as the operating pressure approaches the valve set pressure. Relief-valve leakage rates are addressed in industry standards, and acceptable leakage rates are defined (for example, in API Standard 576, “Inspection of Pressure Relieving Devices”). Where such leakage rates are unacceptable, the user may choose soft-seated or pilot-operated relief valves. Both options require higher investment and may still have restrictions, such as the availability of suitable O-ring material with sustained performance characteristics when exposed to the process media, as well as pilot-valve leakage and corrosion or plugging, and so on.

Rupture discs offer reduced leakage rates, and designs are available with virtually leak-free construction. The installation of rupture discs upstream of the relief valve eliminates emissions in a simple and cost-effective manner.

Allow for in-situ testing of relief valve.The acceptable use of relief valves to protect installations is linked to the need for periodic calibration of these safety devices. Depending on the local regulatory requirements, such calibration may be required annually. Since process shutdown and removal of the relief valve from the process equipment is required for such calibration testing — often to be done at a special test institute or qualified service center — important economic reasons exist to try to extend the calibration intervals. Longer calibration intervals may be allowed by the supervising authorities if the user provides evidence of unaffected set pressure over time.

This can be achieved by regular testing of the relief valve in-situ (that is, without removing the relief valve from the installation) and demonstrating that the valve’s performance is unchanged.

By installing a rupture disc upstream of the relief valve, a limited volume is created, allowing for the controlled introduction of pressure between the rupture disc and the valve inlet from the outside. This pressure (possibly combined with special “pulling force” test and measuring equipment applied to the valve spindle to overcome the spring force and keep the relief valve in closed condition) can be measured and registered as evidence of acceptable valve performance. The relative cost related to adding the rupture disc device is generally far less than what results from the loss of production time when removing and re-assembling the RV.

Considerations for selection

The following guidelines should be considered when selecting a rupture disc upstream of a relief valve:

• The rupture disc cannot interfere with the relief-valve operation. For example, the rupture disc cannot fragment upon bursting, because pieces may obstruct the valve orifice or prevent the valve from fully reclosing. Sufficient distance is required for the rupture disc to open without blocking the relief-valve nozzle (after opening, a single-petal rupture disc may extend beyond the height of the holder reaching into the inlet section of the relief valve)

• To assure proper functioning of the relief valve, the rupture disc device should be “close-coupled” with the relief valve, assuring that the pressure drop during flow at the inlet of the relief valve does not exceed 3% of the valve set pressure, as required. The 3% value is a requirement, and is described in API RP 520. In most cases, this restricts the distance between the rupture disc and relief-valve inlet to a maximum distance of five pipe diameters. This situation is often achieved by installing the rupture disc device directly upstream of the relief valve. Longer distances between the rupture disc and relief valve — by introducing pipe sections or spacers — may result in the creation of reflective pressure waves when the rupture disc opens. This phenomenon can result in undesired fragmentation or re-closing of the rupture disc, and should be avoided

• Since, like the relief valve, the rupture disc is a device that reacts to differential pressure between the upstream and downstream side, measures should be taken to avoid any unnoticed pressure increases in the closed cavity between the rupture disc and relief-valve inlet. Most industry standards and related regulations require that the pressure in the cavity be monitored or vented to the atmosphere. This is commonly achieved through the use of a so-called tell-tale assembly, consisting of pressure gage or indicator, try cock and free vent

Sizing and set pressure

When installing rupture disc devices upstream (at the inlet) of relief valves, the size of the rupture disc should be, at a minimum, the same nominal size of the inlet of the relief valve. Additionally, the rated relief capacity of the relief valve, as stated by the relief valve manufacturer, should be reduced by 10%, or alternatively, reduced to the certified combination capacity value (when the specific valve-disc combination has been capacity-tested and certified by a recognized third party).

The set pressure of the rupture disc device should be set in accordance with the applicable standards and guidelines, as follows:

• ASME VIII Division 1 UG-127 footnote 52 states that the selected pressure should “….result in opening of the valve coincident with the bursting of the rupture disk.” For combination capacity testing, ASME UG-132(a)(4)(a) says: “The marked burst pressure shall be between 90 and 100% of the marked set pressure of the valve.”

• API RP520 paragraph 2.3.2.2.2 states: “…the specified burst pressure and set pressure should be the same nominal value.”

• EN ISO4126-3 paragraph 7.2 states: “The maximum limit of bursting pressure…shall not exceed 110% of the…set pressure or a gauge pressure of 0.1 bar, whichever is greater…” and “The minimum limit…should not be less than 90% of the…set pressure.”

While the statements differ slightly, the basic guidance is the same: make sure the rupture disc’s specified burst pressure and relief-valve set pressure are at the same nominal value (ignoring tolerances). Doing so is relatively easy and meets the intent of each standard.

There may be special cases where it is desirable to have these pressures significantly different. In such cases, the user should carefully evaluate the function of both the rupture-disc and the relief-valve to ensure that there are no adverse effects on the performance of either.

Combination capacity factor

The process of sizing the relief valve is exactly the same whether it is used in combination with a rupture disc or as a standalone relief valve, except for the addition of the combination capacity factor (CCF; known as Fd in EN/ISO 4126-3). This factor represents the ratio of the capacity of the combination to the capacity of the valve by itself.

CCF = Capacity of the combination / Capacity standalone relief valve

The default CCF for most codes is 0.90 (in other words, the combination is assumed to have a capacity equal to 90% of the RV rated capacity, if nothing more is known about the actual capacity). EN ISO 4126-3 adds an additional condition on the use of the default CCF, and requires that the petal(s) of the rupture disc be fully contained within the holder after rupture, in order to use the default CCF. Otherwise, a tested or certified value must be used.

CCF values greater than 0.90 may be used in certain cases where specific testing has been conducted on a particular combination of RD and RV product. This is often referred to as a “certified” combination capacity factor (CCCF). For CCFs on specific RV-RD combinations, see Table 3 here.

Methods for establishing CCCFs vary based on the applicable code, and are summarized as follows:

ASME.

• Testing must be done by an authorized testing laboratory and results registered with the National Board of Boiler and Pressure Vessel Inspectors

• Testing only one size is required to establish a CCCF for a range of other sizes

• Testing with the smallest size and minimum corresponding pressure covers all higher pressures in that size and all larger sizes

EN ISO 4126-3.

• No certifying body or laboratory requirements

• One-size method and three-size method are accepted

• One-size method is applicable to all combinations of the same size and design of rupture disc and relief valve equal to or above the tested pressure

• The three-size method is applicable to all combinations of the same design of rupture disc and relief valve in all sizes equal to or greater than the smallest tested size; and pressures equal to or greater than the appropriate minimum pressure for the size

Both ASME and EN/ISO include requirements for establishing nameplate marking to reflect the capacity (or the CCF) of the combination, model and manufacturer of both the rupture disc and the relief valve. Although these are requirements of both ASME and EN/ISO, in reality, this nameplate is rarely supplied, because the components are generally purchased independently, with neither manufacturer aware of the other.

RD downstream of RV

The following are primary reasons for applying rupture discs downstream of pressure relief valves:

• Prevent corrosion of relief valve

• Prevent fouling or sticking of the relief valve

• Prevent variable superimposed backpressure from affecting the relief-valve operation

• Detect opening or leakage of the relief valve

Prevent fouling and plugging of the relief valve.In situations where relief systems are vented into a common header, the risk exists that blowdown material may enter the vent side of the installed relief valves. Where such vented media can result in either corrosion or polymerization, the external side of the relief-valve mechanism may be affected, resulting in failure to operate when required. By installing a rupture disc with suitable properties at the downstream side of the relief valve, vented media are isolated from the relief valve, therefore avoiding the effects of corrosion and polymerization, and increasing the reliability of the safety system and reducing the need for inspection and maintenance. To ensure that the downstream rupture disc will not impede the proper performance of the relief valve, the burst pressure of the rupture disc should be as low as possible, whereas the provided minimum net flow area of the rupture disc needs to be at least as large as the relief area of the relief-valve outlet.

Prevent corrosion of the relief-valve internals.To avoid corrosion and the resulting need for inspection, maintenance and repair of the relief valve, the use of a downstream rupture disc can be considered. By installing a rupture disc downstream of the relief valve, the rupture disc will act as a chemical seal between the valve outlet and the common header, which could contain potentially corrosive agents from process media released through other emergency relief valves. In this way, corrosion effects of the process media on the valve internals can be eliminated.

Prevent backpressure from affecting relief-valve performance.Where backpressure can be present, its effects on the performance of the relief valve should be considered. Users can do so by selecting relief-valve attributes like balanced metallic bellows, or by using pilot-operated relief valves. These options will like increase cost and will require additional spare parts and maintenance. As an alternative, the use of downstream rupture-disc devices installed at the relief-valve outlet prevents the relief valve from being exposed to backpressure (Figure 5).

 Figure 5. In some cases, it can be advantageous to position a rupture disc at the relief-valve outlet
 

Detect the leakage or activation of relief valve. By detecting the rupture of the downstream rupture-disc device, plant operators can be informed about the upset condition, leading to blowoff. When the interspace between the relief valve outlet and rupture disc is monitored, the leakage of the relief valve can be detected and emissions avoided.

Benefits

While the use of rupture discs at the downstream side of relief valves is relatively uncommon, that arrangement can offer an array of benefits to the plant owner. The acceptable use of this setup has to comply with the following sizing and set-pressure requirements:

• The minimum net flow area of the rupture-disc device installed at the relief-valve outlet needs to be equal to or larger than the relief-valve-outlet relief area

• The burst pressure of the rupture disc needs to be as low as practical to reduce any effect on the relief-valve performance

• Where applicable, the selected rupture disc needs to be capable of withstanding the backpressures expected from the effluent handling system

• The opening of the rupture disc shall not interfere with the relief-valve opening or performance

• The system design shall consider the adverse effects of any leakage through the relief valve, or through the rupture disc, to ensure performance and reliability

• The relief valve may not fail to open at the expected opening pressure regardless of any backpressure that may accumulate between the relief-valve outlet and the rupture disc. The space between the relief-valve outlet and the rupture disc should be vented and drained (or suitable means should be provided to ensure that an accumulation of pressure does not affect the proper operation of the relief valve). Venting, pressure monitoring and selection of low rupture-disc burst pressures are commonly used to meet these requirements

• The bonnet of a balanced bellows-type relief valve shall be vented to prevent accumulation of pressure in the bonnet that can affect relief-valve set pressure

Edited by Scott Jenkins

Author

Roger Bours is the pressure-relief sales manager for Fike Europe (Toekomstlaan 52, B2200, Herentals, Belgium; Email: roger.bours@fike.com; Phone: +32 14-210-031). Bours has held the position for nearly 30 years, and has expertise in pressure-relief solutions for a wide range of applications and industries. He specializes in engineered solutions with extended knowledge in industry needs and requirements. Bours is the author of multiple technical papers, white papers and articles, and regularly conducts workshops on pressure relief applications, requirements and issues. He is active in international standardization committees, including: ISO TC185 “Pressure Relief Devices” (Belgian representative since 1986); CEN0 TC 69 WG10 SG2 “Bursting Disc Devices” (Belgian representative since 1990); CEN TC 69 WG10 SG3 “Bursting Disc Devices in Combination with Safety Relief Valves” (Convenor since 1996); CEN TC 305 WG3 “Explosion Venting Devices & Systems” (Belgian representative since 1998).