API RP 576 5 Causes of Improper Performance

5.1 Corrosion

5.1.1 General Corrosion is a basic cause of many of the difficulties encountered with pressure-relief devices. An understanding of the process/operating conditions need to include and resulting damage mechanisms are required in order to establish and maintain an inspection program of relief valves that yields the highest probability of preventing their damage. Changes in process/operating conditions need to include an evaluation of the corrosion potential of the relief valve. There are several other sources of industry data that specifically identify typical degradation mechanisms for various operating units. Specifically applicable to the refining and petrochemical industry is API 571 covering damage mechanisms. Corrosion often appears as pitted or broken valve parts, deposits of corrosive residue that interfere with the operation of the moving parts, or a general deterioration of the material of the relieving device. Valve malfunction may also be due to sticking of the disc to the nozzle or other disc holder in the guide. This sticking may be caused by corrosion or galling of the metal or by foreign particles in the guiding surfaces. Foreign particles in the guiding surfaces tend to roll metal up, causing severe galling. The use of a bellows can keep the foreign particles away from the guiding surfaces. Sticking of valves illustrates a disc holder that is frozen in the guide as a result of corrosion, e.g. in sour gas services. Corrosion may be slowed or mitigated by the selection of more suitable devices or device materials. Proper maintenance is also a consideration since a leaking valve allows fluids to circulate in the upper parts of the valve, which can contribute to the corrosion of its movable parts. Protective coatings may offer protections against corrosion in some services. Bellows can protect moving parts from the corrosive substance, especially in closed systems.

5.3 Failed Springs

5.3.1 General Spring failures occur in two forms. The first is a weakening of the spring, which causes a reduction in set pressure and the possibility of premature opening. The second is a mechanical failure (complete break) of the spring, which causes uncontrolled valve opening. Although springs may weaken and fail due to the use of improper materials in high-temperature service, failed springs are almost always caused by corrosion. Surface corrosion cracking are the most prevalent of this type of failure in refineries. Surface corrosion attacks the spring surface until the cross-sectional area is not sufficient to provide the necessary closing force. It may also produce pits that act as stress risers and cause cracks in the spring surface and subsequent spring failure. Stress corrosion cracking (SCC) sometimes causes spring failure. The SCC damage mechanism is difficult to detect and predict before spring fails. Hydrogen sulfide (H2S) frequently causes SCC of springs (see NACE MR 0175 and NACE MR 0103 for material recommendations and guidance). Consult the manufacturer to select an appropriate spring in susceptible applications since the material strength, hardness, and heat treatment of the spring can affect its resistance to stress corrosion cracking.

5.6 Galling

5.6.1 General When galling of the metal in the guiding surfaces is not due to corrosion or foreign particles, it is often due to valve chatter or flutter caused by improper piping at the valve inlet or outlet or by severe oversizing of the valve. Galling may also occur if the system operates too close to the set pressure resulting in frequent relieving.

5.9 Improper Handling

5.9.1 General Improper handling can occur during shipment, maintenance, or installation. This improper handling of the relief valve can cause a change of the set pressure, damage lifting levers, damage tubing and tubing fittings, damage pilot assemblies, or cause internal leakage when the valve is in service. Valves are checked for tightness in the manufacturer's plant before they are shipped to the user. Valve tightness is sometimes checked by the user in the maintenance shop before initial use and usually checked after subsequent cleaning, repairing, or testing.

5.1.2 Examples of Preventative Actions for Corrosion

A rupture disk device installed on the inlet and/or outlet of a pressure-relief valve can provide added corrosion protection of the valve internals. In many instances, valves with differing materials of construction can impede or altogether mitigate the effects of corrosion. The use of an O-ring or resilient seat in a pressure-relief valve may stop leakage past the seating surface and eliminate corrosion in the valve's working parts. However, O-ring elastometers may have a limited life under stress due to degradation caused by temperature, corrosive species, aging, or swelling. Bellows seal can be used to protect the spring and the bonnet cavity of the valve from the corrosive loading fluid.

5.6.2 Examples of Preventative Actions for Galling

Correction of improper piping at the valve inlet or outlet will usually stop the action of chatter or flutter (see API 520, Parts I and II). Improper finishing of the guiding surfaces can also result in galling caused by chatter or flutter. Consult the valve manufacturer for recommendations as this is potentially a design and manufacturing issue.

5.8 Improper location, History, or Identification

If not installed at the exact location for which it is intended, a pressure-relief device may not provide the proper protection. To assist in the identification of the devices and to provide information necessary for correct repairs and installation, historical records and specifications should be maintained and referred to when the devices are removed for inspection and repair. Most pressure-relief devices have an identifying serial or shop number placed on the device by the manufacturer or an identifying number tagged, stamped, or otherwise placed on the device by the user. Some users also stamp mating pipe flanges with device numbers. This identification specifies the location of the device and, by reference to the specification record, its limitations and construction.

5.2 Damaged Seating Surfaces

Imperfections in seating surfaces may contribute to improper valve action in service. To prevent leakage of the loading fluid, an optical precision on the order of three light beads/bands according to manufacturer's specifications should be maintained in the flatness of seating surfaces on metal-seated pressure-relief valves (see API 527). There are many causes of damaged valve seats in refinery or chemical plant service, including the following. a) Corrosion. b) Foreign particles introduced into the valve inlet that pass through the valve when it opens, such as mill scale, welding spatter or slag, corrosion deposits, coke or dirt. The particles may damage the seat-to-nozzle contact required for tightness in most pressure-relief valves. The damage can occur either in the shop during maintenance of the valve or while the valve is in service. c) High inlet pressure drop may be caused by improper piping design or lengthy piping to the valve inlet or obstructions in the line and may cause a valve to chatter. The pressure under the disc may become great enough to open the valve. However, as soon as the flow is established, the pressure drop in the connecting piping may be so great that the pressure under the disc fails and allows the valve to close. A cycle of opening and closing may develop, become rapid, and subject the valve seating surfaces to severe hammering, which damages the seating surfaces, sometimes beyond repair. d) Improper handling during maintenance and/or transport, such as bumping, dropping, jarring, or scratching of the valve parts. e) Leakage past the seating surfaces of a valve after it has been installed. This leakage contributes to seat damage by causing erosion (wire drawing) or corrosion of the seating surface and thus aggravating itself. It may be due to improper maintenance or installation such as misalignment of the parts, piping strains resulting from improper support, or inadequate support of outlet piping. Other common causes of this leakage are improper alignment of the spindle, improper fitting of the springs to the spring washers, and improper bearing between the spring washers and their respective bearing contacts or between the spindle and disc holder. Spindles should be checked visually for straightness. Springs and spring washers should be kept together as a spring assembly during the life of the spring. Frequent operation too close to the pressure-relief valve set pressure could cause leakage of process fluid across the pressure-relief valve (simmer) and cycle the pressure-relief valve resulting in seat damage. f) Improper blowdown ring settings. This can cause chattering in pressure-relief valves. The pressure-relief valve manufacturer should be contacted for specific blowdown ring settings. g) Severe oversizing of the pressure-relief valve for the relief loads encountered can cause the valve to cycle (open/close repeatedly), resulting in disc and nozzle seating surface damage. h) Venting liquids across vapor trim pressure-relief valves can cause chatter/cycling/hammer effects with resultant damage.

5.7 Misapplication of Materials

In general, the temperature, pressure, corrosion protection requirements, and the atmospheric conditions of the service determine the materials required for a pressure-relieving device in a given service. Occasionally, severe corrosion or unusual pressure or temperature conditions in the process require special consideration. Manufacturer's can usually supply valve designs and materials that suit special services. Catalogs have a wide selection of special materials and accessory options for various chemical and temperature conditions. Addition of a rupture disk device at the inlet and/or outlet of the valve may help prevent corrosion. The H2S attack on a carbon steel spring in Figure 30 and the chloride attack on an 18Cr-8Ni steel disc in Figure 24 exemplify the results of the misapplication of materials. When service experience indicates that a selected valve type or material is not suitable for a given service condition, an immediate correction that will ensure dependable operation should be made. Great care should be taken to record the identity of special materials and the locations requiring them. An adequate system of records should provide the information needed for the repair or reconditioning of valves in special service and for developing optimum purchase specifications.

5.4 Improper Setting and Adjustment

Manuals by the valve manufacturer provide procedures for proper setting by indicating how to adjust their valves for temperature, backpressure, and other factors. Setting a pressure-relief valve while it is in place on the equipment to be protected may be impractical and should be performed only after special consideration as noted in 6.3. Generally, direct-acting spring-loaded valves should be set in the valve maintenance shop while an appropriate test equipment. During inspection and repair, a properly designed test block facilitates the setting and adjusting of the pressure-relief valve. Pressure-relief valves are designed and certified to operate in specific types of fluid media. Therefore, water, air, steam, or an inert gas such as bottled nitrogen is generally used as the testing medium in the shop, depending on the design of the valve being tested and the requirements of applicable design and testing codes. To ensure that the valve is opening. some overpressure should be carefully applied because an audible leak could otherwise be misinterpreted as the result of reaching the set pressure, making misinterpretation unlikely. The size of the test stand is important since insufficient surge volume might not cause a distinct pop and may cause an incorrect set pressure. Air, gas, or vapor service valves should be set using air or inert gas. Steam service valves should be set using steam. Special attention is needed if the relief valve is placed in superheated steam service to compensate for temperature. Air may be used if suitable corrections are applied in place of steam. Liquid service valves should be set using water. It is important to note what audible or visual indication signifies the set pressure for a specific type of pressure-relief valve. This indication is defined by the manufacturer and is listed in NB-18 and manufacturer's manuals. Consult the manufacturer for the proper technique for setting pilot-operated pressure-relief valves on liquid as the water in the dome area and pilot assembly my create problems when placed in service. Incorrect calibration of pressure gauges is a frequent cause of improper valve setting. To ensure accuracy, gauges should be calibrated frequently on a calibrated dead weight tester. The pressure range of the gauge should be chosen so that the required set pressure of the pressure-relief valve falls within the middle third of the gauge pressure range. Snubbers on pressure gauges are not generally recommended since they tend to clog and produce pressure lag. It may be desirable to use two test gauges during valve testing. Many direct-acting spring-loaded pressure-relief valves have one or more internal rings that can be adjusted. The pressure-relief valve adjusting ring or rings will control the valve blowdown--the difference between the set pressure and the reseating pressure--and valve simmer, depending on the design of the valve being tested. To functionality test the pressure-relief valve and measure the blowdown, similar media properties of the service fluid and adequate flow capacities to fully cycle the valve are needed. Because the density and expansion characteristics of material handled through safety valves are variable and the volume of testing facilities is limited, it is usually impractical to adjust the valve rings and obtain a specific blowdown value on a maintenance sop test block. The rings should therefore be adjusted to obtain a pop on the valve test drum (see manufacturer's maintenance instructions for this adjustment) and then inspected and readjusted for proper blowdown according to the manufacturer's recommendation. This should permit the best average performance characteristics of the valve when installed. Full understanding of terminology is important (see ASME PTC 25).

5.9.2 During Shipment

Most pressure-relief valves have a study appearance that may obscure the fact that they are precise instruments with very close tolerances and critical dimensions. Accordingly, commercial carriers and/or maintenance transport trucks sometimes subject them to improper handling. This may cause a valve to leak excessively in service or during testing. This improper handling may also expose the valve inlet to dirt or other foreign particles that could damage the valve seating surface the first time the valve opens and cause leakage thereafter. Pressure-relief valves should be braced and shipped in an upright position--this is especially true of large valves with low set pressures. When large, low-pressure valves are allowed to lie on their sides, the springs or weights may not exert the same force all around the seating surfaces.

5.9.3 During Maintenance

Pressure-relief valve parts are precision items manufactured to extremely close tolerances. Improper handling can degrade these tolerances, destroying the basic valve alignment on which the fine, exacting performance characteristics of the device primarily depend. Both before and after repairs, improper handling of the completely assembled valve should be avoided. Mishandling of a pressure-relief valve can affect the opening pressure and reseating pressure of the pressure-relief valve during the premaintenance test or after it has been serviced and reset. This should be documented and proper handling procedures should be implemented. Before the valves leave the shop, valve inlets and outlets should be securely covered. Pressure-relief valves with lifting levers should not be moved or carried via the lever, and consideration should be given to wiring the lever go the valve for stability during transportation. Caution--Lifting lever wiring is only used for transport and needs to be removed before installation. Caution--Avoid exceeding the pressure rating of the bellows during a backpressure test as this may damage the bellows.

5.5 Plugging and Fouling

Process solids and contaminants such as coke, sand, or solidified products can sometimes plug various parts of the valve and connected piping. Additionally, monometer service can lead to polymer formation and plugging.

5.9.5 Improper Handling, Installation, and Selection of Rupture Disks

Rupture disk problems are often associated with improper handling, installation, and selection. The following should be considered. a) Ensure the rupture disk is installed in the proper orientation. Some reverse-acting rupture disks will open a significantly higher burst pressure if installed in the reverse direction. b) Once a rupture disk is removed from its holder, the rupture disk should not be reinstalled. Installation in a holder can form an imprint on the disk. Once removed from its holder, it would be difficult to reinstall the disk perfectly in the same imprint. The most likely result will be premature failure below the intended burst pressure. c) Always follow the manufacturer's recommended torque settings when installing the rupture disk in the holder. An improper torque could affect the opening pressure of the disk and in some cases cause nonfragmenting disks to fragment. d) Touching the rupture disk surface could lead to localized corrosion leading to premature failures. e) Disks that become dented or otherwise damaged during installation or handling may open outside of their specified burst pressure tolerance or may not open completely on demand, thereby potentially restricting the relief path. f) Temperature can significantly affect rupture disk opening pressure for some materials. Specifications of appropriate burst temperature should consider ambient heating or cooling if un-insulated and/or untraced. Consult the manufacturer and see API 520, Part 1 for additional information. g) Rupture disks should be installed away from unstable flow patterns to avoid premature failures (see API 520, Part 1, which provides general requirements for installation of rupture disks).

5.10 Improper Differential Between Operating and Set Pressures

The differential between operating and set pressures provides seat loading to keep the pressure-relief valve tightly closed. Due to a variety of service conditions and valve designs, only general guidelines can be given for designing a system. ASME BPVC Section VIII, Division 1 (see Appendix M) and API 520 are useful references. However, individual applications and experience may be relied on.

5.3.2 Examples of Preventative Actions for Spring Corrosion

The following are examples, but are not limited to spring corrosion. a) Spring material that will satisfactorily resist the action of the corrosive agent may be used. b) The spring may be isolated by a bellows. Certain pilot-operated pressure-relief valves have diaphragms or pistons that isolate the pilot spring from the process. c) The spring may be specially coated with a corrosion-resistant coating that can withstand the operating temperature and environment.

5.9.4 During Installation

Valve inlets and outlets should be securely covered before the valves leave the shop. When received for installation, inspection of the openings for foreign materials, shipping stays, and damage should be performed. API 2000, Section 3.7 should be utilized for the requirements for installation of tank venting devices. Caution--Pressure-relief valves are often delivered with shipping stays that stabilize the valve during transport. Such stays shall be removed prior to installation. Caution--Pressure-relief valves should be installed in a vertical orientation, with the disc of a direct-acting valve or unbalanced member of a pilot-operated valve oriented horizontally, such that the disc or unbalanced member moves upward as the valve opens. Other orientations may permit these parts to become misaligned in the guide. ASME BPVC Section VII, Division 1, Appendix M describes under what conditions an orientation other than vertical may be acceptable. Caution--There are weight-loaded valve designs that can be installed on the sides of tanks. Weight-loaded valves may have their weight shipped separate from the valve to protect the pallet seating surfaces during handling. These weights should be installed prior to commissioning the tank. Refer to API 2000, Section 3.7 for requirements for installation of tank venting devices.

5.11 Improper Inlet/Outlet Piping Test Procedures

When hydrostatic tests of inlet/outlet piping are performed, blinds shall be installed. Otherwise, results such as the following might occur: a) the disc holder, guide, spring, and body area on the discharge side of the valve may become foul; b) the bellows of a balanced pressure-relief valve may become damaged by excessive backpressure; c) the dome area and/or pilot assembly of a pilot-operated pressure-relief valve d) the test pressure may exceed the design pressure of the discharge side of the pressure-relief valve.