API 570 Chapter 8 API 576 Inspection of Pressure Relieving Devices
8.1 Introduction to API 576
This chapter is about learning to become familiar with the layout and contents of API 576: Inspection of PressureRelieving Devices. Similar to API 572, API 576 is a well established document (it is still on its 2000 edition) with its roots in earlier documents published by the American refining industry. It is more a technical guide document rather than a code, as such, but it does perform a useful function in supporting the content of API 510. Note the following four points about API 576:
Point 1. It is a well-detailed and comprehensive technical document. Unlike some API codes, which have a very selective approach to their subject, API 576 is one of the best in the quality of the technical information it provides. It is an excellent guide to the practical aspects of pressurerelieving devices.
Point 2. API 576 introduces specific API terminology on the types of pressure-relieving devices. These relate to the definitions used for the following terms: safety valve, relief valve and safety relief valve. These API definitions are technically consistent in themselves but can be different to those used in other codes (e.g. BS/EN/DIN).
Point 3. Similar to API 572, it refers to a few related codes that are not in the API 510 exam syllabus (mainly API 527 covering seat leakage testing; see API 576 section 2 on page 1 of the code). As with previous related codes, you need to know that these additional codes exist but you do not need to study them for the API 510 examination.
And finally, the most important point. Like API 572, API 576 is all text and technical descriptions, accompanied by explanatory photographs of a fairly general nature. It contains no calculations. This means that many examination questions about API 576 in the API 510 certification exam may be closed book. The downside to this is that API 576 contains many separate technical facts, giving a large scope for the choice of exam questions.
Again, similar to API 572, appendix A of API 576 contains specimen work order/inspection report sheet/test record formats. While these undoubtedly contain sound guidance on the format of reports they are not a particularly suitable subject matter for multiple choice exam questions. All this means that you need to concentrate firmly on the technical (rather than administrative) content of API 576. We will look at some of the more important areas as we work through the code.
8.2 API 576 sections 3 and 4: types (definitions) of pressure-relieving devices
Remember that API 576 uses specific definitions for the various types of pressure-relieving devices and that these may not correspond with those in other codes (or your own knowledge and experience). API 576 sees the situation as shown in Fig. 8.1.
Don’t worry if you find this a bit confusing. Read the following points, which attempt to clarify the situation:
NOTE THESE POINTS ABOUT PRV TERMINOLOGY .
- Note the first annotation on Fig. 8.1; it shows the term PRV as a generic definition covering most practical types of pressure-relieving device, excluding bursting discs. We will use this term PRV in this context throughout the rest of this chapter.
- Now read API 576 section 4.2: safety valve. Note how the ‘pure’ type of safety valve described refers, in the main, to valves used for steam service (it doesn’t actually say this, but that is what it means).
API 576 Inspection of Pressure-Relieving Devices
Now read API 576 section 4.3: relief valve. Note how the ‘pure’ type of relief valve described refers, in the main, to valves used for liquid service (again, it doesn’t actually say this, but this is what it means).
Now read API 576 section 4.4: safety relief valve. Can you see why this has been introduced? Think of it as a generic definition, encompassing both the ‘pure’ types of safety valve and relief valve. The reason for API 576 introducing this term safety relief valve is to take into account the fact that many proprietary designs of PRV can be used on gas/ vapour or liquid service and so can be considered (with a bit of imagination) as both a safety valve and a relief valve, depending on application. Now you see why it is important not to confuse the terminology.
Keep these definitions in mind as you work through the rest of this API 576 chapter and they should start to become clearer.
8.3 Types of pressure-relieving device
Section 4 of API 576 describes the different types of pressurerelieving device. Note how this is supplemented by the various terminology definitions in section 3.
Protective device types: API 576 There are a large number of different types of protective device.
- Normal relief valves
- Safety valves
- Balanced safety relief valves (bellows)
- Pilot operated relief valves
Rupture discs (not re-useable)
- Scored tension
- Reverse acting
- Dead weight (more often seen on tanks)
- Pilot operated valves (diaphragm)
- Spring and dead weight
As we are mostly concerned with internal pressure, we shall concern ourselves mostly with spring-loaded safety valves and rupture discs. Before we look at protective devices, we will look at the various internal pressures that can exist inside a vessel. These are summarized in Fig. 8.2.
Normal maximum operating pressure. This is a bit confusing. It is, essentially, the design pressure based on the ASME VIII code.
Maximum allowable working pressure (MAWP). This is equal to or greater than the nominal design pressure (it is based, again, on ASME VIII and a common calculation parameter included in the API 510 examination syllabus).
Accumulation. This is the pressure above MAWP or design pressure (values defined in ASME VIII).
Overpressure. This may be expressed as either:
- The pressure above the set pressure of a PRV at which full discharge flow is achieved (mass flow kg/s or lb/s).
- A percentage of set pressure.
- The same as accumulation (when based on MAWP).
Note that the actual set pressure of the valve or disc is vital to the performance of the vessel. If it is set too low (at or just above the operating pressure) the valve will lift frequently. If it is set too high (above MAWP) the vessel may become overpressurized. There are a lot of terms for the different operational characteristics of a PRV, which are given in API 576 section 3.4. Two terms that deserve further explanation are back pressure and cold differential test pressure (CDTP):
Back pressure is the pressure that is present downstream of the discharge pipe of the PRV. It can either be present all the time or can build up as the result of flow as the PRV opens. For example, if a valve has a set pressure of 100 psi and is vented into a system that is operating at 25 psi, the valve will not operate until 125 psi.
Cold differential test pressure (CDTP) is used for PRVs on ‘hot’ duty and is an adjusted pressure at which the valve opens on a test stand at room temperature. The CDTP is corrected for both temperature and back pressure.
API 576 section 4: pressure-relieving valve types
Section 4.3: relief valves Relief valves (see Fig. 8.3) are direct spring-loaded valves that begin to open when the set pressure is reached. They open progressively and do not exhibit a pop action. Full lift is obtained with an overpressure of 10 % or 25 % depending on the type of valve. The valve will close after blowdown is complete but at a pressure lower than the set pressure. They are sometimes called thermal relief valves as they may reliev small pressure increases caused by thermal expansion of process liquid. Obtained with an overpressure of 10 % or 25 % depending on the type of valve. The valve will close after blowdown is complete but at a pressure lower than the set pressure. They are sometimes called thermal relief valves as they may relieve small pressure increases caused by thermal expansion of process liquid.
Normally, relief valves have a closed bonnet to prevent the release of product that is toxic, corrosive, flammable or expensive. For tightness of the seat, resilient O-rings can be fitted to replace the conventional metal-to-metal seat.
Applications: mostly used on incompressible liquids. Limitations: relief valves should not be used on the following services:
The term thermal relief valve is sometimes used for this tye of PRV. it simply inducates that it may be thermal expansion of system liquid that causes the causes the PRV to Open to relive the pressure, rather than any serious upset condition
Figure 8.3 A thermal relief valve
- Vapour services including steam, air and gas.
- Systems that discharge into a closed header unless the back pressure build-up has been allowed for.
- As a bypass or pressure control valve.
Section 4.7: pilot-operated pressure relief valves In this type, an auxiliary pressure relief valve called the pilot actuates the main valve. The pilot may be mounted on the same connection as the main valve or separately. The pilot valve opens normally at the set pressure and in turn operates the main valve. Figure 8.4 shows the principle.
Used when a high set pressure and large relief area are required. This type of PRV can be set to a maximum flange rating.
- The differential pressure between the operating and set pressures is small.
- Large low-pressure storage tanks.
- Where very short blowdown is necessary
- To replace bellows type valves to overcome the problems of high back pressure.
- Where pressure can be sensed in one position and the process is relieved at another.
- Where frictional pressure losses in the inlet or outlet pipework are high.
Limitations: Pilot PRVs have limitations on systems where:
- Service duty is dirty (unless filters are fitted in the system).
- High viscosity fluids (small passages in the pilot valve slow down the flow rate).
- The process fluids may form polymers, which cause blockages.
- High temperatures where seals, O-rings or diaphragms may not be suitable.
- The process may attack the seals, O-rings or diaphragms.
- Corrosion build-up affects the operation of the pilot valve.
Rupture disc devices
Rupture (or bursting) discs comprise a thin disc of material that has a known bursting pressure, held within a special holder. The actual disc can be made in a number of different configurations (see Fig. 8.5):
- Reverse acting
Conventional (often called direct acting) discs. The pressure acts on the concave side of the disc.
Scored tension loaded rupture disc. The pressure acts on the concave side but the disc is cut or scored by mechanical means during fabrication and is made of thicker material. The objective is to achieve a more accurate, reliable burst pressure.
Composite rupture disc. These are flat or domed, made in metallic or non-metallic material, with multipiece construction. The top section is slit and the burst pressure is controlled by the combination of the top and the underlying sections.
Reverse-acting rupture disc. These are domed but the pressure is on the convex side. The disc can be ruptured by a number of methods:
- Knife blades
- Knife rings
- Score lines
Graphite rupture disc. A flat disc of graphite impregnated with a binder material. It bursts by bending or shear.
8.4 API 576 section 5: causes of improper performance
This section of API 576 contains reasons for pressurerelieving devices (mainly PRVs) failing to work properly. The section is not particularly well structured but does contain good technical details. Much of the section is taken up with the corrosion/damage mechanisms that affect PRVs. Here is a summary of the content.
Section 5.1: corrosion Because PRVs are on the same duty as the pressure vessels they protect, they are subject to the many causes of corrosion that also occur in the vessels. Some typical (refinery industry) examples given in section 5 are:
- Acid attack on carbon steel due to a leaking valve seat
- Acid attack on a stainless steel inlet nozzle
- Chloride corrosion on a stainless steel nozzle
- Sulphide corrosion on a carbon steel disc
- Chloride corrosion on a stainless steel disc
- Pitting corrosion on stainless steel bellows
- Sour gas (H2S) attack on a monel rupture disc
By careful selection of the correct materials within a PRV, most corrosion problems can be overcome. The correct maintenance of the valve also stops any potential leakage allowing corrosive processes into those parts that could rapidly deteriorate.
As previously mentioned, bellows are used to give protection to the valve spring and discharge side of the valve. Also, placing a rupture disc directly under a PRV gives added protection to the valve components.
Section 5.2: damaged seating surfaces Because there is metal-to-metal contact between the valve disc and nozzle, this area must be extremely flat, as any imperfections will lead to a process leak. The seating surfaces must be lapped to produce a finish of two to three light bands (0.000 034 8 in). Figure 8.6 shows the principle.
Section 5.3: failed springs Safety valve springs fail in two distinct ways:
Gradual weakening, which can cause the valve to open ‘light’. This may be caused by:
- Improper material choice for the spring
- Operating at temperatures too high for the material
- Corrosion, leading to cracks and failure
Catastrophic failure of the spring, so that the valve opens and jams, preventing it from closing. The normal cause of this type of failure is stress corrosion cracking (SCC).
Section 5.4: improper setting and adjustment Several factors can affect the setting and adjustment of a PRV:
- Not following the manufacturer’s instructions.
- Testing the valve with the wrong medium.
Water, air or nitrogen is frequently used:
- Gases generally produce a definite ‘pop’ and are generally used for vapour service.
- Water is generally used for liquid service.
- Steam service should be tested with steam, to replicate its temperature and flow characteristics.
- Incorrect pressure gauge calibration. Gauges should be tested with a calibrated dead weight tester. The test pressure should fall within the middle third of the test gauge.
- Blowdown rings not correctly set.
Section 5.5: plugging and sticking When working on a fouling fluid, the inlet pipe to a PRV can become completely blocked. The fouling can be caused by a large number of refining industry processes that give solid particles such as coke and iron sulphide.
Section 5.6: misapplication of materials Occasionally, the material of construction of a PRV is not suitable for the process duty. Hydrogen sulphide and chloride attack are typical examples.
Section 5.7: improper location, history or identification A valve may not provide the required protection if is not located at the correct location. A record should be maintained of the history of the valve including the specification, any repairs, installation details, etc.
Section 5.8: rough handling PRVs are manufactured and maintained to a commercial seat tightness standard given in API 527. Rough handling may change the set pressure or otherwise cause damage to the valve.
Section 5.9: improper differential between operating and set pressures In use, a PRV should be kept tightly closed by having a reasonable margin of difference between the operating and set pressures. The design of the system governs the operating and set pressures, and references to the guidelines are found in ASME VIII.
8.5 API 576 section 6: inspection and testing
This section contains the main inspection and test activities traditionally used on PRVs. It describes (in roughly chronological order) the stages shown in Fig. 8.7.
The Purpose of all theas tests is to minimize the chance of:
- Failure of the pressure boundary (or envelope)
- Failure to lift at the correct prassure
- Leakage across the seat in Service
- Failure to reseat after lifting
Figure 8.7 PRV inspection and test activities
8.6 API 576 familiarization questions