Why use safety valves according to API STD 526
The need for use, selection, sizing, specification and, lastly, the purchase of valves usually arise within a multidisciplinary project; where several other very important instruments and equipments are allocating human resources that may even be working on parallel and independent projects and almost always with very lean HH. The result of this combination of factors is little time to sizing and specify a valve, be it for control or safety. This leads many engineers not to question why they adopt some requirements, especially when it is clear in the standard of the final customer that you have to follow it. When it comes to adopting API STD 526 in relation to the specification of safety valves, it is not very different.
Petrobras, which is one of the biggest motivating companies for the development of engineering in Brazil, has defined in its instrumentation standard N-1882D - Criteria for Elaboration of Instrumentation Projects; that safety valves must, among several requirements:
- Have the discharge orifices according to API STD 526;
- Be selected and sized according to API STD 520 Pt 1;
- Have its tightness in accordance with API STD 527;
- Have its upstream and downstream pipeline installation project according to API RP 520 Pt 2.
API STD 526 requires the same above mentioned, so it´s clear that N-1882 was elaborated using as a basis, the API STD 526 guidelines and the adoption of this constructive standard proves to be practical since it meets at least four items of the N-1882D. This generated a culture within Brazilian engineering to adopt API STD 526 as a requirement in the design of the safety valve at least as a recommended practice.
The purpose of this short article is to present what the API 526 is about; why we must adopt and use this standard and also, because we must not follow blindly the requirements and guidelines of this standard. By reading this article you will be able to answer the following questions:
- What is the scope of API STD 526?
- What defines API 526 in relation to safety valves?
- What are the advantages of adopting and using API STD 526?
- Is a valve according to API STD 526, better than a valve that does not follow this construction standard?
API STD 526 is a standard with the objective of guiding engineers, manufacturers, end users and buyers in the selection, specification, manufacturing standardization and purchase of safety valves. These valves can be operated by spring with or without balancing bellows (conventional or self-operated); or operated by a pilot valve (pilot-operated).
Figure 1 - left: Spring-operated safety valve according to API STD 526; right: Series 821 pilot-operated safety valve (Courtesy Leser)
API STD 526 defines minimum criteria as:
- The orifice discharge area and its
- The distances between centers of the flanges and the faces of the opposite flanges;
- The dimensions and pressure class of the valve, inlet and outlet connections;
- The materials as function of pressure and temperature during relief;
- Pressure and temperature limits;
All valves according to API STD 526 must have inlet
and outlet flanged connections in accordance with ASME B16.5. The pressure and
temperature limits are defined in tables 3 to 16 for spring-operated valves and
in tables 17 to 30 for pilot-operated valves. It is worth noting that, even
though the flanges are in accordance with ASME B16.5; we must not adopt the
pressure x temperature limits of this standard, because these limits are higher
than those defined by API STD 526.
The use of
API STD 526 criteria for the constructive design of safety valves has some
It refers to the possibility of exchanging the safety valve of one manufacturer for another without compromising the protection of the equipment or requiring greater interference in the equipment or pipeline. This is possible for a few reasons.
The first one is the standardization of the discharge orifice areas, which defines maximum flow capacity values very close even for different manufacturers. The orifices are designated by letters that start in "D" and go up to "T". If one manufacturer “A” presents as a protection solution for an equipment, a safety valve 1D2 (one inch for inlet connection, orifice D and two inches for outlet connection) and manufacturer “B” presents another valve 1D2; we can say that “A” and “B” have interchangeable solutions.
Another reason that allows interchangeability is that API STD 526 also defines the distance from the face of the input flange to the center of the output flange; the distance between the face of the outlet flange and the center of the inlet flange and the connection patterns of the inlet and outlet flanges including the diameters and pressure classes themselves. In this way, it is possible to replace one valve with another with practically zero impacts on the piping and on existing equipment. Example: To replace a 1D2 valve from manufacturer A with a 1D2 valve from manufacturer B, there is no need to change flanges or pipe rearrangements.
Interchangeability also brings the benefit to the end user not being dependent on a single supplier that could, for different reasons, stop its commercial operations in a country, complicating or stopping the supply of valves or spare parts.
It is worth noting that, different manufacturers supplying interchangeable valves for the same application; may have different maximum certified discharge flow rates. The discharge orifice area calculations for API STD 526 valves are according to API STD 520 Part 1, but these are preliminary calculations that use recommended discharge coefficient (Kd) in the equations described in the items 5.6 to 5.10 of API STD 520 Part 1 as a starting point for making calculations possible. Each manufacturer has its certified discharge coefficient. Fortunately, most manufacturers have a certified discharge coefficient lower than that determined by the standard, which results in an area larger than the effective one defined by API STD 526. However, as not all manufacturers are; it is prudent then, when purchasing the valve, the instrumentation engineer requires the sizing report with the value of the maximum discharge flow certified from the manufacturer that is selling the solution.Table 1 - Designation of the orifices according to API STD 526 and their respective areas
All valves according to API STD 526 must have inlet and outlet flanged connections in accordance with ASME B16.5. The pressure and temperature limits are defined in tables 3 to 16 for spring-operated valves and in tables 17 to 30 for pilot-operated valves. It is worth noting that, even though the flanges are in accordance with ASME B16.5; we must not adopt the pressure x temperature limits of this standard, because these limits are higher than those defined by API STD 526.
Figure 2 - Distances from the center of the flanged connections to the opposite faces “a” and “b” standardized by API STD 526 - Courtesy Leser
API STD 526 defines the materials that must be used in the main parts of the safety valve such as body, bonnet, spring, flange and nozzle depending on the temperature and pressure that the valve is subjected at the opening moment.
Materials that are normally used in the manufacture of valves of all types such as carbon steel ASME SA-216 Gr. WCB and stainless steel ASME SA-351 Gr. CF8M are placed as minimum requirements for safety valves to be used in certain parts of the valve like body and the bonnet. This guarantees valves with similar performance, weight, mechanical strength and corrosion resistance from different manufacturers.
The requirements above does not restrain any supplier from adopte another material to produce his valve, that for many reasons, may be economically more attractive, since, it has higher pressure x temperature limits than those materials standardized by API STD 526. After all, the Instrumentation Engineer has to evaluate any material used regarding chemical compatibility with the process fluid.
API STD 526 also establishes that valves manufactured in accordance with this standard, must also comply with API STD 527. This last standard deals with the seat lekage while the valve is closed; or in other words; while the pressure scenario in the vessel or pipe is normal. All valves from all suppliers that will be involved in a purchasing process will have the same performance related to the seat leakage.
Although API STD 526 standardizes important parts of the valve, it does not defines all parts. It is still possible to find opportunities for innovation to deliver to the user, the best cost-benefit solution without neglecting the pressurized system protection that is so important.
The scope of the standard makes it clear that API STD 526 is a standard that should be used to purchase a safety and/or relief valve and defines minimum criteria to be met in order to be successful.
By standardizing all these criteria and requiring all suppliers of safety valves to produce a large amount of valves in accordance with API STD 526, a favorable scenario for bidding is created, cause the solutions become very similar. This way, the manufacturers will compete to get the lowest price.
It also facilitates the analysis of the engineer who always spends a good HH with the technical advice of the valve supply proposals. With a wide standardization, the time and cost necessary to evaluate, adjust and approve technical solutions is reduced. The standard itself presents in its appendix A, a data sheet containing the minimum information that must be specified in order to allow a technical and commercial consultation to manufacturers for the supply of spring and pilot operated safety valves. Annex B shows the minimum information that must appear on the nameplate.
Reduction of delivery time
Standardization allows the manufacture and strategic stock of nozzles, flanges, bonnets, bellows, caps and other components. In this way, it´s possible to assemble, test and ship the safety valve in the shortest possible time. It is not always necessary to cast parts or make more complicated adjustments that are not foreseen in the standard.
Not always API STD 526
For small equipments such as pump sealing systems, pump recycles, air tanks for control valves, pig receivers and launchers, among others, that have a small volume of fluid to be relieved to normalize their pressure; using compact small valves is advantageous as they are easier to install. Usually threaded with ¾ ”NPT inlet and 1” NPT outlet, they are cheaper and have a discharge orifices with areas smallers than the D equivalent of API STD 526, which avoids making use of a oversized valve just to meet a construction standard. Oversized pressure safety valve can lead the valve to chattering, as the flow may be too small to be able to open the safety valve at once and the disc will again hit the inlet nozzle seat repeatedly, damaging the assembly and compromising the tightness or worse, compromising performance expected at the time of opening.
With simple design and with fewer parts, the safety valves for thermal relief (or compact), allows a good cost reduction. Since others legal requirements such as the National Board flow certificate and / or the ASME authorization certificate for the manufacturing site be part of the supply documentation; there is no greater cause for concern for the end user.
If the user still wants to replace an existing safety valve that does not comply with API STD 526, with another one according to API STD 526, he will have to evaluate whether the installation follows the recommendations of API RP 520 Pt 2. This evaluation that already has a cost, can lead to the conclusion that the pipes and connections need to be replaced and / or rearranged which would mean even more costs.
If costs is always an important factor in the projects; we can also verify that a valve according to API STD 526 does not always present the best cost benefit for the customer.
There are some good reasons for large companies to adopt API STD 526, if not as a requirement, at least as a recommended practice. All the criteria to be followed and the advantages presented, make API STD 526 kind of a guide to acquire a safety valve with the best cost benefit. It should not be forgotten, however, that the ultimate purpose of a safety valve is to open to guarantee the relief of a system that is in a pressurization scenario and to close when the situation is normalized. The construction according to API STD 526 does not replace the correct sizing and the need for ASME authorization certificates for the factory and the National Board certificate regarding the flow capacity of the tested model for the manufactured site.
Hamilton Maranhão – Instrumentation Engineer
Salvador - Brazil; July 2020
API STANDARD 526 Flanged Steel Pressure Relief
Valves SIXTH EDITION, APRIL 2009
API RP 520 Part I - Sizing and Selection EIGHTH EDITION, DECEMBER 2008
N-1882 D - Criteria for Elaboration of Instrumentation Projects