boiler safety valve setting formula pdf quotation

Boiler explosions have been responsible for widespread damage to companies throughout the years, and that’s why today’s boilers are equipped with safety valves and/or relief valves. Boiler safety valves are designed to prevent excess pressure, which is usually responsible for those devastating explosions. That said, to ensure that boiler safety valves are working properly and providing adequate protection, they must meet regulatory specifications and require ongoing maintenance and periodic testing. Without these precautions, malfunctioning safety valves may fail, resulting in potentially disastrous consequences.

Boiler safety valves are activated by upstream pressure. If the pressure exceeds a defined threshold, the valve activates and automatically releases pressure. Typically used for gas or vapor service, boiler safety valves pop fully open once a pressure threshold is reached and remain open until the boiler pressure reaches a pre-defined, safe lower pressure.

Boiler relief valves serve the same purpose – automatically lowering boiler pressure – but they function a bit differently than safety valves. A relief valve doesn’t open fully when pressure exceeds a defined threshold; instead, it opens gradually when the pressure threshold is exceeded and closes gradually until the lower, safe threshold is reached. Boiler relief valves are typically used for liquid service.

There are also devices known as “safety relief valves” which have the characteristics of both types discussed above. Safety relief valves can be used for either liquid or gas or vapor service.

Nameplates must be fastened securely and permanently to the safety valve and remain readable throughout the lifespan of the valve, so durability is key.

The National Board of Boiler and Pressure Vessel Inspectors offers guidance and recommendations on boiler and pressure vessel safety rules and regulations. However, most individual states set forth their own rules and regulations, and while they may be similar across states, it’s important to ensure that your boiler safety valves meet all state and local regulatory requirements.

The National Board published NB-131, Recommended Boiler and Pressure Vessel Safety Legislation, and NB-132, Recommended Administrative Boiler and Pressure Vessel Safety Rules and Regulationsin order to provide guidance and encourage the development of crucial safety laws in jurisdictions that currently have no laws in place for the “proper construction, installation, inspection, operation, maintenance, alterations, and repairs” necessary to protect workers and the public from dangerous boiler and pressure vessel explosions that may occur without these safeguards in place.

The American Society of Mechanical Engineers (ASME) governs the code that establishes guidelines and requirements for safety valves. Note that it’s up to plant personnel to familiarize themselves with the requirements and understand which parts of the code apply to specific parts of the plant’s steam systems.

High steam capacity requirements, physical or economic constraints may make the use of a single safety valve impossible. In these cases, using multiple safety valves on the same system is considered an acceptable practice, provided that proper sizing and installation requirements are met – including an appropriately sized vent pipe that accounts for the total steam venting capacity of all valves when open at the same time.

The lowest rating (MAWP or maximum allowable working pressure) should always be used among all safety devices within a system, including boilers, pressure vessels, and equipment piping systems, to determine the safety valve set pressure.

Avoid isolating safety valves from the system, such as by installing intervening shut-off valves located between the steam component or system and the inlet.

Contact the valve supplier immediately for any safety valve with a broken wire seal, as this indicates that the valve is unsafe for use. Safety valves are sealed and certified in order to prevent tampering that can prevent proper function.

Avoid attaching vent discharge piping directly to a safety valve, which may place unnecessary weight and additional stress on the valve, altering the set pressure.

but not more than 3% above working pressure. Also the safety Valve re-sits at 5% below working pressure. There are 2 safety valves. Both the safety valves are to be set at a pressure not exceeding 3% of the normal working pressure (stamped on name plate). Let us say the working pressure of the boiler is 7 bar. 3% of 7 bar is (7+0.21) = 7.21 bar. which means, both the safety valves have to be set at a pressure not exceeding 7.21 bar. Apart from that, let us say one of the valves have been set at a pressure say 7.15 bar (assuming two digit decimal even though the practice is not referring to 2 digits), then the other valve must be set at a pressure range of 10% within this. i.e the other valve setting must be 7.16 bar. I used two digit decimal only for understanding the precision of calculation. The second safety valve must be set within 10 % of the setting of the first safety valve. If any one has any other understanding please do discuss.

A safety valve is a valve that acts as a fail-safe. An example of safety valve is a pressure relief valve (PRV), which automatically releases a substance from a boiler, pressure vessel, or other system, when the pressure or temperature exceeds preset limits. Pilot-operated relief valves are a specialized type of pressure safety valve. A leak tight, lower cost, single emergency use option would be a rupture disk.

Safety valves were first developed for use on steam boilers during the Industrial Revolution. Early boilers operating without them were prone to explosion unless carefully operated.

Vacuum safety valves (or combined pressure/vacuum safety valves) are used to prevent a tank from collapsing while it is being emptied, or when cold rinse water is used after hot CIP (clean-in-place) or SIP (sterilization-in-place) procedures. When sizing a vacuum safety valve, the calculation method is not defined in any norm, particularly in the hot CIP / cold water scenario, but some manufacturers

The earliest and simplest safety valve was used on a 1679 steam digester and utilized a weight to retain the steam pressure (this design is still commonly used on pressure cookers); however, these were easily tampered with or accidentally released. On the Stockton and Darlington Railway, the safety valve tended to go off when the engine hit a bump in the track. A valve less sensitive to sudden accelerations used a spring to contain the steam pressure, but these (based on a Salter spring balance) could still be screwed down to increase the pressure beyond design limits. This dangerous practice was sometimes used to marginally increase the performance of a steam engine. In 1856, John Ramsbottom invented a tamper-proof spring safety valve that became universal on railways. The Ramsbottom valve consisted of two plug-type valves connected to each other by a spring-laden pivoting arm, with one valve element on either side of the pivot. Any adjustment made to one of valves in an attempt to increase its operating pressure would cause the other valve to be lifted off its seat, regardless of how the adjustment was attempted. The pivot point on the arm was not symmetrically between the valves, so any tightening of the spring would cause one of the valves to lift. Only by removing and disassembling the entire valve assembly could its operating pressure be adjusted, making impromptu "tying down" of the valve by locomotive crews in search of more power impossible. The pivoting arm was commonly extended into a handle shape and fed back into the locomotive cab, allowing crews to "rock" both valves off their seats to confirm they were set and operating correctly.

Safety valves also evolved to protect equipment such as pressure vessels (fired or not) and heat exchangers. The term safety valve should be limited to compressible fluid applications (gas, vapour, or steam).

For liquid-packed vessels, thermal relief valves are generally characterized by the relatively small size of the valve necessary to provide protection from excess pressure caused by thermal expansion. In this case a small valve is adequate because most liquids are nearly incompressible, and so a relatively small amount of fluid discharged through the relief valve will produce a substantial reduction in pressure.

Flow protection is characterized by safety valves that are considerably larger than those mounted for thermal protection. They are generally sized for use in situations where significant quantities of gas or high volumes of liquid must be quickly discharged in order to protect the integrity of the vessel or pipeline. This protection can alternatively be achieved by installing a high integrity pressure protection system (HIPPS).

In the petroleum refining, petrochemical, chemical manufacturing, natural gas processing, power generation, food, drinks, cosmetics and pharmaceuticals industries, the term safety valve is associated with the terms pressure relief valve (PRV), pressure safety valve (PSV) and relief valve.

The generic term is Pressure relief valve (PRV) or pressure safety valve (PSV). PRVs and PSVs are not the same thing, despite what many people think; the difference is that PSVs have a manual lever to open the valve in case of emergency.

Relief valve (RV): an automatic system that is actuated by the static pressure in a liquid-filled vessel. It specifically opens proportionally with increasing pressure

Pilot-operated safety relief valve (POSRV): an automatic system that relieves on remote command from a pilot, to which the static pressure (from equipment to protect) is connected

Low pressure safety valve (LPSV): an automatic system that relieves static pressure on a gas. Used when the difference between the vessel pressure and the ambient atmospheric pressure is small.

Vacuum pressure safety valve (VPSV): an automatic system that relieves static pressure on a gas. Used when the pressure difference between the vessel pressure and the ambient pressure is small, negative and near to atmospheric pressure.

Low and vacuum pressure safety valve (LVPSV): an automatic system that relieves static pressure on a gas. Used when the pressure difference is small, negative or positive and near to atmospheric pressure.

In most countries, industries are legally required to protect pressure vessels and other equipment by using relief valves. Also, in most countries, equipment design codes such as those provided by the ASME, API and other organizations like ISO (ISO 4126) must be complied with. These codes include design standards for relief valves and schedules for periodic inspection and testing after valves have been removed by the company engineer.

Today, the food, drinks, cosmetics, pharmaceuticals and fine chemicals industries call for hygienic safety valves, fully drainable and Cleanable-In-Place. Most are made of stainless steel; the hygienic norms are mainly 3A in the USA and EHEDG in Europe.

The first safety valve was invented by Denis Papin for his steam digester, an early pressure cooker rather than an engine.steelyard" lever a smaller weight was required, also the pressure could easily be regulated by sliding the same weight back and forth along the lever arm. Papin retained the same design for his 1707 steam pump.Greenwich in 1803, one of Trevithick"s high-pressure stationary engines exploded when the boy trained to operate the engine left it to catch eels in the river, without first releasing the safety valve from its working load.

Although the lever safety valve was convenient, it was too sensitive to the motion of a steam locomotive. Early steam locomotives therefore used a simpler arrangement of weights stacked directly upon the valve. This required a smaller valve area, so as to keep the weight manageable, which sometimes proved inadequate to vent the pressure of an unattended boiler, leading to explosions. An even greater hazard was the ease with which such a valve could be tied down, so as to increase the pressure and thus power of the engine, at further risk of explosion.

Although deadweight safety valves had a short lifetime on steam locomotives, they remained in use on stationary boilers for as long as steam power remained.

Weighted valves were sensitive to bouncing from the rough riding of early locomotives. One solution was to use a lightweight spring rather than a weight. This was the invention of Timothy Hackworth on his leaf springs.

These direct-acting spring valves could be adjusted by tightening the nuts retaining the spring. To avoid tampering, they were often shrouded in tall brass casings which also vented the steam away from the locomotive crew.

The Salter coil spring spring balance for weighing, was first made in Britain by around 1770.spring steels to make a powerful but compact spring in one piece. Once again by using the lever mechanism, such a spring balance could be applied to the considerable force of a boiler safety valve.

The spring balance valve also acted as a pressure gauge. This was useful as previous pressure gauges were unwieldy mercury manometers and the Bourdon gauge had yet to be invented.

Paired valves were often adjusted to slightly different pressures too, a small valve as a control measure and the lockable valve made larger and permanently set to a higher pressure, as a safeguard.Sinclair for the Eastern Counties Railway in 1859, had the valve spring with pressure scale behind the dome, facing the cab, and the locked valve ahead of the dome, out of reach of interference.

In 1855, John Ramsbottom, later locomotive superintendent of the LNWR, described a new form of safety valve intended to improve reliability and especially to be tamper-resistant. A pair of plug valves were used, held down by a common spring-loaded lever between them with a single central spring. This lever was characteristically extended rearwards, often reaching into the cab on early locomotives. Rather than discouraging the use of the spring lever by the fireman, Ramsbottom"s valve encouraged this. Rocking the lever freed up the valves alternately and checked that neither was sticking in its seat.

A drawback to the Ramsbottom type was its complexity. Poor maintenance or mis-assembly of the linkage between the spring and the valves could lead to a valve that no longer opened correctly under pressure. The valves could be held against their seats and fail to open or, even worse, to allow the valve to open but insufficiently to vent steam at an adequate rate and so not being an obvious and noticeable fault.Rhymney Railway, even though the boiler was almost new, at only eight months old.

Naylor valves were introduced around 1866. A bellcrank arrangement reduced the strain (percentage extension) of the spring, thus maintaining a more constant force.L&Y & NER.

All of the preceding safety valve designs opened gradually and had a tendency to leak a "feather" of steam as they approached "blowing-off", even though this was below the pressure. When they opened they also did so partially at first and didn"t vent steam quickly until the boiler was well over pressure.

The quick-opening "pop" valve was a solution to this. Their construction was simple: the existing circular plug valve was changed to an inverted "top hat" shape, with an enlarged upper diameter. They fitted into a stepped seat of two matching diameters. When closed, the steam pressure acted only on the crown of the top hat, and was balanced by the spring force. Once the valve opened a little, steam could pass the lower seat and began to act on the larger brim. This greater area overwhelmed the spring force and the valve flew completely open with a "pop". Escaping steam on this larger diameter also held the valve open until pressure had dropped below that at which it originally opened, providing hysteresis.

These valves coincided with a change in firing behaviour. Rather than demonstrating their virility by always showing a feather at the valve, firemen now tried to avoid noisy blowing off, especially around stations or under the large roof of a major station. This was mostly at the behest of stationmasters, but firemen also realised that any blowing off through a pop valve wasted several pounds of boiler pressure; estimated at 20 psi lost and 16 lbs or more of shovelled coal.

Pop valves derived from Adams"s patent design of 1873, with an extended lip. R. L. Ross"s valves were patented in 1902 and 1904. They were more popular in America at first, but widespread from the 1920s on.

Although showy polished brass covers over safety valves had been a feature of steam locomotives since Stephenson"s day, the only railway to maintain this tradition into the era of pop valves was the GWR, with their distinctive tapered brass safety valve bonnets and copper-capped chimneys.

Developments in high-pressure water-tube boilers for marine use placed more demands on safety valves. Valves of greater capacity were required, to vent safely the high steam-generating capacity of these large boilers.Naylor valve) became more critical.distilled feedwater and also a scouring of the valve seats, leading to wear.

High-lift safety valves are direct-loaded spring types, although the spring does not bear directly on the valve, but on a guide-rod valve stem. The valve is beneath the base of the stem, the spring rests on a flange some height above this. The increased space between the valve itself and the spring seat allows the valve to lift higher, further clear of the seat. This gives a steam flow through the valve equivalent to a valve one and a half or twice as large (depending on detail design).

The Cockburn Improved High Lift design has similar features to the Ross pop type. The exhaust steam is partially trapped on its way out and acts on the base of the spring seat, increasing the lift force on the valve and holding the valve further open.

To optimise the flow through a given diameter of valve, the full-bore design is used. This has a servo action, where steam through a narrow control passage is allowed through if it passes a small control valve. This steam is then not exhausted, but is passed to a piston that is used to open the main valve.

There are safety valves known as PSV"s and can be connected to pressure gauges (usually with a 1/2" BSP fitting). These allow a resistance of pressure to be applied to limit the pressure forced on the gauge tube, resulting in prevention of over pressurisation. the matter that has been injected into the gauge, if over pressurised, will be diverted through a pipe in the safety valve, and shall be driven away from the gauge.

There is a wide range of safety valves having many different applications and performance criteria in different areas. In addition, national standards are set for many kinds of safety valves.

Safety valves are required on water heaters, where they prevent disaster in certain configurations in the event that a thermostat should fail. Such a valve is sometimes referred to as a "T&P valve" (Temperature and Pressure valve). There are still occasional, spectacular failures of older water heaters that lack this equipment. Houses can be leveled by the force of the blast.

Pressure cookers usually have two safety valves to prevent explosions. On older designs, one is a nozzle upon which a weight sits. The other is a sealed rubber grommet which is ejected in a controlled explosion if the first valve gets blocked. On newer generation pressure cookers, if the steam vent gets blocked, a safety spring will eject excess pressure and if that fails, the gasket will expand and release excess pressure downwards between the lid and the pan. Also, newer generation pressure cookers have a safety interlock which locks the lid when internal pressure exceeds atmospheric pressure, to prevent accidents from a sudden release of very hot steam, food and liquid, which would happen if the lid were to be removed when the pan is still slightly pressurised inside (however, the lid will be very hard or impossible to open when the pot is still pressurised).

Is your Section 1 Boiler Safety Valve Body Wall Eroding?With many of these valves being original installation, making it is possible that  75%of all current Boiler valves were installed in the 1960’s*, theanswer is probably yes.

Body wall deterioration can occur over time in any process, and is an often overlooked factor in the review and repair of a safety valve population. Other issues related to the body wall thickness factor are that the availability of old design parts is declining, and the nozzle height of older models is different than what is available today.

The National Board Inspection Code does not address the wall thickness of Safety Valves as a component of the VR Program. This could lead to failure if the wall thickness is not sufficient for the pressure / reaction force that is acting on it. If this feature is never checked in accordance with a standard, it could prove to be the feature that causes a failure.

Additionally, inspecting the body wall thickness of your safety valve population helps to minimize the risk associated  with the current long lead time for new Safety Valves. Anticipating outages before they can occur helps to position end users to be their most competitive.

ASME B16.34, Table 3B showcases the ASME Valve Body Minimum Wall Thickness* calculations per diameter and pressure class.  Utilizing this table will help better inform the end user of the state of their valve body wall, leading to prevention of a failure.

As the experts in the service, maintenance, and repair of Safety Valves, Setpoint Integrated Solutions stands by as a trusted partner to inspect your safety valve population. We look forward to helping you identify issues before they occur while improving safety and mitigating costly downtime.

*Citation of ASME 6.1.1 of B16.34-2013 regarding Wall Thickness, which states: “For inspection purposes the wall thickness of valve bodies at the time of manufacture, except as indicated in paras. 6.1.3 through 6.1.7, 6.2, and 6.7, shall be no less than the minimum values “tm”either as shown in table 3A or table 3B or calculated using the equation shown in Mandatory Appendix VI (which yield essentially the same result). Linear interpolation may be used for wall thickness values intermediate to those listed or calculated. See Nonmandatory appendix B, section B-5 for an explanation of the interpolation procedure. The minimum thickness requirement for the body wall is applicable only as measured from internal walled surfaces. Minimum wall thickness shall not include liners, linings, or cartridges.

A blowdown valve’s main function is to control a continuous or intermittent flow of steam or fluid under high differential pressure. When installed in a system, they drain solid contaminants from the fluid. In this article, you will learn the working principle of a blowdown valve, review a blowdown valve diagram, and its use in boiler and compressor applications.

In industry, blowdown valves often attach to equipment where water is the working fluid. Usually, this water contains suspended solid impurities. As mechanisms such as vaporization or drafting occur in the system, the concentration of undissolved solids increases and hampers system performance. How? Some of these deposits may accumulate and form blockages that disrupt the flow. Also, the solids could deposit on the surface of the equipment and impact heat transfer. This reduces the efficiency of heat exchangers and designed cooling measures. Thus, blowdown valves along drain lines allow for the removal of these contaminants.

It is a common practice to use two blowdown valves in series. One acts as the seal valve, while the other is the main blowdown valve. Customarily, the seal valve opens first when draining, and closes last. However, to minimize erosion of the valve disk faces and seats, both may open simultaneously and rapidly.

In addition, care should be taken to avoid trapping scale and rust particles within the valve by briefly reopening it, after it has been shut. Especially if there was resistance when closing it initially. One important parameter in the operation of blowdown valves is the blowdown percentage. It refers to the ratio of the quantity of blowdown water (Vblowdown) to that of the feedwater (Vfeed) as the formula below shows.

The classification of blowdown valves depends on either the valve location or working interval. Generally, blowdown valves are installed at the surface or the bottom of the equipment, depending on the speed of solid impurities precipitation. Also, the valve may operate continuously or intermittently. Usually, surface blowdown valves operate in steady state, while bottom blowdown valves work intermittently.

This is suitable for applications where the rate of solid impurities precipitation is relatively slower. For a simple design of this type of valve, a pipe inserts near the surface of the water level. Then, water, along with impurities, goes through the pipe continuously as the valve is normally in the open position.

On the other hand, a more sophisticated design uses a swivel joint with a short length of pipe suspended on a float. Thus, it removes oil floating on the water surface. Typically, surface blowdown valves find use in equipment where a significant amount of vaporization exists. This is because as vaporization occurs, the contaminants, precipitate and remain on the water surface. Also, the outlet of these valves often feeds into a flash tank and provides heating for heat exchangers.

As the name implies, this type of blowdown valves installs at the bottom of the equipment. They are opened periodically to enable the evacuation of accumulated solid impurities and sludge. Unlike the surface blowdown valve, this type does not operate in a steady state. This is because prolonged opening decreases the water level quickly, thereby risking a shutdown of the equipment. A basic requirement of this valve is to provide tight shut-off even after repetitive blowdown operations. Also, the drainpipe diameter should be large enough so that the slug does not clog and block the flow.

The diagram above is that of a manually operated blowdown valve. Of importance, the orifice maintains fluid velocity below levels that could damage the trim. Also, the stem mates to the orifice for proper control, while the open yoke enables the operator to see the position of the plunger in the valve body. A long stroke length of the stem enables the prevention of water hammer, which occurs if the valve is opened or closed too quickly. At the exit, the angle of the orifice is intentionally made divergent, to minimize downstream piping erosion and noise.

Blowdown valves are common in boiler and compressor systems. In compressors, they serve to depressurize the gas in the system at critical times such as shutdown, restart, or in the case of an emergency. In boilers, they see more frequent use, where there could be both bottom blowdown valves, and a surface valve, in some cases.

Generally, in boilers, blowdown valves remove both suspended solids and sludge from the surface and bottom respectively. As a result, it prevents the foaming at the water surface which leads to unstable water levels and excessive passing on of liquid in the steam. When blowdown water leaves a boiler, it does so at high temperatures, creating a safety concern. For example, a boiler working at 100 psig typically discharges around 338 ℉. Thus, engineers must ensure controlled discharge occurs into a flash tank prior to disposal into drainage. Or, engineers may repurpose the heat elsewhere within the facility, perhaps to increase feedwater temperature.

**Update to this article, June 7, 2022 : If you found this article helpful, here is a link to another article I recently found that does a nice job explaining the topic: ENGINEERS BEWARE: API vs ASME Relief Valve Orifice Size – Petro Chem Engineering (petrochemengg.com)

If you"ve ever sized/selected a Pressure Safety-Relief Valve (PSV) using vendor sizing programs or good-old hand calculations, you"ve probably run into a very strange anomaly: Why does a PSV orifice size change between American Petroleum Institute (API) and American Society of Mechanical Engineers (ASME) data sets? What is an "effective" orifice area? How do I know which standard to use when selecting a PSV?

Usually, this issue is one of curiosity and doesn"t affect the end result of what valve is chosen. Common practice is to default to API sizing equations and parameters, and only use ASME data sets for situations outside of the API letter-designations. But what if I told you that approach is likely causing you to oversize about 10% of your PSVs and their respective piping systems?

Too often, we leave that third part out of the process, and simply calculate relief loads and select valves using API techniques, without ever checking our selection against certified ASME data. Proper application of these standards is the first key point of this article:

Initial sizing and valve selection is done using API equations, and final valve selection and certification is done using ASME-certified coefficients and capacities.

The BPVC is an enormous code, and not reviewed in detail here. On the subject of PSVs, it basically says that a PSV must be capable of relieving the required load, and it must be tested in a specific manner to be certified to do so. If a valve is tested per the specific directions in the BPVC, it will be ASME certified and receive an ASME UV stamp.

The first thing API does is attempt to standardize physical PSV sizes and design, and it does so in API RP 526, which is targeted at PSV manufacturers. API provides pre-defined valve sizes, with letter designations D through T (API 526). It also defines other details directed toward valve manufacturers (such as temperature ratings). All of this is intended as minimum design standards, and manufacturers are free to exceed these parameters as they wish.

The second thing API does is provide standardized equations and parameters to use when trying to figure out just what size of a PSV one needs for a particular scenario. The equations account for design parameters that ASME doesn"t speak to, such as specific fluid properties, backpressures, critical flow, two-phase flow, and many other aspects of fluid dynamics that will affect the ability of a particular valve to relieve a required load.

API sizing equations are by nature theoretical, standardized, and use default or "dummy" values for several sizing parameters that may or may not reflect the actual values for any specific valve.

API RP 520 very clearly talks about this, and emphasizes that the intended use of its equations is to determine a preliminary valve size, which should be verified with actual data. API intends PSV sizing to be a two-step process, but we are often unaware of this because we (gasp) don’t read the full standard, and/or rely solely on vendor sizing software that hides the iteration from us. See API 520, part 1, section 5.2 for further explanation.

When valves are built, they are built to the API RP 526 standard, however, as one might imagine, when valves are actually tested and certified, the results don’t match up identically to the theoretical values that were calculated. This is where API and ASME intersect; we switch from calculations (API) which were used as a basis to design the valve, to actual empirical data (ASME) to certify the valve. When a valve manufacturer gets the UV code stamp that certifies the valve orifice size and capacity, it is based on actual test results, not API sizing standards. And ASME (which came first) does not have tiered letter designations. The typical D, E, F, etc. sizes we refer to are strictly an API tool, and ASME’s capacity certifications are completely independent of them!

3. A third-party Engineer (you), trying to select a PSV, runs a sizing calculation using API 520 equations on ABC Valve Company"s sizing software, gets a result that requires 4.66in2 to relieve the load, and is now thoroughly confused on what size valve to select.

If one selects the API data set on the sizing software in this example, it will automatically eliminate N-orifice valves as an option, and bump the user up to a P-orifice. However, if one simply selects the ASME data set, the N-orifice valve magically reappears as an option. How can this be? Will the N-orifice work or not?

Digest that for a moment. If you’ve sized and purchased more than a dozen PSVs, chances are you have inadvertently selected a PSV a full size larger than you needed to, in a situation much like our example, simply because you chose a PSV based on its API “rating” rather than its real, certified, stamped ASME rating. If that was a small valve, impact was probably nil. But what if this happened on a valve that resulted in selecting a 8x10 PSV when you could have used a 6x8?

If you’re like me, that answer isn’t very satisfying. Why on earth is this so confusing? How can you simply hit a button on the sizing program and a different size of valve is suddenly acceptable? The key lies how the main coefficient of discharge, Kd, is handled, and how capacities are determined.

There are several K values used in API calculations, all of which have generic values defined in API 520 that can be used for preliminary sizing. These are the numbers used in initial sizing calculations to get us close, then (if we do this correctly) replaced with the actual/tested/empirical/ASME values when we get a certified valve. Remember, anytime you hear “certified” or “stamped”, think ASME.

Let’s take the numbers from the example above, which came from an attempt to size a valve for liquid relief. API says to use a value of Kd=0.65 for liquid relief. If one uses the API data set on the vendor software, then the calculation stops here, and you get a required area of 4.66in2. When you select a valve, you’re comparing that to the API effective (actual) area of an N orifice, which is 4.34 in2, which is obviously too small and you’d logically step up to a P orifice. However….

Remember that the API N-orifice area is just the benchmark, a minimum requirement, and may or may not (most likely not) reflect the actual area of a real-life PSV. Once a valve is selected, all of those K values and capacities should be replaced with actual ASME-certified K values, also determined by testing, that are specific to each valve model, and the calculations performed again.

Normally, ASME-certified K-values are smaller than the API dummy values, driving up the required orifice area. So valve manufacturers have to over-design their valves to make up for it, resulting in ASME-certified areas and capacities that typically exceed the benchmark API ones. The end result of all this?

It (almost) all boils down to one sneaky little sentence in the ASME BPVC which mandates a 10% safety factor on the empirically-determined Kd that “de-rates” the valve (see ASME BPVC Section VIII, UG-131.e.2). This tidbit seems to be a little-known fact that is key to proper PSV sizing and selection, because as engineers we often pile safety factors upon each other and oversize our equipment. I cannot highlight this enough:

I mentioned above that ASME K values are nearly always lower than API values, due to this 10% de-rating. The PSV in our example scenario has a determined Kd of 0.73, which is adjusted down by 10% for a final AMSE Kd of 0.66, slightly higher than the dummy API value (that just means that this particular valve proved it could do about 11% better than the minimum theoretical flow calculated by API when it was tested). So, for our valve in question, the Required ASME area is slightly less than the API area. This is atypical, but not unheard of, and again points to the importance of checking the ASME ratings of any valve you select, and comparing against the API benchmarks.

When you choose to use the ASME data on a specific valve, it’s not just the Kd sizing factor that changes; the actual orifice area and therefore the capacity of the valve also adjusts to empirical, certified values. You can generally expect both values to increase over the API values.

Why is this? Simply that any given real-world valve is usually over-designed so that it will meet and exceed the required minimum capacity of its corresponding API size. What a simple concept, but so often overlooked by engineers!

*Note: this is data from a real case; the specific PSV make/model is omitted. Did you catch the result? The actual, certified capacity of this valve is nearly 13% higher than the generic N-orifice valve, and that includes its 10% safety factor!

With this adjusted orifice area, we can compare to the ASME certified area (which is always going to be larger than the API area), and we have our final answer for the valve size. Often this will not result in a different choice of valve, but sometimes, as in the example case, it will allow us to use a valve with an API letter designation that did not appear large enough based on its API effective area. This can save time and money for our plants by preventing over-sizing valves, leading to smaller piping systems to support them. And remember, the ASME values are empirical and have a 10% safety factor built in, so we don’t need to worry about cutting the design too close; the conservatism is already built in to the method. We can choose the Brand X N-orifice valve and sleep well at night!

Avoid simply defaulting to the API data set for the final “rating” or data sheet when selecting a PSV. Use API sizing calculations as they are intended: for preliminary valve selection. Then switch to the ASME data set. This will often (but not always, remember, it"s valve-specific) result in two differences: