In order to establish the set pressure correctly, the following terms require careful consideration:
For more information, please visit CHT TECK.
The MAAP is often expressed as a percentage of the MAWP.
For steam using apparatus, the MAAP will often be 10% higher than the MAWP, but this is not always the case. If the MAWP is not readily available, the authority responsible for insuring the apparatus should be contacted. If the MAAP cannot be established, it must not be considered to be higher than the MAWP.
There are two fundamental constraints, which must be taken into account when establishing a safety valve set pressure:
In order to meet the first constraint, it is necessary to consider the relative magnitudes of the percentage overpressure and the percentage MAAP (expressed as a percentage of the MAWP).
There are two possible cases:
For example, if the safety valve overpressure was 5%, and the MAAP was 10% of the MAWP, the set pressure would be chosen to equal the MAWP. In this case, the relieving pressure (equal to the set pressure + 5% overpressure) would be less than the MAAP, which is acceptable.
Note: that if the percentage MAAP were higher than the percentage overpressure, the set pressure will still be made to equal the MAWP, as increasing it above the MAWP would violate the second constraint.
For example, if the safety valve overpressure was 25% and the percentage MAAP was only 10%, making the set pressure equal to the MAWP means that the relieving pressure would be 15% greater than the MAAP. In this instance, the correct set pressure should be 15% below the MAWP.
The following table summarises the determination of the set point based on the first constraint.
Table 9.3.1
Determination of the set pressure using safety valve overpressure and apparatus MAAP
Unless operational considerations dictate otherwise, in order to meet the second constraint, the safety valve set pressure should always be somewhat above the normal working pressure with a margin allowed for the blowdown. A safety valve set just above the normal working pressure can lead to a poor shut-off after any discharge.
When the system operating pressure and safety valve set pressure have to be as close as possible to one another, a 0.1 bar minimum margin between reseat pressure and normal operating pressure is recommended to ensure a tight shut-off. This is called the ‘shut-off margin’. In thiscase, it is important to take into account any variations in the system operating pressure before adding the 0.1 bar margin. Such variations can occur where a safety valve is installed after pressure reducing valves (PRVs) and other control valves, with relatively large proportional bands.
In practically all control systems, there is a certain amount of proportional offset associated with the proportional band (see Block 5, Control Theory, for more information regarding proportional offset). If a self-acting PRV is set under full-load conditions, the control pressure at no-load conditions can be significantly greater than its set pressure. Conversely, if the valve is set under no-load conditions, the full-load pressure will be less than its set pressure.
For example, consider a pilot operated PRV with a maximum proportional band of only 0.2 bar.
With a control pressure of 5.0 bar set under full-load conditions, it would give 5.2 bar under no-load conditions. Alternatively, if the control pressure of 5.0 bar is set under no-load conditions, the same valve would exhibit a control pressure of 4.8 bar under full-load conditions.
When determining the set pressure of the safety valve, if the PRV control pressure is set under no-load conditions, then the proportional offset does not have to be taken into account. However, if the PRV control pressure is set under full-load conditions, it is necessary to consider the increase in downstream pressure as a result of the proportional offset of the PRV (see Example 9.3.1).
The amount of pressure control offset depends on the type of control valve and the pressure controller being used. It is therefore important to determine the proportional band of the upstream control valve as well as how this valve was commissioned.
Example 9.3.1
A safety valve, which is to be installed after a PRV, is required to be set as close as possible to the PRV working pressure. Given the parameters below, determine the most suitable safety valve set pressure:
PRV set pressure: 6.0 bar (set under full-load conditions)
PRV proportional band: 0.3 bar operating above the PRV working pressure
Safety valve blowdown: 10%
Answer:
Since it is necessary to ensure that the safety valve set pressure is as close to the PRV working pressure as possible, the safety valve is chosen so that its blowdown pressure is greater than the PRV working pressure (taking into account the proportional offset), and a 0.1 bar shut-off margin.
Firstly, the effect of the PRV proportional offset needs to be considered as the PRV is being set under load conditions; the normal maximum working pressure that will be encountered is:
6.0 bar + 0.3 bar = 6.3 bar (NWP)
By adding the 0.1 bar shut-off margin, the safety valve set pressure has to be 10% greater than 6.4 bar. For this example, this means that the safety valve’s set pressure has to be:
For most types of safety valve, air or gas setting is permissible. A specially constructed test stand is usually employed, allowing easy and quick mounting of the safety valve, for adjustment, and subsequent locking and sealing of the valve at the required set pressure.
The most important requirement, in addition to the usual safety considerations is that instrument quality gauges are used and a regular calibration system is in place. All safety valve standards will specify a particular tolerance for the set pressure (which is typically around 3%) and this must be observed. It is also important that the environment is clean, dust free and relatively quiet.
The source of the setting fluid can vary from a compressed air cylinder to an intensifier and accumulator vessel running off an industrial compressed air main. In the latter case, the air must be clean, oil, and water free.
It is worth noting that there is no requirement for any sort of capacity test. The test stand simply enables the required set pressure to be ascertained. Usually this point is established by listening for an audible ‘hiss’ as the set point is reached. When making adjustments it is imperative for both metal seated and soft seated valves that the disc is not allowed to turn on the seat or nozzle, since this can easily cause damage and prevent a good shut-off being achieved. The stem should therefore be gripped whilst the adjuster is turned.
There is a fundamental difference in the allowable setting procedures for ASME I steam boiler valves. In order to maintain the National Board approval and to apply the ‘V’ stamp to the valve body, these valves must be set using steam on a rig capable not only of achieving the desired set pressure but also with sufficient capacity to demonstrate the popping point and reseat point. Thismust be done in accordance with an approved, and controlled, quality procedure. For ASME VIII valves (stamped on the body with ‘UV’), if the setter has a steam setting facility, then these valves must also be set on steam. If not, then gas or air setting is permissible. For liquid applications with ASME VIII valves, the appropriate liquid, usually water, must be used for setting purposes.
In the case of valves equipped with blowdown rings, the set positions will need to be established and the locking pins sealed in accordance with the relevant manufacturer’s recommendations.
In order to ensure that the maximum allowable accumulation pressure of any system or apparatus protected by a safety valve is never exceeded, careful consideration of the safety valve’s position in the system has to be made. As there is such a wide range of applications, there is no absolute rule as to where the valve should be positioned and therefore, every application needs to be treated separately.
A common steam application for a safety valve is to protect process equipment supplied from a pressure reducing station. Two possible arrangements are shown in Figure 9.3.3.
The safety valve can be fitted within the pressure reducing station itself, that is, before the downstream stop valve, as in Figure 9.3.3 (a), or further downstream, nearer the apparatus as in Figure 9.3.3 (b). Fitting the safety valve before the downstream stop valve has the following advantages:
• The safety valve can be tested in-line by shutting down the downstream stop valve without the chance of downstream apparatus being over pressurised, should the safety valve fail under test.
• When the testing is carried out in-line, the safety valve does not have to be removed and bench tested, which is more costly and time consuming.
• When setting the PRV under no-load conditions, the operation of the safety valve can be observed, as this condition is most likely to cause ‘simmer’. If this should occur, the PRV pressure can be adjusted to below the safety valve reseat pressure.
• Any additional take-offs downstream are inherently protected. Only apparatus with a lower MAWP requires additional protection. This can have significant cost benefits.
It is however sometimes practical to fit the safety valve closer to the steam inlet of any apparatus.
Indeed, a separate safety valve may have to be fitted on the inlet to each downstream piece of apparatus, when the PRV supplies several such pieces of apparatus.
The following points can be used as a guide:
• If supplying one piece of apparatus, which has a MAWP pressure less than the PRV supply pressure, the apparatus must be fitted with a safety valve, preferably close-coupled to its steam inlet connection.
• If a PRV is supplying more than one apparatus and the MAWP of any item is less than the PRV supply pressure, either the PRV station must be fitted with a safety valve set at the lowest possible MAWP of the connected apparatus, or each item of affected apparatus must be fitted with a safety valve.
• The safety valve must be located so that the pressure cannot accumulate in the apparatus viaanother route, for example, from a separate steam line or a bypass line.
It could be argued that every installation deserves special consideration when it comes to safety, but the following applications and situations are a little unusual and worth considering:
• Fire - Any pressure vessel should be protected from overpressure in the event of fire. Although a safety valve mounted for operational protection may also offer protection under fire conditions,such cases require special consideration, which is beyond the scope of this text.
• Exothermic applications - These must be fitted with a safety valve close-coupled to the apparatus steam inlet or the body direct. No alternative applies.
For more Safety Valve Suppliersinformation, please contact us. We will provide professional answers.
• Safety valves used as warning devices - Sometimes, safety valves are fitted to systems as warning devices. They are not required to relieve fault loads but to warn of pressures increasing above normal working pressures for operational reasons only. In these instances, safety valves are set at the warning pressure and only need to be of minimum size. If there is any danger of systems fitted with such a safety valve exceeding their maximum allowable working pressure, they must be protected by additional safety valves in the usual way.
Example 9.3.2
In order to illustrate the importance of the positioning of a safety valve, consider an automatic pump trap (see Block 14) used to remove condensate from a heating vessel. The automatic pump trap (APT), incorporates a mechanical type pump, which uses the motive force of steam to pump the condensate through the return system. The position of the safety valve will depend on the MAWP of the APT and its required motive inlet pressure.
If the MAWP of the APT is more than or equal to that of the vessel, the arrangement shown in Figure 9.3.4 could be used.
This arrangement is suitable if the pump-trap motive pressure is less than 1.6 bar g (safety valve set pressure of 2 bar g less 0.3 bar blowdown and a 0.1 bar shut-off margin). Since the MAWP of both the APT and the vessel are greater than the safety valve set pressure, a single safety valve would provide suitable protection for the system.
However, if the pump-trap motive pressure had to be greater than 1.6 bar g, the APT supply would have to be taken from the high pressure side of the PRV, and reduced to a more appropriate pressure, but still less than the 4.5 bar g MAWP of the APT. The arrangement shown in Figure 9.3.5 would be suitable in this situation.
Here, two separate PRV stations are used each with its own safety valve. If the APT internals failed and steam at 4 bar g passed through the APT and into the vessel, safety valve ‘A’ would relieve this pressure and protect the vessel. Safety valve ‘B’ would not lift as the pressure in the APT is still acceptable and below its set pressure.
It should be noted that safety valve ‘A’ is positioned on the downstream side of the temperature control valve; this is done for both safety and operational reasons:
Also, note that if the MAWP of the pump-trap were greater than the pressure upstream of PRV ‘A’, it would be permissible to omit safety valve ‘B’ from the system, but safety valve ‘A’ must be sized to take into account the total fault flow through PRV ‘B’ as well as through PRV ‘A’.
Example 9.3.3
A pharmaceutical factory has twelve jacketed pans on the same production floor, all rated with the same MAWP. Where would the safety valve be positioned?
One solution would be to install a safety valve on the inlet to each pan (Figure 9.3.6). In this instance, each safety valve would have to be sized to pass the entire load, in case the PRV failed open whilst the other eleven pans were shut down.
As all the pans are rated to the same MAWP, it is possible to install a single safety valve after the PRV.
If additional apparatus with a lower MAWP than the pans (for example, a shell and tube heat exchanger) were to be included in the system, it would be necessary to fit an additional safety valve. This safety valve would be set to an appropriate lower set pressure and sized to pass the fault flow through the temperature control valve (see Figure 9.3.8).
Oil and gas production involves managing large volumes of hydrocarbon fluids, often under high temperatures and pressures. Due to this, production systems—composed of wells, wellheads, pipelines, vessels, pumps, and compressors—must be equipped with robust protection mechanisms to prevent failures caused by overpressure.
Among these, relief valves are the most vital components for overpressure protection within a pressurized system. Their importance is such that they are commonly referred to in the industry as “Ultimate Safeguards.”
Relief valves safeguard oil and gas production systems by preventing ruptures caused by excessive pressure. They help shield personnel, the environment, and infrastructure from catastrophic outcomes that can be caused by uncontrolled hydrocarbon release. They contribute to operational efficiency by ensuring that pressure excursions are safely handled while preserving the integrity of production systems.
Beaumont Manufacturing and Distribution Company (BMD) is a trusted relief valve supplier to the oil and gas industry, with a range of safety relief valves that are well-suited for overpressure protection of production equipment.
This guide will help you to select the correct relief valves for your oil and gas applications. It covers the working principles, key selection criteria, regulatory aspects, proper installation, and maintenance practices.
A relief valve is a pressure control device that opens to relieve excess pressure and recloses once conditions normalize. A conventional relief valve has an inlet nozzle, a movable disc, and a spring that sets the pressure threshold. By balancing forces, fluid pressure exceeding the spring’s set limit opens the valve, safely releasing excess fluid. When pressure falls, the spring reseats the disc, closing the valve. This mechanism prevents overpressure and ensures safe system operation.
BMD’s RV10 relief valves are precision-built for overpressure protection of production equipment and designed for longevity, efficiency, and reliability in the field. RV10 relief valves are manufactured in accordance with the ASME BPVC, and capacity tested and certified by the National Board.
Relief valves in oil and gas operations prevent overpressure in equipment like separators, compressors, and pipelines. These situations could arise, for example, due to blockages, surges, thermal expansion, or control failures. By opening at preset pressure levels, relief valves safely release excess fluid into a relief header system, protecting equipment from rupture and ensuring operational safety.
Choosing the right relief valve involves evaluating several key factors to match the valve with system requirements effectively. Some key factors are:
Operating Pressure and Set Pressure: To avoid leakage due to simmering action when the maximum operating pressure approaches the relief valve set pressure, sufficient margin must be provided between the two. For most applications the operating pressure should be no more than 90% of the set pressure. If vibration is present or pressure surges are expected then it should be even lower.
Flow Capacity (Sizing Considerations): Relief valves must be sized for the worst-case relieving scenario (governing case). When sizing a relief valve the required capacity, set pressure, relieving temperature, and fluid properties should be specified in order to properly select the correct size valve. In some cases multiple valves may be used in parallel. Flow capacity calculations are performed per API and ASME codes
Type of Fluid/Gas Being Processed: The physical and chemical properties of the fluid being handled impact the size of relief valve needed, as well as choice of materials. All operating and relieving scenarios must be considered while evaluating the fluid properties. For example, crude oil may be light or viscous, there may be fouling asphaltenes or waxy substances. Natural gas may contain corrosive acid components. Gas with free water may form hydrates.
Temperature and Environmental Conditions: Relief valves must retain their integrity at conditions to which they are constantly exposed as well as at relieving conditions. High temperatures need heat-resistant components, while cryogenic systems demand materials suited to extreme cold. Environmental factors such as ambient temperature, humidity, precipitation, desert conditions, and offshore service, will affect the choice of materials.
Material Compatibility: It must be ensured that the materials used in the relief valve are compatible with the process fluid at all operating and relieving conditions. For example, relieving scenarios may be at high temperatures and pressures whereas operating conditions may be mild. Corrosion resistance is essential. BMD relief valves are manufactured using different materials as shown in Figure 1.
Overpressure risks in the oil and gas industry encompass a wide range of operating conditions. BMD’s RV10 series of spring-loaded relief valves span a set-pressure range of 15- psi and temperature range of -50 oF-400 o F.
Available in different models to suit varying requirements, RV10 safety relief valves are an ideal choice for overpressure protection of compressors, scrubbers, separators, pipelines and other systems where overpressure protection may be required.
BMD’s RV10 Relief Valves incorporate a non-rising stem design that gives the disk full guidance, while opening and closing. This is coupled with a soft seat design that ensures long lasting set pressure repeatability and bubble tight shut-off. The shorter stem makes the valve compact and ideal for tight spaces.
The RV10 is ASME-certified for gas and liquid service and is available in NPT and flanged connections. The flanged relief valve sizes meet API 526 dimensions. The RV10 is manufactured in accordance with the ASME Boiler and Pressure Vessel Code, capacity tested and certified by the National Board, and meets the requirements of Sec on VIII, Division 1 of the ASME Code.
Relief valve design and operation are subject to rigorous regulations established by various organizations. For oil and gas installations within the USA, the most important national standards are as follows:
API Standard 520, Part 1 Sizing, Selection, and Installation of Pressure-relieving Devices Part I—Sizing and Selection.
API Standard 520 Part II, Sizing, Selection, and Installation of Pressure-relieving Devices Part II—Installation.
API Standard 521, Pressure-relieving and Depressuring Systems.
API Standard 526, Flanged Steel Pressure-relief Valves.
API Standard 527: Pressure Relief Valve Seat Tightness.
ASME Boiler and Pressure Vessel Code (BPVC).
Regular inspections, testing, and documentation are critical for demonstrating compliance during audits.
Relief valves should always be installed and maintained according to manufacturer guidelines and industry standards. Position the valve vertically. The inlet piping should be short and direct, without pockets. Ensure nameplate details are visible. Outlet piping should be free, draining away from the valve to prevent liquid accumulation. Provide proper supports for valve and piping considering reaction forces.
Relief valves are not an end user serviceable device. They must be worked on by a certified VR shop. They should be tested annually at minimum to verify set pressure and that the valve is free to operate.
Selecting the right safety relief valve manufacturer is crucial for system performance and safety.
Quality Certifications and Industry Reputation
Choose a supplier who values meeting the highest industry standard, and with proven expertise in industry-specific valves. Ensure they meet quality standards and certification like ISO- and ASME certification, showcasing compliance and a commitment to quality management and excellence.
A strong reputation backed by customer reviews and industry recognition is essential.
Technical Support Availability
Since relief valves are critical to the safety of oil and gas production facilities, assurance of prompt and reliable technical support from suppliers is very important.
Selecting the right relief valve involves assessing key factors like operating pressure, material compatibility and selecting a manufacturer who is reputable and values the highest quality standards.
Choosing a high quality relief valve for your oil and gas system is vital to the integrity of your system, and the safety and protection of your environment and personnel. Properly specified, installed, and maintained relief valves ensure safety, operational integrity, and compliance with stringent industry standards.
The company is the world’s best Open Cooling Tower supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.