Due to the diversity of safety valves and the diversity and complexity of pressure systems, when selecting a safety valve, the impact of factors such as temperature, pressure and state of the medium phase within the system must be considered. Gradually determine the safety valve's nominal pressure, pressure-temperature rating, spring working pressure grade, nominal diameter and basic shape. Finally, determine the model of safety valve to be selected.
I. Selection of Nominal Pressure
Nominal pressure and set pressure are different concepts, which must be paid attention to when determining and selecting safety valves. The nominal pressure PN is a rounded number used as a pressure-related reference code, expressed in digits.
In safety valves, the nominal pressure refers to the highest pressure that can be withstood at the inlet of the safety valve. It is related to materials and temperature. The nominal pressure of the flange at the outlet of the safety valve is generally one to three levels lower than that at the inlet, which must be noted in the selection.
When determining the rated pressure of a safety valve, the rated pressure must be greater than the set pressure. Ideally, the pressure when the safety valve is fully open should not exceed the pressure rating of the safety valve. The rated pressure series of safety valves in China are 0.25, 0.6, 1.0, 1.6, 2.5, 4.0, 6.3, 10, 16, 32, 40MPa (Note: Safety valves with PN less than 1.0 generally use cast iron valve bodies and are not recommended for use in pressure vessels).
II. Pressure-Temperature Classification
When selecting a safety valve, the impact of temperature must be considered. As the temperature increases, the maximum allowable working pressure at the same rated pressure decreases correspondingly. The nominal pressure of the valve must be determined based on the medium to be protected, the valve material, the working temperature and the maximum working pressure.
The maximum allowable working pressure of the valve at various temperatures can be calculated using the formula below or selected from GB/T 9124-2000, “Technical Conditions for Steel Pipe Flanges”.
Max point =PN(σ) t /(σ) 200
where
- (σ) t – The allowable stress value of the material at the design temperature t ℃, MPa;
- (σ) 200 – The allowable stress value of the material at -200 ℃, MPa;
- Max point – Maximum permitted working pressure, MPa;
- PN- Nominal pressure, MPa
III. Determination of the degree of spring working pressure
After determining the rated pressure of the safety valve, the spring-loaded safety valve also needs to select the working pressure degree of the spring. The spring working pressure grade refers to the allowable working range of the selected spring. Exceeding the working range may cause the safety valve to malfunction. The set pressure range of the spring-loaded safety valve is the working pressure degree of the spring.
The set pressure of the safety valve is adjusted by changing the compression amount of the spring, and the various performances of the safety valve are also controlled by the spring. Each spring can only function within a certain set pressure range. Exceeding this range requires changing the spring, so safety valves with the same rated pressure are divided into different degrees of working pressure according to the setting pressure adjustment range designed by the spring.
| PN | Spring working pressure degree (Mpa) | |||||||
| 1.6 | >0.1-0.25 | >0.25-0.4 | >0.4-0.5 | >0.5-0.6 | >0.6-0.8 | >0.0-1.0 | >1.0-1.3 | >1.3-1.6 |
| 4.0 | >1.3-1.6 | >1.6-2.0 | >2.0-2.5 | >2.5-3.2 | >3.2-4.0 | |||
| 6.3 | >2.5-3.2 | >3.2-4.0 | >4.0-5.0 | >5.0-6.3 | ||||
| 10.0 | >4.0-5.0 | >5.0-6.3 | >6.3-8 | >8-10.0 | ||||
| 16.0 | >10-13.0 | 13.0-16.0 | ||||||
| 32.0 | >16-19.0 | >19-22.0 | >22-25.0 | >25-29.0 | >29-32.0 | |||
From the point of view of ensuring the performance of the safety valve, the lower the working pressure range limit of the spring, the more it can guarantee the performance of the safety valve. The currently most common classification method is shown in the table. Users can refer to it when selecting the spring for the safety valve.
4. Determination of the nominal diameter of the safety valve
1. Nominal Diameter DN
The Nominal Diameter DN represents the size of all piping fittings in the system in numerical terms, which is a rounded reference value and does not completely correspond to the actual diameter size in numerical terms.
The specification of the safety valve is divided by the nominal diameter, and the nominal inlet and outlet diameter of the safety valve is different depending on the model. The nominal outlet diameter of the fully open safety valve is generally one size larger than the nominal inlet diameter, while the nominal inlet and outlet diameters of the micro-open safety valve are generally the same.
Please refer to the table below for the nominal diameter series of safety valves (unit: mm)
| 1 | 15 | 100 | 350 | 1000 | 2000 | 3600 |
| two | 20 | 125 | 400 | 1100 | 2200 | 3800 |
| 3 | 25 | 150 | 450 | 1200 | 2400 | 4000 |
| 4 | 32 | 175 | 500 | 1300 | 2600 | |
| 5 | 40 | 200 | 600 | 1400 | 2800 | |
| 6 | 50 | 225 | 700 | 1500 | 3,000 | |
| 8 | 65 | 250 | 800 | 1600 | 3200 | |
| 10 | 80 | 300 | 900 | 1800 | 3400 |
2. Determination of Nominal Diameter
The nominal diameter of the safety valve must be determined based on the safety discharge volume, therefore, the safety discharge volume must first be calculated, and then the flow diameter of the safety valve is calculated based on the volume of safety discharge, taking into account the discharge coefficient of the safety valve, backpressure conditions, etc.
When determining the diameter of the safety valve, the most basic principle is to choose a safety valve whose nominal flow rate is larger and as close to the safety discharge volume as possible. The required discharge volume (safety discharge volume) to prevent excessive overpressure when abnormal overpressure occurs is determined by the working conditions of the system or equipment and the causes of overpressure, among other factors.
3. Flow Diameter
For safety valves, the flow diameter is the minimum transverse diameter of the valve seat throat, and the flow diameter directly affects the discharge capacity of the safety valve. When selecting a safety valve, first calculate the minimum discharge area required by the protected system based on the safety discharge volume of the pressure system, and then calculate the flow diameter d 0 . Next, determine the nominal diameter of the safety valve. See the table below
Nominal diameter DN and flow diameter d 0 (unit: mm)
| DN | 15 | 20 | 25 | 32 | 40 | 50 | 65 | 80 | 100 | 150 | 200 | |
| d 0 | Complete survey type | 20 | 25 | 32 | 40 | 50 | 65 | 100 | 125 | |||
| Low Lift Type | 12 | 16 | 20 | 25 | 32 | 40 | 50 | 65 | 80 | |||
V. Selection of safety valve type
When selecting a safety valve, in addition to determining the rated pressure, pressure-temperature, spring working pressure level and rated diameter, it is also necessary to choose the appropriate safety valve structure and the material of the valve body and main internal parts according to the working environment.
Basic principles for selecting safety valve types
(1) Use full lift safety valves to discharge gas or vapor.
(2) Use full or low lift safety valves to discharge liquids.
(3) Keyed safety valves can be used to discharge steam or air.
(4) For safety valves used for gases with a setting pressure greater than 3.0 MPa and a temperature above 235°C, consider using safety valves with heat sinks to prevent the discharge medium from directly corroding the spring.
(5) Use safety valves with open caps when the discharge medium may leak into the atmosphere. Use closed safety valves when leakage to atmosphere is not permitted.
(6) For the emission of highly toxic, strongly corrosive and extremely dangerous media, bellows safety valves must be selected.
(7) For high back pressure situations, choose back pressure balanced safety valves or pilot operated safety valves.
(8) In some important situations, it is sometimes necessary to install two safety valves as a backup for each other. The inlet and outlet isolation valves of the two safety valves must adopt mechanical interlocking devices to ensure that the required discharge area for the vessel can be met at any time (including during maintenance and inspection periods).
2. When selecting a safety valve, the following items must also be determined
(1) Closed or open:
The bonnet and lid of a closed safety valve are sealed. Its function is, firstly, to protect internal parts from external contaminants such as dust, without requiring airtight performance; secondly, it is used to prevent the spillage of toxic, flammable and other media or to recover media, which is why a tightness test is required. When a closed type is selected and a tightness test on the outlet side is required, this must be specified at the time of ordering. The airtight test pressure is generally set at 0.6 MPa.
Open safety valves, due to their open bonnet, help reduce the temperature in the spring chamber and are mainly used for media such as steam.
(2) Equipped with a lift switch:
If regular opening tests for the safety valve are required, choose a safety valve equipped with a lift switch. When the medium pressure reaches more than 75% of the set pressure, the lift wrench can be used to slightly lift the valve disc from the seat to check the flexibility of the valve opening.
3. Selection of safety valves with special structures
(1) With radiator safety valve.
Used in situations where the temperature of the medium is high, to reduce the temperature of the spring chamber. Generally, when the temperature of a closed-type safety valve exceeds 250°C and the temperature of an open-type safety valve exceeds 350°C, a radiator safety valve should be selected.
(2) Bellows safety valve.
Mainly used in the following two situations:
1) Used to balance back pressure: The effective bellows diameter of the back pressure balanced bellows safety valve is equal to the mean diameter of the valve sealing surface. Before the safety valve opens, the backpressure force on the valve disc is in a balanced state. Changes in back pressure do not affect the set pressure; when the back pressure is variable and its variation exceeds 10% of the set pressure, this type of safety valve must be selected.
2) Used in corrosive media. The bellows isolates the spring and guide mechanism from the medium, thus preventing these important parts from failing due to corrosion by the medium.
After determining the nominal pressure, pressure-temperature rating, spring working pressure rating, nominal diameter and basic type of the safety valve, the safety valve model to be selected can finally be determined.
4. Selection example
The selection of a safety valve is based on the specific parameters of the working conditions. Now, we will explain the process of selecting safety valves for three different states in a xylene gasket unit process system with an example of 45×10 4 t/a (using spring-type safety valves as an example).
(1) Used for gas media
1) Process parameters
- Tag Number: SV-501
- Operating Temperature: 40℃
- Set opening pressure: 0.98 MPa
- Backpressure: Atmospheric
- Design discharge (considered as nominal discharge W R ): 4660 kg/h
- Medium: Purified compressed air.
2) Selection steps
- Select the applicable formula: Since the medium is purified air, the gaseous medium calculation formula must be used.
- Check the gas properties table: The adiabatic index K of air is 1.40.
- Find the C value corresponding to the K value, where the C value is 2.7.
- Determine the flow of the medium: (According to GB/T12241-2005 to judge whether the gas flow is critical or subcritical) The specific judgment formula and symbol definitions are found in the standard.
- Calculate the displacement according to the relevant formula.
- Choose the model A42Y fully open safety valve, with a nominal discharge coefficient of 0.75. The nominal flow of the safety valve is W R =Weight×0.75
- Substituting into the equation above results: 6213 = 7.49d 0 2 d 0 =28.8(mm)
- When d 0 =32, the corresponding nominal diameter of the fully open safety valve is DN50 (P N is 1.6 MPa).
- Determine the material: Since the medium is purified compressed air, carbon steel valve body can be used.
3) Determine the model
Through calculation, the A42Y16C-DN50 fully open safety valve can be selected, completing the selection process.
(2) For half steam
1) Process parameters
- Tag Number: SV-407
- Operating Temperature: 350℃
- Set opening pressure: 2.64 MPa
- Backpressure: Atmospheric
- Designed discharge: 34,450 kg/h
2) Selection steps:
Select the applicable formula: Since the medium is steam, the inlet pressure is the set opening pressure of 2.64 MPa <11 MPa, and the operating temperature is 350 ℃; refer to the superheated steam coefficient table, the saturated temperature of steam at an absolute pressure of 2.74 MPa is 230 ℃, and the corresponding superheated steam correction factor is 0.87, use the steam formula for calculation.
Determine the relevant calculation formula: Wtsh=5.25AP d Ksh.
Use a fully open safety valve model A48Y (for steam), with a nominal discharge coefficient of 0.75.
The nominal flow of the safety valve Wrsh=Wtsh×0.75.
Substitute into the above equation, obtain: Wtsh×0.75=5.25×d 0 2 ×π/4×(2.64×1.03+0.1)×0.87, therefore d 0 =67.
Check the related tables, when d 0 =80, the corresponding nominal diameter for the fully open type safety valve is DN125.
When the working temperature of the carbon steel valve body is 350℃ and the opening pressure (absolute pressure) is 2.74Mpa, the rated pressure of the selected safety valve is 4.0Mpa.
3) Determine the model:
Through the above steps, A48Y40—DN125 can be selected.
Note: The nominal diameter DN125 is special and is generally not used. If DN150 is chosen, then the corresponding throat diameter of the safety valve is d 0 =100, which is much larger than the calculated d 0 (67), causing frequent jumps of the safety valve (when the selected throat diameter of the safety valve security is very high).
