Both relief valves and safety valves protect equipment by releasing fluid when the pressure in piping or equipment rises too high, but the criteria for choosing between them come down to “the fluid you want to protect” and “the opening action you require.” In general, a relief valve is suited to non-compressible liquid systems such as water and oil, where you want the valve to open gradually and release excess pressure; a safety valve is suited to compressible fluids such as steam and gas, where you want the valve to open wide and discharge all at once the moment the set pressure is reached. In some applications, such as boilers and pressure vessels, the installation of a safety valve is required by law, so the surest approach is to narrow down your choice starting from the application, the fluid, and the relevant regulations.
Key points of this article
- As a basic guide, choose a relief valve to release liquid gradually, and a safety valve to discharge a large volume of steam or gas instantly.
- The differences can be organized around three points: the “target fluid,” the “opening action (proportional lift / pop action),” and “whether the law requires it.”
- Because names and combined types (relief safety valve) also exist, if you are unsure it is safest to consult with us and provide your design conditions.
Differences in how relief valves and safety valves work
Difference in opening action (lift characteristics)
A relief valve typically opens with the valve disc lifting gradually only by the amount that exceeds the set pressure, opening proportionally in response to the rise in pressure. It releases excess pressure little by little to keep the internal pressure of the equipment near the set value, and closes again once the pressure drops. Because liquids have low compressibility and the pressure falls simply by releasing a small amount, this gradual action is well suited to them.
A safety valve, on the other hand, is characterized by “pop action,” in which the valve disc opens all at once when the set pressure is reached. Because steam and gas are highly compressible and their pressure will not readily drop unless a large volume is discharged instantly, the valve opens fully at once to discharge and then closes once the pressure falls. Discharge noise and reaction force also tend to be large, so the design of the discharge destination (the exhaust pipe) is important as well.
Difference in target fluid and application
The first thing to check in selection is the fluid of the system you want to protect. In liquid systems such as water, hot water, hydraulic oil, and chemical solutions, relief valves are often used to release pressure rises caused by thermal expansion or by pump shut-off operation. Countermeasures against thermal expansion in hot-water supply and storage equipment are a representative example.
For compressible fluids and high-pressure systems such as steam boilers, compressed air, various gases, and pressure vessels, safety valves are used because a reliable, large-volume discharge is required when abnormal pressure rises occur. A major difference from liquid systems is that these may be subject to legal requirements for installation and performance, such as the Industrial Safety and Health Act and regulations concerning boilers and pressure vessels.
Be careful with names and combined types
In practice, terms such as “relief valve,” “safety valve,” and “relief valve (relief)” are used interchangeably at job sites and across the industry, and the same valve is sometimes called by different names. In addition, standards such as JIS include a type corresponding to a “relief safety valve,” which opens proportionally yet also has a rapid-opening characteristic near the set pressure, and this may be used for protection on both the liquid and gas sides.
For this reason, it is important not to judge by name alone but to confirm the specification based on design conditions such as fluid, set pressure, discharge volume, and applicable regulations. Note that a difference in name does not necessarily mean a difference in operating principle.
Selection criteria to keep in mind
Set pressure, discharge capacity, and bore size
For both safety valves and relief valves, you need to choose an appropriate set pressure (start-to-discharge pressure) relative to the maximum working pressure of the equipment. For safety valves in particular, it is a prerequisite that the discharge capacity (required relief capacity) be sufficient to release all of the volume generated under the assumed abnormal conditions; if the capacity is insufficient, there is a concern that the pressure will not drop fully. The bore size should basically be selected from the discharge volume, and care must be taken not to decide it based on the connection bore size alone.
Materials, connections, and discharge destination
Choose the materials for the valve disc, seat, spring, and so on according to the type, temperature, and corrosiveness of the fluid. Where the discharged fluid is led (atmospheric discharge or recovery), the presence or absence of back pressure, and the risk of freezing or condensation are also factors that affect the stability of operation. Because the optimal solution for these varies with the conditions, they should be examined for the system as a whole rather than applying a single fixed answer.
Frequently asked questions
Q1. Can relief valves and safety valves be used interchangeably?
A1. Because their operating characteristics and applicable conditions differ, in principle you should avoid treating them as the same and diverting one for the other. In particular, for pressure vessels and boilers subject to legal regulation, a safety valve that meets the required performance is necessary. If you are unsure how to decide, please consult with us and provide your design conditions.
Q2. Is it impossible to use a safety valve in a liquid system?
A2. It cannot be said to be impossible outright, but because the pressure of a liquid drops with a small discharge, a proportionally opening relief valve is more suitable in many situations. Since this varies with the application and set pressure, selection according to the conditions is a prerequisite.
Q3. How should the set pressure be decided?
A3. In general, it is set within a range that does not exceed the maximum working pressure of the equipment, while securing a margin from the normal operating pressure. Because the appropriate value differs depending on the equipment specifications and regulations, treat this as a guide and confirm each case individually.
Q4. Why is the discharge capacity (relief capacity) important?
A4. Because if the volume generated under abnormal conditions cannot be fully discharged, there is a concern that the pressure will not drop completely. For steam and gas safety valves in particular, selecting a bore size that meets the required relief capacity is essential.
Q5. Is periodic inspection necessary?
A5. Because valves can develop seat leakage or sticking with age, periodic operation checks and maintenance are recommended. The inspection interval differs depending on the operating conditions and relevant regulations.
Q6. The naming varies at our job site.
A6. Terms such as “relief valve,” “relief valve (release),” and “safety valve” are easily used interchangeably. It is surest to confirm by specification (fluid, set pressure, discharge volume, applicable regulations) rather than by name.
Summary
Relief valves and safety valves are chosen according to a difference in purpose: whether to release liquid gradually, or to discharge a large volume of steam or gas instantly. The axes for judgment are the three points of “target fluid,” “opening action,” and “legal requirements,” and organizing these as design conditions will leave you with less uncertainty.
If you find yourself thinking “I cannot judge whether the bore size is appropriate,” “I want to determine the required capacity from the discharge volume under abnormal conditions,” “I want to select an equivalent product for an existing relief valve or safety valve,” or “I cannot settle on a specification depending on whether it is a steam system or a liquid system,” please use Yoshitake’s technical and selection consultation together with your design information such as fluid, pressure, temperature, and connection conditions. We will support you in choosing a specification suited to your application, in line with your practical conditions.