The “safety” of a safety valve refers to its ability to reliably open and release excess pressure to the outside when the internal pressure of a pressure vessel exceeds the set pressure, thereby preventing damage to the vessel or piping. At the heart of this is the “discharge capacity”: only when the valve can discharge a greater volume than the steam or gas entering the vessel can it stop the rise in internal pressure. When selecting a safety valve, the top priority is to confirm whether this discharge capacity is sufficient for the conditions of the target vessel.
Key Points of This Article
- A safety valve’s safety is determined by “reliable operation at the set pressure” and by “discharge capacity that releases the required volume.”
- The basic principle of selection is to choose a model with a discharge capacity equal to or greater than the volume of fluid fed into the vessel (the inflow rate).
- Confirm that the set pressure, connection bore, and fluid conditions match, together with the design pressure of the vessel.
How a Safety Valve Protects a Pressure Vessel
In a pressure vessel, the internal pressure may continue to rise due to heating or an external supply. When the pressure reaches the predetermined set pressure, the internal pressure overcomes the spring force pushing up the valve disc, the valve opens, and the fluid is released. When the internal pressure drops, the valve closes again, keeping the vessel within its normal pressure range.
The premise of “safety” is that this sequence of operations occurs reliably at the intended pressure. As a basic rule, the set pressure is determined within a range that does not exceed the vessel’s design pressure (maximum allowable working pressure), and once a pressure has been set appropriately, it should not be changed casually based on on-site judgment.
Discharge Capacity and the Criteria for Selection
Consider Discharge Capacity Based on the “Inflow Rate”
Discharge capacity is the volume of fluid that a safety valve can release per unit of time. The first thing to confirm in selection is the maximum inflow rate expected for the vessel (the generation rate for steam, or the supply rate for gas). If the discharge capacity of the safety valve is below this inflow rate, the valve will not be able to fully suppress the rise in internal pressure even when it opens. Conversely, if the required volume is secured, the internal pressure can be capped near the set pressure.
For this reason, selection uses the discharge-volume chart or the calculation formula in the catalog to choose a model and bore that meet the required volume under the conditions of set pressure, fluid type, and temperature.
Match the Set Pressure, Bore, and Fluid Conditions
Even if the discharge capacity is sufficient, the valve cannot deliver its inherent performance unless the set pressure, connection bore, and target fluid are matched. The set pressure is set at or below the vessel’s design pressure and at a value sufficiently higher than the normal operating pressure. The connection bore is determined by the required discharge capacity; if the bore is too small, discharge cannot keep up, while an excessively large bore can affect the stability of operation.
Because the compatible model and the calculation of the required volume vary depending on whether the fluid is steam, air, water, or another gas, organizing the actual operating conditions before making a selection contributes to safety.
Points Easily Overlooked During Selection
If the back pressure (the pressure on the discharge side) or the resistance of the discharge piping is large, the valve may not discharge the intended volume. In addition, the installation orientation, ambient temperature, and foreign matter contained in the fluid also affect operation. The figures are merely guidelines under fixed conditions, so it is important to confirm whether they match the actual operating conditions.
Frequently Asked Questions
Q1. How should I determine the discharge capacity?
A1. First, ascertain the maximum inflow rate into the vessel. Then choose a model that provides a discharge capacity able to release that volume or more under the set pressure and fluid conditions. The charts and calculation formulas in the catalog serve as a guide.
Q2. Is it acceptable for the set pressure to be the same as the vessel’s design pressure?
A2. As a basic rule, the set pressure is determined within a range that does not exceed the vessel’s design pressure (maximum allowable working pressure). Secure a sufficient margin from the normal operating pressure as well. Confirm the details according to the specifications on the vessel side.
Q3. Is a larger safety valve bore safer?
A3. Not necessarily. The bore is determined by the required discharge capacity, and an excessively large bore can affect the stability of operation. It is important to choose a bore that suits the conditions.
Q4. Can the same safety valve be used for steam and for air or gas?
A4. The calculation of the required discharge volume and the compatible model differ depending on the fluid. Make your selection after confirming the fluid actually used along with its temperature and pressure conditions.
Q5. Is it acceptable to change the set pressure once it has been set, while in operation?
A5. Because the set pressure is a value determined on the premise of protecting the vessel, avoid changing it casually on site. If the operating conditions change, review the selection conditions again from the start.
Q6. How should I think about environments with high back pressure?
A6. If the back pressure on the discharge side or the piping resistance is large, the valve may not discharge the intended volume. Consider the model and bore after taking the effect of back pressure into account.
Summary
A safety valve for a pressure vessel ensures safety by combining “reliable operation at the set pressure” with “a discharge capacity that exceeds the inflow rate.” In selection, the basic approach is to confirm the maximum inflow rate, set pressure, connection bore, and fluid conditions together with the vessel’s design pressure. Since the figures are guidelines under fixed conditions, always confirm their consistency with the actual operating conditions.
If you find yourself thinking, “I don’t know how to calculate the required discharge capacity,” “I know the inflow rate but can’t judge the appropriate bore,” or “I want to select an equivalent to an existing safety valve,” please organize your operating conditions and consult Yoshitake. We will assist with model selection tailored to your fluid, pressure, and vessel conditions from a technical standpoint.