Safety Valves & Level Indicators

Safety Valves & Level Gauges are two fundamental and mandatory instrumentation components that, despite having different functions, complement each other to ensure a pressurized vessel system operates safely, efficiently, and under control. These two elements work like the protective shield and sensory organs of a pressure vessel: Safety valves are the last line of defense, activating in the event of unexpected overpressure in the system to prevent disasters; while level gauges provide real-time information on the amount of product inside the vessel, enabling both process control and efficiency, and proactively preventing dangerous situations such as overfilling. Therefore, for manufacturers with high engineering standards like Cryotanx, the selection, sizing, and installation of the Safety Valves & Level Gauges to be integrated onto the cryogenic storage tanks or other pressure vessels they design, is of at least as critical importance as the manufacturing of the vessel itself.

Safety valves are the most important safety component in a pressurized system. They are mechanical guardians that wait silently and unnoticed during normal operation, but in a moment of danger, they activate within seconds to ensure the protection of a facility, the environment, and most importantly, human life. Their operating principle is intentionally based on a simple and reliable mechanism: they rely on fundamental laws of physics, not on complex electronic or pneumatic systems that are prone to failure.

A typical spring-loaded safety valve consists of a disk held closed by a calibrated spring. The pressure inside the vessel applies an upward force to this disk. Under normal operating conditions, the spring’s downward force is greater than the pressure’s upward force, and the valve remains tightly sealed. However, if the internal pressure begins to rise dangerously due to a fault in the system (such as a blocked outlet, an external fire, or an uncontrolled chemical reaction), the force applied by the pressure on the disk overcomes the spring’s force. This critical pressure is called the “set pressure.” When the set pressure is reached, the disk opens suddenly and fully, discharging the excess gas or vapor inside the vessel to a predetermined safe discharge line. This rapid discharge causes the system pressure to drop quickly. When the pressure drops to a safe level (reseating pressure), the spring force becomes dominant again and automatically closes the valve.

This simple but vital function is mandated by all international pressure vessels standards, such as EN 13445 / AD 2000 / ASME coded vessels. The correct selection and sizing of a safety valve is an extremely serious engineering task. This process is based on a “worst-case scenario” analysis. Engineers must evaluate all potential events that could cause the pressure in that vessel to rise to dangerous levels.

For example, in the event of an external fire, calculations are made for how much heat the tank will be exposed to and how quickly the liquid inside will vaporize and create pressure. As a result of this calculation, the amount of gas that must be discharged per second or per hour (the required discharge capacity) is determined. The capacity of the selected safety valve must be greater than this calculated value. Otherwise, even if the valve opens, a disaster could occur because the discharge rate would be slower than the rate of pressure increase.

These calculations are defined in detail in international standards such as API 520/521. Cryotanx performs this detailed risk analysis for every pressure vessel system it designs and ensures the absolute safety of the system by selecting safety valves of the appropriate capacity and type for each potential scenario. Especially in critical systems, two safety valves and a changeover valve (to isolate them) are installed in the system against the possibility of one valve’s maintenance or failure. This way, one valve can be taken for maintenance without stopping the tank’s operation, while the other continues to protect the system.

This is an indicator of how vital the integrity of Safety Valves & Level Gauges is.

If safety valves are the protective shield of a system, then level gauges are its eyes to the outside world. Knowing how much product is inside a sealed and opaque vessel is a fundamental requirement for a safe, efficient, and profitable operation. Operating a facility without level gauges is like driving a car blindfolded; it is both dangerous and impossible. The importance of these instruments can be summarized under three main headings: process control, inventory management, and safety.

In terms of process control, real-time level information is mandatory to prevent a pump from running dry and burning out when the tank is empty, or to feed the correct amount of raw material to a reactor. Inventory management is critical for a business to know how much raw material or final product it has, to track stock levels, and to plan when to place new orders. In terms of safety, one of the most basic tasks is to prevent the overfilling of a tank. Overfilling can lead to the waste of valuable product through spillage, environmental pollution, and even a dangerous situation by exceeding the tank’s design pressure. Therefore, a reliable level gauge is indirectly a proactive safety tool. There is a wide variety of level gauges technology on the market, designed according to the characteristics of different applications and fluids.

One of the most basic and traditional types is “reflex or transparent glass gauges.” These consist of high-pressure resistant armored glass, mounted on the side of the vessel, allowing the liquid inside to be seen directly. Reflex types, thanks to their prismatic glass structure, make the level easy to read by showing the liquid-filled part as dark and the gas-filled part as bright.

A more modern and safer solution is “magnetic level gauges.” In this system, a float containing a powerful magnet is placed inside a bypass chamber mounted on the side of the vessel, moving with the liquid. This magnet triggers an indicator made of red/white magnetic flaps on the outside of the chamber or an electronic transmitter.

The biggest advantage of this system is that the process fluid remains completely enclosed within a metal chamber and never carries the risk of leaking to the external environment. Therefore, it is frequently preferred for hazardous or valuable fluids and in systems like Cryotanx‘s cryogenic storage tanks.

In facilities where industrial automation is common, “differential pressure (DP) transmitters” are the most popular solution. This device measures the pressure difference between the bottom and top points of the tank. This pressure difference is directly proportional to the liquid height, as the density of the liquid is known.

DP transmitters send the level information as a continuous electronic signal (4-20 mA) to the control room, enabling remote monitoring and control of the process.

In addition to these, more specialized technologies such as radar, ultrasonic, capacitance, and radioactive are also available. The selection of the right technology depends on many factors, such as the type of fluid, its temperature, pressure, and the application’s requirements. It is at this point that the selection of Safety Valves & Level Gauges becomes an engineering decision that determines the overall performance of the system.

Although Safety Valves & Level Gauges may seem like separate entities on a pressure vessel, they actually work in perfect synergy for the holistic safety and control of a system. We can define the relationship between these two components as proactive and reactive safety layers. Level gauges constitute the proactive, or preventive, safety layer. An operator continuously monitors the tank’s fill level by watching the level gauge. In modern systems, when the level reaches a specific danger limit (e.g., 95% full), a transmitter connected to the level gauge sends an alarm signal to the control system.

This “high-level alarm” warns the operator of a potential overfill risk and gives them time to stop the filling. This is a step aimed at preventing the danger before it even occurs. Safety valves, on the other hand, represent the reactive safety layer, or the last resort. Let’s assume that all proactive measures have failed—perhaps the operator didn’t notice the alarm, the filling valve malfunctioned, or another unexpected event occurred—and the tank continues to overfill, causing its internal pressure to rise dangerously. At this exact moment, the safety valve, which requires no human intervention or electronic signal, activates purely mechanically and relieves the excess pressure, protecting the physical integrity of the tank.

Therefore, while a reliable level gauge tries to ensure that the safety valve never has to do its job, the safety valve waits there as a final assurance in case something goes wrong despite everything. This duo ensures a system is both smart and robust. In cryogenic systems, which are Cryotanx‘s area of expertise, the selection and application of Safety Valves & Level Gauges involve additional challenges. The materials for these instruments, which will operate at extremely low temperatures like -196°C, must be chosen from special stainless steel or bronze alloys that do not become brittle at these temperatures. Special designs may be required at the discharge outlets of safety valves to prevent moisture from the atmosphere from freezing and forming an ice plug. To measure the level of cryogenic liquids, special technologies like capacitance-type level gauges provide more accurate results due to the liquid’s low dielectric constant.

In an integrated system like a skid-mounted unit, Cryotanx designs these two critical components as an integral part of the system. The Safety Valves & Level Gauges are factory-installed, calibrated, and tested along with the entire system. This means the customer is provided not just with a tank, but with a complete, safe, and holistic solution, ready to operate with all its safety and control instrumentation. Finally, periodic maintenance and testing are vital for the continued reliability of these two devices. Safety valves must be removed at specific intervals (usually 1-3 years), recertified by checking their set pressures and tightness in test workshops. The calibration of level gauges must also be regularly checked and they must be cleaned. This maintenance discipline ensures that the system’s “silent guardians” and “eyes” are always on duty.