Safety Valves in Process and Storage Tanks

Safety Valves in Process and Storage Tanks: Importance and Use

What is a Safety Valve?

Safety valves are self-actuated valves found in pipes, closed vessels, tanks and compressors, and other pressurized systems such as pressure regulation stations. They are designed to open automatically at a preset pressure. When open, they allow partial discharge of gas, vapor, or liquid in the system, thereby preventing excessive pressure buildup above safe limits. The valve closes again when pressure drops to a safe level, thus preserving the integrity of the piping system and the process fluid.  

What is Overpressure?

In this article, we are referring to overpressure that occurs when a system containing liquid and/or vapor (gas) under pressure (tank, vessel, pipe fitting, pipeline, process unit, etc.) exceeds its safe limit or design pressure. A means must be provided to discharge pressure above the safe or design pressure limit.   Overpressure can arise from four main causes: 1. Process failure: An example of this would be a compressor that continues to feed pressure into a closed vessel (like blowing too much into a balloon). In some cases this can also be referred to as a "blocked outlet". 2. Excessive heating (fire condition): This occurs when external fire around a tank or pipe causes gas/vapor or liquids to expand, increasing internal pressure. 3. Thermal expansion: When a closed section of a pipe filled with liquid is exposed to heating from the sun or an increase in ambient temperature, the liquid expands, causing a slow but significant increase in internal pressure. If not relieved, rupture or leakage can occur in the vessel or pipes. 4. Chemical reaction: Some chemical reactions can generate pressure. Such an accident can occur so suddenly that only a rupture disk can respond fast enough to prevent overpressure.  

What is Overpressure Protection?

Overpressure protection refers to the discharge (venting) of a fluid (liquids and/or vapors) from a system to the atmosphere or another system so that pressure does not exceed a specified value. This is achieved with a pressure relief device that opens at a predetermined pressure. The purpose of these valves is to prevent injury to personnel and damage to property. Depending on their purpose, there are two basic types of overpressure protective devices used to relieve overpressure. These are reclosing (pressure relief valves) and non-reclosing (rupture disks).   The need for overpressure protection is typically determined by codes or regulations that set minimum and/or special requirements for pressurized systems (tanks, vessels, pipes, etc.) containing pressure relief devices. The two most frequently referenced organizations publishing requirements are the American Society of Mechanical Engineers (ASME) and the American Petroleum Institute (API).  

What Types of Safety Valves Are There?

There are three basic types of pressure relief valves. 1. Lever-loaded safety valves are valves in which direct weight is used to keep the valve closed. Since these valves require weight for their set pressure, their set pressures are limited by the available weight. Significant overpressure (typically 100% or more) is required to fully lift the lever, i.e., to fully open the valve. Lever-loaded valves are also called pressure/vacuum vents or pilot pressure breakers. They are used for both pressure and vacuum applications (see Figure 1). (e.g., Anderson Greenwood Type 96A Vacuum Breaker, Enardo 950 series PVRV).   2. Direct-acting spring-loaded safety valves are valves in which the valve disk or stem is held closed by a spring (see Figure 2). As pressure under the disk increases, the upward force pushing the disk increases. When this force equals the downward force of the spring, the valve is at its opening threshold. Any additional increase in pressure causes the disk to move upward and the valve to open. (e.g., Crosby (JOS, JBS, JLT, 800, 900, BP)) 3. Pilot-operated safety valves are valves in which the main valve is held closed by the system pressure itself. Pressure acts upward on a piston on the seat area (nozzle-seat contact area) and is called the dome area. On the top of the piston, it acts downward (see Figure 3). The piston area is larger than the seat area, which causes the downward force to be greater than the upward force. A pilot controls the pressure on the main valve piston. (e.g., Anderson Greenwood Series 200, 400, 500, 700, 800, 900 and 5100, piston-type POPRVs.) The most important functional difference between a direct-acting spring-loaded safety valve and a pilot-operated safety valve occurs as the set pressure is approached. In all direct-acting spring-loaded valves, as system pressure approaches the set pressure, the valve closing force decreases. As system pressure approaches the set pressure, the upward system force and the downward seat/spring force nearly balance, resulting in a condition known as "simmer" which can damage the seat, cause leakage, product loss, and high valve maintenance costs. The system pressure must exceed the spring force/pressure by approximately 10% for the valve to fully open. (Anderson Greenwood Crosby Series 80 pop action valves - Types 81, 83 and 86 - can operate outside this rule. Please contact us for details.) In pilot-operated safety valves, the opposite occurs. As the set pressure is approached, the sealing force of the main valve is maximized due to the greater downward closing force developed by the system pressure acting on the piston area versus the upward opening force produced by the system pressure acting on the seat area. When the pilot senses that the set pressure has been reached, it opens and reduces dome pressure, causing the main valve to open and begin to lift. No simmer occurs in the main valve. A negligible amount of simmer may occur in the pilot valve due to the use of soft seats.  

What is Storage Tank Protection?

Tank blanketing is an inert gas blanket maintained in the vapor space of a pressurized liquid storage vessel along with a vacuum breaker to protect the storage and product without drawing in ambient air. It is necessary to prevent the tank from collapsing due to atmospheric pressure acting on the external walls of tanks where excessive vacuum can form. Using an inert or product-compatible gas instead of ambient air will prevent the formation of potentially explosive mixtures in the tank and deterioration of the tank product. Additionally, the presence of a blanket gas in the tank vapor space can significantly reduce emission leaks, as when the pressure relief valve opens, it will likely discharge mostly blanket gas rather than potentially hazardous and harmful product vapors. [caption id="attachment_143187" align="alignleft"] Figure 1: Rapid cooling[/caption] [caption id="attachment_143188" align="alignnone"] Figure 2: Hurricane (External high pressure)[/caption]    

The Importance of Vapor Pressure Control

• Control of vapor space pressure (above the liquid) is one of the keys to safety. • Pressures that are too high or too low can collapse a tank. • Vapor space pressure changes due to: - Product intake or discharge from tank, - Temperature changes.  

The Effect of Product Inlet and Outlet

• When Product Enters the Tank; - Vapor space pressure increases, - Excess pressure must be relieved. • When Product Exits the Tank; - Vapor space pressure decreases, - Low pressure must be compensated.  

The Effect of Temperature (Increase or Decrease)

• As the product inside the tank cools, vapor condenses; - Tank pressure drops, - Low pressure must be compensated. • As the product inside the tank heats up, vapor expands; - Tank pressure increases, - Excess pressure must be discharged.    

Pressure/Vacuum Relief Valves (Vents/PVRV)

• Vacuum relief; - Tank internal pressure is detected, - If tank pressure < atmospheric pressure; air is drawn into the tank, - This condition returns the tank to proper operating pressure. • Pressure relief; - Tank internal pressure is detected, - If tank pressure > set pressure (safe operating pressure) vapor/gas is released from the tank, - This condition returns the tank to proper operating pressure.    

What is the Purpose of Tank Blanketing / Blanketing?

Isolates the product inside the tank from atmospheric air and moisture. - Longer storage life, Higher quality products. Protects stored product from degradation and contamination - Less oxidation, Less emissions. Prevents harmful product vapors in the tank from being released into the atmosphere. - Less product loss, Prevents flammable gases. Minimizes the hazards of oxygen by reducing its concentration in the vapor space. - Increases safety.     Mehmet Berk Güven Field Sales Engineer Pressure Management Emerson Automation Solutions