A pressure sensor failed on a CIP cycle of a medium size dairy processing facility, last spring, just before an automated CIP cycle was to begin. The conclusion of the CIP cycle would be that a hot caustic solution was trapped in a closed loop between two automated ball valves. As a result, the pressure of the hot caustic solution began to increase because of the expanding nature of the hot caustic solution. Within less than twenty seconds the pressure increased above the design limit of the piping.
The only reason the pipe did not burst and that there was not a plant-wide evacuation was because of a compact spring loaded device that is mounted at the outlet of the tank: a stainless steel safety relief valve. As a result, the valve opened at 80 psi and vented a small amount of the caustic solution into the containment drain and reseated as soon as the pressure dropped. The entire event lasted less than one minute and the production was back to normal by that afternoon. The valve went unnoticed for years but performed its function exactly as it was intended to do. Anyone that specifies fluid systems within the food, pharmaceutical or chemical processing industries must therefore understand how the valve operates and why the "stainless steel" material is not an option in the hygienic industry.
The Fundamental Purpose: Automatic Overpressure Protection
An automatic pressure releasing device that opens up when the system pressure exceeds a certain set point (the static upstream pressure) to adjust the system by discharging liquid, gas or vapors until it reaches below its reseating point after correcting this condition. Unlike rupture discs which can only be used one time, safety relief valves are designed to operate continuously over the lifetime of the entire system.
The safety aspect of these valves allows them to open very quickly and completely (pop operation) in competitive gaseous or vapour service, and 'relief' from high pressure service to an equivalent degree of proportional opening depending on the rise in respective pressures usually experienced with liquid services, through providing a continuous rise in liquid or vapour service due to removal of product from either a ruptured or leaking pipe or operator error.
Most safety relief valves can handle, compressible and incompressible fluids equally, making them the most widely used safety relief device within Process Industries. All safety relief valves are subject to third party certification, including ASME Boiler and Pressure Vessel Code (BPV) Section VII, API 526 and 3-A Sanitary Standards for sanitary applications.
Why Stainless Steel Is the Default Material for Hygienic Service
Material for a stainless steel safety relief valve is selected according to the same requirements as the complete sanitary fluid path: having a corrosion-resistant surface capable of withstanding aggressive chemical cleaning, pitting, and crevice corrosion under chloride-containing conditions and having a surface that can be electropolished to resist biofilm adhesion. Most hygienic safety relief valves are made from 316L stainless steel, which adds molybdenum to the base 304 alloy to provide greater pitting resistance than the base 304 alloy alone.
In ultrapure water systems in pharmaceuticals and semiconductors, the 316L may have low sulphur content and be manufactured from vacuum-melted heats to produce material free from inclusion. The internal spring, which determines the set pressure of the valve, is typically manufactured from high-quality spring steel and when used in very corrosive conditions may also be made from Hastelloy or Inconel alloys. The elastomer seals, which are typically made from EPDM, Viton, or PTFE, must be FDA compliant and able to withstand the repeated SIP cycles at 130°C. Our comparison of 304 vs 316 stainless steel explains why the incremental cost of 316L pays for itself in extended service life and reduced contamination risk.
Safety Valve, Relief Valve, Safety Relief Valve: Navigating the Terminology
The two words are commonly confused, however when specifying devices the difference is important.
- Safety Valve: A valve that operates by a spring mechanism which instantly opens its full port (pop-action) to allow flow through it, is typically used with gases, such as steam and compressed air. It's when the pressure reaches its set point that the valve pops open quickly and remains open until the system pressure drops below the defined reseating setpoint.
- Relief Valve: The opening begins gradually in relationship to the increase in pressure over the set point. This type of valve operates primarily with incompressible fluids (liquids). The valve does not fully open until it reaches a higher pressure than the set point, usually 10% or 25% higher.
- Safety Relief Valve: This device will serve as either a safety or a relief valve, depending on how it is used in that particular application. In the case of sanitary processing, the majority of all valves sold are safety relief valves, allowing for the safe handling of steam in cleaning & sanitizing all valves using SIP (for safety). Also allowing for for the handling of other liquids based on the CIP cleaning method in sanitary applications.
This dual function is important to maintain high standards of cleanliness in the industrial application. The valve must be installed to allow for the use of both liquid and gas cleansers, with a valve that will perform properly, i.e., not leak or chatter, when installed in an industrial environment where it must open cleanly on both liquid and gas. If you are looking at the broader context of how sanitary components work together to maintain a cleanable system, our introduction to what a sanitary fitting is provides the foundational principles that apply to every component, including safety relief valves.
The 3% Rule and Sizing Your Relief Device
A safety relief valve will be selected by an engineer based on calculating the required relieving capacity which represents how much fluid will be released through the valve as a result of excess pressure to ensure that the pressure of the pressure vessel does not exceed its maximum allowable working pressure (MAWP). According to the ASME Code for application of a safety relief valve in a single valve installation for a pressure vessel, the valve must limit any increase in pressure caused by fire to 110% of MAWP; for multiple valves, the first valve allows an increase in pressure to 110% and any additional safety relief valves can allow the pressure to increase to 116% before opening.
The 3 percent rule, frequently quoted concerning pressure relief valves, is not a requirement set forth in any one code but rather serves as a practical guideline established by API and as part of process safety practices. The guideline states that the inlet pressure drop from the protected equipment to the inlet of the valve should not exceed 3 percent of the valve's set point pressure when the valve is at full capacity.
When the 3 percent limit is exceeded, the pressure relief valve will chatter (open and close rapidly), which causes damage to the seat, spring, and Potentially fracture the valve body. In sanitary applications, with the valve immediately mounted on a short nozzle off a tank or pipe, it is normally straightforward to meet the 3 percent limit. The pressure drop calculation must be verified in cases where the inlet piping is too long. Chattering of a stainless steel safety relief valve is particularly damaging; the combination of hard seat and polished disc becoming galling surfaces when subject to vibration, will require the valve be replaced if the seat is damaged from chattering.
PSV vs. PRV: A Functional Guide
Two commonly confused valve types in the industrial field are the Pressure Safety Valve (PSV) and the Pressure Relief Valve (PRV). The distinction between the two valve types is that a PSV allows for the rapid opening of a valve to relieve excessive pressure in gas or vapor applications, while a PRV allows for a gradual opening of a valve in liquid applications. Safety relief valves are designed as a combination of both types of valve opening actions and are often referred to as PSV valves by process engineers that are responsible for a critical safety function on a system.
From a practical perspective, if the valve tag on a sanitary safety relief valve is manufactured as a PSV designation, it has typically been "qualified" via design and testing for gas service, whereas a PRV designation has typically been qualified for service in liquid applications. However, the manufacturer has the ability to verify product dual certification. Therefore, as best practice to prevent confusion, when specifying purchase requirements utilize the term "safety relief valve" and specify each aspect of the fluid phase (liquid, gas or both phases).
Are Stainless Steel Valves Always the Right Choice?
A stainless steel safety relief valve provides distinct advantages in hygienic applications; notably, its resistive properties to corrosion, ease of cleanliness, and compatibility to tubular and fittings designed to be 316L, a common material used by many of the pipe manufacturers that supply Hygienic usage. If you were in a dairy processing facility using hot nitric acid for passivation, after only months a carbon steel or brass valve would be totally destroyed.
Additionally, using a carbon steel or brass valve in a water-for-injection loop used for pharmaceutical applications would create enough trace iron that the conductivity measuring devices would not work correctly. There are limits to the corrosion resistance inherent to stainless steel. Both carbon steel and brass will corrode quickly at a much slower rate compared to stainless steel. The reason is that, while stainless steels have much greater resistance to corrosion than other types of metals, stainless steels will still corrode in extreme conditions, such as a high concentration of chlorides, or by being subjected to certain types of organic acids. For extremely corrosive service conditions, duplex stainless steels, or nickel alloys will typically be utilized; however, duplex stainless steels and nickel alloys are rarely used for the standard sanitary service.
The most important consideration is whether or not the entire valve body is made from true stainless steel. Some low-cost valves are constructed from a stainless steel body and use a chrome-plated carbon steel as a spring which will corrode and ultimately fail at the location of the spring. Manufacturers of sanitary and most reputable processors provide complete traceability; specifically, they provide the material traceability and the material used for every wetted component. Therefore, when you purchase a sanitary safety relief valve from a supplier such as Eagle Fittings, the body, trim, spring and seals on the valve will be documented and traceable back to the mill heat number.
Integrating Safety Relief Valves into Hygienic Process Lines
Typically, food and beverage processing (dairy), as well as pharmaceutical processing, does not pipe a safety relief valve onto a distant pipe rack; instead, the safety relief valve is typically piped directly to the pressure vessel or a critical transfer line. Because of the requirement for a crevice-free/drainable connection, the Tri-Clamp connection is typically used as the connection for sanitary safety relief valves. Additionally, the valve body must be designed in such a way as to provide no horizontal surfaces or dead legs, which will allow the product to pool. The valve must also be designed so that after a discharge event, the valve can be either CIP cleaned-in-place or manually cleaned based on its location and process. For a complete view of how these devices fit into a BPE-compliant pharmaceutical piping system, our guide to pharmaceutical equipment pipes and fittings covers the design principles that apply to every component, from safety valves to the final fill nozzle.
Frequently Asked Questions
What is the 3% rule for pressure relief valves?
A guideline called the "3 percent rule" stipulates that non-recoverable pressure drop through the inlet piping between a protected vessel and a safety relief valve shall not exceed 3% of the safety relief valve's set point when flowing full open. Exceeding this value may cause the valve to chatter or rapidly cycle between being fully open and fully closed, resulting in damage to the valve components such as the seat and spring; therefore, following the 3 percent rule will promote stable operation of valves while relieving.
What is the difference between a safety valve and a safety relief valve?
Safety valves that open quickly by popping are typically installed on piping systems dealing with gases/steam (compressible fluids), while safety and relief valves can operate on either gas/steam or incompressible fluids (liquids). This allows the flexibility for the valve to be used for either purpose, making the relief valve the common vehicle in sanitary (cleanable) processing applications for SIP (Steam In Place) or CIP (Clean In Place) on the same line pipe.
Are stainless steel valves good?
Stainless steel valves are ideal for environments requiring resistance to corrosion, the ability to maintain a clean surface, and the ability to work with existing stainless pipes. In the hygienic industry, 316L stainless steel has been used as the preferred metal; it will not pit when exposed to cleaning solutions. Moreover, electropolishing will yield a surface with minimal bacterial adherence. A stainless steel valve's performance is determined by the precision of the manufacturing process, the alloy grade used in producing the valve, and the material tracking information provided by the producer.
What's the difference between PSV and PRV?
Pressure safety valve (PSV) refers to valves designed to quickly and fully open when subjected to pressure exceedences in gas/vapor applications. By contrast, Pressure Relief Valve (PRV) refers to valves designed to gradually open based on an increase in pressure and used in liquid service applications. Safety relief valves are hybrid valves which perform both functions and are referred to as either PSV or PRV depending on their primary rating fluid. The capability of the valve can be verified by the nameplate and testing certification associated with that valve.
A stainless steel safety relief valve is one of the unsung heroes of process engineering until a pressure gauge (that you hopefully have installed) goes into the red. At this point in time, it becomes the only boundary between an intact system and an explosive failure. In a hygienic processing application, the stainless steel body, FDA-compliant seals, Tri-Clamp connection and documented set pressure all function as one safety device that must work, every time, upon initial installation. The valve that protects your pasteurization skid, bioreactor or clean steam line should also include the necessary material certifications, surface finish data and test results that will help you sleep at night. Eagle Fittings provides safety relief valves with these qualifications and documentation to give you peace of mind that the safety relief valve protecting your sterile process from high pressure is built with known provenance and material traceability.
