Pressure vessel accidents caused 127 fatalities between 2000 and 2010, according to the National Board of Boiler and Pressure Vessel Inspectors. It’s a worst case scenario – one of your mechanical systems malfunctions and someone gets injured.

A typical manufacturing facility has many safety systems, controls, and procedures to prevent injuries from happening. One often-overlooked component is pressure safety devices. These “last resort” devices exist in the background and are designed to do their job only under the most extreme circumstances. In order for these devices to continue to provide the safety measure for which they were installed, you and your facility manager should understand how and why your pressure safety devices are installed.

What is a pressure safety device?

Pressure safety devices protect your systems and equipment from overpressure situations. If you have a pressurized tank with a rated pressure limit of 125 psig*, and the system pressure increases above 125 psig, a pressure safety device is there to relieve the excess pressure in a controlled manner and protect the vessel from rupturing. Many pressure-rated vessels are provided with pressure safety devices pre-installed.

Two common pressure safety devices

A large portion of pressure safety devices fall into two main types:

1. A Pressure Safety Valve (PSV) is sometimes called a Safety Relief Valve (SRV) when used for vapor relief, or a Pressure Relief Valve (PRV) when used for liquid relief (not to be confused with a Pressure Reducing Valve). These valves can open when pressure is too high (above set point) and close when the pressure returns to normal (below set point).
2. A rupture disk or burst disk (sometimes called a Pressure Safety Element, or PSE). Rupture disks are small, typically metal disks mounted inside a vent pipe, specifically designed to rupture or burst at a certain pressure. Below that set point pressure, the system operates normally and there is no flow in the relief pipe. Above the set point pressure, the rupture disk releases and the excess pressure is relieved into the vent line. A failed rupture disk must be replaced.

Why you should understand your pressure safety devices

You might be asking, “Why should I be concerned about this? The safety devices were installed with the systems or equipment. Everything was stamped and passed inspections. Why would there be a problem now?”

Here are six scenarios in which you may have a problem with your safety relief devices and not even know it:

1. Your discharge piping runs too far. Even though your safety device is sized correctly for your pressure vessel, the discharge piping from the outlet of the device could affect its operability. Chemicals or high-energy systems such as steam need to be vented to a safe location. Long discharge piping runs will increase the back pressure on the relief system when the valve opens. If the back pressure is great enough, the relief system may not be able to function properly. The solution to this may be a shorter pipe route, or a larger discharge pipe.
2. Multiple relief devices tie into a single discharge pipe. When multiple relief devices tie into a single discharge pipe, most mechanical codes will require that the relief piping is sized so that all the devices can flow their full capacity simultaneously. While all the relief devices may be properly sized, combining the discharge pipes together can compromise the safety of the overall system. Analysis with a flow modeling program such as AFT Fathom can easily verify whether the combined discharge piping system can handle all the scenarios that may occur.
3. Your system has expanded or changed. As processes at your facility change, or the facility itself grows, the conditions used for the original design of your mechanical systems may change as well. Projects that modify mechanical systems should include a review of the associated pressure safety devices. If the design conditions used to select the devices are no longer valid, a new calculation—and possibly a new device—is required.
4. Your safety device may be too old. Pressure safety valves are built to sit idle for years at a time until called upon in an emergency. Environmental conditions can nonetheless affect the ability of the valve to do its job over time. Many facilities include pressure safety valves in their preventative maintenance programs with a replacement scheduled every five to ten years. A white paper by Exida (a process safety and security company) noted the useful life for pressure relief valves as 4 to 5 years. With no moving parts, rupture disks are less likely to become ineffective over time, but corrosion could result in an unintended release.
5. The safety device rating is not appropriate for your system. The pressure set point of a safety relief device is typically stamped on the device itself. It is often easy to verify whether the current set point is appropriate for your system. However, it is surprisingly common to find an existing safety valve whose set point is well above the maximum allowable working pressure (MAWP) of the vessel the valve protects!
6. A shut-off valve is blocking your safety relief. Most mechanical codes will require that safety reliefs be connected to the system with no isolation valve that can prevent safe discharge of an overpressure situation. In some cases, a valve is installed inadvertently. A valve in front of a safety relief device is particularly worrisome. Since the safety relief does not operate under normal conditions, a shut-off valve could be left closed without the system, or anyone else, noticing. In an overpressure situation, the safety relief valve has been rendered useless.

Given the important role that safety relief devices play in your mechanical systems, periodic review of their age, condition, settings, and system functionality is well worth your time and effort.

Find out more

Applied Engineering has provided field surveys, calculations, and documentation for hundreds of safety relief devices in water, steam, compressed air, refrigeration, and other systems. Please contact us if you would like to find out more!

Doug Walker, PE, is an Associate with Applied Engineering Services.

*psig = pounds per square inch gauge