Did you know that liquid trim valves can be certified for vapor? 

There is often confusion around the required trim and certification when selecting a new safety-relief valve. For example, when there are both vapor relief scenarios and liquid relief scenarios, is a dual-certified PRV required? What about when the service is liquid but the controlling scenario is vapor? In the past, we have recommended “dual-certified” PRVs when there are both liquid and vapor relief scenarios, but these valves can be difficult to obtain or could have long lead times.  When might there be alternatives? In this discussion we will look at what liquid trim means and when different trims may be suitable for different relief possibilities.

The COVID-19 Challenge

As the COVID-19 disease became a pandemic in the span of mere weeks in USA (months for the world), most of us who work in the process safety/engineering field has probably asked, or been asked:

• What kind of precautions have you taken to minimize the COVID-19 spread through:
  • Human contact
  • Computer equipment
  • Shared office spaces
  • Common areas
• How do you determine “non-essential” personnel that should not have access to your facility?
• What is your contingency plan if/when travel or access is restricted?
• How can you keep normal work efficiency for these “non-essential” personnel working offsite?
• What kind of resources do you need to work offsite?

Did you know, OSHA does not allow block valves on the inlet of relief valves installed on air receivers in compressed air systems?

OSHA CFR 1910.169(b)(3)(ii) states, “No valve of any type shall be placed between the air receiver and its safety valve or valves.”

Did you know, relief devices that protect process pipe may be set to open above the piping design pressure?

Per ASME B31.3-2016, Sections 322.6.3. (see Figure 1) and 302.2.4 (f) (see Figure 2), a thermal relief valve may be set up to 120% of the design pressure of the protected system with the owner’s approval, as long as, the allowable accumulation criteria in 302.2.4.(f) are met. Set pressures lower than 120% of the design pressure may be required depending on the allowable accumulation pressures in 302.2.4 (f).

Anyone tasked to start up a unit knows the engineers who designed it rarely miss a thing...

Every bleeder valve is in the right place, spectacle blind turned correctly, and bypass line scrutinized. So why would the flare and relief systems design be any different? The truth is that they are not. Engineering Firms today have perfected the art of throwing waves of engineers on design-build projects. Unfortunately for those engineers, relief and flare system designs are governed by a maze of regulations, codes, standards, and guidelines (collectively known as Recognized and Generally Accepted Good Engineering Practice or RAGAGEP). Experienced Project Managers understand that relief and flare specialists are needed to ensure their projects are safely and properly designed before any construction occurs.

Did you know, the allowable total backpressure for a conventional valve can be larger than 10% of the set pressure? According to API 520-1 §5.3.3.1.3:

"In a conventional PRV application, when the allowable overpressure is 10%, the built-up backpressure should not exceed 10% of the set pressure. A higher maximum allowable built-up backpressure may be used for allowable overpressures greater than 10 % provided the built-up backpressure does not exceed the allowable overpressure."

Did you know, When performing a flare header analysis, it is not uncommon to find pressure vessels that ride on the flare, i.e. they have an open path to the flare header?

These vessels are often degassing drums, maintenance knock out drums, or other similar equipment. While there are practically never any relief loads from these equipment contributing to the global scenarios we typically evaluate, it is still important to ensure that the pressure in the drum does not exceed the code allowable accumulation.

Did you know, under certain circumstances, you need to verify if the Cold Differential Test Pressure (CDTP) is properly compensated for the superimposed backpressure from the flare header?

Have you come across a conventional pressure relief valve relieving to a flare header? Did you know that, under certain circumstances, you need to verify if the Cold Differential Test Pressure (CDTP) is properly compensated for the superimposed backpressure from the flare header?

Did you know, a relief device in liquid service does not always mean that the device is liquid-certified?

Liquid-certified devices are mechanically designed and sized by an engineer to relieve liquid. Per UG-125 of ASME Section VIII, "Vessels that are to operate completely filled with liquid shall be equipped with pressure relief devices designed for liquid service, unless otherwise protected against overpressure."

Did you know to exclude the exit pipe fitting in your PSV outlet pressure drop calculations if your PSV discharges to atmosphere?

If you have performed an outlet pressure drop calculation for a PSV relieving to atmosphere, you may have noticed that often a significant portion of the pressure drop is contributed by the exit pipe fitting. This is because the equivalent resistance of the fitting is high relative to a typical length of pipe.