Essentials of PRV Stability Webinar

AIChE Academy Webinar Presented by ioMosaic

Attendance to this webinar is free.

Could undetected pressure relief valve (PRV) installation instability be putting your facility at risk? Watch this webinar to find out.

In this 60-minute ioMosaic sponsored webinar, gain a better understanding of the risks associated with potentially unstable PRV installations. Then, investigate how to perform an engineering analysis when PRV instability is suspected. Mr. Houston shares how to evaluate historical data and inspection records and presents screening methods for evaluating instability. Because speed of sound estimates are critical, he addresses how to improve your estimating accuracy. Wondering how engineering analysis is being successfully applied in the real world? This webinar includes case studies that will show you while demonstrating state-of-the-art developments in PRV stability.


Our Presenter

Casey Houston

Casey Houston
Senior Partner, ioMosaic Corp

Mr. Houston is a Senior Partner at ioMosaic and brings over 15 years of engineering and process safety experience to his role as a leader of the firm’s Relief Systems consulting group. His work is focused on managing and executing large-scale pressure relief and flare systems design projects for reactive and non-reactive chemical, petroleum and pharmaceutical systems, as well as providing technically sound analysis and documentation for existing process and reactivity hazards. Read more

Webinar Q&A

1. Can you give an example on which fluttering is acceptable?

I would take the fluttering example and look more at the type of fluid of the pressure that I am looking at to really try and translate that into a consequence, and ultimately a risk. So, it might even be that maybe I predicted fluttering for a scenario that is a very low likelihood event and it’s a clean fluid that doesn’t pose any toxic or flammability risks. Therefore, I would have to look at that whole profile before I really make a cut and dry decision on whether or not fluttering would be acceptable.


2. How does the pipe flexibility affect speed of sound impact and how will you use this in a design?

Pipe flexibility decreases the speed of sound. The speed of sound measured using the fluid properties is just the speed of sound in the fluid. Whereas the flexibility or rigidity of the piping and piping supports the system decreases the speed of sound. During our presentation we showed that the Δ P wave term is proportional to the speed of sound in the system. There are different correlations for whether the pipe is anchored at one end, or both ends, or rigidly supported throughout and how to estimate the impact of that on the speed of sound of the system.


3. Do these equations work for liquid too?

Yes. The correlations hold for vapor, liquid and two-phase systems. The important part is making sure you have a good estimate for speed of sound in the fluid and piping arrangement.


4. Can you expand on the wave pressure loss?

The wave pressure loss is a two part equation – the fluid hammer term and the fluid inertia term. The impact on the valve is the wave component of the pressure loss, which is prorated by the tau term and accounts for the pressure losses from our source or reflection point. If we have sudden pipe diameter change that would be a reflection point. We want to quantify by the pressure losses associated with that wave traveling through the linear length of the pipe.


5. If one has 2 different relief design scenarios, one chooses the biggest one but that may cause chattering for other relief scenarios. What would you recommend?

That is a very typical case and if we do have that sort of installation we recommend a multiple valve installation with one valve that is perhaps smaller and set at a lower pressure and then a larger valve set at a higher pressure. We also want to be careful to avoid acoustic interactions between the two because we don’t want them fighting each other and causing each other to be instable so we want those set pressures to have a decent margin between them. We will usually have a smaller lower set valve to account for those smaller scenarios and then a larger valve to account for our design or controlling case.


6. How are acceleration losses due to flashing flow handled in the analysis?

The Δ P wave term is based on the mass flow rate. For the steady-state estimate, you need to average your fluid densities and speed of sound due to the flashing flow.


7. What stability effects could be expected when the inlet piping has a concentric reducer?

This is similar to the previous question that we would be accelerating the fluid through that concentric reducer and may need to consider flashing / condensing and averaging the density and speed of sound.


8. How are acceleration losses due to flash and flow handled in the analysis?

I believe that the answer to the question is that they are ignored and that the delta P wave term is simply the math flow rate. And what you would need to do is get the average of your fluid densities and your speed of sound due to that flashing and flow – is how that would be captured. The acceleration would still be – or recovery on the other side would still be ignored because the frictional component of the wave is still just the frictional losses in a piping system. So that Δ P wave term only includes the velocity head loss effect and the Τ.


9. What stability effects could be expected when the inlet piping has a concentric reducer?

This is similar to the previous question that we would be accelerating the fluid through that concentric reducer. What we’re seeing is that larger diameter piping will actually act as a reservoir and so that might have beneficial effects if we have that larger diameter piping. Regardless of the historical thought that is lowering the frictional losses but it might also give us a little bit of the damping of the fluid as it travels to the valve.