Blowdown in PSV | Most Simple Explanation EVER | Pressure Relief Valve | Safety Relief Valve

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Blowdown in PSV is the difference between set pressure and reseating pressure of a safety valve expressed as a percentage of set pressure.

Blowdown in Pressure Relief valve is Typical defined in codes and standards as -7% and -10%, and is ranging from -4% to -20% depending on the code and service (steam, gas or liquid).

Let us take an Analogy of Bus, to understand the Concept of why blowdown exists. Now Here is a standstill bus , Let us imagine to move the Bus it takes 10 Newtons, i.E to move the bus in motion.

Now imagine this bus is running, If we put exactly the same force in opposite (So lets call it minus 10 Newtons). Will the bus stop right there, No right you might have to put some more efforts to stop it ( Le say -12 Newtons)

Keep this analogy in mind let us see what happens in relief valve. Here is a relief valve on the left side of the picture and the graph of percentage lift (on Y-axis) versus Pressure (On X-axis)

Now we see the system pressure is rising and eventually reaches Set pressure, this would cause the relief valve to open

Now imagine the relief valve is completely open and the process pressure is reducing.

Now like the Bus example when we had put -10 Newtons it was not sufficient to stop the Bus already in Action.

Similarly reducing the pressure to set pressure would not be sufficient to bring the seat of relief valve to rest.

The pressure must be further reduced ( Like our example of Bus to -13 Newtons)

The closing pressure where the relief valve seat rests is lower than the set pressure.

And this difference is called blowdown.

The Next Question is Why is blowdown so IMPORTANT and Why Process Engineers are very particular about Blowdown Percentage.

Let us understand this with a Hypothetic case

Let the set pressure be 100 Bar

And the Operating pressure be 85 Bar

Now under normal operating circumstances at 85 Bar the relief valve is closed.

However, when the pressure rises at 100 bar and beyond the relief valve opens and releases the process fluid to relieve the pressure of the system.

Now Imagine the Blowdown percentage of Valve is 20 %

i.e 20% less of Set pressure ( i.e 100 Bar) = 80 BAR

That means at the 85 BAR (When the process pressure has reduced to normal operating pressure ) the Relief valve is still open.

This is absolutely not desired.

Thus the process department provides percentage blowdown.

For this application we can select a relief valve with Blowdown percentage as 10 % but a blowdown percentage of 20% will not be acceptable.