7 replies to this topic

[bobbobs]

Hi All,
I have come across the term "blow through" recently in PHAs and on p&ids in regards to relief valves. Would someone please explain to me what that term means? For example I have seen it refer to insuring that a relief valve has been sized adequately for steam blow through from a vaporizer. Thanks for any help.

[latexman]

Assume you are using air (or nitrogen) to push a liquid out of a pressure vessel. The liquid level goes lower, and lower, and lower. Eventually the liquid/air interface goes into the bottom nozzle and down the outlet pipe. That is a type of "blow through".

In general "blow through" is when the normal fluid, usually a liquid, that leaves a piece of equipment, somehow changes to something else, usually a gas/vapor. The liquid usually runs out for some reason or the other.

[fallah]

Hi All,
I have come across the term "blow through" recently in PHAs and on p&ids in regards to relief valves. Would someone please explain to me what that term means? For example I have seen it refer to insuring that a relief valve has been sized adequately for steam blow through from a vaporizer. Thanks for any help.

The similar term indicated same phenomena titled "Gas Blow-By".

[bobbobs]

Thanks latexman and fallah. Can blow through also refer to the maximum flow that can occur? A coworker had described to me a blow through situation occurring when there is a control valve failure that moves to the fail open position therefore causing the maximum flow to be seen.

[latexman]

No, that is usually referred to as "control valve fail open" or "regulator fail open". I can see how this failure could lead to a "blow through", but I would group it as a "control valve fail open" since that was the initial failure.

Edited by latexman, 19 January 2010 - 10:19 AM.

[fallah]

Thanks latexman and fallah. Can blow through also refer to the maximum flow that can occur? A coworker had described to me a blow through situation occurring when there is a control valve failure that moves to the fail open position therefore causing the maximum flow to be seen.

Usually the fail position of control valves in the situations may subject to Gas Blow-By should be FC (fail close) to prevent wide opening of the valves leading to maximum flow.

Of course,these valves usually equipped with Internal Mechanical Stop to prevent the valve being wide open in the case of FIC malfunction.

Assume you are using air (or nitrogen) to push a liquid out of a pressure vessel. The liquid level goes lower, and lower, and lower. Eventually the liquid/air interface goes into the bottom nozzle and down the outlet pipe. That is a type of "blow through".

In general "blow through" is when the normal fluid, usually a liquid, that leaves a piece of equipment, somehow changes to something else, usually a gas/vapor. The liquid usually runs out for some reason or the other.

Should we use two phase or gas relief across the PSV for this scenario?

[latexman]

That will depend on the specific scenario and equipment you have. If the transition from liquid to gas is going to be very short, you should look at the liquid case and gas case and that's good enough. However, if the transition from liquid to gas is going to be very long and you anticipate 2 phase flow for a considerable time, then you should look at it on that basis. How long is too long? Let's say you size the PSV for gas or liquid, not 2 phase flow. If the 2 phase transition is over before the overpressure gets to the limit in your design case, typically 10% overpressure, I would stick with the gas or liquid design.