outlet piping pressure drop and built-up backpressure 2

[Mous1747]

the PSV outlet relieves to the atmosphere, so immediately i know the backpressure is 0 psig

however the outlet pipe + fitting will develop a pressure drop of 120 psig

my question is, what is my backpressure ? is it 0 or 120 ?

API 520 1.2.3.3 (J) built-up back pressure is the increase in pressure at the outlet of a pressure relief device that develops as a result of flow after the pressure relief device opens

so my actual question is whether the outlet pipe pressure drop is considered backpressure?

the turbulent flow generated at the outlet fitting is counted as backpressure ?

 

[Mous1747]

i just found this interesting discussion

https://www.eng-tips.com/viewthread.cfm?qid=126305

 

[Mous1747]

https://www.eng-tips.com/viewthread.cfm?qid=181789

after reading the above two threads i am more confused
if someone can summarize an answer that would be very helpful

 

[georgeverghese]

Total backpressure on the PSV exit is 120psig. To develop such a high backpressure, you must be running at high velocity with a small dia exit pipe, or the exit line may be long. What is the PSV setting, and is this a conventional PSV?
At this high velocity, you may have noise / acoustic induced vibration failure risks also- have you checked?

 

[don1980]

mous1747 - There are two components to the total backpressure on a PSV. One is the superimposed backpressure (O psig in this case), and the other is built-up backpressure (120 psig in this case). So, the backpressure (total backpressure) on this PSV is 0 + 120 = 120 psig.

Since this PSV is routed to atm, I'm going to guess that it's a conventional PSV. Conventional PSVs are allowed to have a built-up backpressure that's no higher than the amount of the overpressure. If the PSV is set at the MAWP (and it's not a fire case) then that limit (the amount of overpressure) is 10%. If the PSV is set below the MAWP then the built-up backpressure limit is greater than 10%. The limiting value is the amount of overpressure for that specific installation.

So, here's an example of a case in which this 120 psig built-up backpressure would be acceptable. Suppose the MAWP is 300 psig, and the allowable accumulation is 330 psig. And suppose the PSV is set at 210 psig. This means the overpressure limit for this PSV is 120 psi. Thus this PSV is limited to 120 psi of built-up backpressure. If these were the details for the installation we're discussing, then the installation is perfectly acceptable with 120 psi built-up backpressure. The built-up backpressure is excessively high only when it exceeds the allowable overpressure. So, compare the built-up backpressure for this case(120 psi) against this PSV's overpressure to determine whether it's too high.

 

[Mous1747]

@ georgeverghese : yes it's a conventional PSV set at 615 psi and routed to atmosphere, outlet pipe is 4 in sch 40, 20 ft long and 26815 scfm. so the outlet pressure drop is calculated to be 119.3 psig
which will be 19.4% of set pressure and that's higher than the 10% overpressure limit. No i actually never thought about the acoustic induced vibration.


@don1980 : yes the MAWP of the vessel equals the set pressure of the PSV at 615 psi , so yeah it's only 10% overpressure.

would you change this conventional PSV to bellows type just because this 9.4 % difference ?

 

[Latexman]

Time to talk details with the PSV manufacturer, IMO. They will probably recommend the change.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[Mous1747]

but why the pressure drop due to friction losses in the discharge piping is counted as backpressure?

 

[Latexman]

Put a straw between your lips and blow as hard as you can. What do you feel? That's backpressure due to friction losses.

Atmospheric pressure is so vast it's essentially fixed. Therefore, the pressure on the PSV/mouth end must rise.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[RVAmeche]

A conventional PSV works by a pressure differential across the valve. When the valve opens and tries to relieve at it's capacity, excessive backpressure ( > 10% or manufacturer recommendation), this higher pressure downstream of the valve will limit the flow. Think of it from a pipe flow perspective: you're trying to get the same flow (rated capacity) through the same size area (orifice) with less pressure (due to high backpressure). If you haven't already, API 520 is a very useful document to read when beginning to look at relief valves.

 

[LittleInch]

Errr, 120 psi overpressure is nearly 20% of your 615 psi MAWP. You therfore exceed the allowable pressure. Balanced bellows won't help.

Larger vent pipe I think.

That sure is a big pressure drop for only 20 ft pipe.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

Agreed, you can either select (a) a bigger exit pipe or (b) move out to a bellows operated or pilot operated PSV, with suitable corrections for backpressure in the sizing calcs. A bellows operated PSV setpoint remains fixed at backpressures ranging up to 75%-80% of set pressure (see API520 or 521). The way you've got it now, this conventional PSV will reach the required relief flow only at 615+120 = 735psig, which is way more than the 10% allowable exceedence permitted in relief pressure per ASME.
Depending on which country this plant is located, the company Owners could get dragged into court or industrial tribunal and fined / jailed / forced to shutdown the plant for process safety related Code violations such as this, so get all your calcs and conclusions verified and approved in writing by a senior engineer formally appointed by your Company for checking relief systems modifications.

 

[e43u8]

@georgeverghese
A ballanced bellows PSV can withstand the back pressure almost up to 50% of the set pressure but with considering back pressure correction factor for PSV capacity...
Higher back pressure, even till 100%, would be tolerated by pilot operated PSV...

 

[Latexman]

Mous1747,

I assumed this is gas flow; scfm units. What's the Mach Number in outlet pipe?

Using the data available:
26815/60 = 447 scfs
447/0.0884 = 5055 ft/sec ~ Mach 4.5

What methodology are you using for pressure drop?

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[Mous1747]

yes it's a natural gas and i agree , 6" outlet pipe rather than the existing 4" would take care of this issue but i would prefer just to convert this same valve to bellows type. as per the PSV OEM this valve can be converted to bellows type and should handle this 20% built-up back pressure.

The hydraulic model is calculating the pressure drop in the discharge piping using the relief’s valve max flow capacity, was made by pipe-flo compressible 2007 and Mach# .6895

 

[georgeverghese]

Standard design practice is to keep vent line velocity at <0.7Mach throughout the vent header up till the point of release.

 

[LittleInch]

Mous1747,

Maybe it's me, but I think you're missing / ignoring the big picture here which is this

"and should handle this 20% built-up back pressure"

Yes a BB PSV will prevent this rather high back pressure from interfering with the opening of the valve, but at whatever your max flow case is through this valve, the pressure in the vessel you are trying to protect will be 20% ABOVE its MAWP. Therefore you would potentially be operating outside the recognized maximum pressure limits for the vessel for a full flow relief case. And all for 20 feet of 6" pipe???? A BB relief valve will not solve the overpressure during full flow issue.

The limit in ASME VIII is 10%.

Don 1980 "Conventional PSVs are allowed to have a built-up backpressure that's no higher than the amount of the overpressure. If the PSV is set at the MAWP (and it's not a fire case) then that limit (the amount of overpressure) is 10%. "

Georgeverghese "PSV will reach the required relief flow only at 615+120 = 735psig, which is way more than the 10% allowable exceedence permitted in relief pressure per ASME."

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[don1980]

All - This isn't relevant to the OP's question in this specific case (because the PSV is set at the MAWP) but I'm adding this because I sense there's some misunderstanding about the limit for built-up backpressure. Engineers commonly make the mistake of thinking that conventional PSVs have a fixed 10% limit for built-up backpressure. That's only true when the PSV is set at the MAWP. It's not true when the PSV is set below the MAWP. To understand this, consider the reason for limiting the built-up backpressure on conventional PSVs. Built-up backpressure is limited to ensure that there's a positive force balance on the valve disk. If the built-up backpressure is too high, the force balance becomes negative, causing the valve to re-close prematurely. As long as the built-up backpressure doesn't exceed the overpressure, the force balance is positive. In most cases, PSVs are set at the MAWP, at which point the overpressure is 10% (same as the accumulation). And when PSVs are set at the MAWP the built-up backpressure is indeed limited to 10% (because the overpressure is 10%).

But PSVs are not always set at the MAWP. Suppose the MAWP is 10 barg and the PSV is set at 5 barg. In that case the allowable overpressure is 6 barg (the peak pressure for the vessel is 11 barg). Thus the allowable built-up backpressure is 6 barg (equal to the overpressure), or 120% of set pressure. At first you may be thinking that this can't be right - won't the 120% built-up backpressure cause the PSV to re-close prematurely? The answer is no, because this backpressure is being offset by the overpressure. The pressure inside the vessel is also rising by 6 bar, which exactly offsets the 6 bar on the outlet side. If you do the math, and calculate the pressure drop across the PSV at the two different cases, one in which the PSV is set at the MAWP (10% overpressure) and the other case in which the PSV is set at 5 barg (120% overpressure), you'll see that the dP across the PSV is exactly the same in both cases.

This explains why API 520 Pt I (5.3.3.1.3) says that the allowable built-up backpressure is limited to 10%, or the allowable overpressure, whichever is greater.

 

[chicopee]

The discharge pipe diameter should match or be greater than the diameter of the valve outlet in order for the safety valve to work properly. If the discharge pipe is of the same diameter as the valve outlet than the pipe length should be within some limit which would be listed in the installation instruction or catalog specs. The back pressure is the pressure at the outlet under the valve relieving condition and that back pressure is dependent on the diameter and length of the discharge pipe.

 

[Latexman]

i would prefer just to convert this same valve to bellows type. as per the PSV OEM this valve can be converted to bellows type and should handle this 20% built-up back pressure.

Sounds like a good plan.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.